@ AL - I’m not sure what the differences between diameter and wall thickness make because I haven’t had experience with these and trying to figure or even rationalize would blow my mind. The discussion here, at for me, is why would two axles with the same material (same “stiffness” modulus or modulus of elasticity), same wall thickness and same diameter behave differently? I think the answer is that the modulus of elasticity changes and how it changes is affected by the tensile strength (=hardness) of the axle and the type of load (static, dynamic or cyclic). Also, if you look at the curve in my comment above, there is hysteresis in the modulus curve shown by the open curves associated with cyclic loading. I don’t think anyone can put numbers on it and it appears that automotive engineers are just on the cusp of realizing this phenomena, which karting has used for decades.
Gerard – I’ve found that at least on a OTK kart, that there is a range, even with the same tire. Same class, same day, same session, some are on N and others on H but lap times are very close. I think it depends on what you need to have done to the kart to make it work for you.
@OldFartKarter
Let me put it this way…go out on white Vega’s do 10 laps with U axle(Tony)
come in,change axle to HH and go out again…both axles are 2mm same length 1030.
I am pretty sure you’ll feel a difference.
If you are changing axles at the track, the last the you’d want to do is have to change; bearings, sprocket hub, and rotor hub. You really just want to change, slide out and slide in the new axle.
Should someone expect a difference in handling when switching from a 50mm axle to a 25mm…sure.
@Gerard – Yes, I use axles to tune the chassis and they make a big difference, which is the quandary that I’ve been trying to rationalize. General engineering theory says there should be no difference but my and others kart experience says otherwise. Further up this thread, there is a reference to a tech article referring to a “dynamic” modulus of elasticity, which would explain why axles of the same dimension but different material or heat treat behave differently in the kart.
I have seen guys with full assemblies built and ready to go. move caliper, remove bolts from bearings, break chain. Lift out. Drop in 40mm. bolt together and send it.
Just as an aside, instead of having to mess with the caliper, use an adjustable zip tie to hold down the brake pedal to hold the rear disc in place, & loosen the pinch bolts on the rear disc hub & bearings. Now slide the axles through, & remove the zip tie after retightening the pinch bolts. Easy.
One thing I’ll point out is that plastic deformation of an axle really exists. I put in a CRG S20 axle that had 0.005" runout on the left side, drove it for a day without crashing or hitting curbs, and when I went to remove it (ha) it had 0.040" runout and had to go right to PKT to be straightened. It was operating above its elastic limit; there was no visible cracking on either the bent area or the key holes.
I finally had some time to measure the static deflection of a hard axle (OTK H) and soft axle (OTK Q). I used 20 lbs of weight on a bar that extended 22" past the end of the axle to provide additional bending load.
Thanks for the exercise! I am thinking a laser setup may be more beneficial, you can measure the amplitude and time of the damping, but need a really high resolution laser.
Does this chart help? I will add that I don’t know anything about axles. I change them:
before the race day based on what I believe what will be the best axle given the track layout and conditions of the track
how the kart is handling on the day and to solve an issue
I would say some times if I have a bad problem (Road America last year - kart too flat off the final corner) then change from medium to hard and then cutting the axle did not make a difference. And other times it has helped.
Personally I don’t like changing axles because it is a pain in the ass. And more of a pragmatic issue – I can count on more than one hand how many times I left something loose in the rush to get the axle changed.
Are we sure that 20lbs hanging 22" from the fulcrum is enough force that is representative from actual racing? I think that we should check the force on the axle doing 1.8 G @ xx speed and get that comparison somehow.
That’s where the fun starts I think. Trying to measure\estimate lateral and horizontal forces on the axle while going through a turn. Oh, longitudinal too…
Damping: Yes, there is damping or hysterisis of the stiffness modulus (Modulus of Elasticity) this would account for why hard and soft axles behave differently. One way damping can be measured is by how quickly motion decays due to an impulse input (like a hammer blow). The faster the decay the more damping. See below.
A way to do this would be to mount an accelerometer to the end of the axle connected to a storage oscilloscope, then hit the end of the axle with a soft blow hammer and measure the response. I don’t know if we would see any difference, I believe that the damping occurs at higher stress levels and under cyclic loading, at least according to the material that I’ve read. Both of which is not possible with the test rig that I have.
More Load: How much force is put into the end of the axle shouldn’t matter, it will deflect more but both axles will deflect the same and proportional to load. Except maybe if we are measuring damping.
I believe what this tiny experiment likely proves is that it isn’t the stiffness of the axles that makes them behave differently. Therefore the only element that could affect the axle response is damping. Much like a suspension in a car that consists of a spring and damper.
This knowledge doesn’t change our tuning strategy but at least it provides some logic as to why.
Is there guidance from AMV about when to use which axle?
It is entirely possible that some karts respond less and differently to axle changes. I can feel the difference and somewhat predict the change in a OTK kart but couldn’t in other karts. (The other caveat is OTK is 75% of my kart experience) Also the track might matter, as the axle might help in some areas but hurt in others, the change should be viewable in the data.
