Originally Posted by
Wilfred Laurier
I believe the strength of the hollow axles is comparable to solid axles because (a) in bending, the material near the outside diameter of the axle provides much more strength than the material closer to the inside, and (b) even if the hollow axle is slightly weaker, the skewer fills that space and adds the same amount of strength as the amount of metal missing from the axle. Unless you have exotic Ti or (*shudder*) aluminum skewers, which are both generally weaker materials than most steels for a part of the same geometry.
The skewer also loads the axle in compression, which has to be overcome to bend the axle. This compression is why bearings tighten up when the skewer is applied, the axle actually shortens under the compression, while the skewer stretches. Bolts also stretch when they are torqued; in some critical applications the bolt stretch is measured instead of implying it from the fastener torque.