Tech Talk: Karting Chassis Setup Part 13

We have seen in the last twelve issues the many parameters that can be altered on a kart to obtain different set-ups, with every variation leading to different chassis performance. The three main changes brought about by variations in set-up are the forces acting vertically on the tyres, the footprint of the tyres on the track and the elasticity of the tyres and the chassis. In general, the stronger the vertical force, the higher the grip on that particular tyre, and the greater the tyre footprint the higher will be the grip. Reduced elasticity (high stiffness) will cause a bouncing effect that will vary the vertical forces on the tyres and make driving the kart difficult. Too much stiffness will also transmit every bump straight through to the driver with no absorption by the chassis. In this issue we will look at two common track conditions you will encounter and we will try to indicate the correct variations in set-up for best performance. However, all the indications given must be read considering the fact that the chassis will need to be balanced overall and this will depend on the track surface and grip as well as the style of track (fast with many straights or slow with a lot of turns).

The set-up guidelines here must only be taken as a rough indication and should be tried and tested on the track. Dry conditions with low grip With dry track conditions you will of course be using slick tyres. All the set-ups we are talking about will only work with tyres in good condition. Use a medium/high pressure inflation value depending of course on the manufacturer’s recommended pressure range. Check the tyre pressures every time you end a session to verify that the final tyre temperature is the right working temperature. Of course every time you change the set-up of the chassis the temperature of the tyres will vary differently during the session. Since track grip is not very high the rear end width will have to be narrower than the maximum permitted width of 140cm. However, the value should not really be any lower than 137 or 136cm. If this does not seem low enough then you probably need to alter some of the other parameters instead.

Front end width on the other hand should be set at a high value since this will give more front grip. However, as we have seen in a previous article, too wide a front end will lead to good grip in mid corner and on the exit but lead to a delay entering corners. So 46 First in the World for 45 years KARTING magazine watch out! The front and rear of the chassis can be raised if track conditions are really low grip, this will help increase adherence of both front and rear tyres to the asphalt. To raise corner entry grip you should increase caster angle. Regulate camber so that when you sit in the kart the front tyres are flat on the asphalt, meaning camber is zero with the weight of the driver included. The rear axle should be medium to stiff since a stiffer axle gives greater rear grip. Seat position should remain the same since the balance of the chassis, or the weight distribution, is determined by the seat position and should be the same and correct in all track conditions. Rear wheel hubs should generally be long to increase the stiffness of the rear axle even more. Torsion bars at the side, rear and front tend to give more grip. So you should use all of them if the track has low grip. Dry conditions with high grip In hot conditions and over the course of a race weekend, the track’s grip increases. These kind of conditions help keep the kart on track but it often becomes extremely difficult to use the grip to its full potential.

Chassis will tend to jump about during cornering and acceleration exiting turns will suffer with too much grip from the four tyres. The main objective on a high grip track is to make the chassis as ‘free’ as possible, using the grip but avoiding the kart sticking to the ground. Solutions will be more or less the opposite to the ones used on low grip tracks. Inflate tyres to the minimum value indicated by the tyre constructor. Widen the rear end to the maximum value of 140cm. This will make the kart slide smoothly during a curve with no bouncing (must be avoided) and the revs will stay high thanks to a slight skidding during the curve. Narrow the front end and reduce caster. Camber must be positive so the tyre print is reduced. These solutions will give less grip and better acceleration exiting corners since the front tyres will not be stuck to the asphalt, especially when the steering wheel is still slightly turned. The height of the front and the rear of the chassis can be reduced to a minimum and torsion bars can generally be eliminated, although on this point we sometimes find the opposite is true depending on the kind of chassis being used. The rear axle should be of medium/soft stiffness. You should though use the stiffest axle possible without having a ‘bouncy’ chassis. This, together with a wide rear end, will avoid bouncing, especially on bumpy tracks, and the chassis lifting up on two wheels when cornering. It is better to have the widest possible rear end with a stiff axle than a narrower rear end with a softer axle. Stiff axles always tend to give better performance if they do not bring excessive ‘side effects’ like the ones just indicated. Finally, rear hubs are usually kept long since they make the rear axle work better, although it is difficult to explain why. Front hubs should also be long since they tend to ‘free’ up the front tyres when exiting a corner.