Should motorcycle armour be plastic?
Motorcycle “Armour” has two functions, impact absorption, and abrasion resistance. Metal is not going to absorb much impact. Foamed high density plastic is very good at absorbing impact, so it is ideal. It is not so good at abrasion resistance, so this is provided by the leather or kevlar outer garment. To some extent a dual density insert will protect from abrasion by having a tough outer layer bonded to a foamed, energy absorbing inner layer. In the case of some back protectors, you have a rigid plastic outer, with a backing of impact absorbing foam. The rigid, or semi-rigid layer both protects from abrasion, and also helps absorb impact by spreading a localised impact, like a kerb edge over a greater area for the foam to absorb the shock.