The mad science of rebound in car crashes
How to crack the kooky code of crashes, plus the mad science of stamping sheetmetal, and the black art of becoming a Jedi master with nuts and bolts
EDIT: @6:46, I meant 'elastic zone'. Sorry - zigged when I should have zagged, conversationally. Apologies.
Last episode, when we covered advanced high strength steels >> I asked you to explain why cars rebound off the wall in crash tests. After all, the car is designed to crumple, right? The crumpling absorbs the kinetic energy. So if the energy gets absorbed, why the bounce?
Here’s a representative sample of what you said:
The wrong answers, and why
Some people, of course, tried for a correct answer, and missed the target, emphatically.
"A deformable barrier on the test..." - iVlogBuzz
That does not explain the bounce - the deformable blue face is just a crushable aluminium honeycomb that’s there to simulate the crushable face of a car you have a clipping head-on with. The car would still bounce if all it hit was bare concrete.
"Equal and opposite reaction" - Peter harper
Newton’s third law was very popular in the answers - but this is true of all collisions. During the collision the force the car exerts on the block equals the force the block exerts on the car. It doesn’t account for the ‘why’ of the bounce.
"The concrete block will also have some elasticity." - Equiluxe1
Totally incorrect. The block is chosen specifically for its non-participatory properties. It’s completely unaffected by the collision for all non-trivial analysis.
"The more I look at it the more convinced I am that the front tyre is a major contributor to the bounce back." - Gabriel Rd
I guess it does push back a bit - but also only a trivial factor. The car would still bounce, about the same, if you threw it at the wall minus the tyres.
"isn't inertia part of the energy that cannot be lost?" - Kevin Reid
No - it’s not - inertia is a property intrinsic to everything with mass. It’s like this: a two-kilo brick is weightless in space, right? But if you throw it at someone’s head - in the International Space Station, and it hits them, it’s still going to be deadly, potentially. The reason is: Inertia.
"Conservation of momentum." - David P
Clearly the momentum of the car is not conserved. It doesn’t bounce off at the same speed, and it’s moving in a different direction. So that’s absolutely not it. And yeah, conservation of momentum is universal (at least in Newton’s universe) but where you draw the boundaries for your system really matters.
The right answer: Why the car bounces back
Here’s the answer to why the car rebounds - the full answer, supported by applied science.
The bounce occurs because of the overall elasticity of the entire structure of the car. The bits that are smashed and permanently deformed, plus the bits that are not - like the passenger cell, hopefully, the better to leave you shaken but not shredded in the aftermath.
The entire structure is compressed by the massive inertial load of the crash. Some of it permanently and some of it not. I think we can all agree on that.
To some extent, the structure is like a giant spring. Some of the kinetic energy that is not absorbed by crumpling parts of the car permanently is stored in the elastic strain of the structure.
When the car slows down enough, its acceleration tapers off, the inertial load into the wall drops, and the defacto spring of the structure unloads.
That’s what pushes the car back off the wall. It’s important to understand that the bits that are permanently bent, and the bits that are only elastically deformed, both unload elastically - like springs.
So if you told me it was just the passenger safety cell unloading, you were absolutely on the right track with elasticity, but only halfway there.
The crumple zones stay bent, but when the inertial load comes off, they unload elastically too.
