When a car and a truck crash, the car seems to get damaged a lot more. Why does the car get more damaged then the truck in the collision?

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Why does the car get more damaged then the truck in the collision?

Mostly for practical reasons.

When cars were first being built, they were simply horse carriages with engines. Over time, these evolved into the first really modern cars like the Model T. These still had many vestiges of the carriage era. For this question, the key one is that the chassis of the car was separate from the body.

In most cases there was a framework of steel in a rough H or 8 shape as seen from above, with the wheels and engine mounted to it. The body and interior were then placed on top. In the case of the Model T, you could buy several entirely different bodies for the same frame.

The skins of the body had no structural needs beyond holding themselves together when you slammed the door and such, so they could be relatively light weight. When you take apart a car from the 1950s pretty much everything comes off until you're left with a big go-cart. This basic construction method was used into the 1960s.

At that time, techniques that had been used in the aircraft industry started pushing into the automobile world. Aircraft designers had noted that if you shape a sheet of metal properly, it becomes very strong in certain directions. For instance, if you roll it into a tube, it becomes extremely resistant to bending. So now you don't need to have a structure at the rear of the plane, just make a tube of thin metal and mount the tail to it. You just removed all the framing from the back of the plane and saved a bunch of weight.

So then you started seeing the removal of the car's frame and its replacement by carefully bent pieces of the skin of the car. For instance, the "A pillars", the bits on either side of the front window that often have airbags in them now, used to be a separate piece of metal. But now it's just a rolled up bit of the skin. This is way lighter, because all you have is the skin, not the skin AND a framework under it. Better yet, you can carefully tailor the skin to respond in certain ways depending on how you shape it, which is why cars crush up when hit - they're supposed to, the energy going into bending the metal is not going into your body.

So... back to your question. Trucks aren't built like this. They still have frames. Look in the wheel well of any pickup and you can see it, typically a big black-painted chunk of steel.

So when a car and truck meet, the force of the collision in the car goes into unrolling all those bits of metal, whereas in the truck it goes right into a couple of huge I-beams. So the car is going to lose, every time.

Now that sounds bad, but its not. Consider a car and a truck hitting a steel wall going the same speed. Both stop "instantly". In the case of the car, that energy goes into the body work, folding it up. You walk away. I seriously hope you never experience the same thing in a truck, it can be non-nice.

So are trucks less safe? No, but not for the reason you might think. When a truck hits a car it folds the car up, and thus the truck gets half the advantage. Additionally, the reason trucks have a frame is so they are way stronger, so they can manage a lot more impact without you ending up with the engine in your lap.

  • Interesting indeed, I always thought of this from a momentum/kinetic energy point of view, but the reality is simpler - trucks are just built tougher than cars. Interesting paper at ncbi.nlm.nih.gov/pubmed/24821629 showing that car vs. unibody SUV crashes are in fact less often fatal than car vs. body-on-frame SUV crashes, for both the car and SUV driver. – Nuclear Wang Nov 8 at 20:23
  • Curiously, A/B/C pillars have grown substantially thicker over the decades too. – Criggie Nov 8 at 23:20
  • Then how come when they crashed a body-on-frame 1959 Chevrolet Bel Air vs a unibody 2009 Chevrolet Malibu, the Malibu fared far better? Unibody is going to be much better because crashes come from any direction with different forms, whereas a body-on-frame only has an advantage when the forces directly transfer into the frame elements. – user71659 Nov 8 at 23:47
  • @user71659 - body-on-frame does not imply stronger strength. Trucks vs. cars imply stronger strength. Plus, we've done a lot of engineering in those 50 years, the outcome should not be surprising. – Maury Markowitz Nov 9 at 14:08

Conservation of momentum causes the car to lose more velocity and hense kinetic energy. Where does this kinetic energy go ? Into heating and deforming the car.

From Maury’s reply I’d like to add that cars are built to crumble on impact for the passengers‘ safety. The truck on the other hand is built like a tank. It will spend lots of time on building sites and will take a good beating during its lifetime. It is not designed to crumble.

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