Another thing that I think is an interesting conclusion would be metal fatigue and the fatigue limit of the metal - some steel alloys don’t have an ultimate fatigue failure and that’s usually a hallmark of ferrous materials and titanium whereas a lot of non-ferrous materials (such as the Intrepid with weird cracks everywhere) have a failure-based fatigue life where, eventually, the material will crack.
Here’s a pretty good chart for the tensile strength of 4130 and how, once it hits it’s fatigue limit, it loses strength.
This comparison of steel and aluminum (which can be assumed to be similar for materials like aluminum such as the non-magnetic material in that intrepid) will eventually just go to failure as they don’t have an endurance limit. The highest stresses will be around the bends on the front clip, struts, and where welds happen along the long sections of main rail. That said, you can approximate spring rate with Young’s Modulus and relative displacement. Meaning Y(delta L/ L) is loosely Hooke’s law (I read a paper about this a while ago - I might be off slightly). Y= Stress/strain so as the material has less stress from being fatigued for the same strain it will get lower - meaning the spring rate will get lower and it will get softer. IMO, this falls much more in line with anecdotal evidence I’ve heard about the kart on track.
Something else to take into consideration in the design of bending parts is that there’s so much more than purely stiffness - for all intents and purposes, “rebound” shouldn’t exist as karters talk about it. Energy loading vs unloading should be linear as far as bending is concerned - but it obviously isn’t if you’ve driven axles with different rebounds. The mini kart axle I designed last year was the same stiffness as an incredibly popular axle mini kart axle from Europe but had a completely different rebound because it was using a different material, wall thickness, etc. This is because our rules allow different things in the axle as opposed to european rules. There’s quite a lot to play with outside of stiffness alone. Since we’re undamped, a lot more things matter such as natural frequency and resonance.
I set up an FEA of a kart chassis earlier in school - what I found out was, more or less, without a bunch of specialized information it’s hard to really tell what’s going to happen. Biggest stresses were around bends, etc. I really only focused on torsional rigidity since modelling weight transfer through a fiberglass seat would be… Difficult.
The highest stresses on the kart come at the welds, not on the tubes. Look at the factory welds vs general production welds. Might get a little more insight on to the special karts there as well. I know a lot of factories will do karts specific to tracks as well. Maybe one track is rougher so it needs a more “dull” material, whereas another track might be glassy smooth and single apex turns so it might need a more reactive kart. Quasi-damping is pretty cool and makes a lot of sense in reality but I can’t make much sense of it myself in theory. I know qualitatively what materials behave as what within karting and have working theories to why, but I can’t really give anything definite.
I’ve gotta disagree on the karts come in higher right away thing - I table karts at the end of every day at major races (Pro Tour, Winter Tour, etc) and the karts I table the night before to be perfect often come back perfect (within 1 mm) if they don’t hit anything. This is true for Compkart, Parolin, Energy, Praga, OTK, any of the brands I’ve personally worked with in the last few years. Part of that “pop up” the first session may be auxiliary stress in the frame from assembly - if the kart has stresses in it that, when agitated, release themselves. It’s a pretty common tactic for stress relieving welded parts on a vibration plate and maybe it’s showing up here. I assume karts are stress relieved after welding but maybe not.
At the end of the day, I think that the majority of this comes down to economics and not engineering. The factory karts are usually a different material that wears out faster that they can’t sell to the public because of the life expectancy of a kart, as mentioned above, but I think the teams selling off karts in non factory settings has more to do with whenever they get their money out of them. In three races of rental fees most teams will have paid off their cost on the kart - whatever else is left is profit. You’ll make more off the last two races in rentals than you’ll get as an increased return on selling the kart used as a 1 race versus a 3 race kart. At a certain point, rental customers expect fresh equipment, so this is an ideal spot to do it.