Do different stiffness axles do anything?

This definatley worth more discussion.
It is great to see you guy’s have a good handle on material elastic/ plastic characteristics.
This could also segway nicely into a discussion on axle stiffeness vs hardness as the stress-strain diagrams play there part in explaining soft axles dont flex more than hard axles of a same thickness…I know this statement will stir some emotions
Engineer waiting for robust discussion…

I’m still not entirely convinced axles do anything. Maybe I raced before it was a big deal but when I raced Euros for TopKart in 2001-2 we never changed an axle for tuning. When I raced Euro for CRG in 2006 we never changed an axle for tuning. When I raced KZ for a British manufacturer I tried different axles but found no change. I only tried them because I was tuning for a British CRG team as well and we constantly changed them there.

If I could find somewhere where Davide Fore or Marco Ardigo said axles definitely make a difference then I’d go with it, but I can’t. They aren’t going to say the opposite because their manufacturers make a lot of money selling axles. Man, in the 2000-02 period I raced a Mitox in the UK (yes while being a works topkart driver in europe) and we only moved the front in and out, that was all it ever needed.


I’ve noticed differences when changing axles. For some tracks in my area, I’ve taken different axles to the track and I’ve noticed differences using them. When I had a Velox SK1, they had a range of different axles from Super-Soft to Hard and I spent a few days at the track swapping them out and trying them.

I swapped from a medium to a soft axle over the weekend at Spanaway and noticed some handling changes.
Now ask me to explain it, and you’ll get nowhere. lol

No worries, Nik.

I did have some ideas of what each stiffness level of axle should do, because I tend to ask a ton of questions. Also, when I first started testing axles, I would take big swings of changes to see if I noticed a difference. My Velox especially had a large range of axles from super-soft to hard that let you see what the difference felt like.

I think the thing for me though is that an axle change, especially with club racing, is a bit of a last resort, if trying different hubs, tire pressures and widths aren’t working for me. 9 out of 10 times, I can adjust something on the kart without having to pull out an axle, and I get the result that I’m looking for. (Unlike last weekend, where changing to a soft axle seemed to be the only way to get the kart to stop hopping.)

1 Like

I think the whole theory starts of on a bad foot with the words soft and hard. It’s probably more like springy and less springy.

But either way I started to lose the faith when i went from working on CRGs and Parolins where soft provided more “release” to OTK where hard provided more “release”. This really pricked up my BS monitor.

And no one has got close to explaining how cutting the axle shorter makes a difference.

One benefit to being a newish guy in the US is people don’t know who I am and I sit there quietly while they try to BS my face off.

Well it sounds to me that you’re being more ‘particular’ on terminology, rather than if different ’
axles work or not. @tjkoyen - What do you think on axles?

1 Like

No, no, no, don’t drag me into this…

Haha joking… I think I mentioned it above, but I’ve done a lot of axle testing you like had Davin where I changed back-to-back and noted how the kart felt.

I did a day of rain testing once where I went through all the axles Merlin had until we zero’d in one the that made the kart work best in the rain.

The only reliable testing experience I can offer is what I personally have done. My drivers go the same speed no matter what axle is in the kart… I would say that I often notice a much larger difference in the kart when going to softer axles than when going to harder axles. I think there is a difference in the material make-up but I tend to agree with Nik when he says axles aren’t “hard” or “soft”, they’re “springy” and “less springy”.

Merlin used to have a yellow axle (hard) and a white axle (also hard) which were supposedly the same “stiffness” but the rebound in the white axle was less. I can tell you with 99% confidence that those two axles WERE different in some way. I could never make the kart stable with the yellow axle, it always hopped with it in. The white axle was much more stable so that’s the one I chose every time when going stiffer on the axle.

My powers of deduction would tell me that that is probably how all axles are. There aren’t stiffer or softer axles, just changes in the rates that they spring back.

I’ve also seen how tire wear can change dramatically with different axle compounds. One session the kart will be shredding the outside rear, and the next the graining cleans up with an axle change. So I think there is SOMETHING happening, it isn’t all an elaborate marketing scheme.

Regarding @NikG’s comment on cutting the axle or short axles… The short N is our standard baseline axle now in the tent. This could all be placebo, but we largely believe that the short axle helps the rear dig better in the corner. If this is actually true, my guess is that it has something to do with the relationship between the end of the axle and where the force is being applied inside the hub. The shorter axle will place more force in a different part of the hub than a longer axle will at the same track width. Something to the effect of, “getting the same result of narrowing the rear track without actually narrowing it” I guess. Whether or not this is making a difference, I’m not sure, but at this point all our N axles have been cut anyway so it’s not like we can do much about it short of ordering all new axles.

For the record, I’ve changed axle twice this season in my kart, and one of those was to a Q for the rain. So 95% of the time I’m on the N axle. My overall feel about axles is that there are so many variables you’re affecting by changing it and there’s too many unknowns, that it’s easier to change a bunch of other proven adjustments around the axle instead. I’d rather stick to things I can easily quantify. Also, changing axles is hard work and I’m lazy.


All I know is…

A 50mm CRG medium was great for leveraging the control arms down on my P2 Volvos to replace the balljoints :smiley:

Joking aside… I’m thinking of splitting this into topics for chassis and axles. We’re at about 50+ posts now…

Although metallurgy applies to both, I think the outcome and (debatable) effect with axles is different enough to warrant separating it.

Rock the Vote: Should we split this topic to separate axles and chassis?

  • Yes: Split the topic into separate ones axles and chassis
  • No: Don’t split the topic.

0 voters

Interesting. We have H in our chassis. It was standard length up until about the weekend before MRP when we cut it. I had been using an N last year, then about halfway through we grabbed an H and kept that in there. We haven’t even thought about changing the axle yet this year.

I have noticed a difference in axles types, especially now I drive an OTK. Deflection is not the reason that the axle feels different. If you analyes the loading on the axle due to vertical weight bearing down minus the restoring moment of the wheel try to rotate under itself, actual deflection is minuscule. However the material type and hardness does effect greatly the natural frequency and dampening of the axle (the rate at which the axle restores itself to orginal shape), which I and other engineers believe changes the feel. This dampening characteristic I believe is the key.
Every kart I have owned (albeit only 4 karts) gets looser (or more excited/alive) with a hard axle.

My 2 cents for what its worth.


So my question to both of you (@DavinRS) is, did you have a preconceived notion of what the axle might do and did you know it was being changed and what to?

I ask because psychology is critically important and, for the most part, if someone is told it will give them more rear, then hey presto it will give them more rear.

I’ve done the test of swapping axles without telling the driver and found no difference, this is why I ask.

Please don’t take it as a slight on either of your abilities.

So Damping Ratio as you so correctly point out, could maybe have an effect. But for carbon steels it is very similar (0.05%). Damping is inversely related to modulus and modulus doesn’t really change much in the steels we encounter. Sure these are effected for different metals, even stainless vs carbon steel but not from one grade of carbon steel to the next where Youngs Modulus basically doesn’t change.

Believe me, I want it to be so, I just struggle to reconcile the physics which might just mean i’m not smart enough.


Great, this is the discussion i was hoping for. The placebo effect is strong in our sport. I too though it was crap, as my engineering teachings agreed with what you have said. To be fair it was probaly only my 3rd kart and the one i have now i could feel the difference. As my skill level was no where good enough with the first two. Both chassis i have had to run my hard axle due to previous “racing incidences” and i find the karts different mid corner, Jittery of you will. Hence provide “alive” comment.
Still highly possible it is because i installed it so i knew it was there and supposedly different that i felt the effect.
My engineering ramblings were the conclusions of an engineer far smarter than me, and to be fair we were clutching at straws as it frustrates the heck out of us that the physics dont match the testimonials of the many. Engineers hang up, have to now how.


Wow, pushed reply before I re-read my statement, my grammer sucked. Sorry. Haha

I missed a few points when I split this thread into a separate one for axles, so the conversation is a little jumbled. I was hoping that forum platform software individually tagged each post so that it could keep them in order, even when you pulled them over out on sequence, but I guess not… :frowning: #adminsad

But, at least we have a thread on axles now…

1 Like

Interesting axle article to boost this thread back up.

1 Like

Interestingly enough, I decided to do a quick calc on deflection on a axle under load. steady state at the apex. i.e 60/40 weight split 165kg kart weight so approx 100kg on outside wheel 2.0g restoring moment from the wheel trying to tuck under. And the deflection is in the order of 0.5mm. insignificant in regards to the the tyre deflection. I also dont buy widening the rear track you loose grip because of deflection. I believe this is more to do with the effective tipping line angle of the kart. picture this extreme. If a kart only had one wheel in the centre at the front, irrespective of where that wheel was point, this could infact be dead straight. The kart will try and follow a line which bisects the centre of the tires looking from above. As you can imagine with only one front wheel in the centre this could be quite an extreme angle. (we have all problay seen video of the british Reliant Robyns, besides tipping over that had nasty oversteer.) Obvious this angle less on a kart, but the same fundemental applies. upon exit the extra width in the rear makes the kart feel free because the lever arm length is greater (centre of tyre to centre of mass length) as you accelerate the force from the outside rear tyre tries to rotate the kart about the centre of mass.

Back to the dampening of axles, I forgot that i had a research document from 1966, that states the dampening characteristics of steel is relative to the surface shear stress. i was going to post a screenshot of the graph but couldnt work out how to attach. Davin help?

Ramble over… enjoying this constructive discussion

Ok found it. Feeling dumb right about now

1 Like

Don’t ever feel dumb about asking for help. That’s why we’re here. :wink:
stares at old screenshot

Ohhhhh tasty oldskool chart. It does show there are differences but look how small the % is to begin with.

So I was back in kart this weekend and I tried some axles. I tried a standard, a short standard and a harder axle. Now, I am out of practice, I do not have the feel for setup yet that I once had. But I couldn’t tell a difference, I tried really hard to find a difference. Times would indicate I went faster from once axle to the next (prefinal to final Sunday) but the kart was bent up half an inch on the one side from the first corner on.

In actual fact I never went outright faster then I did on Friday afternoon on ancient tires with the kart as standard (admittedly there was Friday night, Sat morning rain).

So you know what this means, i have to do another race…

How “soft” are soft axles? I went from a hard Freeline axle to a soft, and after 3 sessions it was bent already!!! No curbs, or hard hits anywhere. If they are infact this delicate, this is sad…Anyone have any experience?

1 Like