The seat and its supports are often underestimated by chassis tuners and drivers, but on the contrary such elements are a fundamental part of the chassis and determine a lot of the stiffness characteristics of karts.
Various seats and supports
The structure of the seat and its connections to the chassis generate a system which varies sensibly the chassis stiffness. Moreover the material of the seat can vary and so also the stiffness can vary. Glass fibre, Kevlar, carbon or mixtures of these elements, with also different thicknesses of the layers, can generate variations in the torsional flexibility of the chassis in its rear part, since the seat itself will change its stiffness.
For example a carbon fibre seat is, with the same thickness, three times as stiff compared to a glass fibre one. A Kevlar seat is on the other hand much more elastic if compared to a glass fibre one. Costs are also to be considered and, as usual, carbon fibre seats cost at least double a glass fibre one.
With the use of supports it is possible to preload the chassis on one or both sides. Such load will be positioned on the rear axle bearings. Such preload will be obtained by using length of supports of just a few mm superior to the distance between the holes in the seat, where the support is screwed, and the lateral axle bearings. Supports can be bent and twisted together to obtain something very similar to a spring. Straight supports are extremely stiff. In fact a straight support works in compression on the rear axle where it is linked. Such supports are very stiff in compression. On the other hand the same support bent, works in flexion, so has a much higher elasticity and, as said before, works well as a spring, deforming when a sudden bump is hit by a rear tyre, but still creates downward force when it is needed to increase rear grip.
Effects of supports on chassis behaviour
The stiffer the seat and its supports and the faster will be the diagonal load transfer to the wheels. In fact the weight of the driver whose effect is also increased by the centrifugal lateral force when running along a bend, is transferred to the rear tyres through the seat and its supports. If no supports are present the chassis will first bend and then only part of the weight will transfer to the rear tyres.
Usually it is best for better performance and faster reaction of the kart to have a quick weight transfer, which means good seat and supports stiffness. On the other hand a very stiff seat and supports system does not absorb any irregularity of track surface. We must not forget that if initially caster of the front internal tyre transfers load to the rear external tyre, it gives back load to front wheels as soon as it is the only rear tyre that touches the ground.
The stiffness of seat and supports permits the rear part of the chassis not to flex too much, so we will have a good lift of the internal rear tyre during a curve. If the chassis bends too much instead, both rear tyres will keep touching the ground generating great under-steer.
On the right side of the chassis under the seat we always have two tubes, that are not only connected by the welded tubes, but also by the engine mounts. This determines the fact that the right side of the chassis is stiffer then the left one.
Acting on the seat supports we balance stiffness on the softer left side. If we use an old softened seat, with holes too big for the screws, we will have a low performance chassis. There is nothing worse then a chassis with its seat moving all around. It is also useless to try to set up the chassis with all sorts of regulations, if the area where most of the weight is concentrated deforms easily in any direction.
It is for this reason extremely important to well fix the seat to the chassis with well dimensioned holes, possibly hardened by metal rings, maybe glued to the seat with some resin.
Only when grip is extremely low (dirty or wet tracks) we can use rubber rings that absorb all the forces without loosing connection between the seat and its supports. The best solution, even though more complex, is to have the correct seat for different track conditions. Also some sponge can be used to increase comfort, but with no excess, otherwise we would loose the feeling between our body and the seat-chassis. In fact it is always with your “bottom” that you feel your chassis and understand how to drive it.