Tech Talk: Chassis Set-Up Part 15

Set-up is often determined by the kind of track we are racing on. Indeed, different surfaces and different track shapes require particular chassis set-ups to generate best performance. When we describe a track as being fast or slow, what we are referring to is the average speed of the kart running around it, either long straights and large radius corners for a fast track, or short straights with tight corners for a slow one. Slow tracks Slow tracks are not necessarily easier to drive on, in fact they often are more difficult to interpret than fast ones. Many of the corners, most of them slow ones, must be taken at low speeds so that even though we adapt the gearing to this situation the engine struggles because revs tend to be lower.

David Goode -  Formula #34C
Masters ‘O’ Plate winner David Goode with the NKRA’s Jane Andrews

Also any slight mistake by the driver reduces even further the speed of the kart and the revs of the engine. As we already know, 2-stroke engines perform well in a particular rpm range (the central part of the torque curve). Outside of this range, either lower or higher revs, the torque of the engine is poor and the push by the engine is extremely weak. To avoid a drop in engine revs when taking slow and tight corners the chassis must be set up so that the rear and front tyres have enough grip for traction and steering but do not stick to the surface too much which would lead to a braking effect on the chassis. A very important aspect of kart functionality is in fact allowing the rear tyres to slide so that the revs of the engine can be kept higher. Also, front tyres must not have too much grip otherwise when accelerating out of a corner they will tend to have a braking effect on the kart, slowing it down. As already seen in past articles, to reduce grip on rear tyres the first step is to widen the rear end up to the maximum value permitted.

A second, more complex step, but often even more effective, is to change the rear axle for a softer one. An axle which bends more permits better sliding of the rear tyres and better performance on slow tracks. Front tyres will have less grip if we reduce front end width (the opposite of rear tyres). This is surely the first step. But front hub angles are also very important as we know. Reducing caster angle ‘frees’ front tyres during the corner and helps acceleration exiting the corner. Camber angle can also be increased to reduce the tyre print on the ground and so reduce grip. However, this of course gives faster and less uniform front tyre wear. Caster meanwhile is often used in slow tracks with tight bends because it gives extremely precise steering movements. Fast tracks Fast tracks are somewhat easier to interpret than slow ones. Much depends on engine performance and, sometimes, on the driver’s courage in certain fast corners to keep his right foot down and his left foot up! The concept with fast tracks is of course the opposite to that with slow tracks.

Grip must be as high as possible, particularly when running along large radius and fast bends. Engine revs will be always high and what is really needed is grip sufficient to keep the kart on the track even at very high speeds. Rear end width will be narrow, but most of all the rear axle should be hard. Front track will be extremely wide and camber will be set to have a zero value with the driver sitting in the kart so as to have the maximum tyre print on the tarmac. Not much caster will be required because the high speed and strong centrifugal force acting on the chassis and driver when running along fast and large radius bends will already give a good lifting effect on the inside rear tyre. A good general balance Chassis set-up is always a matter of good balance of all the kart’s parameters.

The indications given for slow and fast tracks will always have to be evaluated and chassis setup will always be balanced to give the best performance. On slow tracks grip must be reduced so as to have good revs when running a bend and reduced braking effect of the front tyres when exiting the turn. On the other hand, too little grip will give a chassis that will be “loose” at the rear when running along the bend and will have not enough grip at the rear tyres when accelerating. If front grip is too low, understeer will also be present. There will be a particular combination of front and rear grip that will give the best performance on a particular track. Tracks also usually have fast and slow sections, so even when running on a fast track, not all the track will be fast and there will be once more the need for a balance in grip between the front and rear tyres. Finally, there is always an important connection between front and rear end set-ups, since if front grip is high so must be rear grip, otherwise the chassis will spin. The best situation is when the chassis slides sideways on all four tyres. We will see in coming issues not only different set-ups for different surface types, understanding how to recognise such surfaces just by looking at them, but also how temperature changes conditions and how a set-up can be, for example, critical because too much grip makes the chassis bounce.