Ian, in terms of how I would define a hard/medium/soft axle would be as my axle supplier does, a soft has the thinnest wall thickness and a hard has the thickness and a medium is somewhere in the middle.
The basics of the Black Art of Axles:
Values of Young's Modulus, E
Steel, mild (hardened) 201.4 GPa
Steel, tool (hardened) 203.2 GPa
Steel, mild, low carbon 210 GPa
Steel, mild or tool 212 GPa
Steel, stainless 215.2 GPa
F = force
L = length of axle
E = Young's Modulus
I = moment of inertia of axle
With a single end load and a fixed single support on cantilevered axle, the equation for deflection (D) is:
D = FL3/3EI
Therefore we can conclude that since the modulus does not vary significantly and it is the only variable effecting deflection with all other geometric properties held constant the deflection of any axle made of just about any steel will deflect the SAME amount for a given load for all practical purposes.
Also as you can see yield and tensile strengths are not included in the equation. Therefore they play no part in the amount of deflection as long as the material is in the elastic region.
As the moment of inertia of a axle, is determined by the axle's cross-sectional shape and it’s wall thickness, now given that the cross-section has not changed then to my mind the only factor that has changed (in my post above) is that the hubs will have a reduced wall thickness of around 5mm thereby changing the axle’s moment of inertia and in my parlance ‘softening’ the axle.
It will of course also change the axles damping characteristics but that’s a whole other can of worms!
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