Bingo! Axle is a big change and affects a ton of variables so it’s usually a last resort change for me. Short N is what I run 95% of the time.
#1 - I am a Mech Engineer that has 25 years experience in the pump and compressor industry. I now work in sales, but still regularly get consulted for input on difficult technical field issues and failure analysis. But don’t hold that against me…
#2 - The modulus of elasticity of all steels is in the range of 28.5-31 x10^6 psi. “Strong” steels are not “stiffer”…they just can be deformed further before they yield. Even radically different alloys like Martensetic SS, Austenetic SS, and high alloy steels have the very close to the SAME MODULUS. So from a static deflection perspective, all steels are the same. There is no such thing as more or less flexible steel when you are in the elastic range (before you bend…or go plastic) Even look at Maraging Steels…the most exotic high strength steels in existence. 18Ni Mar 350 Rockwell C = 57. Tensile = 342,000 psi. Yield = 336,000. YES…those stress numbers are correct. Here is the kicker. Modulus = 30x10^6 psi. Pretty much the exact same as 1018 steel. So if you get a $10,000 18Ni Mar 350 axle made, it wont feel any different, but you will NEVER bend it, if you can still afford to race.
|Steel Grade of Tubing (US Spec)||Tensile||Yield||Hardness||Modulus (10^6 psi)|
|1008 CW||49300||41300||55 RB||29|
|1010 CW||52900||44200||60 RB||29|
|1018 CW||63800||53700||71 RB||29.7|
|1020 HW||60900||50800||68 RB||29.7|
|1026 CW||71100||60200||78 RB||29|
|A106 Gr. C (High Temp Fluid Handling)||70000||40000||100 RB||30.4|
|J524 (good for bending)||45000||25000||65 Brinnel|
Modulus are all in the 29-30 x 10^6 psi range… and heat treatment does not change it much either. It is what it is.
#3 - Hardness of steels, in the correct materials engineering sense, has nothing to do with what we are talking about. Hardness of a steel is determined by striking it with a die of some sort (such as a small very hard round ball, etc), and measuring the size of the indentation when the steel, meaning it has yielded and gone plastic. In other words…localized bending.
#4 - What about damping (the propensity of a material-structure to not spring back quite as hard as you pushed on it) you say? Well all steels have relatively low damping, but I will acknowledge there is not lot of data on damping.
Here is one paper, from the Library of Congress - https://stacks.cdc.gov/view/cdc/10399/cdc_10399_DS1.pdf
You will note there looks to be some difference between the damping of different steels. At 5000 psi (maybe a good number when your kart loads up and hops/jumps in a corner) specific damping runs from 0.2 for 1045…to 2.0 for 1020…so the lower strength steel has more damping (Page 12). Heat treatment does increase damping some…such as for 1144…0.4 to 4.4! (Page 14)
But even these numbers are low. Grey Cast Irons (flake micro structure) have Specific Damping ranges in the 15-25 range! Have you ever seen a machine tool body made from steel? Do you know why? It is not because machine tool mfgs are cheap…it is because CI is the ideal material for large precision machine frames that you want to run smooth. Lots of damping, but expensive to move. And they are brittle materials…which are not a good idea for applications like a kart axle.
That is really all there is to it. You have Modulus (the “spring constant” of steel)…which never changes much. There is some range in damping. So if there indeed is a difference in kart axles, it can only come from one of two areas:
Making the wall thicker or thinner, which no one seems to be doing. For instance, ALL (6) Birel-Freeline Axles are 50mmx2mm, the 2mm being wall.
So if you feel a difference, it must be due to damping…say 1045 annealed vs 1144 heat treated.
There is simply nothing else.
I enjoyed reading that a lot. I feel both smarter, and a lot less so!
I found another damping reference which shows that Martensetic SS (410/416/420 SS) has the highest damping of any steel.
Schetky and Perkins1 have studied the damping properties of metals and alloys. They identified the malleable magnesium as the most damping metal (specific damping capacity - 49%), followed by the alloy inkramut and sonoston - 40%, Nitinol - 40% saylentelloy - 40%,high-carbon cast iron - 19%, nickel (net) - 18%, technically pure iron - 16%, martensitic stainless steel - 8%, gray cast iron - 6%, mild steel - 4%, ferritic stainless steel - 3%,malleable cast iron with spheroidal graphite - 2%, medium carbon steel - 1%, acousticstainless steel - 1%, aluminum - 0.3%, nickel-based alloys - 0.2%, titanium alloys - 0.2%,brass, bronze - 0.2%.
Marin Vujcich posted something similar in the paper he referenced, where it shows 12% chrome having high damping. 12% Chrome = 410 SS = Martensetic SS. So when I see kart mfgs making axles out of 410 or 416 SS (8% damping)…I will think they are actually onto something. Or if I see axles with different wall thicknesses…I will say OK…that is a valid design change. Changing hubs? That also will impact stiffness and behavior. But changing from a steel with specific damping of 1.2% to 4% when the Modulus is the same? I can’t believe you could ever feel the difference when you consider damping of the tire, etc.
The way to tell is to buy some 4130 (very low damping) and heat treat it to Rockwell 32 C, and make some axles with 1.75, 2.00 and 2.3 mm walls. Then do the same with 410 SS (very high damping…and strong)… Then test. The problem is, I do no know where to get 410/416 tube. It might be easier to buy pump shaft (PSQ is very cheap), and have it gun drilled prior to centerless re-grinding. The gun drilling would not be cheap…
I would like to point out that we change the rear track by 5mm and you feel it, which corresponds to roughly 2% of the working fulcrum length of the axle. So small stiffness changes could feasibly be the same. When you are at100% of adhesion is stable the 101% is a slide.
I will say track width affects lateral load transfer more than anything less though.
When you change track with on the rear, it’s not so much an axle behavior thing that changes the handling, but how the load is transferred from the inside to outside.
Narrower track width, more load transfer.
Wider gives less load transfer.
What makes is especially interesting is with karts, people confuse lateral load transfer with grip. Moving the wheel in, doesn’t actually give more grip. It gives less.
BUT, you’ve increased the lateral load transfer, hence the inaide wheel picks up easier and gives the impression that you have more grip.
5mm is a very large change if you review the math. I think we all intuitively under estimate the significance of “just a 5mm change”. It is actually quite significant. A 2% change in working length of an overhung axle (beam) by moving in the hub/wheel will increase the overhung beam stiffness by about 8-10%.
I am not trying to get into engineering overall kart dynamics. It can not be done. I am a good enough Engineer to know that NO Engineer is smart enough to do that. Vehicle dynamics on car, where you can control each corner independently is diabolically complex. A kart? An order of magnitude more so. So you use trial and error, testing, and experience. One bad habit us engineering types have is dismissing correct answers that were arrived at while not using engineering principles.
But we can break down specific isolate parts of the kart…and apply engineering principles to them. That is why I was looking in detail at axles. The physics just to not back up the notion of softer and stiffer axles being a factor, because there is no way to make them stiffer of softer unless you change wall thickness, which I see no one doing.
There are (or at least were) different wall thickness being used by sone manufacturers. There was also a spate of axles shearing which might have ended that.
@Alan_Dove might be able to talk more about that.
Sorry wasnt comparing the change with axle stiffeness just stating that small changes can be felt when you are near the limit. So small changes in damping will also be felt when at the grip limit.
I think that is an issue in cars as well. We look at static dimensions and don’t fully understand how the effect plays out dynamically. Mix in how one adjustment affect others it coordinates with and we get confused.
I hear 2 basic outlooks from people - in regards to chassis tuning.
1 - each kart is different.
2 - all karts are the same
I know I’m not good enough to say what is right. I do find #2 a more comforting scenario. I tire quickly of the VooDoo approach and think there has to be some basic correlations that are universal. I just want my world to make sense to me.
I have been out of the game for a long time. I recall back in the 90s seeing different axle wall thicknesses, but back then, changing axles to impact handling was not as well known. Today…everyone talks about it, but now the axle walls seem to be all the same. For instance, Birel/Freeline has 5 or 6 axles, and they all have the same 2mm wall. I am just not seeing that there are enough different materials out there to get a real range of behavior being claimed without changing the wall, hence my comments.
I could see going safely down to 1.25 mm wall…IF you have the right material, the right heat treat, and most of all get rid of stress concentrations with a inset spline type mounting system for hubs and carriers. But it would cost $ that could only be justified at the factory level.
Maybe one day I will do some dynamic response testing. I have some pretty sophisticated multi channel vibration analysis equipment (CSI/Emerson https://www.emerson.com/en-us/catalog/ams-a2140) which would allow me to quantify damping characteristics. Mount up an axle on a rigid base…and whack it with a modal hammer, and see the dynamic response…
Good point. At the limit, the smallest factors become significant.