Crumple Zone - Function

Function

Crumple zones work by managing crash energy, absorbing it within the outer parts of the vehicle, rather than being directly transmitted to the occupants, while also preventing intrusion into or deformation of the passenger cabin. This better protects car occupants against injury. This is achieved by controlled weakening of sacrificial outer parts of the car, while strengthening and increasing the rigidity of the inner part of the body of the car, making the passenger cabin into a 'safety cell', by using more reinforcing beams and higher strength steels. Impact energy that does reach the 'safety cell' is spread over as wide an area as possible to reduce its deformation. Volvo introduced the side crumple zone with the introduction of the SIPS (Side Impact Protection System) in the early 1990s.

When a vehicle and all its contents, including passengers and luggage are travelling at speed, they have inertia / momentum, which means that they will continue forward with that direction and speed (Newton's first law of motion). In the event of a sudden deceleration of a rigid framed vehicle due to impact, unrestrained vehicle contents will continue forwards at their previous speed due to inertia, and impact the vehicle interior, with a force equivalent to many times their normal weight due to gravity. The purpose of crumple zones is to slow down the collision and to absorb energy to reduce the difference in speeds between the vehicle and its occupants.

Seatbelts restrain the passengers so they don't fly through the windshield, and are in the correct position for the airbag and also spread the loading of impact on the body. Seat belts also absorb passenger inertial energy by being designed to stretch during an impact, again to reduce the speed differential between the passenger's body and their vehicle interior. In short: a passenger whose body is decelerated more slowly due to the crumple zone (and other devices) over a longer time survives much more often than a passenger whose body indirectly impacts a hard, undamaged metal car body which has come to a halt nearly instantaneously. It is like the difference between slamming someone into a wall headfirst (fracturing their skull) and shoulder-first (bruising their flesh slightly) is that the arm, being softer, has tens of times longer to slow its speed, yielding a little at a time, than the hard skull, which isn't in contact with the wall until it has to deal with extremely high pressures.

The final impact after a passenger's body hits the car interior, airbag or seat belts is that of the internal organs hitting the ribcage or skull due to their inertia. The force of this impact is the way by which many car crashes cause disabling or life-threatening injury. Other ways are skeletal damage and blood loss, because of torn blood vessels, or damage caused by sharp fractured bone to organs and/or blood vessels. The sequence of energy-dissipating and speed-reducing technologies—crumple zone — seat belt — airbags — padded interior—are designed to work together as a system to reduce the force of the impact on the outside of the passenger(s)'s body and the final impact of organs inside the body. In a collision, slowing down the deceleration of the human body by even a few tenths of a second drastically reduces the force involved. Force is a simple equation: Force = mass X acceleration. Cutting the deceleration in half also cuts the force in half. Therefore, changing the deceleration time from .2 seconds to .8 seconds will result in a 75 percent reduction in total force.

A misconception about crumple zones sometimes voiced is that they reduce safety for the occupants of the vehicle by allowing the body to collapse, therefore risking crushing the occupants. In fact, crumple zones are typically located in front of and behind the main body of the car (which forms a rigid 'safety cell'), compacting within the space of the engine compartment or boot/trunk. The marked improvement over the past two decades in high speed crash test results and real-life accidents also belies any such fears. Modern vehicles using what are commonly termed 'crumple zones' provide far superior protection for their occupants in severe tests against other vehicles with crumple zones and solid static objects than older models or SUVs that use a separate chassis frame and have no crumple zones.

They do tend to come off worse when involved in accidents with SUVs without crumple zones because most of the energy of the impact is absorbed by the vehicle with the crumple zone — however, even for the occupants of the 'worse off' car, this will still often be an improvement — as the result of two vehicles without crumple zones colliding will usually be more hazardous to both vehicle's occupants than a collision that is at least partly buffered.

Another problem is 'impact incompatibility' where the 'hard points' of the ends of chassis rails of SUVs are higher than the 'hard points' of cars, causing the SUV to 'override' the engine compartment of the car. In order to tackle this problem, recent Volvo SUV/off-roaders incorporate structures below the front bumper designed to engage lower-height car crumple zones.