Repairs to underbody damage of

Monocoque vehicles

The underbody is the foundation on which the

vehicle is built. It is of the utmost importance,

therefore, first to determine the extent of the collision

damage to the underbody and then to correct

it. The repairer must be able to determine what,

if any, damage exists before attempting a repair. It

is also necessary to determine whether any underbody

misalignment exists which requires repair

before attempting to correct body alignment and

damage. The majority of body repair shops have

alignment and repair equipment capable of handling

this type of repair. Although at present the

majority of British manufacturers are producing

motor vehicle bodies designed and built on mono

or unitary construction principles, there are still

some special sports models which have independent

chassis construction, in addition to the

available European and American vehicles which

are based on composite construction. Consequently

body repair establishments must make provisions

to deal with any such vehicles which may be

brought in for repair.

The main idea of straightening underbody damage

is to exert force in the reverse direction from

that of the collision impact. Alignment and repair

equipment permits several pulling set-ups to be

made at one time. This type of equipment allows

the vehicle to be aligned using either brackets or

a measuring system. Repairs are carried out using

a combination of pulling equipment, body jacks,

chains, clamps, wood blocks and welding equipment.

The severity of the damage decides the

method of repair and techniques to be used. In the

case of a vehicle which has received extreme

damage, and in which the underbody has twisted

badly, the method of repair is that the vehicle

must be mounted on to the repair system and the

exact amount of distortion established by using

either brackets or a measuring system. Whilst the

vehicle is still mounted on the system, the underbody

must be pulled back into alignment first and

then checked against the manufacturer’s data, especially

the critical suspension and engine mounting

positions.

First rough out the damage but do not remove

any panels unless it is necessary. Body damage and

alignment should, wherever possible, be corrected

at the same time as any underbody damage. Work

can be made easier by stripping off any body

panels or mechanical parts which might interfere

with accessibility to the damaged area. As side

and cross members are usually formed from lowcarbon

steel, heat may be used to relieve stresses in

badly creased areas. However, when these members

are made from high-strength steel, heat must

not be used because of its weakening effect; the

metal must be repaired in a cold state. For lowcarbon

steel, heat may be applied with an oxyacetylene

welding torch. Only a very little heat

is required to bring the colour of the heated

member to a dull red, at which time it is at a proper

temperature to be worked. Start to heat the buckled

portion or damaged area well out near the edge.

Pan the flame over the entire buckled area so that it

is heated uniformly. Best results can be obtained

by moving the flame in a circular motion until

the entire area is heated. Heating well out towards

the edges of the damaged area first will remove the

chill from the surrounding area, thus preventing the

damaged area from cooling too rapidly.

456Repair of Vehicle Bodies

Pulling equipment can be used for heavy external

pulling by anchoring it to the under-body

members using special bolt-on clamps or chain

attachments and pulling the damaged section back

into line. Where it is difficult to obtain an anchor

point owing to inaccessibility, a substantial metal

tab is welded on to the member; this can be used

for pulling and later removed. In some cases a

combination of pulling and pushing is needed when

correcting damage, and this requires a heavy-duty

body jack to be used in conjunction with pulling

equipment. Wood blocks can be used to spread the

load to prevent distortion. As the repair proceeds

with a combination of pulling, pushing and heating,

continuous checks should be made for alignment,

as it is sometimes necessary to overcorrect to

allow for springback in the metal structure. A final

alignment check should be made after the tension

is released.

Wheel alignment (steering

Geometry)

Over the years there has been increased awareness

of the importance of wheel alignment, and particularly

four-wheel alignment, as an integral part of

the major crash repair procedure. Straightening a

vehicle involves two separate alignment operations.

First, the body must be aligned, making sure that

suspension mounting points are in the correct

position. Once the vehicle is jigged, in many cases

a mixture of old and new suspension components

is fitted. Secondly, a full geometry check

must be undertaken to satisfy the repairer that the

car is within the manufacturer’s tolerances after

the repair.

Wheel alignment has an important role to play

in the overall safety of the modern vehicle, and

proper testing equipment in this area is now vital.

Much of the new technology caters for what is now

regarded as four-wheel or total alignment. Modern

vehicles have become far more sophisticated. They

may have active rear suspension, which can also

be computer controlled. These set-ups have an

element of steering built in to complement the

work done by the front wheels: the system is commonly

known as rear wheel steering or four-wheel

steering. Rear wheel alignment with a fractional

variance from the manufacturer’s setting can now

be critical to a car’s handling and steering. It is not

normal for cars to veer to left or right: suspension

systems are designed and set up by the manufacturer

so that the car will travel straight, regardless

of road crown.

The main purpose of wheel alignment is to allow

the wheels to roll without scuffing, dragging or

slipping on the road. The proper alignment of

suspension and steering systems centres around

the accuracy of six control angles: camber, caster,

steering angle inclination (SAI), scrub radius, toe

(in and out), and turning radius (Figure 14.93).

Inspection and test drive

Thorough visual inspection and a test drive on a

straight flat road should show whether the car

needs to go for wheel alignment. The basic checks

are:

1 Measure ride height.

2 Does the vehicle appear level or is there any

sagging?

3 Are the tyres properly inflated and of correct

size and specification?

4 Is there any uneven wear on the tyres?

5 Low-speed wobble may show up a separated

tyre tread.

6 High speed may reveal steering wear and odd

noises, indicating loose or damaged parts.

Pre-alignment check

This breaks down into two areas: tyre inspection

and steering component inspection.

Tyre inspection, front and rear, is as follows:

1 Check for any unusual wear patterns: camber,

toe wear, inflation wear, cupping.

2 Check for any physical problems: ply separation,

dry rot.

3 Check tyre size and type: same size side-toside,

same brand and tread pattern. Do not mix

radial-ply and bias-ply tyres.

4 Tyre pressure and wheels: check pressure and

set to specifications; check for bent or eggshape

rims.

Steering component inspection is as follows:

1 Tie rod assemblies.

2 Idler arm and bushings.

3 Centre link and joints.

4 Manual steering gear: check for excessive play

and leaks, check U-joint at steering shaft.

Major accident damage 457

5 Power assisted steering (run engine): check for

excessive play; inspect all hoses and seals; check

fluid level and belt condition; check power

assisted balance.

6 Ball-joints and wheel bearings.

7 All bushings, front and rear.

8 Coil springs, torsion bars and shock absorbers.

9 Calibrate alignment equipment.

Wheel alignment procedure

Two-wheel alignment is still used in many workshops

as it takes less workshop time, is easier

and requires less expensive equipment. However,

its efficiency relies on the car having two parallel

axles, and front and rear wheels perfectly in line

with each other.

Four-wheel alignment owes much to computer

technology, which can instantly check and define

the alignment of a vehicle. As full wheel alignment

starts from a centre-line reference on the rear axle

and then checks all steering geometry angles on

all four wheels, a simultaneous four-wheel check

will speed up the entire operation. Computer-based

alignment systems use computer power to carry

out the calculations, so that you can be certain of

centralizing the steering wheel and alignment of

the vehicle. This ensures that the steering wheel

spokes are horizontal when the car is travelling

straight ahead (Figure 14.94).