'You don’t make it safe by forcing cyclists to dress for urban warfare'

[[XAP]Bob]

A cycle helmet is, by design, broken after use...

[pjclinch]

By design it is less effective after use, to a degree that it's effectively a one shot item, but if it's cracked it's passed its limits. The difference is between damaged and broken. Arguably just semantics, but "absorbed all there was to absorb" and "absorbed some of what was there to absorb" is, I think, a significant difference.

Pete.

[pjclinch]

The shell doesnt need rigidity, just to keep the padding between head and force. String would suffice in the total absence of shear forces.

If you've got a crack and something pushing on it the only thing preventing the polystyrene moving out of the way rather than compacting will be your floppy shell on one side only. I suspect that will lead to it being pushed aside, widening the crack at the unbounded end. In practise I think this may quite possibly have a big effect on the ability to absorb energy.

Beyond that I suspect in a real world accident the "total absence of shear forces" is a pretty big "if".

The picture that sparked the discussion did have a cracked polystyrene inner still held in place by the outer.

That really doesn't tell us much. With a non-rigid outer you can crack the polystyrene, push it out the way in a spreading 'V' such that the impact force is directed straight to the inside, and afterwards it will still be held at the outer edge by the shell and can be folded back to look much as it was before the blow. So looking at it afterwards doesn't tell you much, especially in the hyper-forensic detail afforded by a snapshot on the Interweb.

It probably was cracked by a pretty minor force and a slightly larger force could have torn it apart before it had the chance to do any more absorption but we didnt get that far.

For some values of "probably". We really can't tell.

Pete.

[[XAP]Bob]

By design it is less effective after use, to a degree that it's effectively a one shot item, but if it's cracked it's passed its limits. The difference is between damaged and broken. Arguably just semantics, but "absorbed all there was to absorb" and "absorbed some of what was there to absorb" is, I think, a significant difference.

Pete.

Yep - I don't think we fundamentally disagree...
I think the absence of crushing is a less bad indication of outright failure than the presence of cracking.

But I'm not sure we've tried putting a cracked helmet through the snell tests...