Car Crash Dynamics: Why Buckling Up Is Important

crash dynamics front impact

We never wake up in the morning and think, “Hmmm, today I think I’ll get into a car crash, seems like a good day for it.” No. We wake up believing we will get to where we are going no problem. And most of us put on our seat belt, just in case. We know car crashes happen. And since we have some idea of crash dynamics, we know crashes are far from pretty.

Crash dynamics looks at the energy involved and what happens during a crash so engineers can better design injury prevention systems like your seat belt or air bag. There are many variables that go into a crash and need to be accounted for when looking at crash dynamics and crash outcomes.

One of the biggest variables is speed. Other variables include road conditions and whether or not the occupant was using a restraining system, car seat or seat belt.

3 Stages of a Crash

Most of us think of a crash as the car hitting something or something hitting the car. Looking at the dynamics of what happens during a crash shows us there are three collisions during a crash.

1. Vehicle collision

This is the collision we are all most familiar with. This is when the vehicle strikes an object. Depending on the object — a tree, a pole, another vehicle stopped or coming at you – your vehicle likely hits this object and comes to almost an immediate stop. Some objects will move when you collide with it so it slows the stop motion.

most common car seat mistakes2. Human collision

When the vehicle comes to a stop, the occupants inside are still moving at the speed the vehicle was moving. The occupants keep moving until something stops them. This something could be designed to stop them while protecting them like seat belt or air bag. Or this something could be the dashboard, steering wheel or windshield (supposing a forward impact crash). If you are an unrestrained occupant and those don’t stop you, you could come all the way out of the car which is a much more deadly situation.

3. Internal collision

The car might be stopped and your body may have stopped but your internal organs are still moving. Crazy, right? I never thought of this until I took the child passenger safety course. But it makes sense.

All the internal organs continue moving toward the point of impact until they collide with something that stop them like other organs or bones. This collision can cause internal bleeding or organ damage.

These collisions all happen very, very quickly.

Just how quickly are these dynamics happening in a crash

These three collisions happen in about six tenths of a second. Let’s imagine you are driving 35 mph and hit a tree:

  1. In the first tenth of a second the center bumper hits the tree and stops, the rest of the vehicle and everything inside keep moving. The vehicle starts deforming.
  2. The car continues to deform, engine parts begin to crush until the engine itself and frame hit the tree and decelerate.
  3. As the frame and body of the car continue to deform, the front dash and windshield has slowed to 20 mph. However the occupants are still moving at 35 mph.
  4. The frame slows to 20 mph, continuing to dissipate crash energy. The occupants are still moving at 35 mph.
  5. The safety belts begin to stretch (they are designed to do this) to decelerate the occupant “over time”. We call this ride down. The frame slows to 15 mph and the occupants slow to 25 mph, unless you are unrestrained, then you are still moving at 35 mph along with any loose objects in the car.
  6. By six tenths of a second the safety belts have stretched to their limit. Occupants have slowed to 10 mph and the dash and windshield stopped. The body, frame and engine continue absorbing crash energy. All the loose objects continue at speed until they hit the front seats, front passengers, dash or windshield.

Any unrestrained occupants continue at speed until they strike the dash or windshield, their body deforms and crushes. Unrestrained occupants internal organs continue at speed until they strike the rapidly decelerating body frame.

Of course crash dynamics vary some depending on the type of crash (frontal, rear-impact, side-impact, rollover or spin) so that the injuries vary by type of crash as well.

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After the 3 collisions

What happens after these three collisions? Continuing with crash dynamics, after the initial three collisions things start rebounding. That essentially means you bounce back toward where  you started. The restrained occupant rebounds from moving forward into the seat belt and their heads continue back to come in contact with the head restraint.

To give you a good visual, watch this great 1970s film from Chrysler Corporation called Dynamics of a Crash.

Of course cars work differently now with crumple zones, airbags and other safety features. But the seat belt remains the most important safety feature to protect occupants in a car crash.

Download our report: Common Car Seat Mistakes and How to Fix Them

How Seat belts and Child Restraints Help

“One role of the vehicle seat belt is to allow the occupant to “ride down” the rapidly changing velocity of the vehicle during the crash, thus spreading the energy of the impact over a longer period of time,” said Arbogast Injury Report.

A restraint has to be strong enough to hold up during extreme crash energy. To determine the amount of restraining force required to safely restrain an occupant we use a simple (simplified) equation: weight x speed = restraining force.

crash dynamics restraining force

And this is why mom can’t just reach over and hold back a child in the front seat when slamming on the brake. You remember those days right? Her arm is not strong enough to restrain the say 70 pound child at even 10 mph. She would need to be able to restrain 700 pounds of force. I mean moms are strong but…

So we use seat belts and child restraints (aka car seats) to restrain us and prevent — or at least reduce the severity of — an injury. The primary job of a restraint is to keep the occupant in the car. Just this greatly increases our chances of surviving a crash.

Restraints also minimize the amount of crash force an occupant feels by absorbing some of the energy like the seat belt or car seat harness material stretching to allow the occupant to “ride down” the crash. This means the stop is slowed instead of immediate. Crumple zones on the vehicle are also meant to help passengers ride down the crash energy by absorbing some of the force and extending the time it takes for the vehicle to stop.

Restraints also help by spreading crash energy over a larger area of the body by contacting several points of the body with a wide strap of material. And they help protect us by contacting stronger parts of our body.

One more reminder

Oh ya and remember at number 6, we said all the loose objects go flying? Keep your car as clutter free as possible. I know, we’re parents. We have to carry lots of stuff. Try to keep heavy, sharp objects in the trunk or otherwise secured. Think of it this way, if you wouldn’t throw it at your child at 30 mph, don’t have it in the car.

By Amie Durocher, Creative Director at Safe Ride 4 Kids and certified CPS Tech since 2004

Copyright 2020 Safe Ride 4 Kids. All rights reserved. You may not publish, broadcast, rewrite or redistribute this material without permission. You are welcome to link to Safe Ride 4 Kids or share on social media.

© kalinovsky | depositphotos
© SafeKids CPS Tech Instructor manual
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