Why cars are designed to fall apart in a crash

HostIf you walked past a bad wreck and saw two cars, one that looked almost perfect and another that was completely smashed in, you would probably think the people in the clean car had the better day.

HostBut it turns out that a car that stays stiff and keeps its shape in a crash is actually a deathtrap. Why is it better for the car to just fall apart?

GuestIt comes down to a trade between force and time. Every car moving down the road has a huge amount of movement energy. When you hit something, that energy has to go somewhere. It can't just vanish. If a car is built to be rock solid and hits a wall, it stops almost instantly. We're talking maybe a tenth of a second. Because that stop is so sudden, the punch of that hit is just huge. It's more than a human body can take.

HostSo by making the car weaker, we're actually making the stop slower?

GuestYeah, that's the goal. Engineers design those front ends to crush like a soda can on purpose. This increases how long the crash lasts by just a few tiny thousandths of a second. It sounds like nothing, but in the world of physics, it's everything. If you can double the time it takes for the car to come to a full stop, you cut the peak force of the hit right in half. By giving up the front two or three feet of the car and letting it get destroyed, you're buying the human body more time to slow down.

HostBut metal is tough. I have a hard time seeing how a thick steel frame just folds up neatly. I would expect it to just snap or maybe even poke through the floor like a spear.

GuestThat's a real worry. If the frame of the car was just a straight, solid beam, it would be dangerous and very hard to guess how it might break. That's why engineers use what they call triggers. These are little notches or holes or even just curves built into the steel at specific spots. They're intentional weak points. When the hit happens, the steel has to bend at those spots first. It forces the metal to fold up like an accordion. This is what they call energy management. As each fold in the metal happens, it uses up a massive amount of that movement energy. It turns it into heat and the physical work of bending the steel. By the time that punch makes its way back to where you're sitting, a big part of the hit has already been soaked up by the car itself.

HostThis still feels like a gamble. If the whole car is designed to fold up, what stops it from folding right onto my legs?

GuestThat's where the second part of the design comes in. A modern car is really two different machines joined together. You have the soft ends that are meant to be crushed, but then you have the middle part. That's the safety cage. It's a rock-hard box made of super strong boron steel. This part is built specifically to not bend or break. If the front of the car is a sponge that soaks up the water, this cage is the glass that holds the people. It makes sure that even if the trunk is gone and the front is flat, the door frames stay the right shape and the dashboard doesn't get shoved into your lap.

HostWhat about the engine? That's a giant block of solid metal. It can't fold like an accordion.

GuestNo, you can't really crush an engine block. So engineers changed how it moves during a wreck. They use special mounts that are designed to shear off. They basically snap on purpose. Instead of the engine being pushed straight back into the cabin, it slides down and goes under the floor. It gets out of the way so the rest of the car can keep crushing and soaking up that power.

HostI'm still stuck on the timing. You said we're only adding a few tiny slices of a second to the crash. Is that really enough to save a life?

GuestIt sounds like nothing to us, but for your heart and your brain, it's everything. This is what people call the second collision. When a car hits a wall, the car stops. But the stuff inside the car is still moving. Even if your seatbelt holds your chest in place, your insides are still moving at the speed you were going before the hit. Your brain is still moving until it hits the inside of your skull. Your heart is still moving until it hits your ribs.

HostSo the car isn't just stopping itself, it's trying to stop my heart gently?

GuestExactly. Every inch that the car crumples acts like an extra bit of braking distance for your heart and brain. By using the crumple zone to make the car interior stop more slowly, engineers make sure the forces are low enough that your organs don't get crushed against your own bones. Every inch of metal that folds up is an inch of room that turns a deadly, instant stop into something you can walk away from.

HostIt's wild to think that the more the car looks like junk afterward, the better it did its job of keeping you from being destroyed.

GuestThose mangled pieces of steel are actually the reason the cabin stays safe and the people inside stay whole.

HostThe next time I see a car with a front end that's totally flattened, I won't think about how weak it was, but how much time it bought for the person behind the wheel.