Absolutely, it is very complex and I've often wondered how they engineer a monocoque chassis to crumple on impact but protect the cabin space. It can't crumple much cos there's a solid lump of metal under the bonnet.
There are definitely 2 areas of crumple - the high impact where the chassis crumples and the low impact where the bumper crumples, but by law shouldn't result in damage to the vehicle - I believe this is up to 5mph unless its different these days.
In a high speed impact, the low-speed crumple will go first, so I suppose that knocks 5pmh off the speed that the main chassis has to absorb - not inconsiderable and could make a difference. Bolting a tubular steel bumper on will add that 5mph back on - plus or less any difference its characteristics have to the std armature.
It will also be illegal in the UK - I'm pretty sure of that - it would be treated like bolting on a steel bull bar. Killers to pedestrians.
Agreed that those MIRA crashes are real world....
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