Chassis straightening

There’s a saying that rubbing is racing, but sometimes the rubbing goes a bit overboard and there is proper contact, either with another kart, the kerb, or some fixed and immovable object that ends up in you having a bent chassis to contend with.

Now, in a lot of instances this is not the end of the world and the chassis can be repaired relatively easily by someone knowledgeable in this field. Unfortunately, there are also instances where one has to bite the bullet and dig deep into the piggy bank to fork out for a new chassis.

Basic material science

To understand what can and can’t be done to fix a bent chassis, one needs to have some basic knowledge of material science. A chassis is made up of steel tubes that are elastic i.e. they have some ‘spring’ in them so that if they are twisted, bent, or deformed in some way (within certain limits of course) due to an externally applied load, they will immediately return to their original shape once this load is removed. Elasticity is that ability of a material of being able to deform under the action of a load and then return exactly to its initial shape once that load is removed. This is best understood by referring to a stress/strain curve for a typical steel as shown in the diagram below.

For simplicity, the stress in the material is proportional to the applied load, and the strain is proportional to the amount of deformation experienced under the applied load. As the load is applied (vertical axis on the graph), the deformation will increase (horizontal axis on the graph). The material will return to its original shape or position, providing one is operating in the region ‘OA’ which is the elastic region of the material.

At some point ‘B’, the material reaches its elastic limit also known as it’s Yield Point and it will then not return to its original shape if the load is further increased. Instead, once the load is removed, the return will be along the dotted line from ‘C’ to the horizontal axis, and a permanent set will be visible on the affected portion of the chassis. The Ultimate Tensile Strength (UTS) of the material is denoted by the point ‘D’ on the graph. In extreme cases if the load is increased to the point ‘E’, the material will have plastically deformed to the extent that it reaches its Fracture Point, and breakage results.

Chassis tubes can also deform plastically or change their elastic characteristics through a process called fatigue where the loads acting on the chassis are not that high, but are applied over an extended period of time which finally leads to failure. As an analogy, try bending a piece of wire such as a paper clip through a small angle. At first it is hard to bend, but by repeatedly bending it many times, it eventually softens (also because of heat generated) and finally breaks because of crystallisation in the metal and loss of its elastic characteristics.

The bottom line is that a kart with a bent chassis won’t handle properly, but the good news is that provided that a tube has not buckled or creased when it has deformed to the extent shown in the picture below, there is every chance that you can bend it back. Once a tube has taken a kink, it is part way to collapsing and it will never have the strength it once had, so more than likely you are on a hiding to nothing and will have to lay out for a new chassis. There are some that might suggest cutting out and replacing pieces of the tubes with welded in pieces, but this is not to be recommended as you never know exactly what tubing was originally used in the manufacture of the chassis and you could end up fitting pieces with mismatched properties, lower tensile strength, and the like.

Chassis materials

Manufacturers are very cagey when it comes to revealing the materials they use for their chassis. They will all tell you their ‘XYZ’ chassis is made of 25mm, 30mm, 32mm, etc. tubing, but they don’t usually tell you the wall thickness used, whether its heat treated after welding, or what their choice of material is. Some, like Haase and Tony Kart for instance, do state that their chassis are made with a Chrome Moly steel, but again they don’t state which grade of Chrome Moly is used. Referring to the table below, one can see that there are some major differences in both the Yield and UTS of the sample steels shown in the table below.

It appears that the tendency in recent times is for manufacturers to opt for a softer chassis rather than a stiffer one as it generally performs better by allowing more flex. This can be achieved in a variety of ways such as using thinner walled tubes, more elastic materials, and other changes in the chassis design. The downside of course is that the chassis tends to have a shorter lifespan with good performance compared to a more rigid one, and is also more easily deformed if it is subjected to an impact. Taking all the above into consideration, one should thus heed the previous warning of cutting out and replacing damaged tubes on a chassis because you are never all that sure if you are using the correct replacement materials, have good welds, what the heat affected zone around the weld will be like, etc.

Tools and setup required

To start with, one needs a rigid type of straightening table something along the lines of that shown on the RHS. This allows the rear axle to be set in the hoops shown near the far end of the table, and the front of the chassis is then suspended on a central pivot that allows measurements to be taken all around the suspected bent chassis as shown the next picture Furthermore, in order to do a proper check, it is preferable to have all the extraneous bits such as the front and rear bumpers, fuel tank, bib, side pods, wheels, motor, and seat removed as this makes the measurement operations far easier to undertake.

A quick fix to reset a portion of the chassis parallel and in-line with the rear axle is often done at the kart circuit simply to get someone back racing in time for the next heat. This unfortunately must be considered as a temporary measure until the kart can be properly jigged up, measured, and straightened as outlined later.

To correct a bend in the chassis, it obviously needs to be over-bent in the opposite direction i.e. there needs to be some plastic deformation. This is usually applied through some form of lever system e.g. a hydraulic press, using suitably shaped bocks on the affected area whilst the rest of the chassis is supported off other blocks on the table. Of course, knowing how much over-bend to apply and where to apply it, is where the experience and craftsmanship of the straightener come into play.

Suggested straightening method

This next portion is by no means intended to be the hard and fast rules of how to straighten a chassis. Each expert in this field will have their own preferred methods and will probably insist that theirs is the one and only way to do the job properly. What is intended, is to give some better insight to the uninitiated on some of the more important aspects that are taken into consideration when the straightening task is undertaken.

It is very important that a visual check is conducted on the stripped chassis before any measurements are conducted. Using a simple straight edge held against the tubes very quickly shows up where any major bends are. This needs to be checked in a few orientations at every spot on the chassis as a ‘vertical’ bend won’t be spotted if the straight edge is oriented to check for a ‘horizontal’ bend and vice versa.
Having found where the major bends are, these need to be rectified first before one gets down to looking at the smaller bends by taking measurements on the table.
Having then set the chassis up on the table, the bearing carriers must be checked for square and flatness as they are often bent upwards by a racing incident.
Next, measure the heights of the king-pins against each other to see if they have been bent upwards. If not, then check the side lengths between the rear axle and king-pins, and also the diagonals from the bearing carriers to the king-pins (having the seat removed is thus essential). This quickly tells one whether the chassis is shorter on one side or the other, and also if it is bent in such a way as to cause ‘crabbing’.
So now you know roughly what you have to deal with. The starting area is to get the rear tubes and bearing carriers sorted out. Then check the bumper support tubes as these may need some adjustment. If the kart has had some rear end damage the whole rear end will need to be brought back into the correct alignment. This can be quite a lengthy job but you cannot attempt to straighten the front of a kart without these operations being carried out first.
The kart’s front tube should sit flat on the front tube support on the table, normally a slightly domed piece. This may not happen as the majority of karts are quite soft, so it needs to be adjusted to sit flat before you can start on the front geometry.
The top of the front yokes need to be levelled up. Also digitally check and reset the caster to the correct angles for the kart model. A word of warning on this - if you use pin gauges with a digital protractor on them, then any misalignment to the centre line of the kart will give you bad readings. It is essential that the readings are taken absolutely in-line with the centre line of the chassis.
Lastly, reassemble any hub parts (if removed) and check the camber and toe angles with an alignment laser such as a Sniper.
Once this is done, the seat and other ancillaries that were removed to start with, can be refitted and you’re good to go.

Weight distribution

The bottom line is that the more precise the measurements are during the straightening operation, the better the kart will handle, and working to tenths of a millimetre is not uncommon. This can easily be verified by ‘cornering’ the assembled kart i.e. putting its wheels onto four accurate scales located on a flat surface – and as a matter of interest, a garage floor isn’t necessarily flat. The top of the scales must be level with each other (use a long level for this) and shim them underneath until the top surfaces are level with each other both front to rear and side to side. Then lift the kart onto the scales and ensure that the wheels are in the middle of the platform of each scale. The driver must then sit in the kart in full kit as they would on race day, and an appropriate amount of fuel as per race day must be in the tank. The front wheels must also be pointing fore/aft as this affects the measurements on the scales. Next, check the readings.

The bottom line is that the weight distribution should be 50/50 left to right, and close to ~43% front and ~57% rear. Moving ballast around the kart, changing the seat position, etc. can all be used to even this distribution out. If moving the seat position to optimise weight distribution, don’t forget to make sure the driver is still comfortable and can properly control the kart otherwise they’ll almost certainly not be as quick. To alter any cross-weight results, the kart will need to be raised or lowered in a particular corner, usually by moving the shims around within the yokes to alter the ride height at the front, or by moving the bearing carriers up or down a hole at the rear. Lifting the axle at any given corner will take weight off that wheel and transfer it to the opposite side, and vice versa. Make small ride height changes and re-measure your cross weights until the kart setup is optimised. As mentioned, the best way of transferring weight from side to side is by moving of the shims or bearing carriers.

There is another pretty crude method that is adopted, but it seems that it is accepted by a portion of the karting fraternity. That method is to put some bend back into the chassis - something you have just spent hours and money on fixing in the first place. By putting the heavier wheel onto a block on the floor and applying weight to the rest of the chassis through someone standing on it, one effectively transfers some weight to the wheel on the other side of the kart. As they say, you pay you money and take your choice!!

How often to check the chassis

It’s the age old question of how long is a piece of string? Some of the top teams have their chassis checked every two or three meetings although most opt for once a season. However, if your driver is a habitual ‘kerb-hopper’ it is worth having it checked as often as the budget allows. Is it worth having a brand new chassis checked out? Well, a chassis direct from the factory should of course be pretty good, but there is talk that for the ultimate performance one can often find a little improvement here and there by going to an experienced chassis straightener.

Chassis scuffing during straightening

Occasionally one hears concerns that the straightening process will damage the surface finish. However, using a bit of common sense, soft pads on the hydraulic press and strategically placed blocks with rags wrapped around the tubes to protect them rarely does much, if anything, to the finish. Given the choice between a chassis that no longer goes around corners and one that bears a couple of scuffs from being properly straightened, the general consensus is that most would opt for the latter.

Emile McGregor - MSA Technical Consultant