UKC

Ive got a 20mm through axle wheel that I want to use on another fork which is 15mm. I can get end caps for my hub that reduces it to 15mm but it seems odd to me that the whole wheel would then only be supported by two metal end caps rather than the whole hub. Is this right?

Thanks 

The bits in the middle aren't doing anything.  Think about inside the bearings - it's only supported by the tips of 8 little balls really.

I definitely agree with that, but the end caps just sit on the outside of the bearings, nothing inside, I can't get my head around it

The axle 'tube' isn't just a rolling surface. It clamps together and brings better rigidity to the lower stanchions, improving overall performance. Think of the three as a 'U' shape moving together. The rims just need sufficient width of contact that the loads don't distort the axle tube.

A bit simplistic, but hopefully it helps.