Most of my 30mm axles have been hollow, but I did have a solid one in the collection. They've all gone now. I've not done any deflection calcs yet (they don't take very long, so maybe tonight), but the geometric variable that governs beam stiffness is "Second Moment of Area". The material variable is "Young's Modulus", and only varies by about 3% across all the grades of steel I've ever worked with. Deflection of hard, medium and soft axles by these calculations will always be the same as one another as long as the dimensions stay the same. Since all the 50mm axles I've ever bought have been in 2mm wall, they must be similar stiffness to one another. It is the hardness that varies, and whether that makes any difference to the price of fish (or handling of a kart) is a bit of a moot point.
Anyhow, first set of figures is Second Moment of Area (I) of a few axle dimension sets, measured in cm^4. If one axle has "I" as double that of another, it will deflect half as much for the same load.
Outside diameter - mm Wall thickness - mm I - cm^4
30 Solid 3.97
30 7.5 3.73
30 5 3.19
50 2 8.70
So even at 2 mm wall thickness a 50mm axle will deflect half as much as a solid 30mm axle. But now for the weird bit: the deflections are not only small, but I'm pretty sure they don't change very much with cornering load. So far it seems they actually change the "wrong" way, with cornering forces reducing the upward deflection of the axle at the hub.
I sold off all my 30mm bits at the end of the summer, and I'm wishing I'd kept one axle set to do some more testing now.
Liam
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