Repairs to monocoque bodies
By pulling
The method of correcting damage by pulling from
the outside is not new, but the application of this
principle has developed into two main methods.
These are direct and vector pulling.
Major accident damage 453
creates a pulling force on the damage. As pressure
is increased the angle will continuously change in
response to the pressure applied (Figure 14.90).
only be achieved by the use of alignment and repair
equipment; visual inspection is not satisfactory and
can lead to inaccuracies. However, the vehicle must
still be considered as a unit and the pulling force
applied must act on the whole of the body and not
just on the damaged parts.
The development of pulling systems has meant
that in dealing with major body repair work it is
possible to get a direct upward or downward pull
to the front, rear or either side of the vehicle using
single or multiple pulling equipment.
When repairing mono constructed bodies, basic
body repair methods and a knowledge of panel
beating is absolutely essential. The combination of
methods always necessary in repairs is as follows:
heavy external pulling, use of body jack, use of
heat (only on low-carbon steels and not on highstrength
steels), use of power tools and cutting out
buckled sections or reinforcing members. Before
any pulling commences, the angle of the pull is
already known, but difficulties can arise in setting
up the precise angle with the equipment. This problem
can be solved by using the principle of parallel
lines. First examine the damaged parts carefully
and take accurate measurements to see in which
direction, and by how much, the damaged parts
have moved. This means that both lateral and vertical
movement must be determined to make sure
the correct reversal of this damage can be achieved.
When this is known, the first pulling must always
start as near to the major impact as practicable.
Heavy external pulls are a must for all severe
mono constructed body damage because the impact
has stressed the body metal. It is important when
pulling damaged sections back to their original
shape not to apply all the force at one place at the
same time. Spread the restoring forces by using
multipulls, which will allow the pulling forces to
be activated from several points on the damaged
section simultaneously. This method of pulling is
more effective than single pulls on severely damaged
vehicles (Figures 14.91 and 14.92). The body jack
can also be used to assist by applying additional
forces at the same time so they help each other in
working the panel back into place.
With all mono constructed bodies, special attention
must be given to certain load-bearing or structural
members such as frame side members, inner
wheel arch panels, cowl areas, cross members and
the floor pan that form a part of the cross members
or box sections. These must be brought back to the
Figure 14.90Vector pulling (Dataliner, Geotronics Ltd )
Figure 14.89Direct pulling (Kroll (UK) Ltd )
The principle involved in a pulling repair is
to apply force to the car body that is opposite to
the force that originally damaged it. With a monocoque
body, the engine, suspension and steering
rack all contribute to the final strength and shape
of the vehicle, so it is important not to remove
them unless absolutely necessary for the type of
repair being carried out.
Before beginning actual pulling, establish exactly
the nature and type of damage sustained. This can
454Repair of Vehicle Bodies
equivalent of their original condition, which means
returned to their original shape without wrinkles or
excess thinning of the metal.
The first step in making any hook-up is to
anchor the pulling equipment into anchor pots, a
rack-system, or an alignment and repair system.
Next, a pulling attachment such as a sheet metal
clamp or pull hook is secured to the damaged
part. Work the damage out of the metal with
a hammer and dolly while the metal is under
tension from the pulling equipment. Use heat
sparingly as required to remove severe wrinkles.
Where sheet metal damage is widespread, several
hook-ups may be required to bring the damage
out gradually to prevent tearing and further
damage.
With the vector method of pulling, parallel
pulling can be carried out in the following way.
First connect the chain to the damaged part of the
car, then hold the chain tightly in the direction you
wish to pull and adjust the pulling unit or ram to
the measured angle. It is not necessary to have the
chain vertically in line with the direction of pull,
but it is important the ram should not be set at an
angle lower than 45 degrees; that the chain links
are set in line without any twist; and that the chain
between the ram and chain anchor point should
always lean towards the car. As soon as tension is
applied to the chain, safety wires must be fitted in
case the clamp or chain tears away from the main
structure of the car.
Figure 14.91Simultaneous multipulls on two vehicles (vector pulling) (Blackhawk Automotive Ltd )
Figure 14.92Simultaneous multipulling using direct
pulling (pulling towers) (Kroll (UK) Ltd )
Major accident damage 455
Rectification of various damage types
Although every accident is different and must be
treated individually, it is possible to generalize and
divide accident damage into several types. Each
type can be pulled and pushed straight in a number
of ways.
For instance, if a vehicle has been in a head-on
collision, and has suffered a straightforward
crumple, secure the undamaged part of the underbody
or chassis leg and pull out the crumple section
in the direct line of damage. If the vehicle was
struck by another, or if it has hit a low obstacle, it
will have sagged. To repair this, secure one end of
the vehicle, provide a support at the point of the
bend, and pull down the raised end. With a twist,
hold and support the undamaged part and apply a
downward force to the front while allowing the
other end to pivot up. A diamond can be difficult.
Hold one side of the vehicle and pull along the line
of the diagonal. The tendency will be for the whole
of the vehicle to move, so hold the opposite corner
to the pull. To repair severe side damage, push out
and stretch the crushed side, It will also help to
pull from the outside; this is because the passenger
compartment is very strong, and repairing damage
to it requires considerable force.