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#TechTalk: Crumple Zones

Posted January 4, 2022

Car safety features have come a long way in the last few decades. Some of the most effective, however, need not be too high-tech. Take crumple zones. 

Also known as crush zones, crumple zones are areas of a vehicle that are designed to take some of the impact in a collision and literally get “crumpled” to accomplish two safety goals: reduce the initial force of the crash and redistribute it before it hits, not only the vehicle's occupants but also other road users, like bikers, nearby pedestrians and other vehicles. 

The best way to reduce the initial force in a crash with a given amount of mass and speed is to slow down the deceleration. This happens when you slam on the brakes for an emergency stop. Notice that the force you experience is greater than when you gradually slow down for a stoplight. 

In a collision, slowing down the deceleration by even a few tenths of a second can drastically reduce the force. Cutting the deceleration in half also cuts the force in half. Therefore, changing the deceleration time from .2 seconds to .8 seconds will reduce the total force by 75%. 

How do crumple zones accomplish this? They create a buffer zone around the perimeter of the car. Certain parts of a car, like the passenger compartment, are rigid and not easily broken or twisted. If these rigid parts hit something, they decelerate quickly, resulting in much force. Surrounding those parts with crumple zones allow the less rigid materials to absorb the initial impact. Once a crumple zone starts to warp, the car decelerates over a few extra tenths of a second.

Crumple zones also help redistribute the impact. All the force must go somewhere-–the goal is to keep it away from the occupants. Think of it as a “force budget” that is spent in varying amounts on the car and its passengers. If a car hits a moving object, some of the force is transferred to that object. If the car glances off something and starts to spin or rolls, much of the force is spent on the spinning and rolling. More force is spent when parts fly off. 

Most importantly, it takes a huge amount of energy to damage a car. Imagine how much force is needed to bend a steel frame, smash body panels, or shatter glass. You wouldn’t want that to hit anyone around you. 

Now, imagine if cars do not have crumple zones. A 975kg Hyundai Accent traveling at 20 KPH or 5.6 meters per second hitting a bicycle would have a force transfer of 5,363.5 Newtons (N). 4000 N can already break a human femur!  

You’ll be thanking the high heavens for crumple zones--and that Hyundai has thoughtfully installed this remarkable structural safety feature on its vehicles, bearing in mind your safety and that of other road users.

Originally posted by Butch Angeles last October 2, 2019 on Facebook.