Re: @AndyS 4 Adam1
I would have thought that not having a massive block of incompressible cast iron or aluminium in between you and the other object limits the amount of energy that can be absorbed
But the engine is held to the frame by three or sometimes four bolts, which are designed to shear in a crash. The (red hot) engine goes under the car (usually, there are some howler designs that push it up through the bonnet so it can come through the windscreen and land in the driver's lap), directed there by the slope of the firewall.
Designs with a conventional gearbox will hopefully dislodge the rear axle or the assembly may attempt to come up through the tunnel cover for a hug.
You don't want solid stuff in the way. The point of the crumply stuff is to streeeeeeeeetch out the time it takes to go from "Yeehaaohchrist!" to zero and thus reduce the force experienced by the occupants (F=ma, a=dv/dt so for a given dv the dt needs to be as long as possible to minimise F). That's why seatbelts are stretchy and airbags leak.
Interestingly, the telescopic steering column is a different case. It collapses because before they did that it was not uncommon for crashing drivers to be killed by a kung-fu punch to the chest by the steering wheel. Then the sun visors and dashboards had to become soft because with no wheel holding back the driver his/her head was free to hit them. The old plywood sun visors were first to go, followed by the wooden dashboard.
And so was born the Nanny State, where a person was no longer free to trepan themselves on the visor or do some amateur dentistry on the old instrument cluster.
Now, of course, the only serious threat to most drivers is the danger of having two small trumpets cold-stamped into their temples when the air-bag deploys and blows off the horn push.
Biting the hand that feeds IT
