Since one of the most important parameters of a kart chassis is the rear axle hardness, and since in the Rok category it can be changed, we decided to carry out a test to verify that theoretical rules referring to chassis set-up were confirmed by track tests.
Start of test: soft rear axle
We started our test with new LeCont CIK H LH tyres, the hard compound homologated by the CIK. These tyres have a superficial tread hardness of 54 DIDC-IRHD compared to the 42 of CIK M LH08 medium tyres we tested some time ago. Since the tyres were hard compound we decided to start with a soft axle, which we know is not the best solution with these tyres as, in theory, a soft axle does not help grip, but on the contrary usually gives a sliding effect to the rear of the chassis. This can help on high grip tracks and with very soft tyres to “free” the kart when exiting corners.
We started our test at the Pista D’Oro track near Rome with not much rubber on the track and consequently quite low grip. We set the tyre pressure at 8psi, quite high for a start, but the cool temperature led us to start with something that could help with finding some grip fast. The set-up was 1400mm rear width and minimum width front. Neutral caster was set. The kart started running with very low grip on the rear tyres. The sliding was very smooth, so quite controllable, but excessive. Only after eight laps did the grip on the rear tyres become acceptable, but still not sufficient. The lap times started at 51.50s and decreased constantly to 50.11s but this was still too slow. Once stopped, the tyres showed a very smooth surface and a total absence of the bulky areas on the tread that usually show evidence of good grip.
The only possibility was to move on to the next step of the test, changing the rear axle from soft, indicated by the letter P, to medium, indicated by the letter U.
Changing the rear axle
Some quick tips on rear axle changing:
• To avoid having the rear axle stuck in the bearing passages, always clean it very well before trying to slide it out.
• Also remember to unscrew all bolts from the axle and loosen disc and rear sprocket carriers.
• Then use a rubber or plastic hammer to hit the rear axle sideways, this will permit to avoid ruining the edges of the axle. In fact this would spoil the axle and also make it impossible for it to slide inside the internal diameter of the bearings.
• Once the axle slides you can help it with an aluminium tube or bar if the edge of the axle gets stuck just inside the internal ring of one of the bearings. Just put the aluminium tube on the edge of the axle inside the bearing and hit the tube on one side with a hammer.
Test with medium rear axle
We pretty quickly changed the rear axle which was perfectly straight so it slid out of the bearings’ internal cylinders easily. Sometimes when the TECHNICAL axle is slightly bent the process is much harder, and we have to be careful to understand which side the axle is bent and make it slide on the other side to help it come out. We proceeded with the same parameters and nothing was changed in the chassis set-up except the rear axle. The kart immediately, from the first laps, showed much better performance with rear grip increased strongly. Also the greater stiffness of the rear axle helped exiting curves, pushing the kart out of the bends thanks to the axle bending and returning to its straight position with greater strength due to the greater stiffness.
The lap times decreased quickly, and after the first two laps times decreased to under the best laps of the previous session with the soft axle. The lap time was 50.02s on lap 3, and decreased down to 48.98s on lap 8. After that the lap times were more or less constant and we stopped to check the tyre treads. They definitely showed a more bulky surface confirming my sensation of greater grip.
It generally helps to have a stiffer rear axle when on hard compound tyres and a low grip track. But with more grip and softer tyres a softer rear axle helps, giving more predictable behavior from the chassis and smoother sliding of the rear tyres. Sometimes in fact too much grip can lead to the chassis remaining “stuck” to the track with loss of speed and slower lap times.