S-Express
Guest
Yes, they don't rotate / are shielded by chains / bits of frame / cranks etc.The risk assessments become very tricky with the presence of a rotating metal disc or several, being ridden at speed, in close proximity.
Motocross and MotoGP riders seem to manage ok - and have done so for many years. Reports of disc-related injuries are minimal to non-existent despite crashes being commonplace, so hardly a serious issue.