[wildmanx]

> a concrete helmet is literally worse than useless.

Is it? It would still absorb energy upon impact. Which, in contrast to popular belief about bike helmets, is its main function. Yes, ordinary bike helmets will also just break. But in doing so, they absorb energy which then won't be absorbed by your skull.

There is no point in a perfectly stiff helmet. It would just transfer the energy right through and you'd crash your skull into the helmet, which doesn't help.

[WMaking]

It may not be as much of a factor at bicycle speeds, but I've read that heavy helmets can dramatically increase whiplash in things like motorsport. The increased mass on your head means higher forces trying to rip it off. Obviously the other benefits of real helmets outweigh that issue, but I'd imagine a concrete helmet with very poor shock absorption and very high mass would not fair so well.

[tromp]

Concrete is pretty stiff. But mostly, wearing such a helmet will make it much harder to move your head around to scan your surroundings, increasing the chance of accidents.