Rear Axle Width

Hi! I currently run a 1020mm Medium Axle (which has been cut down from 1040mm), however I’ve heard some guys at my track (which is quite tight) saying that they’re running their rear axles cut down to 1000mm. I would like to give this a shot, however I would rather get some advice before cutting down my axle any further, as I can’t find much information about the axle width online.

How would the axle width affect the handling of the kart, given that the track width and hub length would remain the same? Would running a shorter axle improve turn in on the kart since the rear track is ‘softer’ as there is less axle inside the hub?

Thanks!

Generally, we run the 1000mm axle to get a little bit more “dig” or “sidebite” when the kart is loaded in the corner. It’s a subtle adjustment that some drivers won’t even notice, but it can help when really fine tuning the way the kart handles load.

If you feel like the kart sets down the inside rear just slightly too soon, a shorter axle might help it dig that little bit extra to keep the load up at apex. Conversely, if you have a really minor hop or overloading issue, extended the axle seems to help settle that down a bit.

For me, shortening the rear axle is a bit similar to narrowing rear track width, but much finer.

Help me picture this; are you saying the wheel hubs are extended passed the end of the axle?

Yes. All modern full-size karts use hubs that extend past the end of the axle, since the introduction and widespread acceptance of 50mm axles (probably about 10 years ago).

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Thank you for that, but I’m afraid I can’t see where the axle ends. I appreciate your efforts.

Yeah I realize that’s not a great pic, I couldn’t find a good one that showed it more clearly.

I believe the Kart Republic rear hubs might have a 50mm hole all the way through, allowing you to slide the hubs further inward if you wanted, but regardless, modern karts don’t really work that narrow anymore unless you’re looking at a really low HP application or super hard tires.

I just walked outside and checked my KR2. They are a pass through design. The hubs are 97mm total length. If you’re running their wheels at 55 1/8” width you’ll have 65mm of axle inside of each hub and have 32mm left on each hub if you wanted to go to flush with the edge of the hub.

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So you’re saying 1/3 of the hub is not supported by the axle? And that makes the difference in the handling?

Correct, more or less. We usually play within a range of 54.5" - 55" for overall rear width, so small adjustments make quite a bit of difference on that.

Regarding the short axle vs. long axle at the same rear width, many drivers would say the shorter axle (less axle in the hub) gives the kart more “dig” mid-corner. It’s a very small difference and could be some placebo effect, but when fine tuning how much the kart is unloading the inside rear wheel, it could make the difference between the kart being flat or hopping.

How that functions, I couldn’t give you a scientific answer. Having less axle in the hub would soften the area within the hub I would think, possibly changing the way the axle flexes on the end only.

If the data doesn’t support the theory, get a new theory (Al Nunley) if you’re lap times show improvement, I may have to change my theory.