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Yes, but you say "but in general the construction of frames today aluminum is stiffer" and my point is that it's impossible to state that accurately without knowing a lot more about the frame design.
I can't say that the KTM frame is stiffer or not comparted to any other frame just because it's made from steel and another is made from aluminum. Material alone is not enough information to make that statement, and simply looking at the geometry of the frame isn't enough either.
I know media and moto people in general repeat it over and over, but that doesn't mean it's true. There's A LOT that's repeated that's flat out wrong in this industry! Unless you've actually run the FEA on each or have physically bench tested the frames (not simply ridden them), it's not possible to know which is stiffer.
From a fatigue strength perspective, steel can have infinite fatigue life below a certain stress level. Aluminum does not share this and will always eventually fail, no matter how low the stress levels. But a fatigue failure will not be evident from "frame stretch" or any appreciable changes in geometry, stiffness, or anything else noticeable aside from a crack.
The story of a frame stretching is always an interesting one. Simply measuring the wheelbase to determine that is wildly inaccurate, particularly if the measurement tool is a sprinter van... There's lots of variables to wheelbase that are constantly adjusted - chain slack as the chain wears, fork height, different aftermarket parts (clamps, linkages, races), different tires, tire wear, different gearing, etc., all change wheelbase.
It takes a lot for a frame to "stretch" an appreciable amount that would effect wheelbase. It's not something that happens slowly over time as is often believed. If it happens at all (it shouldn't), it happens all at once from a massive overload (huge case, massive over-jump to flat, crash, etc.) in which portions of the frame fail (plastic deformation, but not necessarily break). This is a bad thing and the frame is not designed to do that. For normal riding without massive overload, I would expect a frame with 3 hours on it to measure out identically to one with 300 hours (within manufacturing tolerances).
You mean there's no magic frame "break in" period where it reaches a plateau of elasticity in 2 hours of ride time?!
I am so glad we are deep diving frame construction and that ET is going to cause stock KTM450's to soon have 75HP. I have no idea how my 5 lap vet track motos would be on point otherwise....
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I read your blog while taking a break from studying for my FE test. Lots of good information here and helped me brush back up on my definitions as well as graphs. Much appreciated for sharing. My favorite part was the work hardening portion as I didn’t know what alloys can and can’t be heat treated and need to be work hardened instead. I forge knives in my shop at home so heat treatment and tempering really intrigues me. Thanks for the read and distraction from studying.
You're not alone I loved my 97CR250. Had the suspension done by factory connection.
Bike was a rocket ship and probably was the fastest 250 of that time. Holeshot city. 😎
Pit Row
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