OK, I didn’t know for sure, just thought that aluminium alloy would be a bit weak for the splines on the free hub.
It
can be...
I was on a ride with a bunch of people once when we dropped down a steep lane to a bridge over a stream and were faced by a steep climb back up the other side. The rider next to me suddenly shouted '
WTF!' and ground to a halt with his cranks whizzing round at about 120 rpm.
I expected the problem to be with the pawls in his freehub not engaging properly, but the problem turned out to be in one gear only - all the other gears worked just fine. We continued the ride with him avoiding the dodgy gear.
When he got home he took his cassette off and discovered that the problem was this...
(That particular cog had ripped its way through the splines!)