Chapter 9Vehicle Construction

Objectives

• Define the most important parts of a vehicle• Explain body design and frame variations• Compare unibody and body-over-frame

construction• Identify the major structural parts, sections, and

assemblies of body-over-frame vehicles

Objectives (continued)

• Identify the major structural parts, sections and assemblies of unibody vehicles

• Summarize how to classify vehicles by body, engine, and drivetrain configurations

Introduction

• Vehicle construction refers to how a vehicle is made

• Vehicles are a maze of interacting mechanical-electrical systems

• Over 15,000 parts are used in a typical vehicle• Damage to one part can affect the operation of

another seemingly unrelated part• Different types of construction require different

methods repair frame/unibody damage

Crash Testing

• Vehicles must be light, aerodynamic, and yet strong and safe

• Computer-simulated crash testing is used before building a prototype to find weak structural areas

• Critical that passenger compartment is strong enough to prevent injury

• Certified crash tests are done with a real vehicle and sensor equipped-dummies

• Crush zones are built into the frame or body to absorb some of the energy of a collision

Vehicle Classifications

• Vehicle classification relates to the construction, size, shape, number of doors, type of roof, and other criteria of a motor vehicle

• To communicate properly in collision repair, you must understand these basic terms

Vehicle Construction

• Part or component refers to the smallest units on a vehicle

• An assembly is several parts that fit together • Panel refers to a large removable body part• Pan refers to a floor-related component• Chassis includes everything under the body,

consisting of the mechanical systems that support and power the car

(B) With body-over-frame construction, a thick gauge steel frame provides the foundation for holding other parts. This type of construction is commonly used on large trucks and SUVs.

Figure 9-4. Two very different methods used to construct modern vehicles: unibody and body-over-frame construction

(A) Unibody construction welds major body panels together to form the frame for attaching the engine, drivetrain, suspension, and other parts. This type of construction is commonly used on cars.

Full Frames

• Body-over-frame vehicles have separate body and chassis parts bolted to the frame

• Full frame vehicle is heavier - high amounts of energy are absorbed by the frame in a collision

• Torque boxes allow some twisting to absorb road shock and collision impact

• Crossmembers extend sideways across frame rails to support the engine, suspension, chassis

• Full- or partial-frame construction is used on most full-size and some small pickup trucks

Full Frames (continued)

• Frame is an independent separate part• Perimeter frame has a frame rail near the

outside of the vehicle, and is the most common• Ladder frame has long frame rails with a series

of straight crossmembers, and is seldom used• Partial frame is a cross between a solid frame

and a unibody– Sub-frame assemblies are used at the front and

rear while the unibody supports the middle area

Unibody Construction

• Unibody construction uses body parts welded and bolted together– Uses lighter, thinner, high-strength steel alloys

• Body shell is formed by welding sheet metal into a box- or egg-like configuration– Strength is achieved through shape and design

instead of mass and weight

• Stressed hull structure disperses force over the entire body shell

Space Frame

• Space frame vehicle has a metal body structure covered with an outer skin of plastic or composite panels

• Roof and quarter panels may be attached with mechanical fasteners or adhesives

• After a collision a space frame is more likely to have hidden damage, or hidden corrosion

• Support members are bolted to unibody bottom– Needed in high-stress areas to reduce body flex

Figure 9-7. The suspension and braking systems bolt directly to the body on a vehicle with unitized construction.

Joining Parts

• Fastened parts are held together with fasteners• Welded parts are permanently joined by welding• Press-fit or snap-fit parts use clips or an

interface to fit parts together• Adhesive-bonded parts use high-strength epoxy

or special glue to hold parts together• Composite unibody is made of plastics and other

materials– Keeping metal parts to a minimum cuts weight

while increasing strength and performance

Figure 9-8. Note the space frame construction. Composite (plastic) panels fasten to a metal inner body structure. Composite panels can be made flexible to resist door dings and small dents.

Major Body Sections

• Vehicle is divided into three sections– Front, center and rear

• Front section includes everything between front bumper and fire wall– Also called nose section, front clip, “doghouse”

• Center section or midsection includes body parts that form passenger compartment– Also called a “greenhouse”

• Rear section also called tail section, rear clip, “cathouse”

Panel and Assembly Nomenclature

• Panel is a stamped steel or molded plastic sheet that forms a body part

• When panels are joined with other components, the result is an assembly

• Vehicles built for American roads– Left side is steering wheel side

– Right side is passenger side

Front Section Parts

• Cowl is near rear of front section, right in front of the windshield

• Shock towers or strut towers are reinforced body areas for holding upper parts of suspension

• Dash panel, firewall, or front bulkhead is panel dividing front and center sections

• Bumper assembly bolts to front frame horns or rails to absorb minor impacts

Figure 9-12. This top view of unibody construction shows how structural members are added to support the engine suspension and other mechanical systems.

Center Section Parts

• Floor pan is main structural section in bottom of passenger compartment

• Tunnel is formed in floor pan for transmission and drive shaft

• Pillars are vertical body members that hold roof panel in place and protect in case of rollover

• Front pillars extend next to windshield edges– Must be strong; also called A-pillars

• Center pillars or B-pillars, are roof supports between front and rear doors

Center Section Parts (continued)

• Rear pillars, or C-pillars, extend up from quarter panels to hold rear of roof

• Rocker panels or door sills are strong beams that fit at bottom of door openings

• Rear shelf, or package tray, is a thin panel behind rear seat in front of back glass

• Window regulator is a gear mechanism that allows you to raise and lower door glass

• Side impact beams are metal bars or corrugated panels that bolt or weld inside door assemblies

Figure 9-15. Doors normally have strong steel beams under the door skin to protect people during side impact collisions.

Rear Section Parts

• Rear hatch is a larger panel and glass assembly hinged for more access to the rear

• Quarter panels are large side body sections that extend from side doors back to rear bumper

• Lower rear panel fits between trunk compartment and rear bumper

• Rear shock towers hold rear suspension• Inner wheel housings surround rear wheels

Gaskets and Seals

• Various gaskets and rubber seals prevent air and water leakage between body parts

• Seals or weatherstripping are often used around doors and the rear deck lid

• Rubber seal is partially compressed when door or lid is closed to form a leakproof connection

• Rubber gasket often seals stationary glass where it fits into body

Anticorrosion Materials

• Anticorrosion materials prevent rusting of metal parts

• Undercoating is often a thick tar or synthetic rubber-based material sprayed onto underbody

• After performing repairs, you must restore all corrosion protection

Sound-Deadening Materials

• Sound-deadening materials help quiet passenger compartment– Insulation that prevents engine and road noise

from entering passenger area

Engine Locations, Drivelines

• Longitudinal engine mounts to crankshaft centerline front-to-rear– Used by front-engine RWD vehicles

• Transverse engine mounts sideways in engine compartment, its crankshaft centerline extends toward right and left

• Front-engine, front-wheel drive (FWD) has both engine and transaxle in front– Constant velocity (CV) axles extend out from

transaxle to power front wheels

Engine Locations, Drivelines (continued)

• Front-engine, rear-wheel drive (RWD) has engine in front and drive axel in rear

• Rear-engine, rear-wheel drive (RRD) has engine in back – Transaxle transfers power to rear drive wheels

• Mid-engine, rear-wheel drive (MRD) has engine behind front seat

• All-wheel drive uses two differentials for all wheels• Four-wheel drive uses a transfer case to send

power to two differentials and all wheels

Vehicle Sizes

• Compact car is smallest body classification– Normally uses a 4-cynlinder engine

– Lightweight, gets the highest gas mileage

• Intermediate car is medium in size– Uses a 4-, 6- or 8-cylinder engine

– Usually has a unibody construction

• Full-size car is large, heavy and often has a high performance V8 engine– Either unibody or body-over-frame construction

Roof Designs

• Sedan is a body design with a center pillar that supports roof

• Hardtop does not have a center pillar to support roof

• Hatchback has a large third door at back, commonly found on small compact cars

• Convertible has a retractable canvas roof with a steel tube framework, or a removable hardtop

• Station wagon extends roof straight back to rear of body with a rear hatch or tailgate

Vans and Trucks

• Van has a large box-shaped body to increase interior volume or space

• Full-size van normally is front-engine, RWD• Minivan is smaller and often uses front-engine

FWD with unibody construction• Pickup truck normally has a separate cab and

bed, typically with front-engine, RWD

Summary• Vehicle classification relates to construction, size,

shape, number of doors, type of roof, etc. • Three main types of frame construction

– Body-over-frame, unibody, and space frame

• Vehicle commonly divided into 3 body sections– Front section, or nose section

– Center section, or midsection

– Rear section, tail section, or rear clip

• Vehicle sizes: compact, intermediate, full size