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.