How Crumple Zones Work

Looking at photographs with car accidents that took place in the 1950's and accidents in more recent times you'd think that engineers have gone backwards and have made vehicles less safe.
Mercedes-Benz S-Class W222 crash test 6 photos
Photo: Daimler AG
Pininfarina NidoPininfarina NidoPininfarina NidoBMW X1Mercedes-Benz S-Class
Early automobile design theories saw extremely rigid bodies that were very resistant during an accident and didn't allow too many deformations. As a consequence, all the forces were transferred to the occupants, most of the times this being quite fatal.

It wasn't until 1953 that the first crumple zones were implemented on vehicles. Like many other technologies in the automotive work, the company responsible for it was Mercedes-Benz. One of the engineers, Béla Barényi, had studied this problem for quite some time and in 1953, his ideas came to fruition in the "Ponton" (three-box body) Mercedes (model series W120).

In 1967, the Mercedes Heckflosse (also known as the Fintail) was the first production car in the world with “crumple zone” safety features including a safety cage with crumple zones and a trunk that had been made almost 50% bigger.

The Theory

Like it or not, physics has the explanation as to why crumple zones are necessary. Isaac Newton's first law states that an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

If a vehicle is traveling at 50 mph (80 km/h), so are the bodies inside and if this vehicle stops abruptly into a solid wall, the bodies will “feel” the need to keep going in the same direction at 50 mph, unless of course something stops them. What's more, even if the bodies themselves stop, the internal organs will continue to move, thus causing severe injuries.

We're still not out of the physics woods yet. There's another law, the second, from the same Newton saying that force equals mass multiplied by acceleration. Translated to our situation, that of an accident, it means that the force experienced by the automobile and its occupants decreases if the time required by the vehicle to stop increases.

So What Do Crumple Zones Do Anyway?

They work exactly according to the two laws. Placed at the front and the rear of the car, they absorb the crash energy developed during an impact. This is achieved by deformation, something unheard of in the early days of automobile design. While certain parts of the car are designed to allow deformations, the passenger cabin is strengthened by using high-strength steel and more beams.

Second, crumple zones delay the collision. Instead of having two rigid bodies instantaneously colliding, crumple zones increase the time before the vehicle comes to a halt.

What About Very Small Cars?

That's a very good question as small cars don't have room for crumple zones. Take the smart for instance. Where could you possibly have crumple zones on a car like that? Engineers found a solution for that minute vehicle as well.

All generations of the smart are built around a tridion safety cell, a steel housing that combines longitudinal and transverse members that displace impact forces over a large area of the car. Another important component of the smart is the crash box.

“The smart fortwo is designed with steel bumpers at the front and rear that are bolted to the safety cell´s longitudinal beams via slip tubes. They can be replaced after minor collisions at low costs. For parking lot bumps, an impact of less than 2 miles an hour won't affect the crash box at all. Up to about 10 miles per hour, the slip tubes move to keep impact away from the tridion safety cell.

Over 10 miles an hour, the tridion safety cell transmits impact over its entire surface to dissipate energy and protect its occupants (assuming a perpendicular impact involving the entire front width). At the rear of the car, the crash box is also built of steel, which crumples much like the front slip tubes do. At an impact exceeding the severity threshold, the fuel supply to the engine is stopped and the central locking system is automatically unlocked”

Pininfarina Nido Concept

In 2004, Pininfarina's Nido concept showed an alternative to the classic crumple zones. The Nido Concept is composed of 3 primary elements: the cell, the sled and the absorber. In the event of a head-on collision, the vehicle absorbs part of the energy with the deformable front section of the chassis, constructed of two metal struts with internal plastic foam absorbers.

These components are shaped as truncated cones in order to dissipate the energy over the cellular sheet metal firewall, which in turn transfers the energy along the central tunnel and the side members.

The remaining energy, due to the mass of the dummies and the sled, shifts the sled itself forward and compresses the two honeycomb absorbers between the rigid cell and the dashboard of the sled shell, resulting in the gradual and controlled deceleration of the dummies.

The insertion of honeycomb absorber elements between the rigid cell and the sled shell means that, in a collision, the deceleration curve for the sled is lower than the curve for the rigid cell.

Instead of a conclusion

With the advent and continuous improvement of active safety, the crumple zone's role regarding occupant protection has almost disappeared from the public eye, but that doesn't mean it has become less important.

On the contrary, since most carmakers have started improving the basic idea even further, while safety organizations like the IIHS, NHTSA or Euro NCAP have started adding types of crash tests that are much harder to pass. The increasingly draconian pedestrian safety rules in Europe have even changed how many modern cars are designed, influencing the length and height of engine hoods and the angle of grilles.

That said, safety cells are even stronger now, thanks to the increased use of extra strong steel and even Carbon Fiber Reinforced Plastic (CFRP). This has allowed automakers to play more with the design and materials used on the deformable parts of modern cars, so crumple zones are here to stay no matter how many active safety features are invented.
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