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TKART magazine How To | Straighten the chassis
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20 January 2016
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New chassis are obviously symmetrical and with bends, angles and overall tubing geometry exactly as the manufacturer intended, except for microscopic differences due to welding. Over time, though, contacts, accidents, curbs and broken components can cause the frame to get bent out of shape. Sure, when tubes are severely bent there’s nothing to do but replace the chassis entirely. Yet, even minor distortions can seriously compromise kart performance.
Generally speaking, the expression “bent chassis” refers to a permament torsion between rear end and front end. Occasionally, deformation is tied to the C angles, but this happens less frequently and is tougher to fix. This is the thing: the chassis does a lot of work by way of torsion of the longitudinal tubes that connect front and rear end and, for a number of reasons, these can end up twisting excessively, thus causing overall deformation of the frame. How do you know if this is the case? You set the chassis on a flatbed (photo) that is at least 1000 x 800 mm. If everything is in order, both ends will be perfectly parallel to the flatbed - of course, except for the tubing section where the Cs are welded, which are obvsiously curved upwards. The Cs are precisely what will tell you if the chassis is bent: in case of permanent (static) torsion, they’ll have a different height relative to the bed, with obvious alteration of the base set-up and the angles that the front tires form relative to the ground.
Say the front end is twisted clockwise relative to the rear end along the longitudinal axis (looking at it from behind): this will cause the front right tire to be lower than the front left tire. Since on left-hand corners the front right tire is the one that leans in, in this case it will be lower than normal, making it tougher for the rear inside (left) tire to lift off the ground and so producing an understeering effect. So, if you’re experiencing understeering and want to know if it’s because the chassis is bent, this is what you do: place the kart on a flatbed with supports under the rear axle and under the front tube. Using a caliper, measure the height of each C, taking as reference the top portion of the bolt that locks it to the stub axle. If one C is offset compared to the other (higher or lower), even by a few millimeters, it means the chassis is crooked and has to be straightened.
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