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).