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The Rear axle is one of the main elements to influence chassis setup. The stiffness of the axle determines behaviour of rear part of the chassis, making the kart oversteer or understeer, and changes strongly the balances of forces acting on the chassis.
As we know in fact kart behaviour along a curve is based on the possibility for the internal tire to lift off the ground since there is not differential system on rear axle. Now a softer axle bends more and quicker than a stiff one. This means that a kart can reduce grip on rear tires, and increase oversteer with a softer rear axle and vice versa with a stiff one. Very simply, but always considering there must be a balance in kart setup between front and rear stiffness, increasing rear axle hardness gives better grip on rear tires, but difficulty in entering curves, because of too high grip on rear tires compared to front tires. The opposite happens having softer axles.
But which are the parameters that vary rear axle stiffness? They are three, material, thickness of tube wall, diameter of the tube. Along the years chassis have shifted from having full tubes of small diameters (25 mm) to having hallow tubes as rear axles with increasing diameters from 30 mm, to 40 mm, and in recent days to 50 mm. What happens is that a tube which bends with an elastic deformation returns to its initial straight form after the kart exits the curve. This return to initial shape can happen with different speed and promptness. A stiffer axle is quicker in returning to its straight shape, and this determines faster reaction of the kart in having the internal tire back on the ground when exiting the curve. The two rear tires touching the ground give more grip and the possibility to accelerate after the curve. So material of the rear axle determines different setup of the kart. Some materials are surely better then others in their elastic capacity to regain initial straight shape. Much effort is being done by kart constructors to find materials with better elastic characteristics. Thickness of the tube also acts on axle stiffness R with the following formula:
R is proportional to: a * (d22 – d12), where d2 is the Bigger diameter axle gives surely a better look to the chassis. But the practical difference is that for physical reasons, which we will not go deep into, a bigger diameter axle is more reactive than a smaller diameter one. So the shift to larger diameter axles has had the aim of increasing reaction of the chassis exiting curves and giving the possibility of accelerating earlier.
But it is not only stiffness of the axle to act and change setup. We know that rear carriage width also acts on rear tire grip. The wider the carriage, the lower the grip. First of all this is due to the fact that a longer rear axle, and a wider spacing between rear tires, permits greater deformation and bending of rear axle along curves. It is somehow equivalent to having a softer tube. On the other hand another effect is that rear carriage width increase makes the rear tires lift with more difficulty. Rear carriage is flatter on the ground and even though rear grip decreases, eventual sliding of rear tires is smooth and uniform, easily controllable. If we reduce rear carriage width rear axle becomes stiffer, grip increases, but rear carriage is less stable. Internal tire lifts without rear axle bending much. What happens is that external rear tire blocks itself laterally on the ground instead of sliding. But as soon as lateral force is too strong rear external tire suddenly looses grip along the curve, with fast lateral slides of the rear of the kart, which becomes much more nervous and difficult to control. With high drivers this effect is even more evident since centre of gravity is positioned higher and momentum on kart increases. That is why high kart drivers usually tend to have wider rear and front carriages.
Finally now we know which parameters act on rear axle stiffness, amongst these usually the type of material is indicated by a lettering on carved on the axle. Also we know how stiffness acts on kart setup. Finally we also have seen that rear carriage width acts on rear axle flexibility and on general chassis behaviour along a curve.