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?
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.
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).
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.
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.
Hi @tjkoyen,
Operating at that width, say 1390 (54.7") and with something like a 95mm long hub and 1000long axle would leave maybe ~50mm of axle into the hub - is this what you see typically?
Is 50mm (1 axle diamater) about the minimum you would run, or do you have substantially longer hubs?
I always run the OTK medium hub (92mm) and have no issue running rear track at 1390mm on a 1000mm axle. Just always make sure the pinch bolt is over the end of the axle by several mm to prevent it slipping off and you’ll be fine.
Based on what you describe, that a shorter axle allows the inside rear wheel to lift longer, I’d theorise the phenomenon as this: as we turn into a corner, the outside rear hub acts as the fulcrum point for an axle that is lifting the inside wheel.
With less axle contained within that outside hub, less of the potential energy created by jacking the axle can be transferred into the opposite side of the fulcrum point (the axle within that hub), so it remains on the side of the lifted axle (and chassis) to bend it higher and for longer.
…or maybe not. This is just a semi-educated guess. Happy to hear anyone else’s thoughts! Going to try a cut-down axle this weekend me thinks.
Just wondering. Would going from a 95mm hub to a 85mm hub and keeping the rear width the constant have the same effect as cutting the axle 10mm on each side?
The “spring” energy comes from that portion of the axle that flexes. The portion of straight, unloaded axle (length inside the hub) releases no KE; it’s just along for the ride (pun).
I revive this topic because I’ve read a lot, in this forum and in other sites, about cutting the axle, but something sounds strange. I mean, for what I read, it seems that a shorter axle gives only advantages.
I quote an article by kart360.com :
" What a shorter axle will provide:
Allow the kart to react quicker in the rear, allowing for more rotation
Stabilizes kart under braking
Holds rear tire up longer
Decreases grip on exit"
So the question is, why everybody doesn’t use shorter axle? I think there must be a tradeoff, an advantage of standard axle, because every manufacter sell their karts with 1040mm axle, but I can’t understand where is this tradeoff.
I think another important question is, professional drivers do use shorter axle? I’d like to understand if a shorter axle helps only non professional drivers, covering up their driving problems and therefore it’s counterproductive for a professionist, or if is a common use also in professional teams.
I think the thing you have to remember is that this is just another tool used for tuning. Every manufacturer also sells their kart with a certain length hub, or specific stiffness axle too, and in most cases the stock/standard setup will work well for most people in most situations. Are there advantages to changing these things…maybe, maybe not. The four bullet points you mention may not be an advantage in every situation.
In my case, I cut the standard axle to use a different brand wheel as they have a different offset than OEM stock wheels and I had zero adjustments for rear width.
The short answer is that it depends on the chassis, tire, driver, and track conditions. All of these factors are going to affect the balance of the kart. With the KR chassis for example, nearly everyone seems to run a cut axle.
At our last race at Road America, I installed a Medium AMV axle which was 1040mm. Next session I cut it down to 1010 and the results were everything you quoted from the article. Biggest difference was the reactivity of the kart, along with quick and more predictable rotation.
Because sometimes you don’t want the rear to stay up longer or the outside rear wheel to dig harder. It’s a tuning tool, so for example if you are already overloading then outside tire and getting too much inside rear wheel lift, a shorter axle will only make it worse.
It is a small and subtle change between axle length, but I usually use the short axle as my standard setup.
