Contents - Öhlins · The TT44 damper is designed to handle the demanding damping charac-teristics...

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Contents1. Introduction ___________________________________________ 42. How the damper works_________________________________ 5

3-way configuration compression cycle _____________________5HSC configuration compression cycle ______________________73-way configuration rebound cycle _________________________7HSC configuration rebound cycle __________________________7

3. Damping curves terminology____________________________ 84. External adjustments___________________________________ 9

External adjusters summarized ____________________________9Low speed adjusters ___________________________________ 11Reservoir compression adjuster _________________________ 11High speed compression adjuster________________________ 11Hydraulic spring preloader ______________________________ 13

5. Internal adjustments __________________________________ 14Piston _______________________________________________ 14General shim information _______________________________ 14Sealing shim__________________________________________ 15Ring shim and centering shim ___________________________ 15Preload shim _________________________________________ 16Clamp shim __________________________________________ 16Valve stop ____________________________________________ 16

6. Shim stack theory ____________________________________ 17Stack stiffness ________________________________________ 17Stack preload _________________________________________ 17Knee and slope guidelines ______________________________ 18

7. Hysteresis ___________________________________________ 198. Combining main piston and reservoir damping ___________ 219. Damping functions ____________________________________ 23

10. Factory recommended damper set-ups _________________ 2511. Damping guideline ____________________________________ 2612. Work section _________________________________________ 27

Service tools, oil & grease ______________________________ 27Main piston reshimming, basic method ___________________ 28Main piston reshimming, alternate method ________________ 34Reservoir reshimming __________________________________ 35Optional HSC adjuster installation _______________________ 38Optional HSC adjuster spring change ____________________ 41Hydraulic spring preloader refilling _______________________ 41Gas pressure _________________________________________ 41Routine maintenance __________________________________ 41

13. Adjustment and valving charts _________________________ 4314. External dimensions and damper identification __________ 5115. Optional new parts ____________________________________ 5316. Spare parts __________________________________________ 54

Damper body _________________________________________ 54Piston shaft __________________________________________ 55End eye ______________________________________________ 55Reservoir ____________________________________________ 56HSC adjuster _________________________________________ 57Hydraulic spring preloader ______________________________ 58Main piston shims _____________________________________ 59Reservoir shims _______________________________________ 59

17. Addresses ___________________________________________ 63

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Congratulations on choosing theÖhlins TT44 formula car shock ab-

sorber, the most unique and powerfulracing damper available today.

The TT44 damper design is the culmi-nation of two decades of Öhlins success-ful participation in world championshipevents. This damper draws on all theexpertise developed by Öhlins while win-ning more than 60 World Champion-ships. The TT44 damper is designed tohandle the demanding damping charac-teristics needed for all types of tracks,from street courses to super speedways.The ability to create high damping forcesat very short strokes, combined withpowerful adjusters, will give you out-standing performance and offer manynew possibilities.

The Öhlins TT44 features a patentedconcept with a unique double wall de-sign and two adjustable bleed valves tocontrol the flow between these tubes.These valves control the initial compres-sion and rebound damping and arecheck-valved to be completely inde-pendent of each other. They meter theoil flow created by the main piston area,

not the flow created by shaft area dis-placement. This translates to low inter-nal pressure during the compressionstroke.

Even though damping force builds rap-idly, the low internal pressure preventshigh friction from the shaft seal. The re-sult is excellent short stroke/high forceperformance.

The temperature stability is maintainedby using a flow restriction design in thebleed valves that create a turbulent flowat very low piston velocities. Materialswith different thermal expansion rates areused to compensate for the viscositychange of the oil caused by changes intemperature.

Thanks to the unique design of thebleed valves (they are not tapered nee-dles working in a fixed orifice) every step(click) of the adjusters produces equaland predictable changes in force in thenormal operating range. Optimum set-tings are easy to find.

The Öhlins shim system with the ringshim preload device offers infinite com-binations of shim stacks of very hugespectrum of different character with one

and the same piston.The reservoir has a traditional adjust-

able needle bleed valve in parallel withits own shim stack, which works onlyduring the compression cycle. The wholesystem is pressurized by nitrogen gasbehind a floating piston to ensure sepa-ration of the gas and oil.

As an option there is an additionalcompression control adjuster available,providing a total of 4 external damperadjusters. This pressure regulating valve,unique in many ways, controls only thepressure over the main piston. It givesnew possibilities in shaping dampingcurves. For instance “double knee”curves can be produced easily.

In keeping with Öhlins long tradition ofperfection, performance is outstandingand long life is to be expected.

The Öhlins TT44 damper is a racerfriendly damper, easy to set up, dial in,service and rebuild.

All dampers are dyno tested beforethey are delivered to the customer.

Support is always available from theÖhlins factory and Öhlins distributorsworldwide. �

1. Introduction

The Öhlins TT44, the winning damperin formula, sports and touring car racing.

1. Introduction

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2. How the damper works

Figure 1. Oil circuit. 3-way configuration during compression cycle.

Figure 2. Oil circuit. 4-way configuration during compression cycle.


The following description of the func-tion of the damper is divided into four

different situations: compression and re-bound damping cycles with and withoutthe high speed compression (HSC) ad-juster.

The compression damping cycle de-scribes the situation when the shaft-pis-ton unit moves into the damper body,while the rebound damping cycle de-scribes the situation when the shaft-pis-ton unit withdraws from the damperbody.

3-way configuration stands for thebasic TT44 damper with three externaladjusters. The HSC configuration refersto the basic TT44 damper with the op-tional external high speed adjuster in-stalled. It gives a total number of 4 ex-ternal adjusters. Dampers with this con-figuration are often called ”4-ways”.

3-way configurationcompression cycleFor a start we assume that the adjust-able reservoir compression bleed valve(figure 1-E) is fully open. In this case, thepressure on the compression side of thepiston (figure 1-G) will remain almostconstant and the same as the gas pres-sure during the whole stroke, thoughsome small change of the pressure willoccur because of the change of gas vol-ume caused by shaft displacement.

As long as the reservoir compressionbleed valve is set to fully open, only thepressure drop on the rebound side of themain piston (figure 1-H) causes the oil tomove from the compression side to therebound side of the piston. This meansthat the initial nitrogen pressure in thereservoir has to be high enough to han-dle the compression forces. If the pres-sure is too low there is a risk of dumpingmore oil than the shaft displaces into thereservoir and cavitation will occur in therebound chamber.

When track conditions cause the ve-hicle suspension to move, the damperpiston will attempt to move through thedamper oil. In order for the piston tomove, oil must flow from one side of themain piston to the other. In the initial partof a compression stroke, when the ve-locity of the piston is low, the oil flowbypasses the main piston by travellingthrough the adjustable low speed com-pression bleed valve (figure 1-A), thecompression check valve (figure 1-B) andthen flows between the two tubes (fig-ure 1-C). The oil re-enters the main tubeon the rebound side through a port nearthe end cap.

As the piston velocity increases thepressure drop across the main piston willincrease. Observe that it is the pressure

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Figure 3. Oil circuit. 3-way configuration during rebound cycle.

Figure 4. Oil circuit. 4-way configuration during rebound cycle.

drop on the rebound side of the pistonthat causes this, not the increase in pres-sure on the compression side. Depend-ing on the combination of shims, thecompression shim stack (figure 1-D)opens at a certain pressure. This pro-vides a direct pathway for the oil, allow-ing it to pass through the stack to theother side of the piston. Depending onthe shim stack, the opening will be some-thing from abrupt to gradual. As long asthe piston is moving and the bleed valveis not fully closed, some oil will alwaysflow through the bleed valve.

Note: In practice many strokes neverreach a velocity high enough to causeenough pressure drop across the mainpiston necessary to cause the shims toopen.

During the compression stroke, the oildisplaced by the volume of the pistonshaft as it enters the main body is forcedinto the reservoir. This causes a smallincrease in system pressure due to thepiston shaft volume displacement mov-ing the floating piston and thereby com-pressing the nitrogen.

In the passage connecting the mainbody and the reservoir, there is anothercompression valve system (one bleedvalve in parallel with a shim stack) simi-lar to the system found in the main body.By closing off the compression reservoirbleed valve (figure 1-E), the reservoirvalve system contributes to the totalcompression damping.

The reservoir compression bleed valveregulates the flow at low piston veloci-ties. At higher piston velocities, the res-ervoir compression shim stack (figure 1-F) opens to provide a pressure blow off.

The pressure differential across themain piston will build up quicker if thereis some damping from the reservoir, be-cause there is not only a pressure dropon the rebound side of the main pistonbut also a pressure rise on the compres-sion side. This also reduces the risk ofdumping oil into the reservoir. However,the amount of reservoir damping neededto achieve a quick damping force buildup is small. Excessive reservoir damp-ing will cause hysteresis.

High hysteresis is caused because thedamping force from the reservoir is cre-ated by the pressure acting only on thecross section area of the piston shaft.For equal damping force the pressureneeded per unit of damping is muchhigher compared to the pressure neededfrom the much larger main piston. Higherpressure compresses the oil more, eventhe best hydraulic oils have a certaincompressibility, and also expands thedamper slightly more. In a dyno graphthis can be seen as a difference in forceduring acceleration compared to decel-eration. The force will be lower duringacceleration than during deceleration.This is most evident at low velocities. The


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Öhlins TT44 in different configurations: 1. “straight piggyback”, 2. “offset piggyback”, 3. “remote reservoir”, 4. “straight piggy-back” in 36 mm spring configuration, 5. optional HSC adjuster, 6. hydraulic spring preloader.

force differential between accelerationand deceleration is referred to as hyster-esis. One of Öhlins design goals for theTT44 damper is to have the possibilityto run the damper with low hysteresis.See chapter ”Hysteresis” on page 19.

Because of the restriction in the reser-voir, the pressure in the whole damperbody will rise. This will reduce the risk ofcavitation.

The compression damping curve is thesum of the forces created in the mainbody plus those created in the reservoir.See chapter ”Combining main piston andreservoir damping” on page 21.

HSC configurationcompression cycleWhat has already been described hereabout the 3-way adjuster compressioncycle is still valid with the addition of theHSC option, but now there is also a pop-pet valve in parallel with the compressionshim stack of the main piston. The pop-pet valve is pushed against its seat by apreloaded coil spring. The amount of pre-load can be externally adjusted. This pre-load determines what pressure differen-tial across the main piston is necessaryto make the poppet valve open.

The HSC configuration provides anadditional pathway to the other side ofthe piston. The new pathway goesthrough the center of the shaft (figure 2-I)

and then encounters the poppet valve (fig-ure 2-J). As the piston velocity increases,the pressure drop across the main pistonrises. At some velocity the movement ofthe piston creates a pressure drop acrossthe main piston, that equals the predeter-mined pressure required to open thevalve.

After passing the valve, the oil exits thecross drilled holes (figure 2-K) in the shaft,on the other side of the piston. Depend-ing on the combination of shim stack andpoppet valve preload, the poppet valvecan open before, at the same time, orafter the main piston shim stack. Thisdetermines the character of the damp-ing curve.

The opening characteristic of the pop-pet valve is always abrupt, unlike thegradual opening characteristic of theshim stack.

3-way configurationrebound cycleThis cycle is very much the same as thecompression damping cycle (3-way con-figuration) of the main piston.

During the rebound stroke, the pres-sure of the rebound side of the main pis-ton is increasing, while the pressure ofthe compression side is kept almost con-stant. This causes the oil to move backacross the piston. When the piston ve-locity is low, oil will initially flow between


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the two tubes (figure 3-A) and arrive atthe low speed rebound bleed valve (fig-ure 3-B).

After the valve has metered the flow,the oil will open the rebound check valve(figure 3-C) and travel to the compres-sion side of the piston.

When the opening pressure of the mainpiston shim stack is reached, oil will alsopass through the stack (figure 3-D).

To compensate for the displacementchange caused by the shaft leaving thebody, a check valve in the reservoir (fig-ure 3-E) will open so that oil can freelyreturn to the compression side of the pis-ton. This will cause a small decrease insystem pressure, due to the reductionof piston shaft displacement moving thefloating piston and therefore expandingthe nitrogen.

HSC configurationrebound cycleThe rebound damping cycle is the samewhether the damper is of 3-way or HSCtype (figure 4).

The poppet valve of a HSC adjustabledamper (figure 4-J) has the function of acheck valve. It will seal against the valveseat during the rebound stroke and there-fore prevent oil from flowing back throughthe shaft. The oil passage will becomeidentical to the standard 3-way typedamper during the rebound stroke. �

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3. Damping curves terminology

Figure 5. Terminology.

Nose

Slope angle

In order to understand the next part ofthis manual we must all speak the same

language. In the damper industry thereare some slang words used to describedifferent zones to interpret dampingcurves. The three key words are nose(low speed), knee and slope (high speed).Careful study of this section will yield acomplete understanding of these termsand allow you to read damper dynocurves. Later chapters will show how tomanipulate the damper to produce al-ternate dyno curves. This chapter is rel-evant for both compression and rebounddamping curves.

Note: In this part of the manual thespeeds and forces shown are not actualand just chosen for illustration purposes.

Later on, these values will be real whereadjustment and valving combinations arecatalogued. See chapter “Adjustmentand valving charts” on page 43.

Because a high percentage of raceteams have and use Roehrig damperdynamometers, Öhlins has chosen toformat all graph figures in this manual inthe same proportion as the latest Roehrigdyno graph printouts, and in the chapter“Adjustment and valving charts” the fig-ures are of the exact size.

There are different ways of presentingdyno curves produced by a Roehrigdyno.

Note: Unless otherwise indicated, thecurves shown are compression open(measurement during acceleration) andrebound closed (measurement duringdeceleration).

To explain the damping curve termi-nology it is not necessary to include thedamping in the reservoir or the influenceof the optional high speed compressionadjuster at this time.

Take a look at figure 5 and notice thefirst portion of the damping curve thatstarts at 0.0 inches/second damperspeed and ends at about 0.5 inches/sec-ond. This speed zone is called the noseand is also referred to as low speed. Thevalve affecting this part of the curve isthe low speed adjuster. It is always of afixed orifice type (the size of the orifice isnot variable by pressure) and is oftencalled bleed. The design and size of thebleed determines the character andshape of the nose.

See “Low speed compression and re-bound adjusters”, on page 11 for furtherinformation.

The finish of the nose zone coincideswith the beginning of the knee zone. Thispoint is determined by the initial open-ing of the piston shim stack. Its locationin the curve can be found by identifyingwhere the upwards curve first begins tolevel off into the radius that transitionsinto the straight line (in this case about0.5 in./sec.).

The knee portion extends until the shimstack has transitioned from closed toopen (in this case about 1.2 in./sec.).Locating where the knee radius stopsand blends into the straight line identi-fies the end of the knee zone and thebeginning of the slope zone. The lowspeed adjustment in combination with

the shim stack’s properties determinesthe position and shape of the knee.

The slope, also referred to as highspeed, is determined by a combinationof the shim stack stiffness and the size,shape, quantity and placement of thepiston ports adjacent to the undersideof the shims. In most cases the slopewill continue rising in a straight line todamper speeds well beyond those foundon most racing dyno charts. Eventuallythe slope will end and the curve will againturn upwards. This happens when thesize of the piston holes begin to restrictoil flow (piston holes are also fixed ori-fices). The slope angle relative to thehorizontal plane defines the magnitudeof the slope and can be quantified asPound/(Inch/Sec.) or N/(m/s).

See ”Knee and slope guidelines”, onpage 18 for further information.

The nose, knee and slope are keywords to understanding the followingconcepts.

Note: A nose, a knee and a slope canalso be identified in the reservoir damp-ing system. This system is of the sametype with a fixed orifice valve in parallelwith a shim stack. See chapter ”Com-bining main piston and reservoir damp-ing” on page 21.

If the high speed compression adjusteris used a second knee can be achieved.See “High speed compression adjuster”on page 11.

The speed of the damper refers to thespeed of the shaft movements, not to thespeed of the car. �

3. Damping curves terminology

Slope

Knee

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4. External adjustments


Figure 7. Optional high speed compres-sion damping adjuster. - = soft. + = hard.

Figure 6. Standard external adjusters.- = soft. + = hard.

+ -

HSC

-

- +

-+ +

RC

LSC LSRMost dampers that are externally ad-justable have some type of low

speed adjuster. Low speed adjusters arealmost always externally adjustable vari-able orifices that become fixed orificesafter adjustment. In the damper indus-try, these low speed orifices are often re-ferred to as bleeds.

Unlike shim stacks, bleed orifices donot change size in response to changesin pressure. Because oil will always travelto the path of least resistance, it will firstflow through the open bleeds until thereis enough pressure to open any othervalves. Oil flows through the bleeds anytime the damper shaft is moving, andcontinues to flow in parallel with the flowthrough the piston shim stack after thestack has opened.

The most common type of adjustment,when it comes to external high speed ad-justers, is an adjuster that moves the kneeup or down without changing the slope,or just marginally changing it. To achievethis, the amount of force pushing the valve,shim or poppet valve, against its seat isvaried. That is done by changing the pre-load of the spring element; shim stack, coilspring, cup spring etc.

The oil flow that is controlled by theexternal adjusters varies between differ-ent type of dampers. The larger the flowis, the better the conditions will be for apowerful adjuster. There are two reasonsfor that:

A larger flow is easier to control.The tolerance zones of the parts haveto be reduced to keep the precision ifthe flow is reduced.

A larger pressure area, the pressurearea is proportional to the oil flow, willkeep the internal pressure of thedamper at a lower level. This increasesthe damper response and the damperwill build up damping force quicker.

External adjusters summarizedThe TT44-damper from Öhlins comesnormally as a 3-way externally adjust-able damper. The HSC adjuster is thensold separately as a kit. However, forsome cars the dampers are deliveredwith the HSC adjuster already mounted.

Low speed compressiondamping adjuster (LSC)Type of adjuster:Bleed adjuster.Effects:The flow from the main pistonduring compression strokes only.Identification:Gold knob at the headof the cylinder body.

Number of positions:Approx. 38

Low speed rebounddamping adjuster (LSR)Type of adjuster:Bleed adjuster.Effects:The flow from the main pistonduring rebound strokes only.Identification:Silver knob at the headof the cylinder body.Number of positions:Approx. 38.

Reservoir compressiondamping adjuster (RC)Type of adjuster:Bleed adjuster.Effects:The flow from the displacementof the shaft during compressionstrokes only.Identification:Black knob at the topof the reservoir.Number of positions:Normally within 20-25. Dependson the shim stack used.

Optional high speed compressiondamping adjuster (HSC)Type of adjuster:Poppet valve preload adjuster.Effects:The flow from the main pistonduring compression strokes only.Identification:Gold wheel in the end eye.Number of positions:Approx. 55.

Note: All Öhlins external adjusters, in-cluding the optional high speed com-pression adjuster, are “fully hard” whenturned clockwise until they stop.

It is very important that the clicker po-sition is always counted from “fully hard”.The reason is “full hard” is always anabsolute position. “Fully soft” will varydepending on tolerances. In the case ofthe reservoir, “fully soft” will also vary withthe shim stack chosen and in the caseof the optional high speed compressionadjuster the detents will become gradu-ally less pronounced.

Normally the first click and/or detentis counted as “zero” position.

To match damping curves of a pair ofdampers in the dyno, sometimes theclicker numbers will end up +/-1 clickfrom each other.

Because of the high number of clicksof the HSC adjuster, the click positionnumbers can differ even more.

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Figure 8. Influence of the low speed adjusters. The graph represents both the LSC and the LSR.

Figure 9. Influence of the standard reservoir compression adjuster (2.5 mm type). The graph illustrates thereservoir damping only. Note: Different scale.

Figure 10. Influence of HSC adjuster (1 mm wire spring). Here LSC is set to click position 10. No oil passesthe main piston compression shim stack. Note: The base curve is different due to a different shim stack.


Clickposition(from topto bottom)612182430



Without HSC

6 12 18 24 30

0 3

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body. But it only operates during thecompression cycle and meters only theoil driven to the reservoir by the pistonshaft displacement. It works in serieswith the main compression system. Thecompression damping curve becomesthe sum of the damping created in themain body plus the damping created inthe reservoir. See chapter ”Combiningmain piston and reservoir damping”, onpage 21. The reservoir valves are however notidentical to the main valves and there-fore create damping curves with slightlydifferent character. Here, even if the bleedis closed, the pressure build up is verymuch delayed because of hysteresis.See chapter “Hysteresis”, on page 19 formore information.

See chapter ”Adjustment and valvingcharts”, on page 43 for real values.

High speed compression adjusterIn order to extend the performance ofour TT44 damper we offer this additionalcompression control adjuster. With thisfeature we now have three distinct com-pression adjustments and combinedwith the existing rebound adjuster thereare now a total of four adjusters. Thismodification has just a few new parts andcan easily be retrofitted to most existingTT44 dampers.

The concept for this new option is toprovide race team engineers and tech-nicians with more external control overthe compression damping to providemore usable time during practice andqualifying and to provide a system toshape damping curves in a way not oth-erwise possible. Even if the HSC adjusteris used, this does not eliminate the needof reshimming the compression shimstack to optimize the setting.

The main parts required to convert oradd this HSC adjuster are:

New end eye incorporating a windowto give access to the adjustment wheel.New hollow shaft to house the poppetvalve and adjuster mechanism.Threaded insert valve seat.Bronze valve guide.Poppet valve.Valve spring.Valve.Adjuster mechanism with threadedseal cap and affixed adjuster wheel.

No other part than the shaft and theend eye, need be replaced. Installationis quite easy. By following the steps de-scribed in chapter “Optional HSC ad-juster installation”, on page 41, you canavoid pitfalls.

The HSC changes the preload of apoppet valve in parallel with the com-pression shim stack. A coil spring pushesthe poppet valve against its seat. Thepreload of the spring determines the


Just remember maximum clockwise is“full hard” for all adjusters.

Low speed adjustersThe two low speed adjusters LSC andLSR have the same design and areuniquely designed so that in the normaloperating range each click of the knobswill change the damping in equal steps.They are not tapered needles working infixed orifices, where the damping forceincreases progressively per click as theneedle is closed. The adjusters are pow-erful over the whole range making iteasier for you to find optimum settings.

The LSC and LSR have left-handthreads and the knobs move away fromthe body when adjusted clockwise. Donot let the outward motion mislead you.All the way out is full hard!

Both knobs have a range of approxi-mately 38 clicks. To match a pair ofdampers in the low speed area, the clickpositions shall not differ more than 2clicks.

The knobs can be adjusted either byhand or with a small screwdriver. Do notuse too much torque when closing thebleeds completely.

As the adjuster is turned counter-clockwise the clicker numbers get higher.

Temperature stability is maintained inthe low speed area because of theunique design of the bleed valves, whichcreates a turbulent flow at very low pis-ton velocities. Also materials with differ-ent thermal expansion are used to com-pensate for the viscosity change of theoil caused by temperature changes.

Depending on the situation, differentstarting settings are recommended. Gen-erally it is better to start with the adjust-ers a little more open and gradually closethem off. Öhlins recommend setting theLSC to a click position between 5 and15 and the LSR to a click position be-tween 10 and 25.

Low speed adjusters effecton damping curveAssuming the LSC and the LSR are ad-justed the same and the piston velocityis the same and that there is no oil flow-ing through the shim stacks, exactly thesame amount of oil will flow through bothvalves. The LSR has exactly the sameeffect on the nose of the rebound damp-ing curve as the LSC has on the nose ofthe compression damping curve.

Figure 8 shows the influence of theLSC. As described above, it could justas well be a graph showing the influenceof the low speed rebound adjuster. In thegraph the original baseline compressioncurve of figure 6 can be seen togetherwith four alternate curves. These extracurves are achieved by adjusting only thegold knob. The numbers in the graph rep-resent various low speed adjuster clickersettings. The dotted curves pointing up-

wards indicate the theoretical shape ofthe curves if there was only a bleed valveand no high speed shim stack to open.See chapter ”Adjustment and valvingcharts”, on page 43 for real values.

As the bleed is opened more and more,the damping is reduced. The speed atwhere the knee begins increases and thenose is stretched longer and longer. No-tice, the force at where the knee andslope begins is always the same. Theslopes also remain parallel to each other.This is because the shim stack deter-mines the knee and the slope forces andwe have not changed any shims yet.

Conversely, if you wish to keep theslope starting speed constant, the shimstack preload has to be reduced everytime the low speed adjuster is opened.

Reservoir compression adjusterThe RC is the reservoir counterpart tothe gold knob on the main body. It alsoadjusts bleeds and therefore the lowspeed damping of the reservoir. It is atapered needle working in a fixed orifice.

The needle valve is available in twodifferent sizes, one for the 1.5 mm nee-dle seat and one for the 2.5 mm needleseat. The 2.5 mm needle/needle seat isstandard. For more information about the1.5 mm needle, see chapter ”Optionalnew parts”, on page 53.

The RC has a standard right-handthread so it moves into the reservoir bodywhen turned clockwise. In this case “allthe way in” is full hard.

This adjuster has a range of approxi-mately 20 clicks. The full range variesdepending on the reservoir shim stackchosen. The total thickness of the shimstack effects the number of click posi-tions, as the needle seat is also the boltthat clamps the shims. Different stackthicknesses create different needle seatheights. The thinner the stack is, the lessclick positions there will be. The lostclicks will be on the full open side of therange. However, even with the thinneststack possible, there are still plenty ofusable clicks available.

The reservoir adjuster shall be adjustedonly by hand.

Your designated starting clicker settingcan also vary greatly with different styleof shim stacks. Generally a good start-ing position is with the RC with the 2.5mm needle at click position 8 to 12 fornon-preloaded reservoir shim stacks andclick position 1 to 5 for preloaded, lowblow off shim stacks. There will be morespecifics about the different type of shimstacks when we move on to chapter “In-ternal adjustments”.

Reservoir compression adjustereffect on damping curveFigure 9 shows the influence of the RC.In the reservoir there is a valve systemvery similar to those inside the main

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Figure 11. Situation A. The original compression shim stack is used and opens first followed by the HSCadjuster valve opening.

Figure 12. Situation B. A modified stiffer compression shim stack is used and opens first, followed by theHSC adjuster valve opening.

Figure 13. Situation C. The HSC adjuster valve opens simultaneously with the original compression shimstack.


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Figure 14. Situation D. The HSC adjuster valve opens first, followed by a modified compression shim stackopening.


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Shim stack openings

pressure differential to open it.There are two different springs avail-

able for the HSC, one with 0.8 mm wireand one with 1 mm wire. The HSC ad-juster is delivered with the stiffer spring(1 mm) mounted and the softer (0.8 mm)as a supplemental part.

By turning the adjuster wheel clock-wise (viewed from the end of the damperat the shaft side), the preload of the pop-pet valve increases.

As on all adjusters on the TT44, theclicks are counted from maximum clock-wise position (max preload = max force).There are a total number of approxi-mately 55 clicks for both the 0.8 mm andthe 1 mm spring. At the full soft end ofthe adjustment range, when the adjusteris turned fully counterclockwise, there willbe a couple of turns without detentclicks. The adjuster should not be usedin this area.

When the preload of the poppet valveincreases, the space for the valve tomove will decrease, as the preloader(part # 05464-01) moves closer to thevalve. The risk of the valve bottomingagainst the preloader increases withdamper speed and is also higher withthe soft spring (part # 05473-01).

Note: To avoid bottoming of the pop-pet valve, the adjuster should not be setto a click position less than 10.

Turning the adjustment wheel changesthe preload of the poppet valve. This isdone by using a 2 mm diameter pinthrough the window of the end eye. Amaximum of two clicks can be made persweep.

No start setting can be recommended,as it will vary depending on type of car,ratio, shim stacks etc. The HSC adjusterblow off starting point is best set with thehelp of a damper dyno. We recommendclick position 30 for the first “dyno” run.

Generally the blow off point is set nosooner than 2 inches/sec.

High speed compression adjustereffect on damping curveFigure 10 shows the influence of theHSC. As described earlier, two differentsprings are available. By using the 1 mmwire spring you can run a larger adjust-ment range than you can if you use the0.8 mm spring. The minimum force forthe two springs is about the same, butthe maximum force will be higher andthe damping force change per click willbe larger with the stiffer spring. See chap-ter ”Adjustment and valving charts” onpage 43 for real values.

There are several distinct ways to usethe HSC adjuster in conjunction with theother external adjusters and the shimstacks. Keep in mind that the HSC func-tions in some ways exactly like a com-pression shim stack. Both are pressureregulators that control oil flow by open-ing at a pre-determined pressure therebyproviding a path for the oil to flow. Thisadditional pathway allows the oil to reachthe other side of the piston with less re-sistance. Lower resistance alwaysequates to lower damping force. Whetherthese two valves open simultaneously, orone after the other, and which valve opensfirst, is your option.

Following are descriptions of differentmethods of using the piston shim stackin conjunction with the HSC adjusterpoppet valve:

A. The original compression shim stackis used and opens first, followed bythe HSC adjuster valve opening(figure 11).

B. A modified stiffer compression shimstack is used and opens first, followed

by the HSC adjuster valve opening(figure 12).

C.The HSC adjuster valve opens simul-taneously with the original compres-sion shim stack (figure 13).

D.The HSC adjuster valve opens first,followed by a modified compressionshim stack opening (figure 14).

All of these situations have been testedduring the development. However, eachrace team has to decide which configu-ration will best suit their needs.

Hydraulic spring preloaderThe hydraulic spring preloader (“weightjacker”) makes it possible to adjust springpreload from the cockpit. The rate is 0.5mm/turn and the stroke is 6.5 mm.

The spring preloader is designed for2” inner diameter (i.d.) springs andcomes with a 1500 mm long hose. Formore dimensions, see drawing in chap-ter ”External dimensions and damperidentification” on page 51.

The spring preloader can be adjusted byhand to approx. 1200 lbs (5500 N) of load,which is the maximum continuous load.

By changing some of the parts in themaster cylinder, there is a possibility torebuild the spring preloader, so you geta rate of 0.75 mm per turn. However, thiswill increase the torque needed for a spe-cific load. Contact Öhlins or an Öhlinsdistributor for further information.

If the spring preloader needs to betaken apart for whatever reason, it is veryimportant to get all the air out of the sys-tem when it is assembled. If not thestroke will be reduced and the flexing willincrease. See chapter “Hydraulic springpreloader refilling” on page 41 for guide-lines. �

14

We have just learned about the ex-ternal adjusters. We now need to

look inside to see what tools are avail-able to influence the damping. (The twodifferent springs available for the highspeed compression adjuster have alreadybeen covered in “High speed compres-sion adjuster” on page 11).

PistonThe piston (#5415-11), the heart of thedamper, has three ports in each direc-tion, is made out of sintered steel and isspecially developed for racing purposes.Until the end of 1998 it was a machinedsteel piston (#5415-01). The function ofthe machined piston and the sinteredpiston are identical. However, the sin-tered piston is 25 % lighter and the tol-erances are smaller making it easier tomatch dampers.

Thanks to the large piston diameter (44mm), a quick damping response can beachieved.

The piston is flat on both sides provid-ing easy control of the condition of thesealing surfaces.

The rebound side of the piston has amachined groove in the surface that hasno function other than making it easierto identify the rebound side. The com-pression side has no groove.

General shim informationÖhlins shim stack system will offer usalmost endless possibilities.

Depending on where a shim is posi-tioned in the stack it will have differentfunctions and effects on the dampingcurve. The shims are named by theirposition. See figure 15 and 16.

Some information is always valid, nomatter what type of damping curves youare looking for.

Usually the shims in the stack will besmaller and smaller the farther they arepositioned away from the piston. Thefarther away the shims are positionedfrom the piston, the less effect they willhave on the initial part of the dampingcurve (at lower velocities).

Shims with uniform thickness and di-ameters that get smaller in even stepswill reduce stress concentration in theshims, and reduce friction between theshims when flexing. However, this idealis not always possible.

The shims in the reservoir are smallerthan the ones on the main piston. Thei.d. of the shims on the main piston is 12mm, and in the reservoir the i.d. is 8 mm.The standard thicknesses are the same:0.15, 0.20, 0.25 and 0.30 mm. See chap-ter “Spare parts“, on page 54 for the dif-ferent o.d:s available.

With the diameters available for the

5. Internal adjustments


Figure 15. A typical Öhlins main piston shim stack.

Figure 16. A typical reservoir shim stack with the standard cupped valve body (part #01244-01).

Valve body

Preload reduction shim

Sealing shim

Clamp washer

Preload shims

Washer

Clamp shim

Centering shim

Ring shim

Sealing shim

Main piston

Preload shims

15


main piston it is possible to create stackswith 2, 4 and 6 mm increments of diam-eter change in even steps:

For 2 mm spacing use:38,36,34,32,30,28,26, etc.For 4 mm spacing use:38,34,30,26, etc.For 6 mm spacing use:38,32,26, etc.

With some combinations it is not pos-sible to have even spacing all the way tothe clamp shim. In those cases it is de-sirable to reduce the spacing as theshims get closer to the clamp shim.

A similar approach is used concern-ing shim thickness. Although uniformthickness is desirable, many times it isimpossible to achieve a particular damp-ing force without mixing the thicknesses.If this is the case Öhlins suggests thelarger shims be thinner and progressivelythicker as the shims become smaller.

Sealing shimAfter flowing through the piston, the oilfirst encounters the sealing shim. Thisshim closes the piston ports and mustalways be large enough to completelycover them. For this reason the diam-eter of the sealing shim is not variable.The TT44 main piston design requires aminimum sealing shim diameter of 38mm for the compression side and 36 mmfor the rebound side.

In the reservoir the sealing shim has adiameter of 18 mm.

As the sealing shim does cover theports, it also acts as a check-valve whenthe direction of oil flow reverses. Thethickness of this shim can be adjustedto give more or less overall damping. Thenumber of sealing shims can also bemultiplied for additional force but withsome compromise in low damper speedsensitivity. On the main piston the seal-ing shim is never preloaded so a thick-ness change here affects mainly theslope accompanied by a slight changeto the knee height.

When using the optional cup type mainpiston, see chapter “Optional new parts”on page 53, or the standard valve body(part # 01244-01) in the reservoir, eventhe sealing shim can be preloaded. Seefigure 18.

Ring shim and centering shimThe next shims are the centering shimand the accompanying ring shim. Theseshims are only used if preload is desired.

The outside diameter (o.d.) of the mainpiston centering shims is 34 mm asstandard for both the compression andthe rebound stack.

If a ring shim system is used in the res-ervoir this diameter is 15 mm. See be-low for a description of how the stack ispreloaded if the standard valve body isused.

The ring shim is positioned concentricto the centering shim and has the samei.d. as the o.d. of the centering shim (34mm). The o.d. of the ring shim is the sameas the sealing shim. Because of the dif-ferent o.d.:s of the sealing shims usedon the compression and the reboundside, the two ring shims also have differ-ent o.d.:s, 38 mm for compression and36 mm for rebound.

Ring shims with i.d. 30 mm and o.d.34 mm are also available. In using thesering shims, their o.d. will be smaller thanthe sealing shim and this will result in theknee of the damping curve getting morerounded. Figure 17 illustrates preloadedcompression and rebound stacks.

The reservoir ring shim i.d. is 15 mmand the o.d. 18 mm.

Ring shims are available in 0.20, 0.25and 0.30 mm thicknesses. Centeringshims are available in thickness 0.15,0.20, 025 and 0.30 mm. This will give amaximum preload of 0.15 mm.

The static preload is calculated by sub-tracting the thickness of the centeringshim from the thickness of the ring shim.Example:

A 0.30 mm ring shim minus a 0.20 mmcentering shim equals a 0.10 mmpreload.

As the centering shim is an additionalshim in the stack the overall slope of thedamping curve will be slightly increasedif ring shims are used (the stiffness of thestack increases). To minimize this sideeffect, the same static preload can beachieved by using a thinner centeringshim with a thinner ring shim, whichequals the same preload. For example a0.15 mm centering shim can be used

together with a 0.20 mm ring shim, in-stead of a 0.20 mm centering shim anda 0.25 mm ring shim.

If you require a higher preload than 0.15mm, a secondary ring shim combinationcan be used. (This is only valid for themain piston.) To do this, another shimwith the same o.d. as the sealing shimmust be used as a divider between thetwo ring shim combinations. It is not nec-essary that both ring shim combinationsare identical. The total preload can becalculated by combining the individualpreload at each ring shim.

Note: With 0.30 mm thick preloadshims we recommend that the maximumtotal preload is kept below 0.20 mm atthe main piston and 0.10 mm in the res-ervoir. For thinner shims more preload isacceptable. In either case more preloadcan be tried but the shims need to bemonitored for permanent bending.

If the standard valve body is used inthe reservoir, the preload is achieved bythe offset between the shim sealing seatand the center land where the preloadshims are clamped (see figure 18). Nomi-nally this offset is 0.40 mm. This distanceis reduced to the wanted preload by add-ing a stack of shims (preload reductionshims). For this purpose use only shimswith o.d. 12 mm. Example: 0.05 mm pre-load is built into the stack by adding1x0.20 mm and 1x0.15 shim mm givinga total height of 0.35 mm. 0.40 minus0.35 equals 0.05 – the desired preload.

If you require negative preload (a two-stage stack) on the main piston, it is pos-sible on both compression and reboundside. Figure 19 illustrates a negative pre-loaded compression shim stack. Nega-tive preload means that there is a gapbetween the ring shim and the followingshims in the stack. These following shimsare sometimes called the second stack.Negative preload is achieved by using

Figure 17. Cross section of compres-sion and rebound shim stacks with ringshim configuration. The preload in thefigure is exaggerated.

16

Figure 18. Cross section of preloadedreservoir shim stack with standardcupped valve body. The preload in thefigure is exaggerated.

Figure 19. Negative preloaded com-pression shim stack with a rebound shimstack without preload. The negative pre-load in the figure is exaggerated.


an extra shim that together with the cen-tering shim has a thickness exceedingthe ring shim thickness. This extra shimcan be placed between the centeringshim and the sealing shim or after thecentering shim if you wish to use the stiff-ness of the centering shim in the firststack. This extra shim can be of any o.d.smaller than the centering shim. The firststage stiffness is determined by the firststack only. The second stage stack willhave a stiffness resulting from the wholestack. Generally a gap of 0.05 mm isenough. In some cases more gap canbe used. There are two shim combina-tions that give a 0.05 mm gap. Both com-binations use a 0.30 mm ring shim. Firstcombination uses a 0.20 mm extra shimand a 0.15 mm centering shim for thesum of 0.35 height. Second combina-tion uses a 0.15 mm extra shim and a0.20 mm centering shim. The sum isagain 0.35 mm. The second combina-tion is preferable because the ring shimhas more engagement with the cente-ring shim. See also ”Stack preload” onpage 17 for further information.

Note: Always maximize engagementbetween the ring shim and the centeringshim.

Preload shimA preload shim is just as the name saysalways preloaded, even in its static po-sition. The preloading force produced bythe preloading shims is the result of theirspring rate multiplied by the distancethey are prebent. As the ring shim mustbe followed by a shim of the same o.d.,the first preloaded shim will always havethe same o.d. as the ring. Then comes avariable quantity of shims of variousthickness and diameters. All the shimsfollowing the ring shim except the clampshim will be prebent when the shaft nutis tightened because their centers will be

clamped solidly to the centering shimand their edges will rest on the thickerring shim. The numerical amount of staticpreload is only accurate for shims of thesame o.d. as the ring shim. All smallerdiameter preloaded shims are bent lessas the o.d.’s get smaller. This is whychanging a larger shim has more effectthan changing a smaller shim.

Quite often the majority of shims in thestack are in the preload category.

In the case of the reservoir, when thestandard valve body part is used the 12mm diameter shims under the sealingshim control the preload or lack of pre-load. These shims are only spacers. Allother shims in the stack are bendingshims. So here even the sealing shim canbe preloaded. The same is true of theoptional cup-type main piston with theexception of the additional clamp shim.

The static force produced by the pre-loading shims is determined by their stiff-ness times their static preload.

Clamp shim / clamp washerThe last shim in the compression andrebound stack of the main piston is theclamp shim. Unlike all the previousshims, the clamp shim never bends ormoves. As its name implies, the func-tion of this shim is to clamp the centersof all the other shims. It is really just asolid spacer and when the shaft nut istightened it creates a solid column, thesize of its o.d. through all the other shimsclear to the piston surface.

This solid column determines the ful-crum that all the flexing shims bendabout. The power of the clamp shim isin its variable o.d.. Increasing the clampshim diameter moves the fulcrum outand reduces the amount of unclamped,flexing part of all the other shims. Mov-ing the fulcrum out towards the pistonports effectively reduces the amount of

leverage the oil has on the shim, there-fore increasing the damping. When re-ducing the clamp the opposite is alsotrue.

Varying the clamp shim affects theentire stack. The knee and slope arechanged simultaneously by varying theclamp shim. Changing the clamp shimis very useful if you want to raise or lowerthe damping curve without changing thecharacteristic of the shim stack.

Note: A maximum clamp diameter of23 mm for compression and 26 mm forrebound is recommended.

Sometimes if there are few shims inthe stack, multiple clamp shims can beused as spacers to allow the stack toopen fully without contacting the valvestop. For the same reason we suggest aminimum clamp thickness of 0.30 mm.

The clamp shim diameters are avail-able in 1 mm increments. However, thepercentage of damping change for each1 mm step increases as the diametersincrease. The reason for this is that, eventhough the diameter steps are uniform,the percentage each 1 mm step en-croaches on the remaining unclampedshims will be greater and greater. There-fore a change from a 20 mm to a 21 mmclamp will not be as significant as achange from 21 mm to 22 mm.

In the reservoir there is no clamp shim.Instead the clamp function is integratedin the stepped washer (part # 00641-01),that also has the function of a valve stop.The stepped washer diameter is 10 mm.

Valve stopThis part provides a rigid base for theshim stack to work against. Once in awhile the valve stop is used to limit theamount the shims can bend. It is impor-tant that its surface is always flat andshould be checked occasionally with aprecision straight edge. �

17

In the earlier chapters we have dis-cussed the effect of the external ad-

juster on the damping curve and whatfunction the different shims have. How-ever, we would like to discuss furtherwhat influence different shim stacks haveon the damping curve.

Stack stiffnessShim stack stiffness is the same as shimstack spring rate. In theory the springrate increases progressively the furtherthe shims are flexed, but in practice therate is more or less constant.

The stiffness is difficult to measure,therefore it is never quantified as force/distance [lbs/inch] or [N/m]. The slopeof the damping curve does reveal thestiffness of the stack, therefore the stiff-ness is more often given in force/veloc-ity [lbs s/inch] or [N s/m].

Note: To change the slope of thedamping curve the stack stiffness hasto be changed.

Figure 20 illustrates the effect achievedby altering just the shim stack stiffness.The shim stack is modified incremen-tally to be stiffer and stiffer. Notice theslope lines are no longer parallel. Thestiffer the stack, the steeper the slope.

Changes in quantity, clamp diameterand stiffness of the individual shims allaffect the stack stiffness (spring rate):

The stiffness is directly related to thenumber of shims. Varying the numberof shims is used to change the slopeif just a small change is needed.

The stiffness increases progressivelywith the increase of the clamp diam-eter. A change of the clamp diameter

is the most powerful tool in changingthe slope of the damping curve and isonly used when larger changes areneeded. See “Clamp shim/clampwasher” on page 16.

Changing the stiffness of the individualshims can theoretically be done in twodifferent ways: change o.d. and/or thethickness. Changing the o.d. is sel-dom used except for fine tuning.Changing the thickness is the secondmost effective way of changing theslope. A lot can be gained by under-standing how the stiffness is relatedto the thickness of the shim.Read the next few paragraphs care-fully!

The stiffness of a shim is not linear inrelation to its thickness. If two 0.15 mmshims are stacked together the stiffnesswill not equal one 0.30 mm shim. In factit takes a little more than eight 0.15 mmshims to equal one 0.30 mm shim. Tocalculate how many thin shims it takesto equal one thicker shim the formula isapproximately the same as for compar-ing the stiffness of dissimilar constantsection beams. The procedure is to di-vide the thicker shim by the thinner shimto get the ratio between the thicknessesof the shims. Then if you cube the thick-ness ratio you will have an idea of howmany thin shims it takes to equal onethicker shim.

Example 1:0.30 ÷ 0.15 = 2.002.00 cubed (2.00 x 2.00 x 2.00) = 8.00Hence:0.15 x 8.0 shims = one 0.30 shim

Example 2:0.25 ÷ 0.15 = 1.671.68 cubed = 4.66Hence:0.15 x 4.66 shims = one 0.25 shim

Example 3:0.20 ÷ 0.15 = 1.331.33 cubed = 2.35Hence:0.15 x 2.35 shims = one 0.20 shim

Because the shims do not bend exactlylike constant section beams, this formulagives an answer that is not entirely cor-rect. In reality, the equivalency factor isslightly more than the result yielded bythe formula. Understanding shim equiva-lency is very useful if you are trying to hitspecific damping forces. As you can seethe equivalency factors are mostly unevennumbers. This can be used to your ad-vantage if you need to split the differencebetween adding one identical shim orleaving the stack unchanged. This knowl-edge will also help you select the appro-priate shim thickness for the dampingchange you are looking for.

Stack preloadThe preload of a shim stack is the dis-tance the shims in the stack are prebentby ring shims or cup pistons.

The preload always refers to theamount of prebending of the shim thatis prebent the most. (Depending on theposition in the stack, the shims can havedifferent amounts of prebending. If adouble ring shim arrangement is used,only the shims after the second ring shimwill be bent the sum of both preloads.)

In the case of ring shims, preload is

6. Shim stack theory


Figure 20. Influence of shim stack stiffness.

�Stifferstacks

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

100

200

300

400

500

Velocity (inch/sec)

Force (Pound)

18

Figure 21. Influence of shim stack preload.

Figure 22. Influence of shim stack stiffness with preload compensation.


�Stifferstacks

Preload mm(from topto bottom)0,200,150,100,05

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

100

200

300

400

500

Velocity (inch/sec)

Force (Pound)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

100

200

300

400

500

Velocity (inch/sec)

Force (Pound)

created when the ring shim is thicker thanthe centering shim. In the case of thestandard reservoir valve body or acupped main piston, the preload isachieved by the offset between the shimsealing seat and the center land wherethe preload spacer shims are clamped.

The term negative preload is used inthe case of a double stack, where it tellshow much the shim closest to the pis-ton has to open until it bumps into theremainder of the shims.

Figure 21 illustrates the effect of vary-ing only the ring shim. All the shims inthe stack except the ring shim are leftthe same as the original baseline. Thenumbers shown at the right edge of thefigure represent the amount of preload(the static bending on all the preloadingshims with the same o.d. as the ring shimcaused by the difference in thickness ofthe ring shim and its centering shim). In-creasing the bending of the shims be-

hind the ring shim raises the pressure onthe sealing shim positioned between thepiston and the ring shim. This causes theknee to move higher and higher up thetheoretical low speed curve. Because theknee follows the low speed curve, theknee is not only higher but starts at aslightly higher damper speed for eachpreload change.

The baseline curve is shown as hav-ing a preload of 0.05 mm. This could beachieved with a centering shim of 0.15mm thickness combined with a ring shimof 0.20 mm thickness giving a differen-tial of 0.05 mm. The first broken line couldbe achieved by increasing the ring shimto 0.25 mm, which gives a differential of0.10 mm, etc.

As only the ring shim is changed, thestiffness of the shim stack will not be af-fected (the spring rate of the stack re-mains the same) and the slope lines willremain parallel to each other. However,

for extreme changes in preload, somechange in the slope would be seen, asthe rate of the stack is progressive.

After all this discussion about preload,it is important to remember that Öhlinsoffers as many non-preloaded shimcombinations as those that are pre-loaded.

Knee and slope guidelinesThe position of the knee depends on thestiffness of the stack plus the preload ofthe stack. The slope is controlled onlyby the stiffness of the stack.

The shape of the knee is determinedby the opening characteristic, gradual orabrupt. The more abrupt the openingphase, the sharper the knee will be. Theconverse is also true.

Note: Even with zero preload there isstill some knee as it still takes a rise inpressure to open the shim stack.

The zero preload knee is much less pro-

19

-5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0 4.0 5.0-500

-400

-300

-200

-100

0

Velocity (inch/sec)

100

200

300

400

500

Force (Pound)

7. Hysteresis

Figure 23. Damping force during a complete sine-wave cycle illustrating hysteresis.

7. Hysteresis

Compression

Rebound

----- Deceleration

Acceleration

nounced than the knee from a preloadedstack, but it is still of importance.

When the stack stiffness is increasedand the static preload remains constant,there will still be an increase in pressureapplied to the sealing shim. The stiffershims behind the ring shim are now moreresistant to bending, so more preloadforce is created. This also occurs to alesser extent with non-pre-loaded shimstacks. There will also be an increase inslope as a result of the stiffer shims.

Whether it is changes to preload orstack stiffness that rises or lowers theknee, the starting point will always fol-low the theoretical low speed curve.

Because the low speed curve does notrise vertically this means a higher kneewill occur at a slightly higher damperspeed, and a lower knee at a slightlylower damper speed. The only way to

keep the knee opening speed constantis to compensate by closing the lowspeed adjuster a small amount.

If you desire an increase in slope butnot an increase in knee force, this canbe achieved by increasing the stiffnessof the stack (see “Stack stiffness” onpage 17) and then compensate for theadditional preload force by lowering thestatic preload. The opposite is also valid.

Sometimes compensation can beachieved by changing the diameter ofthe clamp shim while changing thethickness and/or quantity of large diam-eter shims at the same time. This tech-nique works for both preloaded andnon-preloaded shim stacks but is theonly option for non-preloaded shimstacks. The exact proportion for com-pensating is best determined with thehelp of a damper dyno.

Example: a larger clamp diameter to-gether with a thinner sealing shim and asmaller diameter of the following shim willgive more slope with a similar knee.

Figure 22 shows the influence of shimstack stiffness with preload compensation.

If you think the knee force is optimized,a slope change should not be performedwithout including preload compensation.

Also, if you determine the knee needsto be reduced, increasing the slope atthe same time is sometimes a good idea.The converse is also true.

When a steep slope is needed togetherwith a soft opening characteristic a nega-tive preloaded shim stack can be used.This makes the shim stack open smoothlyand quickly to small movements.

See also “Ring shim and centeringshim” on page 15. �

On damper graphs there is always adifference in ascending (accelera-

tion) and descending (deceleration) partsof the curve. We normally refer to this

area as hysteresis. Technically, the termhysteresis is related to energy losses buthere we are actually storing energy asthe damper acts like a spring.

Note: In figure 23, negative velocityillustrates compression, positive re-bound.

This delay in damping force build up

20

of this, hysteresis needs to be kept to aminimum.

The compressibility can be classifiedinto three different groups: elasticity ofthe damper parts, compressibility of theoil itself and compressibility of the gas inthe oil. Also the gas volume causes ahysteresis effect that might be seen onthe dyno curve depending on how wellthe dyno takes care of the gas force com-pensation. The elasticity of the damperitself is linear.

Sine wave damper test

0

0,1

0,2

0,3

0,4

0,5

0,6

0 1 2 3 4 5 6

The compressibility of the oil itself isclose to linear. However, the gas pres-sure will affect this part of the hysteresis.

Some hysteresis comes from the com-pression and expansion of gases in theoil. Gas bubbles in the oil will be com-pressed in a very progressive way andwill act almost like a slack in the system.

The gas in the damper is a result ofdifferent factors. Air enters the damperduring the filling procedure and some airis already integrated with the oil when itis delivered. But even without air therewill be gas bubbles at low pressuressince the oil contains different additivesthat boil at different pressures/tempera-tures.

When the oil is under low pressurethose additives change to a gaseousform which create bubbles. There will beeven lower pressure locally at the valvesthan the theoretical pressure calculatedfrom the damping force or measured bya pressure gauge, ie, due to dynamicpressure. With dynamic pressure, the oilvolume changes caused by movementof the main piston not having an imme-diate pressure response throughout theentire volume of oil. Gas bubbles can beprevented by maintaining the recom-mended nitrogen pressure and by addi-tional damping by the reservoir. Seechapter “Combining main piston and res-ervoir damping” on page 21.

The amount of hysteresis for a certaindamping force can be very different de-pending on the size of the piston (eg,pressure area) and the amount of oil un-der variation of pressure. The larger thepiston is, the easier it will be to reducethe hysteresis. This is explained by theformula F=pA. F is the damping force, pis the pressure and A is the pressurizedarea. For a specified damping force (F) asmaller area (A) will lead to a higher pres-sure (p). The higher pressure will com-press the oil more. This will cause morehysteresis. The piston shaft acts as asmall diameter piston sending oil to thevalve in the reservoir. Because the effec-tive pressure area is very small and thetotal oil volume is large, there will be alot of hysteresis from this portion of thedamping force compared to the damp-ing force produced by the main piston.

How much the delay affects the damp-ing curve is very much related to thestroke and frequency in the test. Whenkeeping the maximum velocity constantand varying the frequency and stroke itis very obvious that with a short strokeand a high frequency the hysteresis de-forms the damping curve more than longstroke and low frequency.

In figure 24, the relation between dis-placement and velocity is shown for a peakvelocity of 5 ips and a frequency of 2 Hz.

Note: The displacement at lower ve-locities is very small and every delay, slackin the system is shown very clearly. �

Figure 24. Stroke-velocity relation in a typical dyno run. Peak velocity 5 ips and fre-quency 2 Hz.

All Öhlins dampers, not just the racing dampers, are dyno testedbefore they are delivered to the customer.

7. Hysteresis

Velocity (inch/sec)

Pos

ition

(inc

h)

can always be seen during a completesine-wave cycle, even if the bleeds areclosed. So this delay is not related toleakage but to the hysteresis in thedamper.

Hysteresis is actually compressibilityin the damper system. This delays pres-sure rise and pressure drop. Hysteresisaffects the performance of the damper.In formula racing where the damperstrokes are short, you are dependent onquick damping force build up. Because

21

8. Combining main piston and reservoir damping

Figure 25a. Combining main piston and reservoir damping. Example 1. (RES-15-05, RC:2.)

Figure 25b. Combining main piston and reservoir damping. Example 2. (RES-15-05, RC:8.)


Piston + reservoir

Main piston only

Reservoir only

Piston + reservoir

Main piston only

Reservoir only

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

100

200

300

400

500

Velocity (inch/sec)

Force (Pound)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

100

200

300

400

500

Velocity (inch/sec)

Force (Pound)

The compression damping is the sumof the damping curves from the main

piston and the one from the reservoir.Both curves consist of a nose, a kneeand a slope.

In the figures 25a, 25b and 25c thecompression damping from the main pis-ton is the same (Baseline compressioncurve).

In the first example the reservoir stackis preloaded 0.05 mm (stack RES-15-05)and the reservoir adjuster is set to clickposition 2.

In example 2, only the click position ofthe reservoir adjuster differs from exam-ple 1. Here it is at click position 8.

In the last example, the click positionis back at click position 2, but now the

preload has increased to 0.15 (stackRES-15-15).

The total damping at a specific speedis calculated by adding together the mainpiston damping and the reservoir damp-ing at that speed.

As mentioned in the chapter abouthysteresis, there will be a difference inforce build up depending on the differ-ent stiffness of the two systems.

Assuming that there is no restriction inthe reservoir, the compression dampingforce is built up by a pressure drop onthe rebound side of the main piston. Byusing a restriction in the reservoir therewill also be a pressure build up on thecompression side of the piston. This re-sults in a quicker pressure build up since

we have a pressure increase on one sideof the piston at the same time as thepressure is dropping on the other sideof the piston. Note that under these cir-cumstances the reservoir restriction re-duces the hysteresis. The amount of res-ervoir damping needed to achieve quickdamping build up is small and all exces-sive reservoir damping will cause highhysteresis.

We can find the exact amount of res-ervoir damping needed to maximize thecompression damping force build up byusing the available pressure areas in sucha way that the flexing of the damper isminimized. We call it “balancing” the res-ervoir damping with the main pistondamping. The method will be described

22

Figure 25c. Combining main piston and reservoir damping. Example 3. (RES-15-15), RC:2.)

force build up, we have to use all areasavailable. By spreading the dampingbetween the main piston and the reser-voir in relation to their area relations, the

damping force build up can be maxi-mized.

The pressure area of the main pistoncan be calculated by first calculating thewhole area based on the o.d. of the pis-ton. However, because of the presenceof the shaft on the rebound side all theoil on the compression side cannot travelacross the piston. To find the pressurearea at the main piston, Apiston, the shaftarea, Ashaft, must be subtracted from thetotal piston area. The subtracted shaftarea represents the oil that is forced intothe reservoir. The formulas below de-scribe the theoretical relation betweenthe total damping, Ftot, and the dampingfrom the reservoir, Fshaft, if maximum pres-sure build up is the goal.

Ftot = Dp Ashaft + 2Dp ApistonFshaft = Dp AshaftFshaft / Ftot = Ashaft / (Ashaft + 2 Apiston )

For the TT44 the ratio Fshaft / Ftot is 0.07.

This means that the damping from thereservoir should be 7% of damping fromthe main piston. Normally we recom-mend a setting, where the reservoirdamping is more like 15% of the totaldamping.

If you study the damping force buildup more carefully, you would find it to bequite complex. However, even if the dis-cussion above is very simplified the re-sult is very useful as a rule of thumb.

Note: There are sometimes situationswhere you are not looking for a maximumbuild up of damping force.

Since the rebound damping is causedonly by the main piston oil flow, no bal-ancing between the main piston damp-ing and the reservoir damping can takeplace. �

The Öhlins four poster rig can reproduce both dynamic and aerodynamicforces and is widely used by race teams and car manufactures.


Piston + reservoir

Main piston only

Reservoir only

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00

100

200

300

400

500

Velocity (inch/sec)

Force (Pound)

in the text below.A damping force is the result of a pres-

sure difference multiplied by actual area.If we want to maximize the damping

23

9. Damping functions

In 1993 Nigel Mansell won the CART championship for theNewman-Haas team with very special Öhlins dampers. The expe-rience gained with these dampers led to the TT44 design.


Historically, dampers were asked onlyto provide a comfortable ride. If you

were lucky, driver controllability was en-hanced at the same time. With the ad-vent of ground effect aerodynamics inthe late seventies, racing engineers dis-covered that damper settings are a valu-able tool for optimizing “aero” effects. Atthe same time the tire companies foundthey needed to redesign their tires to takeadvantage of the downforce created bythe new ground effect aerodynamics.The mechanical grip of these new tiresalso turned out to be extremely sensi-tive to damper settings. These develop-ments doubled the number of duties re-quired of dampers of today. The prioritylist today for racing damper functions is“aero” management, mechanical grip,tire wear, driver controllability and ridecomfort. Dampers have a powerful in-fluence on the performance of your car.

These five damper functions are all in-terrelated but at the same time optimizingone of these functions can sabotageanother. A compromise between func-tion goals is many times unavoidable.Finding the most effective compromiseis the overall goal and will pay dividendson the racetrack.

Comfort, grip and controlThese five goals are so tightly interwo-ven that most of the time it is very diffi-cult to make a damping change and thenproperly assign the performance gain orloss to the correct category. For exam-ple, let us say you have added some ex-tra compression damping to the frontdampers and now the front tires havegained grip. The question is did we cre-ate pure mechanical grip from the tiresor is the gain from improved aerodynam-ics or from better dynamic fore/aft pitch

control or possibly a higher dynamic rideheight or center of gravity which couldalso change dynamic roll centers. It isnot essential to know the exact causeand effect, but it is possible through acleverly planned sequence of subse-quent tests to better isolate the gain andassign it to the right category. If this isachieved, the focus of further testing willbe more on target and the possibility ofa wayward theory will be minimized.

All this may sound too hypothetical butrest assured if you optimize the aerody-namic potential without compromisingthe grip and then find the mechanicalbalance by adjusting the springs, sway-bars, etc. the driver controllability willmost likely be there automatically. Theride comfort may be compromised butdo not be too concerned. Even thoughÖhlins dampers generally produce animproved ride quality, we have found thatdamper settings that give too much com-fort cannot provide optimum grip or con-trollability.

Ground effect carsIn this ground effect age, dampers canmaximize the amount of downforce gen-erated by the underside of the car byassisting in maintaining a constant airgap between the underside and theground. Today we have basically twotypes of formula car ground effect con-figurations, tunnels and flat bottoms.With both types the clearance betweenthe underside and the ground is very criti-cal. Generally there is a ride height “sweetspot” that is favourable for generatinghigh downforce with a minimum of “aero”drag. The problem is that this “sweetspot” is very close to the ground. Gooddamper settings will keep the car at thisideal ride height a higher percentage of

the time through most dynamic condi-tions without allowing the underside tocontact the ground (bottoming).

With both tunnel and flat bottom carsthe center of the downforce is foundwhere the geometry of the undersidecomes closest to the ground. With ei-ther type of car the center of downforcemigrates with any change in pitch anglein relation to the ground caused by brak-ing, cornering or acceleration. This mi-gration of the center of downforce altersthe handling balance by increasing thedownforce towards the direction of mi-gration and reducing the downforceaway from the migration. Therefore,added tire grip will occur at the end ofthe car that moves closer to the ground.

Tunnel cars have far less downforcemigration than flat bottom cars becausethe contour of the tunnel is curved in theshape of a venturi with a raised entry thatcurves down to a short flat area followedby a long, slowly enlarging exit. Tunnelsare generally positioned near the vehi-cle center of gravity. The tunnel flat partis in the closest proximity to the groundand that is where the center of downforceoccurs. When the car pitches fore or aftthis part of the tunnel primarily rocksback and forth and does not raise orlower significantly. Tunnels minimizedownforce migration.

Flat bottom with diffusersOn the other hand, flat bottom cars withdiffusers can have the downforce migratefrom just ahead of the diffuser at the backall the way to the tip of the nose underbraking. For cars with raised noses themigration will essentially stop where theunderside begins to move away from theground. Flat bottom cars are much moresensitive to static and dynamic pitch

24

pression damping allows the tire to movefreer and ride up on top of the pavementgrain, metaphorically similar to “dry aq-uaplaning”. As the compression damp-ing is increased the tire will interlock withthe pavement and grip will increase. Ifthe damping is further increased incre-mentally, eventually the grip will stop im-proving and begin to go down. This ismainly caused by too much pressurefrom the suspension that overheats thetire or compresses it too much, givingunduly high tire load variations. Keep inmind that the suspension pressure thetire feels is the sum of the compressiondamping, the spring rate, the sway-barrate and possibly the torsional rigidity ofthe chassis. If the pressure sum seemsto be optimized for grip but for other rea-sons it is indicated that one componentof the sum needs to be increased, an-other component may need to be re-duced. For instance, a higher spring ratemay be necessary to reduce fore and aftpitching. In order to make the stifferspring work properly the compressiondamping may need to be reduced. Inanother case one car might have less

torsional stiffness in its chassis than an-other. To compensate for this the car withlower chassis stiffness will require morecompression damping to make the sus-pension pressure sum high enough. Anindicator of too much suspension pres-sure is controllable sliding at all speedsand all phases throughout the turns (flatsliding).

Grip and rebound dampingGrip in relation to rebound dampingworks in a slightly different manner. Re-bound damping only occurs after therehas been some compression of thedamper and spring. The pavement grainconstantly causes small wheel move-ments of the suspension system. The re-bound damping controls the expansionin these small displacements. If the re-bound damping is excessive, the expan-sion will be too slow leading to a loss ofgrip. This type of grip loss will be par-ticularly noticeable in rear tire forwardtraction with the application of power.Cornering grip will not be as dramaticallyeffected as forward traction.

If a lot of rebound damping is used thesuspension will be dynamically pumpeddown which can improve the aerody-namic downforce. If there is enough“aero” gain it can more than offset anyloss of grip due to slow rebound recov-ery. When this approach is used compres-sion damping is generally reduced at thesame time to help the pumping down. Wehave seen success with this approach,but today most teams are pursuing thehigh compression, low rebound techniquewith even better results. Both philosophieshave their place. It seems that in theclasses where the downforce potential ismuch less, the proportion between com-pression and rebound damping leans to-wards less compression and more re-bound damping.

Qualifying or raceIn most cases vehicle stability will be quiteacceptable when the damper has beenadjusted for optimum “aero” and gripmanagement. Sometimes “aero” and gripneed to be slightly compromised in orderto adapt to the style of different drivers. Inthe final analysis a car that is more driverfriendly will prevail over a car with ultimategrip that is also nervous.

Sometimes settings that are good forqualifying can be too hard on tires after alot of laps. Our experience suggests slightlymore compliant damper settings for therace than those used during qualifying.

One final word about ride comfort.Harshness is either from a suspensionthat is too stiff to comply with bumps orfrom a suspension that shakes becauseof inadequate damping. Deciding whichcondition exists in your car plus a reviewof your damper settings can guide youin solving harshness problems. �

Today the Öhlins TT44 design is used in touring car racing and haveeven been tested on bikes. The picture is showing a BTCC front strut.


changes than ground effect cars.Damper settings for flat bottom carstherefore need to be biased more to-wards pitch control than the settings fortunnel cars.

Both tunnel and flat bottom cars canalso benefit by keeping the undersideparallel to the ground, side to side, whilecornering. In this case the downforcemigrates from side to side but also willdiminish substantially if the inside of thecar raises away from the ground. For carsthat turn only one direction as on an oval,sometimes higher corner speed can beachieved by increasing the compressiondamping and reducing the rebound onthe right side (outside) and the oppositeon the left side (inside).

Grip and compression dampingCompared to aerodynamics, under-standing the dynamics of tire grip is moreelusive and the perceived rules changefrom one type of tire to another. It seemstire grip is created when the tire ispressed into the track surface enoughto cause the rubber to interlock with thegrain of the pavement. Not enough com-

25

Racecar set-ups as well as track con-ditions can vary in an endless

number of ways. There is no informationavailable about the optimum damper setup for just your car. However, to help you,Öhlins distributors worldwide havespecification cards available with recom-mended damper settings. These settingsare track proven with good results forracecars with their manufacturer’s stand-ard configurations.

Öhlins does not suggest that these set-tings are perfect and cannot be im-proved, but we recommend them as apoint at which to start testing. The spec.cards give information about valving aswell as clicker settings. For most vehicleclasses both road course and oval set-tings are available. Sometimes recom-mendations for springs are also available.

Note: Dampers delivered by the dis-tributor are usually valved and adjustedfor road courses with the exception ofIRL dampers (Indy Racing League). Thedampers can usually be modified by thedistributor to almost any setting the cus-tomer desires.

Updated continuallyThe information on the spec. cards isupdated continually. Make sure you havethe latest version of the spec. cards. Theolder the design date on the spec. cardis, the greater the possibility of a later,improved spec. card.

For quality control, all Öhlins TT44

10. Factory recommended damper set-ups

dampers are dyno tested at least oncebefore they are delivered to the customer.They are first tested at the Öhlins factoryin Sweden. At the Öhlins factory, thedampers are always tested with the set-tings according to the specificationcards. At our distributors the settingsmay be changed by request or updated,then before delivery the dampers aretested a second time.

Standard or modifiedIf track testing indicates that your racecarworks best at clicker positions far awayfrom the spec. card suggestions, this isa clue indicating that the internal shimstacks are not quite right for your appli-cation. Undesirable handling character-istics also suggest that a shim stackmodification is in order.

If your car is modified from “standard”it is difficult to predict how much thedamping needs to be changed from thestandard settings. However, if the modi-fication is to the damper rocker ratio,compensating damping force for a newratio can be calculated. Having intimateknowledge of your damper mountinggeometry is the key to predicting theproper amount of damping forces.

Rockers and click settingsWhen it comes to proper clicker settingsin relation to damper rocker ratios, Öhlinshas some general rules. For racecarswith damper/wheel rocker ratios of

around 1.0, we recommend for lowspeed compression (LSC) a clickerrange of 6 to 12 clicks. For low speedrebound (LSR) 15 to 25 clicks is a goodstarting range.

If your rocker ratio moves the damperslower than the wheel the clickers willneed to be set to lower numbers to givemore low speed damping. The reasonis the wheel has mechanical leverageover the damper and the dampingforces at the damper will end up lesseffective at the wheel. In addition, theleverage also causes the damper pis-ton speed to be lower. Thus the originaldamping must be multiplied by thechange factor and then the new damp-ing force must be moved to a lower pis-ton speed this time dividing the speedby the same factor.

Tightening the clicker will achievemore damping at a lower speed. How-ever, more often than not, shim settingsneed to be changed to compensate forleverage changes.

Conversely, if your rocker ratio movesthe damper faster than the wheel theclickers need to be set to higher num-bers to give less low speed damping andthe damping change factor now needsto divide the original damping and mul-tiply the piston speed.

Öhlins technicians can help you cal-culate damping curves necessary tocompensate for changes in damper/wheel rocker ratios. �

Öhlins distributors worldwide have specificationcards available with recommended damper settings.

10. Factory recommended damper set ups

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