choose, install, test & service brakes. Disc- & drum-brake sign. Brake materials for racing or street. Air cooling & water ceding. Proportioning valves & balance bar Practical da'ta & formulas. Fred Puhn Fred Puhn Registered Professional Engineer 3 1.Corvette rotor-and-caliper assembly by Girlock, modifled for racing by Tilton Engineering. 2.Neal Products brake-pedal, balance-bar, bracket & dual master-cylinder assembly; Airheart master cylinders shown. 3.Tempilaq temperature-sensing paint from Big-Three Industries. 4.Alston proportioning valve. HPBooks@ 11ArlDBOOK 3 Fred Puhn RegisteredProfessional Engineer 4 1.Corvette rotor-ano-caliper assembly by Girlock, modifiedfor racing by Tilton Engineering. 2.Neal Products brake-pedal , balance-bar, bracket & dual master-cylinder assembly; Airheart master cylinders shown. '._ _ _..- '.- --3.Tempilaq temperature-sensing paintfrorn Big"Three Industries; 4.Alston proportioning valve..... I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction3 1Basics................................................. 4 2Drum Brakes.......................................... 15 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Disc Brakes23 4Friction Material....................................... 34 5Hydraulic Systems..................................... 42 6Brake Pedals & Linkages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66 ..........................................7Power Assist77 8Other Types ofBrakes................................. 82 9High-Performance Brakes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88 1 0 Testing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 ........................................1 1Maintenance1 21 ........................................12Modifications137 Trouble-Shooting Guide..............................1 66 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Suppliers List1 69 Reference Tables....................................1 71 Index...............................................174 THANKS Thankstothemany brake suppliers and experts whohelped withtechnicalinformation andphotographs.Special thankstoMacTiltonofTiltonEngineering,BillNeal ofNeal Products, and JohnMoore ofAPRacing fortheirpersonal contributions. Thanksalso to CarrollSmith ofCarroll Smith Consultingforhelpful suggestionsgiven after reading the finished manuscript. SpecialthankstoGarrettVanCamp,VanCampRacingEnterprises,Inc.,25192 Maplebrooke,Southfield,MI48034.Hisheroiceffortsandgreattechnicalknowledge gained over the yearsas a Ford Motor brake-design engineerand race-carbrake-design consultant made a significant contribution to the content and completeness of this book. It could not have been this quality without him. Thanks Garrett. NOTICE: The information contained in this book is true and complete to the best of our knowledge. All recommendations on parts and procedures are made without any guaranteeson the part of the author or HPBooks. Because thequality of parts, materials and methods are beyond our control, author and publisher disclaim all liability incurred in connection with the use ofthis information. Publisher: Rick Bailey; ExecutiveEditor: RandySummerlin;Editorial Director: TomMonroe, P.E., S.A.E.; Senior Editor: Ron Sessions, A.S.A.E.; ArtDirector: Don Burton; Book Design: Paul Fitzgerald; Production Coordinator: Cindy J. Coatsworth; Typography: Michelle Carter; Director ofManufacturing: AnthonyB. Narducci; Photos: Fred Puhn, others noted; CoverPhoto: Bill Keller Published by HPBooks ADivision ofHPBooks, Inc. P.O. Box 5367, Tucson, A2 857036021888-2150 ISBN 0-89586-232-8Library ofCongress Catalog Number 84-62610 01985 HPBooks, Inc.Printed in U.S.A. 2nd Printing 2 Contents Introduction............................................ 3 1Basics.................................................4 2Drum Brakes.......................................... 1 5 3Disc Brakes........................................... 23 4Friction Material....................................... 34 5Hydraulic Systems.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..42 6Brake Pedals &Linkages............................... 66 7Power Assist.......................................... 77 8Other Types of Brakes................................. 82 9High- Performance Brakes.............................. 88 10Testing.............................................104 11Maintenance........................................1 21 1 2Modifications........................................1 37 Trouble-Shooting Guide..............................166 Suppliers List. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..1 69 Reference Tables. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..1 71 Index...............................................174 THANKS Thankstothemanybrakesuppliersandexpertswhohelpedwithtechnicalinformation andphotographs.SpecialthankstoMacTiltonofTiltonEngineering,BillNealofNeal Products,andJohnMoore ofAPRacingfortheir personalcohtributions.Thanks also to CarrollSmithof CarrollSmithConsultingforhelpfulsuggestionsgivenafter readingthe finished manuscript. SpecialthankstoGarrettVanCamp,VanCampRacingEnterprises,Inc.,25192 Maplebrooke,Southfield,MI48034.Hisheroiceffortsandgreattechnicalknowledge gainedover the yearsasa Ford Motor brake-designengineer andrace-car brake-design consultant made a significant contribution to the content and completeness of this book. It could not have been this quality without him. Thanks Garrett. NOTICE: The information contained in this book is true and complete to the best of our knowledge. All recommendations onparts and procedures are made without any guarantees on the part of the author or HPBooks. Because the quality of parts, materials and methods are beyond our control, author and publisher disclaim all liability incurred inconnection with the use ofthis information. Publisher:RickBailey;ExecutiveEditor:RandySummerlin;EditorialDirector:TomMonroe,P.E.,SAE.; Senior Editor:RonSessions,A.S.A.E.;ArtDirector:DonBurton;BookDesign:PaulFitzgerald;ProductionCoordinator: CindyJ.Coatsworth;Typography:MichelleCarter;Director ofManufacturing:AnthonyB.Narducci;Photos:Fred Puhn,othersnoted;CoverPhoto:BillKeller PublishedbyHPBooks ADivisionofHPBooks,Inc. P.O.Box5367,Tucson,AZ85703602/888-2150 ISBN0-89586-232-8LibraryofCongressCatalogNumber 84-62610 1985HPBooks,Inc.PrintedinU.S.A. 2ndPrinting Whendriving down along hill on acrowded freeway, a situation can occur that demands good brakes. Many ofthe cars in this photo are going over 55 mph. If an emergency were to happen, the resulting chain-reaction braking would result in some drivers locking the wheels. Mostofusonlythinkaboutbrakes when a panic stop occurs ahead in traf- ficandallweseearebrakelightsand theundersidesofcars.Thesenear- emergenciesillustratehowimportant brakesaretooursafety.Brakesare alsoavitalpartofhighperformance, as any racer can tell you. Because everyone wants higher per- formanceand safety, brakes deserve a greatdealofattention.Wenotonly wantourcartogo fast,butitshould alsostopquicklyandsafely.Anycar withpowerfulandconsistentbrakes instillsconfidenceinthedriver.It alsoincreasesdrivingpleasure.Bad brakes are terrifying. Ifracingis yourgame,youneedto knowmoreaboutbrakesthanthe problems, too. Italkabout brakesas asystem. This includesfluid,lines,pedals,levers andlinkages,aswellasthebrake units.Wheels,bodyworkandeven theframestructurebecomeapartof thebrakesystemwhentheyaffect brakeperformance.Thisbookcovers each partofthe system and how it re- latestooverallbrakeperformance.It willhelpintheselectionofcompo- nentsifyouprefertodesignabrake system. Thefirstsectionofthebookdeals withparticularpartsofabrake system.Thesecondsection,starting withChapter9,describeshowto design,install,test,maintainand modifyabrakesystemforracing casualdriver.NO matterwhattypeofapplications. ace-barconceptsalso Brakingisessentialinwinningraces. racingyoudo,brakeperformanceisapplytohigh-performanceroadcars areOnaroad course-themost severe duty forautomo- vital.Roadracingismost demandingvehicles.Ialsonotewheret'hereare tivebrakes.Notice smokefromtheright onbrakes,althoughdragracingandimportantdifferencesbetweenracingfronttireoncarthat's braking hardinthe oval-trackeventshavespecialand roaduse.corner. Introduction Whendrivingdownalonghillonacrowdedfreeway,asituation can occur that demands goodbrakes.Manyofthecars inthisphotoare going over 55 mph. If an emergency were to happen, the resulting chain-reaction braking would result in some drivers locking the wheels. Mostof usonlythinkaboutbrakes when apanic stop occurs ahead intraf-ficandallweseearebrakelightsand theundersidesofcars.Thesenear-emergenciesillustratehowimportant brakesaretooursafety.Brakesare alsoavitalpartof highperformance, as anyracer can tellyou . Because everyone wants higher per-formanceandsafety,brakes deservea greatdealofattention.Wenotonly wantourcartogofast ,butitshould alsostopquicklyandsafely.Anycar withpowerfulandconsistentbrakes instillsconfidenceinthedriver.It alsoincreasesdrivingpleasure.Bad brakes are terrifying. If racingisyourgame,youneedto knowmoreaboutbrakesthanthe casualdriver.Nomatterwhattypeof racingyoudo,brakeperformanceis vital.Roadracingismostdemanding onbrakes,althoughdragracingand oval-trackeventshavespecial problems,too. Italkaboutbrakesasasystem.This includesfluid,lines,pedals,levers andlinkages,aswellasthebrake units.Wheels,bodyworkandeven theframestructurebecomeapartof thebrakesystemwhentheyaffect brakeperformance.Thisbookcovers eachpartof the systemandhowitre-latestooverallbrakeperformance.It willhelpintheselectionofcompo-nentsifyouprefertodesignabrake system. Thefirstsectionofthebookdeals withparticularpartsofabrake system.Thesecondsection,starting withChapter9,describeshowto design,install,test,maintainand modifyabrakesystemforracing applications.Race-carconceptsalso applytohigh-performanceroad vehiCles.Ialsonotewherethereare importantdifferencesbetweenracing and road use. Brakingisessentialinwinningraces. These stockcarsarecompetingonaroad course-the mostsevere dutyfor automo-tivebrakes.Noticesmokefromtheright fronttireoncarthat'sbrakinghardinthe corner. 3 Good brakes are essential to overall vehicle performance. Even though engine performance, suspension and body aerodynamics approach perfection, race cars, such as this GTP Corvette, will not be competitive without good brakes. Photo by Tom Monroe. Most ~roductionsedans have drum brakes Brakesystemsaredesignedtodo onething-stopthevehicle.Sounds easy,butproblemsstartwhenbrakes muststopavehiclefromhighspeed in a short distance, and do it over and overagain.Weexpectnofailuresor lossofcontrol.Allbrakesystems shouldstopavehicle.Thedifference betweena good system and abadone ishowwellitwillperformunderthe most adverse conditions. Allvehicleshavebrakes,andthey always did. Ever since man discovered the wheel,stoppingitwasaproblem. Carts,wagonsandcarriageshad brakes,usuallysimple blocks rubbing onawheel.Thisestablishedabasic thathasyetto change,even withthe mostsophisticatedbrakesystem:All brakes,thefrictionmaterialsrub againstmetalsurfaces. Differenttypes ofbrakesarearrangeddifferently,or . .usedifferentmethods offorcingrub- bingsurfacestogether.There are also differencesindissipatingheatonce it is generated. Eitherdrumbrakes ordisc brakes,or a combinationofthe two,are usedon mostvehicles.Thesetermsreferto how friction surfaces are designed and configured. DrumBrakes-Allearlyvehicles useddrumbrakes;manyoftoday's vehiclesstilldo.The rubbingsurface is a metal cylinder called a brake drum, usually made of cast iron. Earlydrumbrakeswereexter- nal-rubbingsurfacewasoutsideof ~-- -ontherearanddiscbrakesonthefront,brakes usefiictionto stop the vehicle.the drum. More moderninternal drum Drum brakes are preferredon rearwheels brakes have the rubbing surface inside because a parking brake is easily adapted. BRAKE TYPES Thislarge drumisontherearofa2-ton t hedrum.Thereareshoes insidethe sedan. Whentwopartsrubtogether,thedrumwithfrictionmaterialattached. resultingfrictiongeneratesheat.InThis friction materialis calledlining. It BasicsI Good brakes are essential to overall vehicle performance.Even thoughengine performance, suspension andbody aerodynamics approach perfection, race cars, such as this GTP Corvette, will not be competitive without good brakes. Photo by TomMonroe. Mostproductionsedanshavedrumbrakes ontherearanddiscbrakesonthefront. Drumbrakesarepreferredonrearwheels because aparking brakeiseasily adapted. Thislargedrumisontherearofa2-ton sedan. 4 Brakesystemsaredesignedtodo onething-stopthevehicle.Sounds easy,butproblemsstartwhenbrakes muststopavehiclefromhighspeed inashort distance,anddoitover and overagain.Weexpectnofailuresor lossofcontrol.Allbrakesystems shouldstopavehicle.Thedifference betweenagoodsystemandabadone ishowwellitwillperformunderthe most adverse conditions. Allvehicleshavebrakes,andthey alwaysdid. Ever since mandiscovered thewheel ,stoppingitwasaproblem. Carts,wagonsandcarriageshad brakes,usuallysimpleblocksrubbing onawheel.Thisestablishedabasic thathasyettochange,evenwiththe mostsophisticatedbrakesystem:All brakes use friction tostop the vehicle. BRAKE TYPES Whentwopartsrubtogether,the resultingfrictiongeneratesheat.In brakes,thefrictionmaterialsrub againstmetalsurfaces.Different types ofbrakesarearrangeddifferently,or usedifferentmethodsof forcingrub-bingsurfacestogether.Therearealso differencesindissipatingheatonceit is generated. Eitherdrumbrakes ordiscbrakes,or acombinationof thetwo,areusedon mostvehicles.Thesetermsreferto how frictionsurfaces aredesigned and configured. DrumBrakes-Allearlyvehicles useddrumbrakes;manyoftoday's vehiclesstilldo.Therubbingsurface isametalcylinder called abrake drum, usually made of cast iron. Earlydrumbrakeswereexter-nal-rubbingsurfacewasoutsideof thedrum.Moremoderninternal drum brakes havetherubbing surface inside thedrum.Thereareshoesinsidethe drumwithfrict ionmaterialattached. This frictionmaterialiscalledlining.It II Honda disc brake is typical of front brakes usedonsmallsedans.Exposedrubbing surface of disc brake aids cooling. isdesignedtorubagainstthedrum withoutburning,meltingorwearing rapidly.Theshoesareforcedagainst theinsidesurfaceofthedrumwhen thedriverpushesthebrakepedal, creatingfrictionbetweenthelining andthedrumsurface.Drumbrakes are covered in Chapter 2. DiscBrakes-Amodernbrake designisthediscbrake.The drumis replacedbyaflatmetaldisc,orrotot; witharubbingsurfaceoneachside. The rotoris usuallymade of cast iron. Frictionmaterialsare insideacaliper, which surrounds the rotor. Disc-brake frictionmaterial-oneoneachside of therotor-iscalledabrakepad,puck orlining.Thiscaliperisdesignedto clamp the pads against the sides of the rotortocreatefriction.Discbrakes are covered in Chapter 3.BRAKE-ACTUATINGSYSTEM Betweenthedriver'sfootandthe wheelbrakesarecomponentsthat translateforcefromthedriverinto frictionforceatthebrake-rubbing surfaces.Icallthistheactuating system. This systemcanbemechani- cal,hydraulic,pneumatic or acombi- nation ofthese.Future vehicles could useelectricsystems.Whateverthe typeofactuatingsystem,the resultis thesame:Whenthedriveroperates the system, brakes are applied. BrakePedal&Linkage-Brake pedalsandlinkagesareintegralparts Earliest automotive brakes were drum type with the rubbing surface on the outside of thedrum.Becausefrictionmaterialsur- rounds the outside ofthe drum,little cool- ingaircontactsthehotrubbingsurface. External drum brakes are simple and easy to service, but have horrible cooling ability. Prewar MG used mechanical brakes. Front brakes are operated by cables that flex as thewheelssteerandmove up anddown. Finned aluminum drums give better cooling than plain cast iron. P = Clamping Force on Rotor II Disc brake operates by clamping rotor between two stationarypads. Rotor turns with the wheel; caliper is mounted t o a fixed part of the suspension, usually the spindle or upright. Axle Centerline of a brake system. The pedalis the fa- miliarleverthatthedriverpushes withhisfoottoapplythebrakes. Regardlessofthetypeofbrake- actuating system used, system applica- tionalwaysbeginswiththedriver operatingapedal-orleverinrare cases.Brake-pedaldesigndetermines thelegforcerequiredtostop the car. Stud Itisalsoafactorindetermining how solid the brakes feel to the driver. Trade-offs are made when designing brakepedals.Longpedalsreducethe pedalforce requiredto stop a vehicle. However,longpedalshavelong travel. They can also feel spongy to the driver.Brake-pedaldesignisdetailed in Chapter 6.-\Rotor Hondadisc brakeistypicalof front brakes usedonsmallsedans.Exposedrubbing surface of disc brake aids cooling. isdesignedtorubagainstthedrum withoutburning,meltingorwearing rapidly.Theshoesareforcedagainst theinsidesurfaceofthedrumwhen thedriverpushesthebrakepedal, creatingfrictionbetweenthelining andthedrumsurface.Drumbrakes are covered in Chapter 2. DiscBrakes - Amodernbrake designisthediscbrake.Thedrumis replacedbyaflatmetaldisc,orroto;; witharubbingsurfaceoneachside. Therotorisusuallymadeof castiron. Frictionmaterialsareinsideacaliper, which surroundstherotor.Disc-brake frictionmaterial-oneoneachside of therotor- iscalledabrakepad,puck orlining.Thiscaliperisdesignedto clampthepadsagainst the sides of the rotortocreatefriction.Discbrakes are covered in Chapter 3. BRAKE-ACTUATING SYSTEM Betweenthedriver'sfootandthe wheelbrakesarecomponentsthat translateforcefromthedriverinto frictionforceatthebrake-rubbing surfaces.Icallthistheactuating system.Thissystemcanbemechani-cal,hydraulic,pneumaticoracombi-nationof these.Future vehicles could useelectricsystems.Whateverthe typeof actuatingsystem,theresultis thesame:Whenthedriveroperates the system, brakes are applied . BrakePedal&Linkage-Brake pedalsandlinkagesareintegralparts Earliest automotive brakes were drum type withtherubbingsurfaceontheoutsideof thedrum.Becausefrictionmaterialsur-roundsthe outsideofthe drum,little cool-ingaircontactsthehotrubbingsurface. Externaldrumbrakesaresimpleandeasy to service, but have horrible cooling ability. p PrewarMGusedmechanicalbrakes.Front brakes areoperatedbycablesthatflexas thewheelssteerandmoveupanddown. Finnedaluminum drums give better cooling than plain cast iron. Caliper P =Clamping Force on Rotor Caliper Mounting Bolt - - - Axle Centerline Wheel Stud ~ R o t o r Discbrakeoperatesbyclampingrotorbetween two stationarypads.Rotorturnswith the wheel; caliper is mounted to a fixed part of the suspension, usually the spindle or upright. of abrake system. Thepedalisthe fa-miliarleverthatthedriverpushes withhisfoottoapplythebrakes. Regardlessofthetypeofbrake-actuating systemused, system applica-tionalwaysbeginswiththedriver operatingapedal-orleverinrare cases.Brake-pedaldesigndetermines thelegforcerequiredtostopthecar. Itisalsoafactorindetermininghow solid the brakes feeltothe driver. Trade-offs aremade when designing brakepedals.Longpedalsreducethe pedalforcerequiredtostopavehicle. However,longpedalshavelong travel.They can alsofeelspongy tothe driver.Brake-pedaldesignisdetailed in Chapter 6. 5 Althoughmostmechanicallyactuated brakes are found on antique cars, some are stillbeingused.Developed byAPRacing foruseoncompetitionrallycars,this moderncaliperismechanicallyactuated. Independent ofthehydraulically actuated brakes, these calipers are used on the rear forhigh-speedcontrolonslicksurfaces. Photo courtesy AP Racing. Inthe'209,Duesenbergintroducedhy- draulicbrakeswithlargefinneddrums. Thiswasprobablythefirstroad carwith enoughhorsepower thatrequired agreat improvement in braking. I tdid not use flex hoses, but instead ran fluid through internal passages in the suspension. Thebrakepedalisconnectedtoa linkagethattransfersforcetotheac- tuating system. This linkage can be as simple asapush/pullrodoperating a singlehydraulicmastercylinder.Or, the linkagemaybea complicated,ad- justablebalance-bar system for chang- ingthe balancebetweenfront and rear brakes.Early-design,mechanical- actuatinglinkageextendsalltheway tothebrakesthemselves.Brake- linkagedesignisdiscussedinmore detail in Chapter 6. MechanicalBrakes-Thesimplest brake-actuatingsystemisamechani- calsystem.Thebrakepedaloperates cablesorrodsthatapplythebrakes whenthepedalispushed.Earlysys- Early race cars had no front brakes. Although this Peugeot was a winnerwith its advanced high-speed dual-overhead-cam engine, it used cable-operated rear-wheel brakes. temsweremechanicalandarestill usedforparkingbrakesonpresent- dayvehicles.Themechanicallinkage movestheshoesoutwardinadrum brake,orclampsthepadsagainsta disc-brake rotor. HydraulicBrakes-Moderncarsuse hydraulic brakes.In a hydraulicallyac- tuatedsystem,thecablesorrodsof the mechanicalsystem are replaced by fluid-filled lines and hoses. The brake: pedallinkageoperatesapistonina mastercylindertopressurizethefluid inside the lines and hoses. Fluid pres- sure in each wheelcylinderforces the frictionmaterialagainstthedrumor rotor.SeeChapter5foradetailed explanation ofhow a hydraulic system works. Pneumat i c Brakes-Inapneumatic, orair-brake,systemthebrakesare controlledbycompressedair.Air brakesaregenerallyusedonlarge commercialvehiclesandtrucks.An advantageofthepneumaticbrake systemissafety.Smallleakscannot causeatotallossofbrakingbecause airisconstantlysuppliedbyacom- pressorandstoredinlargevolume. Pneumaticbrake-systemoperationis described briefly in Chapter 8. BRAKE HISTORY Theearliestbrakeswerederived fromthoseusedonhorse-drawn wagons.Ascarsbecameheavierand morepowerful,theseprimitive brakessoonwereimprovedtothe earlyexternal-typedrumbrakes,all with mechanical-actuatingsystems. Earlybrakeswereontherear wheels only. The major reason for this wasthedifficultyindesigninganac- tuatingsystemonwheelsthatare steered.Engineersavoidedtheprob- lembyomittingfront-wheelbrakes. Anotherreasonfornotusingfront brakes wasconcern that the car might tipoveronitsnoseiffrontbrakes were applied hard! Earlyexternal-typedrumbrakes usedabandoffrictionmaterialout- side the drum. his type ofbrakewas easytodesign,butthe frictionmate- rialpreventedthe drum from cooling. Also,exposedfrictionmaterialswere subjecttodirt,oilandwater contamination. When the brakeshoes wererelocatedinsidethedrum,a moderndrumbrakewasborn.These were first used on the 1902 Renault. Althoughfour-wheelbrakingwas triedearlyinthe20thcentury,most earlycarshadrearbrakesonly. Then, inthe1920sitwasdiscoveredthat frontbrakesaddedgreatlytoacar's stopping ability,andtheywere judged safe.Four-wheelbrakesystemssoon became universal. Mechanical-actuatingsystemswere stillusedonmostcarsuntilthelate '20swhenhydraulicsystemscame intouse.Mechanicalbrakeswere used for auto racing long after hydrau- Althoughmostmechanicallyactuated brakes are found on antique cars, some are stillbeingused.DevelopedbyAPRacing foruseoncompetitionrallycars,this moderncaliperismechanicallyactuated. Independentofthehydraulicallyactuated brakes, these calipers are used on the rear forhigh-speedcontrolonslicksurfaces. Photo courtesy AP Racing. Inthe'20s,Duesenbergintroducedhy-draulicbrakeswithlargefinneddrums. Thiswasprobablythefirstroadcarwith enoughhorsepowerthatrequiredagreat improvementinbraking.It did not useflex hoses, but instead ran fluid through internal passages in the suspension. Thebrakepedalisconnectedtoa linkagethattransfersforcetotheac-tuatingsystem.Thislinkagecanbe as simpleasapush/pullrodoperatinga singlehydraulicmastercylinder.Or, thelinkagemaybeacomplicated,ad-justablebalance-bar system forchang-ingthe balancebetween front and rear brakes.Early-design,mechanical-actuatinglinkageextendsalltheway tothebrakesthemselves.Brake-linkagedesignisdiscussedinmore detail in Chapter 6. MechanicalBrakes-Thesimplest brake-actuatingsystemisamechani-calsystem.Thebrakepedaloperates cablesorrodsthatapplythebrakes whenthepedalispushed.Earlysys-6 Earlyrace cars had no front brakes.Although thisPeugeot was a winner with its advanced high-speed dual-overhead-cam engine, it used cable-operated rear-wheel brakes. temsweremechanicalandarestill usedforparkingbrakesonpresent-dayvehicles.Themechanicallinkage movestheshoesoutwardinadrum brake,orclampsthepadsagainsta disc-brake rotor. HydraulicBrakes-Moderncarsuse hydraulicbrakes.In ahydraulically ac-tuatedsystem,thecablesorrodsof themechanical system are replaced by fluid-filledlines andhoses. The brake'-pedallinkageoperatesapistonina mastercylindertopressurizethefluid insidethelinesandhoses. Fluidpres-surein eachwheelcylinder forcesthe frictionmaterialagainstthedrumor rotor.SeeChapter5foradetailed explanation of howahydraulic system works. PneumaticBrakes-Inapneumatic, orair-brake,systemthebrakesare controlledbycompressedair.Air brakesaregenerallyusedonlarge commercialvehiclesandtrucks.An advantageofthepneumaticbrake systemissafety.Smallleakscannot causeatotallossofbrakingbecause airisconstantlysuppliedbyacom-pressorandstoredinlargevolume. Pneumaticbrake-systemoperationis described briefly in Chapter 8. BRAKE HISTORY Theearliestbrakeswerederived fromthoseusedonhorse-drawn wagons.Ascarsbecameheavierand morepowerful,theseprimitive brakessoonwereimprovedtothe earlyexternal- typedrumbrakes,all with mechanical-actuating systems. Earlybrakeswereontherear wheels only.Themajor reasonforthis wasthedifficultyindesigninganac-tuatingsystemonwheelsthatare steered.Engineersavoidedtheprob-lembyomittingfront-wheelbrakes. Anotherreasonfornotusingfront brakeswasconcernthatthe carmight tipoveronitsnoseiffrontbrakes were appliedhard! Earlyexternal-typedrumbrakes usedabandoffrictionmaterialout-sidethedrum.Thistypeof brakewas easytodesign,butthefrictionmate-rialpreventedthedrum fromcooling. Also,exposedfrictionmaterialswere su bjecttodirt,oilandwater contamination.When thebrake shoes wererelocatedinsidethedrum,a moderndrumbrakewasborn.These were first used on the 1902 Renault. Althoughfour-wheelbrakingwas triedearlyinthe20thcentury,most earlycarshadrearbrakesonly.Then, inthe1920sitwasdiscoveredthat frontbrakesaddedgreatlytoacar's stoppingability,andtheywere judged safe.Four-wheelbrakesystemssoon became universal. Mechanical- actuatingsystemswere stillusedonmostcarsuntilthelate '20swhenhydraulicsystemscame intouse.Mechanicalbrakeswere usedforauto racinglong after hydrau-II Internal details ofearly Duesenberg hydraulic drum brake:Notice fluid passages through axle, kingpin and spindle. Sealing was a problem eventually solved by use of flexible hydraulic lines. Thislittlediscbrakestarteditall-first discbrake used on amass-produced car. Goodyear-Hawleydiscbrakeswereof- fered on Crosley Hotshot and Supersports roadstersintheearly'50s.ACrosley HotshotwonthefirstSebring12-hour endurance race with thesebrakes.In later years,theywere popular for small sports- racing cars. lic systems were developedfor passen- ger cars. The simple mechanicalbrake systemwasreliable,easytounder- stand andmaintain, andnot subjectto suddenlossofbrakingthatcould happentoahydraulicsystemwitha failed line or seal. AfterWorldWar11,discbrakes begantoappear.The firstproduction carwithdiscbrakeswasthe1949 CrosleySupersport.Discbrakeswere usedsuccessfullyonthe24- hours-of-LeMans-winningJaguarin Since the mid-'50s,disc brakes have been highly developed forboth racing and road use. Companies such as JFZEngineered Products have takenbrake development farbeyond the early disc-brake concept. Rotors, pads and brake-mounting hardware have all benefited from rigors of faster race cars. Photo courtesy JFZ Engineered Products. the'50s.Discbrakessoonbecame popularonmanyracecars.Indiana- polis500carsuseddiscbrakesearly too,buttheyhadlittleeffectonthe outcomeofracesonthisfasttrack. OnlyatLeMans,wherecarsmust deceleratefrom180 to30 mphevery lap,anddoitfor24hours,weredisc brakestestedtotheirlimit.Evenin racing,drumbrakesarestillusedin certainclasses,butmostmodernrace cars use disc brakes. Beyondthebasicchangesinbrake design,therehavebeenmanyimpor- tantimprovements.Brakesystems todayareverysafe;andcomplete systemfailureoccursrarely.Modern brakescangoforyearswithlittleor noattentioninhighwayuse,but thereinliesaproblem:Whenreally needed, performance maybe marginal becauseofinattention. This bookwill help youkeep that from happening. FRICTION & ENERGY Frictionisresistancetosliding.Any Internal details of earlyDuesenberghydraulic drumbrake:Notice fluidpassages through axle, kingpin andspindle.Sealingwas a problem eventually solved by use of flexible hydraulic lines. Thislittlediscbrakestarteditall-first discbrakeusedonamass-producedcar. Goodyear- Hawleydiscbrakeswereof-feredonCrosleyHotshot and Supersports roadstersintheearly'50s.ACrosley HotshotwonthefirstSebring12-hour enduranceracewiththesebrakes.Inlater years,theywerepopularfor smallsports-racing cars. licsystems were developed forpassen-ger cars.The simplemechanicalbrake systemwasreliable,easytounder-stand andmaintain,andnot subjectto suddenlossofbrakingthatcould happentoahydrauli csystemwitha failedline or seal. AfterWorldWarII,discbrakes begantoappear.Thefirstproduction carwithdiscbrakeswasthe1949 CrosleySupersport.Discbrakeswere usedsuccessfullyonthe24-hours-of-LeMans-winningJaguarin .. Since the mid-'50s,discbrakeshavebeen highlydevelopedforbothracingandroaduse. CompaniessuchasJFZEngineeredProductshavetakenbrakedevelopmentfarbeyond the early disc-brake concept. Rotors, pads and brake-mounting hardware have all benefited from rigors of faster race cars. Photo courtesy .IFZ Engineered Products. the'50s.Discbrakessoonbecame popularonmanyracecars.Indiana-polis500carsuseddiscbrakesearly too,buttheyhadlittleeffectonthe outcomeofracesonthisfasttrack. OnlyatLeMans,wherecarsmust deceleratefrom180to30mphevery lap,anddoitfor24hours,weredisc brakestestedtotheirlimit.Evenin racing,drumbrakesarestillusedin certainclasses,but mostmodernrace cars use discbrakes. Beyondthebasicchangesinbrake design,therehavebeenmanyimpor-tantimprovements.Brakesystems todayareverysafe;andcomplete systemfailureoccursrarely.Modern brakescangoforyearswithlittleor noattentioninhighwayuse,but thereinliesaproblem:Whenreally needed,performance maybe marginal becauseof inattention. Thisbookwill help you keep that fromhappening. FRICTION &ENERGY Frictionisresistancetosliding.Any 7 - Friction Force Friction between theboxand flooris what makestheboxdifficulttoslide.Ifbox weightorfrictionbetweenthefloorand box increases, so must the force to slide it. Heatisdevelopedonslidingsurfacesas box is moved. twoobjectsincontactwithandtrying tomoverelativetoeachot herhave friction. Itcanbehighorlow depend- ing on t he types of surfaces in contact. Frictionhelpskeepyourfeetfrom slidingout fromunderyou.When you are standing onice,friction is low and itis difficult to prevent slipping. Iftwosurfacesincontactare sliding,t hefrictioncreatesheat.You canconfirmthisbyrubbingyour KINETIC ENERGY OF ROTATION Strictlyspeaking,anobjectwith kinetic energycan be eithermoving i n a straightline or rotating about its owncenterofgravity(CG).Ina speedingcar,kineticenergyis mostly i n the moving car. Unless the cari s spinningdowntheroad,less than10% ofthetotalkinetic energy i s stored i n rotating parts ofthe car. Rotating parts include tires, wheels, brakes,engine anddriveline. Addi- tionalkinetic energystored in these rotatingpartsmustbeabsorbedi n the brakes. However, to make calcu- lationssimpler,Iignorethesmall amountofkineticenergystoredin rotating parts. Athighspeed,kineticenergy stored i n the rotating tire-and-wheel assembliesincreasessignificantly. Ifyouhitthebrakeshardathigh speed,therotatingpartsmustbe stoppedbeforethewheelcanlock andslide.Ittakestimeandpedal efforttostopthisrotatingweight, evenifthecardoesn'tslowatall. Consequently,iti s moredifficultto lockthewheelswhentravelingat higherspeeds.Ironically,this makesacarsaferathighspeedif thedriverpanicsandhitsthe brakes toohard. However, i n racing, itincreasespedaleffortasthe driver tries to reach the traction limit of the tires. Type of EnergyExample HeatEnergy stored i n a hot brake rotor. SoundNoise from exhaust. LightLight from headlights. Stored MechanicalEnergy stored i n a compressed coil spring. ChemicalEnergy i n a gallon of gasoline. ElectricalCurrent from a battery turning a starter. RadiationMicrowave energy i n a microwave oven. KineticEnergy stored i n a speeding bullet. Energy can be changed from one form to another,but it can't be created or destroyed. Here aresomedifferentformsofenergy.Mostformsaretransformedintoheatafterenergy does its useful work. handstogetherrapidlybackand forth. Youcan feel t he warmth.Thi s friction canhelpwarmyourhandsonacold day.I nbrakes,frictionisusedto createheat.The processofcreating heat stops t he car. Theamount offrictionbetween tworubbingsurfacesdependsont he materialsandtheirroughness. Theamount offrictionisdescribedbya number calledt hecoefficientof.friction.Ahighnumbermeans alarge amount offriction;alownumbermeansasmallamount offriction. Readmor e aboutfriction at t he begin- ning of Chapter 4. Energyist he abilitytodo work.A movingcardevelopsenergy.Thefasteritmoves, t hemor eenergyit develops. Thi s type of energy is called ki neti c energy. When speed is doubled,fourtimest hekineticenergyis developed. That is,kineticenergy varies as t he square of speed.To calcu- late kinetic energy of a car, use the fol- lowing formula: W,S2 Kinetic energy = - 29.9 in foot-pounds (ft-lb) W, = Weight of Car in pounds (Ib) S=Speedofcari nmilesperhour (mph) Convert i ngEnergy-Thefirstlawof thermodynamicssays:Energycan neverbecreatednordestroyed. However,energycanbeconverted from one form to another.Differentforms ofenergyare heat, sound, light,storedmechanical, chemical,electricalandradiated. Storedelectricalenergyinabattery willconvertenergyintoheatorlight byconnectingt hebatterytoalight bulb.Storedmechanicalenergyina springcanbeconvertedtokinetic TEMPERATURE & HEAT The difference between temperature andheatmaybeconfusing.Weall arefamiliarwithtemperature,mea- suredi neitherdegreesFarenheit (F) or degrees centigrade (C). Heati saformofenergy.When heati s added toamaterial,its tem- peraturerises;whenheati s removed,itstemperaturedrops. Thus,temperaturei stheeffectof addingorsubtractingheat energy. WhenIsaysomethingheatsup,I meanheatisadded.WhenIsay cool s off,I mean heat is removed. In eithercase,thetemperatureofthe object changes. Imeasurekineticenergyinfoot- pounds(ft-lb). However,i t couldbe measuredi nBritishThermalUnits (BTU's),justasengineersdo.One BTU ISthe amount ofheat it takes to raise the temperature ofonepound ofwaterbyonedegreeFarenheit. OneBTUi sequalto778ft-lbof energy,oroneft-lbequals0.0013 BTU.Althoughonepoundofwater changestemperatureonedegreeF whenoneBTUofheatisadded, othermaterialsdonotreactthe same.Theirtemperaturechangeis differentwhenoneBTUofheatis added. Therelationshipbetweentem- peraturechangeandheat-energy changei sgovernedbyaproperty calledspecificheat.Eachmaterial hasi ts ownspecificheatas shown fortypicalmaterialsintheaccom- panyingtable.Specificheatisthe temperatureriseforonepoundof materialwhenoneBTUofheatis added. Amaterial'sspec~f i cheatis veryimportanttoabrake-design engineerforcalculatingbrake- temperature change for each stop. Ideally,brakesshouldbemade frommaterialswithahighspecific heat.Thiswouldresultinasmall temperature rise for agiven amount ofkineticenergyputintothe brakes.Asmalltemperaturerise means thebrakes wouldhave fewer problems. .. Friction Force Frictionbetweentheboxandflooris what makestheboxdifficulttoslide.Ifbox weightorfrictionbetweenthefloorand box increases, so must the force to slide it. Heatisdevelopedonslidingsurfacesas box is moved. twoobjectsincontactwithandtrying tomoverelativetoeachotherhave friction.It canbehighor lowdepend-ingon thetypes of surfaces in contact. Frictionhelpskeepyourfeetfrom slidingoutfromunderyou.When youare standing onice,frictionislow and itisdifficult toprevent slipping. Iftwosurfacesincontactare sliding,thefrictioncreatesheat.You canconfirmthisbyrubbingyour 8 KINETIC ENERGY OF ROTATION Strictlyspeaking,anobjectwith kinetic energycanbeeithermoving inastraight line or rotating about its owncenterofgravity(CG).Ina speedingcar,kineticenergyis mostlyinthe movingcar.Unlessthe carisspinningdowntheroad,less than10%ofthetotalkineticenergy isstoredinrotatingpartsof the car. Rotatingpartsinclude tires,wheels, brakes,engineanddriveline.A d d i ~ tionalkineticenergystoredinthese rotatingpartsmustbeabsorbedin thebrakes.However,to make calcu-lationssimpler,Iignorethesmall amountofkineticenergystoredin rotating parts. Athighspeed,kineticenergy storedin the rotating tire-and-wheel assembliesincreasessignificantly. Ifyouhitthebrakeshardathigh speed,therotatingpartsmustbe stoppedbeforethewheelcanlock andslide.Ittakestimeandpedal efforttostopthisrotatingweight, evenifthecardoesn'tslowatall. Consequently,itismoredifficultto lockthewheelswhentravelingat higherspeeds.Ironically,this makesacarsaferathighspeedif thedriverpanicsandhitsthe brakestoohard.However,inracing, itincreasespedaleffortasthe driver tries toreachthe tractionlimit of the tires. ExampleType of Energy Heat Sound Energy stored ina hot brake rotor. Noise from exhaust. UghtLight fromheadlights. Stored Mechanical Chemical Electrical Radiation Energy stored in a compressed coilspring. Energy in agallon of gasoline. Kinetic Current from a battery turning a starter. Microwave energy in a microwave oven. Energy stored ina speeding bullet. Energy can be changed from one form to another,but it can't be created or destroyed. Here aresomedifferentformsofenergy.Mostformsaretransformedintoheatafterenergy does its useful work. handstogetherrapidlyback andforth. Youcanfeelthe warmth.This friction canhelpwarmyourhandsonacold day.Inbrakes,frictionisusedto createheat.Theprocessofcreating heat stops the car. Theamountoffrictionbetween tworubbingsurfacesdependsonthe materialsandtheirroughness.The amountoffrictionisdescribedbya numbercalledthecoefficientof friction.Ahighnumbermeansalarge amountoffriction;alownumber meansasmallamountoffriction. Readmore about frictionatthe begin-ning of Chapter 4. Energyistheabilitytodowork.A movingcardevelopsenergy.The fasteritmoves,themoreenergyit develops.Thistypeof energyiscalled kinetic energy.When speed isdoubled, fourtimesthekineticenergyis developed.Thatis,kineticenergy TEMPERATURE & HEAT The difference betweentemperature andheatmaybeconfusing.Weall arefamiliarwithtemperature,mea-suredineitherdegreesFarenheit (F)or degrees centigrade (Cl. Heatisaformofenergy.When heatisaddedtoamaterial,itstem-peraturerises;whenheatis removed,itstemperaturedrops. Thus,temperatureistheeffectof addingorsubtractingheatenergy. When,Isaysomethingheatsup,I meanheatisadded.WhenIsay cools off,I meanheat isremoved.In eithercase,thetemperatureofthe object changes. Imeasurekineticenergyinfoot-pounds(It-I b).However,itcouldbe measuredinBritishThermalUnits (BTU's),justasengineersdo ..One BTUis the amount of heat it takes to raisethetemperatureofonepound ofwaterbyonedegreeFarenheit. OneBTUisequalto778It-Ibof energy,oroneft-Ibequals0.0013 BTU.Althoughonepoundofwater varies asthe square of speed. Tocalcu-late kinetic energy of a car,use the fol-lowing formula: WcS2 Kinetic energy=--29.9 infoot-pounds (It-Ib) W=Weight of Car inpounds (Ib) Sc=Speedofcarinmilesperhour (mph) ConvertingEnergy-Thefirstlawof thermodynamicssays:Energycan neverbecreatednordestroyed. However,energycanbeconverted from one formtoanother. Differentformsof energyareheat, sound,light,storedmechanical, chemical,electricalandradiated. Storedelectricalenergyinabattery willconvertenergyintoheatorlight byconnectingthebatterytoalight bulb.Storedmechanicalenergyina springcanbeconvertedtokinetic changestemperatureonedegreeF whenoneBTUofheatisadded, othermaterialsdonotreactthe sallle.Theirtemperaturechangeis differentwhenoneBTUofheatis added. Therelationshipbetweentem-peraturechangeandheat-energy changeisgovernedbyaproperty call1edspecificheat.Eachmaterial hasitsownspecificheatasshown fortypicalmaterialsintheaccom-panyingtable.Specificheatisthe temperatureriseforonepoundof materialwhenoneBTUofheatis added.Amaterial'sspecificheatis veryimportanttoabrake-design engineerforcalculatingbrake-temperature change for each stop. Ideally,brakesshouldbemade frommaterialswithahighspecific heat.Thiswouldresultinasmall temperature r isefor agivenamount ofkineticenergyputintothe brakes.Asmalltemperatur erise means thebrakeswou l1dhavefewer problems. MELTINGTE'MPERATURE SPECIFIC HEAT Material Water Beryllium-pure Beryllium-QMV Magnesium-AZ31 6-H24 Aluminum-6061-T6 Aluminum-2024-T3 Carbon-pure Titanium-pure Titanium-B120VCA Magnesium-HK31A-H24 Stainless Steel-304 Cast Iron Steel-C1020 Copper-pure Degrees F 32 2340 2340 1100 1080 940 6700 3070 31 00 1100 2600 2750 2750 1980 Degrees CBTU/ lb/F 01.oo 12820.52 12820.45 5930.25 5820.23 5040.23 37040.1 6 16880.1 4 17040.1 3 5930.1 3 14270.1 2 15100.10 15100.10 10820.09 Specificheats ofvarious materials are listed from thehighest tolowest. Specificheat is amount ofheat energy required toraise onepound ofmaterial byone degree Fahrenheit. Materialwithhighest specificheatisnotnecessarily thebestforbrakes. Tobe agood brake material, it must withstand high temperature,conduct heat rapidly andhave a good rubbing surface. energywhenawind-uptoycaris released.Chemicalenergystoredin gunpowderisconvertedintosound, heat and kinetic energy when ignited. Acarmovingdowntheroadhas kineticenergy.Tostopthecar,you mustdisposeofthiskineticenergy. Becauseenergycannotbedestroyed, itmustbeconvertedto another form. Thiskineticenergycouldbeconvert- ed into any of the forms listed on page 8, butconversionto heatis easiest. By forcingfrictionmaterialagainst drumsorrotors,heatiscreatedand thecarslows.Ifthebrakesortires squeal,somesoundenergyisalso produced,buttheamountofkinetic energyconvertedtosoundissmall compared to heat energy. Ifyouhadanelectriccarusing batteries,youcouldbrakethecarby convertingtheelectricmotorintoa generator.Thiscouldbedoneby switchingtheconnections.The motionofthecarwouldturnthe generator,putting electricenergyinto thebatteries.The carwouldslowbe- causepower-kineticenergyinthis case-turnsthe generator. Some elec- triccarsmaintaintheirbatterycharge byusingthisformofbrakingcalled regeneralive braking. Itwouldbewonderfuliffuelcould beputinthegastankbyhittingthe brakes.Itcan't,however,soallthat kineticenergyislost.Ontheother hand,ifyoudrivemoreslowlyoran- ticipateeverystop,youcouldusethe brakeslessand,thus,conservefuel. Trythiswhiledrivingtowork.The fuelenergyusedtomovethecaris lostinheatenergyeachtimeyouhit thebrakes.Byminimizingbraking, you increase mileage. Ifyouslowthecarwithoutusing thebrakes,thekineticenergycanbe changedintotwodifferentforms.If youare driving on a flatroadandtake yourfootoffthethrottle,the carwill slow.Kinetic energy is lost in air drag, friction(heat)inthe engine and drive line, and rolling resistance of the tires. Because theseitems gethot, kinetic energyisconvertedintoheat. However,becauseslowingtakes longer andthe items beingheatedare muchlarger,drive-linecomponents andtiresdon'treachthehightem- peraturesachievedbythe brakes. But, theheatisthere.Putyourhandon yourtiresaftera fastrun onthe high- wayandsee howtheyfeel. Touchthe rear-axlehousingandthetrans- mission.A portion of the powerof the enginewaslostsupplyingtheenergy to heat those parts. Theotherwayyoucanslowacar withoutthe brakesisbycoastingup a hill.The carlosesenergytodragthe sameasonaflatroad,buti t slows quicker. The kinetic energy is convert- edintopotetltioletiel,gyasthecar climbs the hill. Potentialenergyisjustanother formofstoredmechanicalenergy.It isincreasedwhen a weightisraisedto agreaterheight.Asthecarclimbsa hill,some ofitskineticenergyiscon- vertedto potentialenergy. This poten- tialenergycanbechangedbackinto kineticenergybyallowingthecarto coastdownthehillwiththeengine shutoff.Thespeedatthebottomof thehillwillbelessthantheoriginal Carparkedattopofhillhaszerokinetic energy.However, its position attopofthe hill gives it potential energy. This potential energyischangedintokineticenergyas car coasts down hill. Aftercoasting halfwaydownthehill, car has lost half ofits potential energy, but has gainedkineticenergy.Kineticenergyin- creases as car speed increases. All potential energy ofcoasting caris con- vertedinto kinetic energy atbottom ofthe hill where carreaches maximum speed and kinetic energy. speed when youbeganto coast upthe hill because some energy is lost in fric- tionanddraggoingupthehill,plus the energy lost going down the hill. MELTING TEMPERATURE SPECIFIC HEAT MaterialDegrees FDegrees CBTU/1 b/F Water3201.00 Beryllium - pure234012820.52 Beryllium-QMV23401282045 Magnesium-AZ 31B-H2411005930.25 Aluminum-6061-T610805820.23 Aluminum -2024-T39405040.23 Carbon-pure670037040.16 Titanium-pure307016880.14 Titanium-B 120VCA310017040.13 Magnesium-HK 31 A-H2411005930.13 Stainless Steel-304260014270.12 Cast Iron275015100.10 Steel-C1020275015100.10 Copper-pure198010820.09 Specificheatsofvariousmaterialsarelistedfromthehighesttolowest.Specificheatis amountofheatenergyrequiredtoraiseonepoundof materialbyonedegreeFahrenheit . Materialwithhighestspecificheatisnotnecessarilythebestforbrakes.Tobeagood brakematerial,it mustwithstandhightemperature,conductheatrapidlyandhave agood rubbing surface. energywhenawind-uptoycaris released.Chemicalenergystoredin gunpowderisconvertedint osound, heat and kinetic energy when ignited. Acarmovingdowntheroadhas kineticenergy.Tostopthecar,you mustdisposeofthiskineticenergy. Becauseenergycannotbedestroyed, itmustbeconvertedtoanother form. Thiskineticenergycouldbeconvert-edintoanyof the formslisted onpage 8,but conversion toheat iseasiest.By forcingfrictionmaterialagainst drumsorrotors ,heatiscreatedand thecarslows .Ifthebrakesortires squeal ,somesoundenergyisalso produced,buttheamountofkinetic energyconvertedtosoundissmall compared to heat energy. Ifyouhadanelectriccarusing batteries,youcouldbrakethecarby convertingtheelectricmotorin toa generator.Thiscouldbedoneby switchingtheconnections.The motionofthecarwouldturnthe generator,puttingelectricenergyinto thebatteries.Thecarwouldslowbe-causepower- kineticenergyinthis case - turnsthegenerator.Someelec-triccarsmaintaintheirbatterycharge byusingthisformofbrakingcalled regenerative braking. Itwouldbewonderfuliffuelcould beputinthegastankbyhittingthe brakes.Itcan't ,however,soallthat kineticenergyislost.Ontheother hand,if youdrivemoreslowlyoran-ticipateeverystop,youcouldusethe brakeslessand,thus ,conservefuel. Trythiswhiledrivingtowork.The fuelenergyusedtomovethecaris lostinheatenergyeachtimeyouhit thebrakes.Byminimizingbraking, you increase mileage. Ifyouslowthecarwithoutusing thebrakes,thekineticenergycanbe changedintotwodifferentforms.If youaredriving on aflatroadandtake yourfootoff thethrottle,thecarwill slow.Kineticenergyislostinair drag, friction(heat)inthe engine anddrive line, and rolling resistance of the tires. Becausetheseitems gethot,kinetic energyisconvertedintoheat. However,becauseslowingtakes longerandtheitemsbeingheatedare muchlarger,drive-linecomponents andtiresdon'treachthehightem"-peratures achievedbythe brakes.But, theheatisthere.Putyourhandon yourtiresafter afastrun onthe high-wayandseehowtheyfeel .Touchthe rear-axlehousingandthetrans-mission.Aportion of thepower of the enginewaslostsupplyingtheenergy toheat those parts. Theotherwayyoucanslowacar withoutthebrakesisbycoastingupa hill.Thecarlosesenergytodragthe sameasonafl atroad ,butitslows quicker. The kinetic energy isconvert-edintopotentialenergyasthecar climbs the hill. Potentialenergyisjustanother formofstoredmechanicalenergy.It isincreasedwhen aweightisraisedto agreaterheight.Asthecarclimbsa hill,someof itskineticenergyiscon-vertedtopotential energy.This pote n-tialenergycanbechangedbackinto kineticenergybyall owingthecarto coastdownthehillwiththeengine shutoff.Thespeedatthebottomof thehillwillbelessthantheoriginal Carparkedattopofhillhaszerokinetic energy.However,itspositionattopofthe hillgives it potential energy"This potential energyischangedintokineticenergyas car coasts down hill. Aftercoastinghalfwaydownthehill ,car has lost half of its potential energy, but has gainedkineticenergy.Kineticenergyin-creases as car speed increases. Allpotential energy of coastingcar iscon-vertedinto kineticenergy atbottom of the hillwhere car reaches maximum speedand kinetic energy. speedwhenyoubegantocoastupthe hillbecause some energy islost infric-tionanddraggoingupthehill,plus the energy lost going downthehill. 9 Temperature (" F) / Brakes Released Temperatureof Rubbing Surface Interior ofRotor Material Temperature of Interior of Drum or Rotor Interior ofDrum Rotor Cross Section % Rubbing SurfaceDrum Cross Section T, = Maximum temperature reached atrubbing surface. T, = Average temperature after stop. T, = Starting temperature before stop. I During a stop, brake rubbing-surfacetemperature increases more rapidly than interior tem- perature of a drum or rotor. Eventually, temperatures equalize after brake is released. Aver- age brake temperatureoccursat apointbetween rubbing-surfaceandinterior tempera- tures before much cooling takes place. Potentialenergyismeasuredin units of foot-pounds (ft-lb). Potential- energychangeequalstheweightof the objectmultipliedbythe change in height.Ifa3000-lbcarcoastsupa 100-ft-highhill,i t gains300,000ft-lb ofpotentialenergy. Ifall this energy is convertedbackintokineticenergy, the speedofthe carcanbecalculated from the formula on page8. Bychang- ingtheformulausingalgebra,i tcomes out as follows: inmiles per hour Ep = Potentialenergy ofcar in foot-pounds (ft-lb) W,= Weight ofcar inpounds (Ib) For our example. Speed = 3000 Obviously,thisismuchfasterthan wouldhappenifyoutriedi t witha real car.Thedifferencebetweenthis sampleproblemandarealtestisthe kinetic energy lostin drag and friction. Howhotdobrakesgetduringone stop? -Becausethebrake-rubbing surfaces are heated byfriction,it is im- portantto know whatthe temperature isafterone stop.P.highsurface tem- peraturecancausefadeor damageto abrake.Theproblemisthatheatis constantlybeingtransferredfromthe friction surface of the drum or rotor to airandcoolerinteriormetal.This makest heexactsurfacetemperature difficulttocalculate.However,itis easyto calculatethe averagetempera- tureofa drum or rotorafterone stop. Thedifferencebetweensurfacetem- peratureand average temperatureof a drum or rotoris shown above. Incalculatingthe averagetempera- tureofadrumorrotor,youmust makesome assumptions.Thismakes t he calculationeasierandtheanswer more accurate. Tests have shown that t he following assumptions are valid: Assumethatallheatenergyfrom the stop flowsinto the drum or rotor. Infactthisisanaccurate assumption because the frictionmaterialinsulates the restof the brakefrom heat and the metaldrumorrotorisaverygood heat conductor. Assumedragonthecarfromall sourcesiszero,includingtheeffects ofairdrag,rollingresistanceand engine braking. This is a good assump- tion forstops below100 mphbecause airdragissmallcomparedtobraking forces. Ignorekineticenergystoredin rotating partsofthe car. This assump- tionandthe assumptionofzerodrag Theheavieradrumorrotor,thelowerits temperaturerise duringasinglestop.De- signedfora3000-l bsportscarpowered bya300- HPengine,thislargedrum weighs 22 Ib. causeerrorsinoppositedirections. Thus,theerrorinthebrake- temperature calculation is small. Ignorecoolingofbrakesduring stop.Heatflowintorotorordrum materialisrapidcomparedtocooling time. The first step is to calculate the tem- peraturechangeofthe drum or rotor. Anyincrease intemperature is known astetnperaturerise.Temperaturesare alwaysrelatedtoenergyby change-notbythe absolute values of energyor temperature. Itisimportant tothinkofthisasachangeinenergy causing a change intemperature. Tem- perature riseinthe brakeis causedby akinetic-energyreductioninthe moving car. Foraparticularstop,figurethe changeinkineticenergyusingthe formulaon page8. This resultsinthe following relationship: Kc =Kineticenergychangeinfoot- pounds= K,- K,infoot-pounds K,= Kineticenergybeforethe stop i n foot-pounds K,=Kineticenergyafterthestopi n foot-pounds Obviously,ifthecarcomestoa complete halt,KA iszero. The change inkineticenergyisusedtocompute the temperatureriseofthe brake.For theweightofthebrake,useonlythe weightofthedrumsand/orrotors. Thetemperaturerisemustbeadded to the temperature of the brake before thestoptoobtainfinalbrake temperature. Temperature (OF) ,/ Brakes Released Temper at ure of Rubbing Surface of Drum or Rotor '\..Temperature of "I nterior of Drum or Rotor Mater ial Rubbing Surface Interi or of Rotor _ Materi al Interior of Drum Material Rotor Cross Section Ts L---__~ - - - - - - - - - - - - - - - - - - - - - - - - - -~ Time of StopDrum Cross Section T m =Maximum temper ature reached at r ubbing surface. Ta =Average temperature after stop. T s=Starting temperatur e before stop. During a stop, brake rubbing-surface temperature increases more rapidly than interior tem-perature of a drum or rotor.Eventually, temperatures equalize after brake is released. Aver-agebraketemperatureoccursatapointbetweenrubbing- surfaceandinteriortempera-tures before much cooling takes place. Potentialenergyismeasuredin units of foot-pounds(ft-Ib).Potential-energychangeequalstheweightof theobjectmUltipliedbythechangein height.Ifa3000-1 bcarcoastsupa 100-ft-hi ghhill ,itgains300,000ft-lb of potentialenergy.If al l thisenergyis convertedbackintokineticenergy, thespeedof thecarcanbecalcul ated fromthe formulaon page8.Bychang-ingtheformulausingalgebra,it comes out as follows: f29.9E: Speed=y ~ - - - pinmilesper hour We (mph) Ep =Potential energy of car in foot-pounds(ft-Ib) We =Weight of car inpounds (Ib) For our example, , . , . . " . . ~ " ; " - ~ . . . . . , . . . . . , . . . - , . -Speed=(29.9)(300,000)=55h 3000mp. Obviously,thisismuchfasterthan wouldhappenif youtri edit witha real car.Thedifferencebetweenthis sampleproblemandarealtestisthe kinetic energy lostin drag andfriction. Howhotdobrakesgetduringone stop? -Becausethebrake-rubbing surfaces are heated by friction,itisim-portanttoknowwhatthetemperature isafteronestop.P. highsurfacetem-peraturecancausefadeordamageto 10 abrake.Theproblemisthatheatis constantlybeingtransferredfromthe frictionsurfaceof the drum or rotor to airandcoolerinteriormetal.This makestheexactsurfacetemperature difficulttocalculate.However,itis easytocalcul atetheaverage tempera-tureof a drum or rotor afterone stop'. Thedifferencebetweensurfacetem-perature andaveragetemperatureof a drum or rotor isshown above. Incalculatingtheaveragetempera-tu reofadrumorrotor,youmust makesomeassumptions.Thismakes thecalculati oneasierandtheanswer moreaccurate.Testshaveshownthat the foll owing assumptions are valid: Assumethatallheatenergyfrom thestopfl owsintothedrumorrotor. Infactthisisanaccurateassumpt ion becausethefrict ionmaterialinsulates the rest of thebrakefromheat and the metaldrumorrotorisaverygood heat conductor. Assumedragonthecarfromall sourcesiszero,includingtheeffects ofairdrag,rollingresistanceand engine braking. This is a good assump-tionforstopsbelow100mphbecause airdragissmallcomparedtobraking forces. Ignorekineticenergystoredin rotatingpartsof thecar. Thisassump-tionandtheassumptionof zerodrag Theheavieradrumorrotor,thelowerits temperatureriseduringasinglestop.De-signedfora3000-lbsportscarpowered bya300-HPengine,thislargedrum weighs 22 lb. causeerrorsinopposi tedirections. Thus,theerrorinthebrake-temperature calculation is small. Ignorecoolingofbrakesduring stop.Heatflowintorotorordrum materialisrapidcomparedtocooling time. The firststepisto calculate thetem-peraturechangeof thedrumorrotor. Anyincreaseintemperatureisknown astemperaturerise.Temperaturesare alwaysrelatedtoenergyby change -notby the absolutevalues of energyortemperature. It isimportant tothinkof thisasachangeinenergy causing achange intemperature. Tem-perat ureriseinthebrakeisca usedby akinetic-energyreductioninthe moving car. Foraparticularstop,figurethe changeinkineticenergyusingthe formulaonpage8. Thisresultsinthe followingrelationship: Ke=Kineticenergychangeinfoot-pounds=Ks- KAinfoot-pounds Ks=Kineticenergybeforethestopin foot-pou nds KA =Kineticenergyafterthestopin foot-pou nds Obviously,ifthecarcomestoa completehalt ,KA iszero. Thechange inkineticenergyisusedtocompute thetemperatureriseof thebrake. For theweightof thebrake,useonlythe weightofthedrumsand/ orrotors. Thetemperaturerisemustbeadded tothetemperature of thebrakebefore thestoptoobtainfina lbrake temperature. Thetemperatureriseofthebrakes is calculated as follows: Kc Temperature rise = - 77.8 W,in degrees Fahrenheit (F) W, =Weightofalltherotorsand drums in pounds Forexample,let'scomparethe temperatureriseinthebrakesofa 3500-lbsedanstoppingfrom60rnph tothetemperatureriseofa3500-lb stock-car'sbrakesslowingfrom120 rnph to 60 mph. Firstlet'sfigurethetemperature riseforthe sedan. Assumethe brakes weigh5Ibeach,foratotalbrake weightof20Ib.Calculatethechange inkineticenergyslowingfrom60 rnphtostop. From page8, the kinetic energy of a moving car is: W,S2 Kinetic energy= - 29.9 For our sedan traveling at 60 mph: K, = (3500 lb)(60 1nph)~/29.9 = 421,000 ft-lb. K, = Kinetic energy before stop Kineticenergyafterthestop= 0, becausecarnowhaszerospeed. Changeinkineticenergy= 421,000 ft-lb.Fromtheaboveformulatem- perature rise of the brakes is: (421 000 ft-lb) Temperature rise = (77,i)(20 Forstockcarslowingfrom120 to60 mph, change in kinetic energy is: K, = (3500 I bI (l 20 mphI2 29.9 = 1,686,000 ft-lb K, = (3500 l b)(60 rnphI2 29.9 = 421,000 ft-lb K, = 1,265,000 ft-lb. K, = Kinetic energy after stop K, = Kinetic energy before stop Kc = Kinetic-energy change Noticehowmuchgreaterthis changeinkineticenergyiscompared tothesedan.Eventhoughthespeed reductionwasthesame60mph,the speedreductionoccurring atahigher initialspeedresultedinmuchgreater energyputintothebrakes.The tem- peratureriseforthestockcarinthe race is: Temperaturerise=(1,265,000ft- lb)/(77.8)(20Ib) = 81 3F (434C). However,becauseoflesscooling Tires on thiscarhave reached theirmaximum coefficient offriction. Although a carstops faster if the wheels are not locked, most drivers hit the brakes too hard during a panic stop. This test,being conducted at Goodyear's San Angelo, Texas, test track, is to see how tires react during a panic stop. Photo courtesy Goodyear. time,racing on a medium-speedtrack withshortstraightsmightresultin higherbraketemperaturesthana high-speedtrackwithlongstraights. Bothtemperatureriseperstopand coolingtimearecriticaltobrake performance.Rotorsweighingmore than5 Ibwouldnormallybeusedon a 3500-lbracecar.Heavierrotors reduce the temperature rise per stop. DECELERATION Decele~.a/ion isameasureofhow quicklyacarslows.Deceleration meansslowingthecar-acceleration means speeding it up. Bothaccelerationanddeceleration aremeasuredinunitsofgravity-g's. One g is the force exertedbyan object duetogravityattheEarth's surface-howmuchanobjectweighs whileatrest.One gisalsoameasure ofaccelerationordeceleration-22 rnphpersecond.Zerogoccursina weightlessenvironment. Acceleration is positive; deceleration is negative. Once the brakes are applied,a car is stoppedbythe frictionforcebetween thetiresandtheroad.During braking,frictionactsonthetiresina directionoppositetomovement.The higherthedeceleration,thegreater this friction force becomes. Maximum possibledecelerationoccursatthe maximumcoeflicientof fiictionbe- tween the tires and the road. This hap- pens justas the tires are about to skid. Oncetireslosetractionandskid, decelerationdrops.Readonforan explanationofcoefficientoffriction, or simply ,f,.ictiot7 coeJficient. INERTIA FORCES Morethan200yearsago,SirIsaac Newtonwrotethebasiclawrelatinga forceonanobjecttoits acceleration. Simply stated, Newton'sLaw is: F Acceleration of an object=in g's F=Unbalancedforceonobjectin pounds W= Weight of object in pounds Inthisformula,theforceFcauses acceleration. Ifa carweighing3000Ib hasabrakingforceof- 1500 Ib,the stopping forceis- 1500 Ib+ 3000 Ib = -0.5g. Ift he force pushesina directionto cause the object to speed up, the force ispositiveandaccelerationisaposi- tivenumber.Iftheforcecausesthe objecttoslow,theforceisnegative andaccelerationisnegative.Wecall the negative accelerationdecele~.arion. TheforcereferredtoinNewton's Lawisunbalancedforce.Thatmeans ifthe force is not resistedbyopposing force,t heobjectisfreetomove,or accelerate.IfIpushonatreewitha force of100 Ib,the tree willnotmove. The 100-lb force is resistedbythe tree rootswitha100-lb force,sothereis nounbalancedforceonthetree.To causeacceleration,eitherpositiveor negative,the force on the object must be unbalanced. Tounderstandhowunbalanced forces workon acar,assume youare drivingacaronadragstrip.Atthe start,youletouttheclutchandpush downonthe accelerator,andthe fric- tionforcebetweenthetiresandroad pushesthecarforward.Atthestart, Thetemperatureriseof thebrakes iscalculated as follows: Kc Temperature rise=---77.8 WB indegrees Fahrenheit (F) WB =Weightofalltherotorsand drums inpounds Forexample,let ' scomparethe temperatureriseinthebrakesofa 3500-lbsedanstoppingfrom60mph tothetemperatureriseofa3500-lb stock-car'sbrakesslowingfrom120 mph to60mph. Firstlet'sfigurethetemperature riseforthesedan.Assumethebrakes weigh5Ibeach,foratotalbrake weightof 20lb.Calculatethechange inkineticenergyslowingfrom60 mphtostop.Frompage8,thekinetic energy of a moving car is: WS2 KinetiC energy=_c_ 29.9 For our sedan traveling at 60mph: KB=(3500 Ib)(60 mph)2/29.9 =421,000 ft-Ib. KB=Kinetic energy before stop Kineticenergyafterthestop=0, becausecarnowhaszerospeed. Changeinkineticenergy=421,000 ft-Ib.Fromtheaboveformulatem-perature rise of the brakes is: ._(421,000 ft-Ib) Temperature rise - (77.8)(20 Ib) =270F (132C) Forstockcarslowingfrom120to60 mph, change inkinetic energy is: K=(3500 Ib)(1 20 mph)2 B29.9 =1,686,000 ft-Ib K=(3500 Ib)(60 mph)2 A29.9 =421,000 ft-Ib Kc=KB- KA Kc=(1,686,000)- (421,000) Kc=1,265,000 ft-Ib. KA=Kinetic energy after stop KB=Kinetic energy before stop Kc=Kinetic-energy change Noticehowmuchgreaterthis changeinkineticenergyiscompared tothesedan.Eventhoughthespeed reductionwasthesame60mph,the speedreductionoccurringatahigher initialspeedresultedinmuchgreater energyputintothebrakes .Thetem-peratureriseforthestockcarinthe race is: Temperaturerise=(1,265,000ft-Ib)/(77.8)(20Ib)=813F (434C). However,becauseoflesscooling Tiresonthiscarhavereachedtheir maximum coefficient of friction.Although acarstops faster if the wheels are not locked, most drivers hit the brakes too hard during a panic stop. This test,being conducted at Goodyear's San Angelo, Texas, test track, is to see how tires react during a paniC stop. Photo courtesy Goodyear. time,racingon amedium-speedtrack withshortstraightsmightresultin higherbraketemperatu resthana high-speedtrackwithlongstraights. Bothtemperatureriseperstopand coolingtimearecriticaltobrake performance.Rotorsweighingmore than5Ibwouldnormallybeusedon a 3500-lbracecar.Heavierrotors reduce the temperature riseper stop. DECELERATION Decelerationisameasureofhow quicklyacarslows.Deceleration meansslowingthecar-acceleration means speeding itup. Bothaccelerationanddeceleration aremeasuredinunitsof gravity-g's. One gisthe forceexertedbyanobject duetogravityattheEarth ' s surface- howmuchanobjectweighs whileatrest.Onegisalsoameasure ofaccelerationordeceleration - 22 mphpersecond.Zerogoccursina weightlessenvironment.Acceleration ispositive; deceleration is negative. Once thebrakes areapplied,a car is stoppedbythefrictionforcebetween thetiresandtheroad.During braking,frictionactsonthetiresina directionoppositetomovement.The higherthedeceleration,thegreater this frictionforcebecomes. Maximum possibledecelerationoccursatthe maximumcoefficientof /rictionbe-tween the tires andthe road. This hap-pens just asthetires are abouttoskid. Oncetireslosetractionandskid, decelerationdrops.Readonforan explanationofcoefficientoffriction, or si mply frictioncoefftcient. INERTIA FORCES Morethan200yearsago,SirIsaac Newtonwrotethebasiclawrelatinga forceonanobjecttoitsacceleration. Simply stated, Newton's Law is: Acceleration of anobject=~ing's F=Unbalancedforceonobjectin pounds W=Weight of object inpounds Inthisformula,theforceFcauses acceleration.If acarweighing3000Ib hasabrakingforceof-1500 lb,the stopping forceis-1500 Ib-7- 3000Ib =-0.5 g. If theforcepushesinadirectionto causetheobjecttospeedup,the force ispositiveandaccelerationisaposi-tivenumber.Iftheforcecausesthe objecttoslow,theforceisnegative andaccelerationisnegative.Wecall the negative acceleration deceleration. TheforcereferredtoinNewton's Lawisunbalanced force.Thatmeans if the forceisnotresistedbyopposing force,theobjectisfreetomove,or accelerate.IfIpushonatreewitha forceof 100 Ib,thetree willnot move. The 100-lb force isresistedbythe tree rootswitha100-lbforce,sothereis nounbalancedforceonthetree.To causeacceleration,eitherpositiveor negative,the forceonthe objectmust be unbalanced. Tounderstandhowunbalanced forcesworkonacar,assumeyouare drivingacaronadragstrip.Atthe start,youletouttheclutchandpush downontheaccelerator,andthefric-tionforcebetweenthetiresandroad pushesthecarforward .Atthestart, 11 air drag is zero,so the forward force is notresistedbyanything but drive-line frictionandtiredrag,orrolling resistance.Theforwardforceis almostunbalanced.Consequently,ac- celerationis maximum at the start. Ast hecargainsspeed,airdrag increases.Itopposes the forward force on the tires thatis trying to accelerate thecar.Theunbalancedforceisthe forwardforceminustherearward force.Asspeedincreases,theunbal- ancedforcebecomessmallerbecause of air drag. Consequently, acceleration also gets smaller. Neartheendofthedragstrip,the carismovingso fastthatthe air-drag forceapproachesthe forwardforceof thetiresagainsttheroad.Assuming enginerpmandtracklengtharen't limitingfactors,speedwillincrease until no force unbalance exists and ac- celerationbecomeszero.The carhas reachedits maximumspeed. The only waytoacceleratethecartoahigher speed would be to increase the forward force(more enginepower),orreduce therearwardforce(lessairdrag). Everyrace-cardesignerknowsthat morepowerorlessdragwillincrease car speed. Inaddition tothe basicrelationship betweenforceandacceleration, Newtoncame up withother important lawsofnature.t l ediscoveredthat everymovingobjecthasinertia.That is, an object always moves at the same speedandinthe same directionuntil actedonbyan unbalanced force. Iner- tiaiswhatkeepstheEarthmoving around t he Sun and keeps satellitesin orbit.Thereisnoairdraginouter space,so once anobjectis atspeed, it keepsmovingforever.Onlyaforce can change its speed or direction. AcaralsoactsaccordingtoNew- ton'slaws.Onceitismoving,acar wantstocontinueina straightline at the same speed.Everypartofthe car anditspassengersalsowanttokeep moving. When the brakes are applied, aforceisappliedtothecarbythe tires, causing deceleration. Becausepassengerstendtocon- tinueatthesamespeed,theywill moveforwardintheseatandstrike theinstrumentpanelunlessheldby restraintdevices,legsorfriction againsttheseat.Thisforwardforce thattends to "throw"a passengerfor- wardduringbrakingiscalledinertia ,force.Inertiaforceactson everything inacarasitsspeedchanges. - Friction Forces on Tires- --- Tirefrictionforcesareexternalforcesthatcausedeceleration. Theroadpushes on the tiresin theopposite direction ofmotion. lnertia force of the car acts in the same direction as motion and equals friction force. Therefore,ifacaraccelerates,the inertiaforceactstowardthe rear;ifi tdecelerates,theinertiaforceacts forward. Inertiaforceing'sisequaltocar's accelerationordeceleration.Inertia forcesareeasytocalculateinpounds if you know the acceleration. W = Weight of theobject in pounds a = Acceleration of the object in g's Inertiaforce ismeasuredin pounds. If thecarisdecelerating,theinertia forceisnegative-itactsinaforward direction. Tryvisualizingwhathappenswhen a car decelerates. Itis easiertovisual- izetheforcesifyouimaginea driver tryingtostop so quicklythathelocks thewheels.Imagineacarskidding withall four wheels locked and smoke pouringoffthetires.Assumethe car weighs3000Ibandthecoefficientof lnertiaforcesactoneverypartofacar. Thispassengerresists inertia forceonhis bodywithhisarmsagainstthedash, feet against thefloor, andseatbelt aroundhis waist.lnertia force on his hat is not resist- ed byanything, soit flies forward into the windshield. Acceleration of car = F/Wc F = Unbalanced forceon car in ~ounds frictionbetweenthetiresandroadis Wc = Weight of car in pounds 0.7. The friction force on all four tires Acceleration of car=00 lb)/(3000 is:Ib) = -0.7 g. Friction force = p FN Theunbalancedforcehasaminus p= Coefficient offrictionbetween two signbecausei t actsi nadirection sliding surfaces F, = Forcepushingthetwosurfaces opposite to the car. Remember: Nega- togetherin pounds tiveaccelerationmeansdeceleration; it causes the car to slow. In this example, -- - - ~~-~ Becausedecelerationis0.7 E.everv - ,p= 0.7 and FN= 3000 Ibpartof the car has an inertia force on it Friction force= (0.7)(3000 Ib) = 2100 of0.7g. The inertiaforce on a 200-lb Ib.passengeris: lnertia force = Wa This frictionforceonthe tirestries tow=Weightofthepassengerin decelerate the car. The decelerationis pounds easytocalculatefromNewton'slaw, a= Accelerationofthepassengerin page11:g's airdragiszero,sothe forwardforceis notresistedbyanything but drive-line frictionandtiredrag,orrolling resistance.Theforwardforceis almostunbalanced.Consequently,ac-celeration is maximum at the start. Asthecargainsspeed,airdrag increases.It opposes the forwardforce onthetiresthatistrying toaccelerate thecar.Theunbalancedforceisthe forwardforceminustherearward force.Asspeedincreases,theunbal-ancedforcebecomessmallerbecause of air drag.Consequently, acceleration also gets smaller. Neartheendof thedragstrip,the carismovingso fastthattheair-drag forceapproachestheforwardforceof thetiresagainsttheroad.Assuming enginerpmandtracklengtharen't limitingfactors,speedwillincrease untilno forceunbalance exists and ac-celerationbecomeszero.Thecarhas reacheditsmaximumspeed.The only waytoacceleratethecartoahigher speed would be toincrease the forward force(moreenginepower),orreduce therearwardforce(Jessairdrag). Everyrace-cardesignerknowsthat morepowerorlessdragwillincrease car speed. Inadditiontothebasicrelationship betweenforceandacceleration, Newton came upwith other important lawsofnature.Hediscoveredthat everymovingobjecthasinertia.That is,anobject alwaysmoves atthe same speedandinthesamedirectionuntil acted on byan unbalanced force.Iner-tiaiswhatkeepstheEarthmoving aroundtheSunandkeepssatellitesin orbit.Thereisnoairdraginouter space,soonceanobjectisatspeed,it keepsmovingforever.Onlyaforce can change its speed or direction. AcaralsoactsaccordingtoNew-ton'slaws.Onceitismoving,acar wantstocontinueinastraightlineat thesamespeed.Everypartof thecar anditspassengersalsowanttokeep moving.When thebrakes are applied, aforceisappliedtothecarbythe tires, causing deceleration. Becausepassengerstendtocon-tinueatthesamespeed,theywill moveforwardintheseatandstrike theinstrumentpanelunlessheldby restraintdevices,legsorfriction againsttheseat.Thisforwardforce that tends to"throw" a passenger for-wardduringbrakingiscalledinertia force.Inertiaforceactsoneverything inacarasitsspeedchanges. 12 .. Inertia Force Friction ForCes onTires Tirefrictionforcesareexternalforcesthatcausedeceleration.Theroadpushesonthe tiresinthe opposite direction of motion.Inertia force of the car acts in the same direction as motion and equals friction force. Therefore,ifacaraccelerates,the inertiaforceactstowardtherear;if it decelerates,theinertiaforceacts forward. Inertiaforceing'sisequaltocar's accelerationordeceleration.Inertia forcesareeasytocalculateinpounds if you know the acceleration. W=Weight of the object inpounds a=Acceleration of the object in g's Inertia forceismeasured inpounds.If thecarisdecelerating,theinertia forceisnegative-itactsinaforward direction. Tryvisualizingwhathappenswhen acardecelerates.Itiseasiertovisual-izetheforcesif youimagineadriver tryingtostopsoquicklythathelocks thewheels.Imagineacarskidding withallfourwheelslocked and smoke pouringoff thetires.Assumethecar weighs3000lbandthecoefficientof frictionbetweenthetiresandroadis 0.7.Thefrictionforceon allfourtires is: Friction force =f.LFN f.L=Coefficient of frictionbetween two sliding surfaces FN=Forcepushingthetwosurfaces together inpou nds In this example, f.L=0.7 and FN=3000lb Frictionforce=(0.7)(3000Ib)=2100 lb. This frictionforceonthetirestries to deceleratethecar.The decelerationis easytocalculatefromNewton'slaw, page11: Inertiaforcesactoneverypartofacar. Thispassenger resistsinertia forceonhis bodywithhisarmsagainstthedash,feet againstthefloor,andseatbelt aroundhis waist.Inertia force on his hat isnot resist-edbyanything,soit fliesforwardinto the windshield. Acceleration of car =FIW c F=Unbalanced force oncar in pounds We=Weight of car inpounds Accelerationofcar=(-2100 Ib)/(3000 Ib)=-0.7 g. Theunbalancedforcehasaminus signbecauseitactsinadirection oppositetothe car.Remember:Nega-tiveaccelerationmeansdeceleration; it causes the car to slow. Becausedecelerationis0.7g,every part of the car has aninertia force on it of 0.7g.Theinertiaforceona200-lb passenger is: Inertia force=Wa W=Weightofthepassengerin pounds a=Accelerationofthepassengerin g's If there were some wayt o support a car ati tscenterofgravity(CG),i t wouldbe balanced.Rotatethecart oanother position,suchasonaside,andit would remain balanced.TheCG is theonly sup- port point where balance exists regardless of position. Inertia force = (200 Ib)(-0.7g) = -1 40 Ib. Theminussignmeanstheforceis forward.Ifthereislittlefrictionbe- tweent he passenger andt he seat,the 140-lbinertiaforceactsonhisseat belt or legs. In this example,t he car decelerates atonly0.7 g. Witht he wheelslocked andsliding,thecoefficientoffriction islowerthanitsmaximumpossible value.Maximumdeceleration of a car pis determined bymany factors, includ- ingtires,aerodynamicforces,road conditionandbrake-systemdesign. Mostcarsonstreettirescanreach about0.8-gdecelerationondry pavement.Race cars can decelerate at well over 1 g. WEIGHT TRANSFER A car'sinertiaforce acts at its center of graviry,or CG,of thewhole car, The CG is the point about whichthe entire carisbalanced.Ifyoucouldhangthe carbyacableattachedatitsCG,the car wouldbalance in any position. TheCGisthecenterofallthe weight. All the inertia forces on the in- dividual parts oft he car addedtogeth- erarethesameasasingleinertia forceforthewholecaractingatits CG. Becauseacar'sCGisalways abovetheroad,inertiaforcefrom brakingalwaystriestoloadthefront tiresandlifttherears.Thiseffectis calledweight transfer. Weighttransfermeansthatthe fronttiresareloadedmoreduringa stop,andthereartiresare unloaded. Afront-wheel-drive car,such as this VW,has majority of weight at front. When brakes are applied hard, more weightis transferredt o the front.Thenose dropsand tail rises during braking due t o weight transfer. Ff = vertical force on front tires (Ib) F, = vertical force on rear tires (Ib) wc P YYCgFf =Wc-F, +- Xcg = horizontal distance from front axle to ICG (in.) I = wheelbase length (in.)F,= wc xc;- WCP Ycg II Ycg = CG height (in.) Wc = Car weight (Ib) p. = Coefficient of friction Weight transfer=Wc f i Ycq I Ignoringaerodynamicforces,theseforcesactonacarduringhardbraking.Maximum weight transfer can be calculated if ti re coefficient of friction is known. The weightofthe wholecardoes not change.Weightaddedtothefront tiresissubtractedfromthereartires duringweighttransfer.Theforces actingonacarduringbrakingare shownintheaccompanyingdrawing. Inthis simple illustration, aerodynam- ic forces are notshown. Aerodynamic forceschangetheamountofthe forces,butnotthebasicprincipleof weight transfer. Becauseweighttransferloadsthe fronttires,additionalfrictionforce canbedevelopedbythefronttires beforetheyskid.Toproducethis extrafrictionforce,front brakes have toworkharderthaniftherewasno weighttransfer.Atthe same time,the rearbrakescandolesswork. Typically,frontbrakessupplyabout two-thirds ofthetotalbraking force in ahardstop. The ratioisevenhigher onanextremelynose-heavycar.Be- cause the frontbrakesdo mostofthe work,theyneedto belarger thanrear brakes.The forcesareusuallyhigher on frontbrakes,so theymustabsorb more heat energy. If there were somewaytosupport acar at itscenterofgravity(CG),itwouldbe balanced.Rotatethecartoanother position,suchasonaside,anditwould remainbalanced.TheCGistheonly sup-portpoint where balance exists regardless of position. I nertia force=(200 Ib) (-0. 7 g) =-140Ib. Theminussignmeanstheforceis forward.Ifthereislittlefrictionbe-tweenthepassenger andthe seat,the 140-lbinerti aforceactsonhisseat belt or legs. Inthisexample,thecardecelerates atonly0.7g.Withthewheelslocked andsliding,thecoefficientof friction islowerthanitsmaximumpossible value.Maximumdecelerationof acar lisdeterminedbymany factors,includ-ingtires,aerodynamicforces,road conditionandbrake-systemdesign. Mostcarsonstreettirescanreach about0.8-gdecelerationondry pavement.Racecarscandecelerate at wellover 1 g. WEIGHT TRANSFER Acar's inertia forceactsat itscenter of gravity,or CG,of thewhole cal'.The CG isthepoint about whichthe entire carisbalanced.If youcouldhangthe carbyacableattachedatitsCG,the car wouldbalance in any position. TheCGisthecenterofallthe weight.Allthe inertia forces on the in-dividualpartsof the car addedtogeth-erarethesameasasingleinertia forceforthewholecaractingatits CG.Becauseacar'sCOisalways abovetheroad,inertiaforcefrom brakingalwaystriestoloadthefront tiresandlifttherears.Thiseffectis calledweight transfer. Weighttransfermeansthatthe fronttiresareloadedmoreduringa stop,andthereartiresareunloaded. Afront-wheel-drive car,such asthisVW,hasmajority of weight at front.Whenbrakes are appliedhard,moreweightistransferredto the front.Thenosedropsand tailrisesduring braking due to weight transfer. F,=vertical force on front tires Ob) F,=vertical force on rear tires (Ib) Xeg=horizontal distance from front axle to CG(in.) I =wheelbase length (in.) Yeg =CGheight (in.) We =Car weight (Ib) fJ.=Coefficient of friction t ~ I F=W_ F+ We fJ.Y cg ,e ,I F=WcXCg_Wc fJ.Yeg 'II Weight transfer=WefJ.Yea I Ignoringaerodynamicforces,theseforcesactonacarduringhardbraking.Maximum weight transfer can be calculated if tire coefficient of friction is known. Theweightof thewholecardoesnot change.Weightaddedtothefront tiresissubtractedfromthereartires duringweighttransfer.Theforces actingonacarduringbrakingare shownintheaccompanyingdrawing. Inthis simple illustration, aerodynam-icforcesarenotshown.Aerodynamic forceschangetheamountofthe forces,butnotthebasicprincipleof weight transfer. Becauseweighttransferloadsthe fronttires,additionalfrictionforce canbedevelopedbythefronttires beforetheyskid.Toproducethis extrafrictionforce,frontbrakes have toworkharderthaniftherewasno weighttransfer.Atthe sametime,the rearbrakescandolesswork. Typically,frontbrakessupplyabout two-thirdsof thetotalbraking forcein ahardstop.Theratioisevenhigher onanextremelynose-heavycar.Be-causethefrontbrakesdomostof the work,theyneedtobelargerthanrear brakes.Theforcesareusuallyhigher onfrontbrakes,sotheymustabsorb more heat energy. 13 BRAKING LIMITS Withamodernbrakesystem,how good can brakes be?What determines thelimitstobrakeperformance? Whatmakesyourcarstopquicker than the next car, or vice versa? Therearelimitsthatdetermine howquickacarcanstop.Someof theselimitscanbealteredbydesign or maintenance,so only the basic laws ofnaturelimita car'sstopping ability. Brake-performance limits are: 1.Force 2.Deflection 3.Wear 4. Temperature 5. Tire traction Abrakesystemshouldbedesigned andmaintainedsothattiretraction determineshowquicklyyourcarcan stop.Ifanyoftheotherfourlimits keepyoufrom stopping quicker,your brakes are not adequate. Forcelimitmeansthedriver pushesashardaspossiblewithhis footandthecarcan'tstopany quicker.Inotherwords,ifthedriver couldpushharder,the carwouldstop quicker.Thislimitcanbealteredby Geoff Brabham is at the traction limit of his Toyota Celica in this turn. Combination of brak- ing and cornering forces causes tires to slip and point at extreme angles to the direction of travel.When caris atits tractionlimit, anyattempttocorner, accelerate orbrake harder will throw it into a skid. reducing master-cylinder size, putting on differentlining,usingpower-assist brakes,orothermethods.Idiscuss how to reduce the force a driver has to exert laterinthe book. In some cases, aforcelimitoccurswhenthebrakes gethot. This iscalledbrake fade.The answerhereisdissipatingheat.Per- hapstheforcelimityou'veencoun- teredisreallyatemperaturelimit. How to handle excess heat is discussed in Chapters 10 and12.Deflectionlimitisreachedasthe brakepedalstops atthefloororstop. Thismeansthepedalismovingtoo fartogetmaximumefficiencyfrom thebrakes.Adeflectionlimitcanbe eliminatedbydesignchanges suchas stiffeningthe pedal-support structure, increasingmaster-cylindersize,in- stallingstifferbrakehoses,changing tostiffercalipers,orothermodi- fications.Maintenancecaneliminate adeflectionlimitifairistrappedin the brakelines. Wearlimitwon'thappenwhen brakesarenew.However,iffriction materialiswornexcessively,itmay bewornoutjustwhenyouneedthe brakesmost-suchasat theendofa long race. Wear limits can be eliminat- edorreducedbychanginglinings, usinglargerbrakes,orbydissipating heat. Brake wearis discussedin Chap- ters 4 and12. Temperaturelimit:Brakescannot absorbthefullpowerofanengine continuouslywithoutsometimeto cool.Whenthetemperaturelimitis reached,youcanreachaforcelimit, deflectionlimit,orgreatlyincrease thewearatthesametime.Other thingscanhappen,too,such as com- pletedestruction. ofthe brakes or total collapse of a structural part. Excessive temperatureisacommoncauseof brake problems. Traction limit: Ifbrakes are proper- lydesigned and maintained,and don't gettoohot,theonlystopping limitis tjretraction.Ifyoutryto stop quicker thanthetractionlimitallows,the wheelslockupandthe tires skid. The tractionlimitisalwaysthelimitwith goodbrakes,butitcanbeincreased throughcorrectadjustmentofbrake balance.Adjustingbrakebalanceis discussedinChapter10.Modifica- tionstoallowbrake-balanceadjust- ment are describedin Chapter12.BRAKE FADE Brakefadeislossofbrakingdue to overheating.It cancauselo'nger pedaltravelasthebrakesget hotter-maybetothepaintwhere thepedalgoestothefloor.Pedal effortmayalsoincreaseasheat bu'ildsup,eventothepoint where pushing withmaximumforce won't lock thewheels!Manytimes,fade causesacombinationofboth longerpedaltravelandincreased pedal effort. Whateverfadeis,thedriveris facedwith a panic situation.Brake fadetypicallyoccursattheworst posSiblemoment-goingdowna longhillpulling a trailer,at the end ofalongrace,orduringapanic stopfromfreewayspeedinheavy traffic.Thisis when youneed your brakesthemost.Consequently, brakefadeisalwaysscary,and often dangerous. BRAKING LIMITS Withamodernbrakesystem,how goodcanbrakesbe?What determines thelimitstobrakeperformance? Whatmakesyourcarstopquicker than the next car, or vice versa? Therearelimitsthatdetermine howquickacarcanstop.Someof theselimitscanbealteredbydesign ormaintenance,so only thebasic laws of naturelimitacar ' s stoppingability. Brake-performance limits are: l. Force 2.Deflection 3.Wear 4.Temperature 5.Tire traction Abrakesystemshouldbedesigned andmaintainedsothattiretraction determineshowquicklyyourcarcan stop.Ifanyoftheotherfourlimits keepyoufromstoppingquicker,your brakes are not adequate. Forcelimitmeansthedriver pushesashardaspossiblewithhis footandthecarcan'tstopany quicker.Inotherwords,ifthedriver couldpushharder,thecarwouldstop quicker.Thislimitcanbealteredby 14 Geoff Brabham is at the traction limit of his Toyota Celica inthis turn.Combination of brak-ing and cornering forces causes tires to slip and pOint at extreme angles to the direction of travel.Whencarisatitstractionlimit,anyattempttocorner,accelerate orbrakeharder will throw it into a skid. reducingmaster-cylindersize,putting ondifferentlining,usingpower-assist brakes,orothermethods.Idiscuss how to reduce the forcea driver hasto exert laterinthebook.In some cases, aforcelimitoccurswhenthebrakes gethot .Thisiscalledbrake fade.The answerhereisdissipatingheat.Per-hapstheforcelimityou'veencoun-teredisreallyatemperaturelimit. How to handle excess heat isdiscussed inChapters 10 and12. Deflectionlimitisreachedasthe brakepedalstopsatthefloororstop. Thismeansthepedalismovingtoo fartogetmaximumefficiencyfrom thebrakes.Adeflectionlimitcanbe eliminatedbydesignchangessuchas stiffeningthepedal-support structure, increasingmaster-cylindersize,in-stallingstifferbrakehoses ,changing tostiffercalipers,orothermodi-fications.Maintenancecaneliminate adeflectionlimitifairistrappedin the brakelines. Wearlimitwon'thappenwhen brakesarenew.However,iffriction materialiswornexcessively,itmay bewornoutjustwhenyouneedthe brakesmost-suchasattheendof a long race.Wear limits canbe eliminat-edorreducedbychanginglinings, usinglargerbrakes,orbydissipating heat.Brake wearisdiscussedinChap-ters 4 and12. Temperaturelimit:Brakescannot absorbthefullpowerofanengine continuouslywithoutsometimeto cool.Whenthetemperaturelimitis reached ,youcanreachaforcelimit, deflectionlimit,orgreatlyincrease thewearatthesametime.Other thingscanhappen,too,suchascom-plete destruction of thebrakes or total collapseof astructuralpart.Excessive temperatureisacommoncauseof brake problems. Traction limit:If brakes are proper-lydesignedandmaintained, and don't gettoohot,theonlystoppinglimitis tiretraction.If youtrytostopquicker thanthetractionlimitallows,the wheelslockupandthetires skid.The tractionlimitisalwaysthelimitwith goodbrakes,butitcanbeincreased throughcorrectadjustmentofbrake balance.Adjustingbrakebalanceis discussedinChapter10.Modifica-tionstoallowbrake-balanceadjust-ment are described in Chapter 12. Drum Brakes Adjusting Lever Retainer hoe Hold-Down Moderndrumbrakeautomaticallyadjustsshoesoutwardasfrictionmaterialwears.Drumisnotshown.DrawingcourtesyChrysler Corporation. Mostcarshaveusedinternaldrum brakesoverthe years.Theycontinue tobeusedontherearofmostroad cars. Eventhoughdrumbrakesshare commonfeatures,detailsmaydjffer. Eachhasametaldrum,usuallycast iron.Thedrumrotateswiththe wheel.Withinthedrumarebrake shoes linedwithfrictionmaterial. This material,consisting ofvarious organic andmetalliccompounds,isthebrake lining.*Thebrakeshoesaremoved againsttheinsideofthe drumbypis- tonsinsidethewheelcylinders.Hy- draulicfluidunderpressureinthe wheelcylindersmovesthepistons. Wheelcylindersandbrakeshoesare mountedonametalbackingplate. Thisbackingplateisboltedtothe car'saxlehousingorsuspension upright. Cable Guide Shoe Hold-down PartsAutomatic-Adjuster Parts Bendix duo-servo rear brake is typical of drum brakes found on American cars. Brake fea- tures automatic adjusterandhighservoaction.Includedis linkage t o operate shoes from the parking-brake cable. Drawing courtesy Bendix Corp. DrumBrakes Backing Hole Covers !; ) Hold-Down Pin or Rod Wheel Cylinder Assembly &( Anchor-Pin Plate fJ Secondary aReturn Spring Cuprr;; Springr Retainer 2 Moderndrumbrakeautomaticallyadjustsshoesoutwardasfrictionmaterialwears.Drumisnotshown.DrawingcourtesyChrysler Corporation. Mostcarshaveusedinternaldrum brakesovertheyears.Theycontinue tobeusedontherearof mostroad cars. Eventhoughdrumbrakesshare commonfeatures,detailsmaydiffer. Eachhasametaldrum,usuallycast iron.Thedrumrotateswiththe wheel.Withinthedrumarebrake shoes linedwithfrictionmaterial.This material ,consisting of variousorganic andmetalliccompounds,isthebrake lining.Thebrakeshoesaremoved againsttheinsideof thedrumbypis-tonsinsidethewheelcylinders.Hy-draulicfluidunderpressureinthe wheelcylindersmovesthepistons. Wheelcylindersandbrakeshoesare mountedonametalbackingplate. Thisbackingplateisboltedtothe car'saxlehousingorsuspension upright. Paing-Brake Strut Shoe Hold-down Parts Adjuster cable!""Adjuster-Lever Spring I rgIiUlil4S---' Cable Guide Pivot Ad"t" JUS109uAdjusting Screw Automatic-Adjuster Parts Bendix duo-servo rear brakeistypical of drum brakes found onAmerican cars.Brake fea-turesautomaticadjusterandhighservoaction.Includedislinkage to operate shoes from the parking-brake cable.Drawing courtesy Bendix Corp. 15 r Wheei Cylinder Pressurized HydraulicFluid \TrailingShoe r Hvdraulic Force Friction Force on Trailing Shoe I L ~ h o e Pivots I - Drum Rotation Singleleading-shoedrumbrake-sometimescalledaleading-Shoes in this single leading-shoe brake both pivot toward the right and-trailing-shoebrake-hasone leading and one trailing shoe. Aswhenactedonbyfrictionforces.Althoughwheel-cylinderforce drumrotatesclockwise,frictionforceonleadingshoeforcesitkeepsbothshoesagainstthedrum,frictionforcesmodifypres- againstdrum,creatingservoaction,orforcemultiplication.Drumsureexertedbyeachshoe.Leading-shoepressureincreases; rotation tends to reduce shoe-to-drum force of trailing shoe.trailing-shoepressuredecreases.Thereislittleoverallservo action withthistypeofbrake, as friction-forceeffectsonshoes cancel out each other. SERVO ACTION Therearemanyvariationsof simpledrum-brakedesign.Designs differintheamountofforce- multiplication,orservoaction.Imagine drivingonecarwithstandardbrakes andanotherequippedwithpower- assistbrakes.Theamountofpedal forceisgreatlyreducedwithpower- assistbrakes.Servoactionactsmuch likepowerassist - i t reduces the force requiredonthebrakepedalfora givenamountofbraking.However, servoactionoccurswithinthebrake itself. LeadingorTrailing-Tohelpunder- standservoaction,let'slookathow brakeshoesaremounted.Imaginea brakeshoepivotingat one endanda wheelcylinderpushingontheother. There are twowaystomount abrake shoe, as leaditillg shoe or as trailing shoe. Thisdependsonwhichendofthe shoepivotsinrelationtodrum rotation.Ifthebrakedrum