Let's talk about axles. One of the biggest changes you can make to a kart is to change axles. The axle is such a hugely influential part of the kart since it spans the width of the rear of the frame, connects multiple rails in the kart and in every way possible the kart has been developed around a solid rear axle with no differential. For a long time everyone took the axle for granted, it was there just so you had somewhere to bolt the brake disc, wheels and sprocket hub. You changed it when you bent it but like almost every part of the modern racing kart, we learned that changing the axle material can fix, or cause, many setup issues.

For the longest time axles were pretty straight forward, "need more grip?", put in a stiffer axle. If the track had a lot of rubber and you needed to free up the kart, put in a softer axle. Over the last few years the material the karts are made from has evolved and karts are running much more caster than they used too. This has changed how some chassis brands are tuned. I see it more in the 50mm axle karts than the 40mm and 30mm size axle karts and classes.

One thing to mention is that axles in a certain range, like 50mm axles, will almost all have the same wall thickness (e.g. 2mm). The difference in how the axle behaves comes down to chemical composition and the heat treating process the tubing went through (like annealing or normalizing, as an example) which changes the stiffness and mechanical properties of the axle. So it's very important to realize that wall thickness alone is not always a reliable indicator of how stiff an axle may be. If an axle isn't clearly marked it could be anything!

How the axle works in the kart - As you turn the steering wheel the kart transfers weight diagonally, picture an X. If you are turning left, the karts weight goes to the left front and the right rear and takes weight off the right front and the left rear. This allows the kart to unload the inside rear tire (and axle) and the kart basically goes through corner with the inside rear tire off the ground (or at least much lighter) and has a "differential".

This next part is my speculation and theory, could be right, kind of right, kind of wrong, or very wrong, but it's the best I got! In most karts putting in a stiffer axle makes it harder for the kart to lift the inside rear tire and produces more rear grip with the stiffer axle. So if we need less grip we went to a softer axle which allowed the inside rear tire to lift up easier and make the kart "free" through the corner and with the softer axle the kart would twist more and not use the tires as efficiently. With some of the latest kart models this is now completely opposite, now we need to put in a stiffer axle to free up the kart.

Older designs were generally stiffer frames and to free them up we needed to soften the kart as much as possible. Newer models are softer overall and transfer weight differently. So putting a stiffer axle in some of the newest models allows the kart to transfer weight better, it's less of a "wet noodle" with a stiffer axle. The softer axle just increases chassis flex even more and the kart doesn't transfer at all. It's another way to do the same thing, unload the inside rear tire of the kart. But you seem to run out of softer axles a lot quicker trying to free up a stiff kart than harder axles trying to free up a soft kart. As I mentioned earlier, this seems to be more of a 50mm axle phenomenon than with a 30mm or 40mm axle.

At the end of the day it comes down to the same thing no matter what the theories say, you gotta test it on the track! Despite my theories above, I have had a "stiff" kart that was loose so we threw a stiffer axle at it, long hubs, more seat stays etc. and it never got better. So if what we were doing didn't work it was time to go the other way. Soft axle, short hubs, take out the middle bearing, less seat struts and now the kart actually HAD rear grip and was fast. That might be the biggest take away from this entire article, if what you are doing isn't working then it's either, A. Not the problem or B. Do the opposite!

Good luck, keep testing and take good notes!

Mark Dismore Jr.