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Tech Tuesday: Chassis Straightening

Tech Tuesday: Chassis Straightening

Of all the components that go together to make a kart, if you had to choose a single thing that was deemed most important what would you go for?

It would be hard to pick anything other than the chassis, as all the other components rely on it in order to do their job properly. It fulfils multiple roles, everything from being a mounting point through providing the karts suspension.

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Alignment and flexing

Quite obviously a kart chassis is designed to flex, we all know that it’s by changing these flex characteristics with torsion bars etc that the chassis is tuned to work for different tracks and weather conditions. The nature of their steel tube construction means that this is possible, as it can be moved a certain distance and will then return to its original position. Watch slow-motion footage of a kart through a bend and you can see the chassis flexing this way and that as it performs.

What is straight?

Take a kart at 150kg with driver, the chassis is perfectly straight and the tyres on each ‘axle’ are of the same diameter and inflated to the same pressure. With the driver sitting in the kart at least theoretically, when put on corner weight scales they should be reading exactly 32.25kg for each of the front wheels and 42.75 kg for each of the rears 43% front 57% rear.  Measuring the distance between the centre points of the front and rear axles should reveal that it’s exactly the same left to right too. All four tyres are pressing into the track surface with an equal amount of force and the handling will be consistent.

Take the kart out for a track session and provided that the chassis isn’t flexed beyond a certain amount (the yield point) when dropped back onto the scales, the weights and measurements should remain the same.

However, abnormally high loads like an accident or constantly banging curbs can take the chassis tubes past this yield point. Once it has passed this distance it puts a set is in the tube and it won’t come back to its previous position. Bounce the kerbs hard again and if the yield point is passed once more the deformation will increase and so on.

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Bent?

Now take our 150kg kart when the chassis that has taken some hammering or a bit of a thump. Now the scales say 34kg right hand front 30.5 right front, while at the rear the figures are similarly skewed, but in the other direction. The chassis has twisted, use it like this and it will over-use (and potentially over-heat) the more heavily laden tyres and under-use those which are more lightly loaded. Take the between axle measurements once more and possibly the wheel-base will be different one side to the other. The numerical discrepancies are the least of your problems, a kart with a bent chassis won’t handle.

The good news is that provided that a tube has not buckled or creased when it has deformed there is every chance that you can bend it back. While the old trick of one wheel up on a tool box and bounce has its place but only as an emergency measure get you out for the Final, it is crude and reserved for the situations when nothing else is an option. By far the best option is to have a chassis properly jigged and set straight. The right company can deal with a surprising amount of bend and give you back a chassis that will once more perform as it should.

How bent and still be rescued?

Merely taking a nearly new chassis and jigging it would tell the whole story of what can be achieved by the proper methods. What we need was something far more difficult. Enter this poor little 1964 Tecno which had taken a heck of a thump after a steering failure; to the extent that wheel base on the left side was shorter than the right by best part of a centimetre. You didn’t need corner weight scales to tell you that it was twisted merely lifting the front wheels in turn told you all you needed to know, one side was far heavier than the other. Because of its rarity chassis replacement was not an option it needed to be rescued. Step in Adrian Beddall of B.R.T. Quietly confident that he could turn the chassis around and get rid of the woes of the last 50 years.

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Where do the experts start?

It’s little use attempting to do delicate corrections when the basics are not right, so BRT started with a simple straight-edge to determine any bend in the rear of the chassis, this is corrected by the tubes being steered this way and that using a combination of an overhead press that was part of a sort of sliding gantry affair, a ‘free’ jack that could be put anywhere and occasionally close fitting mandrels slid inside the frame tubes to allow bending moment to be applied to exactly the right area.

Only when the tubes are dead straight do they even consider taking measurements determine twist and lozenge plus measuring (and eventually correcting) in a couple of other areas which Adrian politely asked me not to mention as they are his ‘trade secrets’ and he reckons that it’s work in these areas that make the difference between a chassis that’s nearly-right and a chassis that is really-right.

Determining these values means the use of a proper surface table and dial gauges, as amateurs we don’t stand a chance without this sort of kit. Taking measurements on what even what appears to be a perfectly flat concrete floor won’t give properly accurate results.

How accurate?

What do we mean by accurate?  Well consider this…. BRT work to within 0.125 of a mm.

This level of accuracy is especially relevant for lower power classes like Cadet where having a chassis that is completely consistent in left and right hand bends it critical for finding the last few tenths or even hundredths of a second.

The correction is done by accurately packing the chassis in various positions to allow the hydraulic press to be applied to specific areas. Although the chaps are very experienced in applying just the right amount of tweak to these particular areas they are constantly looking at the heights of the chassis tubes relative to one another constantly checking that the movement has not affected other areas.

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When you get the chassis back

If you have opted for the full service not only will the chassis be straight but the back axle will be fitted in the correct position and the front stub axles will be set up for toe in and camber, All you have to do is bolt the rest of the bits back on and go racing.

How often?

Some have their chassis at least checked every meeting, others every two or three although most opt for once a season. 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 new chassis trued? While Adrian admits that a chassis direct from the factory will be “pretty good” he’s prepared to say that for the ultimate performance they can often find a little improvement here and there.

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Damaging the chassis?

Occasionally owners express concerns that the correction process will mar the surface finish. I asked Adrian about this and he contests that merely correcting the sort of set that a chassis takes from day to day use rarely damages the surface finish, even accident damage which will often require more extreme measures with the common sense use of the soft pad on the hydraulic press and strategically placed blocks of soft-wood and rags wrapped around the tubes to protect them rarely does much if anything to the finish. Mind you most owners given the choice between a chassis that no longer goes around corners and one that bears a couple of scuffs from being properly straightened will opt for the latter every time!

What can’t be straightened?

Once a tube has taken a kink it is part way to collapsing and will never have the strength it once had, here you are most likely on a hiding to nothing. Depending on how far you want to go there may be ways and means of dealing with this but that, as they say, is another story.

How much does all this cost?

All kart chassis are different so it’s rather like asking how long a piece of string is. However to give you an idea a simple check and straighten of an otherwise good chassis can be achieved in an hour or so and the guys can get an awful lot achieved in three hours. Even the daunting task of taking the Tecno chassis from bent to straight only took just under five hours, considering what was achieved in that time and the fact that there were two people working on the chassis for the entire time the charge of £180 – £40 for the first hour and £30 for every hour after that seems entirely reasonable.

This article was first published in Karting magazine in August 2015 and was written by Mary-Ann Horley

 

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