Automotive Technology Explained: Part 3 - Impact Force Redistribution
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Automotive Technology Explained: Part 3  -  Impact Force Redistribution

Modern cars are safer now than they have ever been, and although there is as yet no car that offers its occupants complete protection against the effects of impact forces, the advent of technologies that manage these forces have greatly reduced the number of fatalities in traffic accidents.

Although the dividing line between safety systems and technology that makes cars safer is very narrow, the engineering aspects that go into impact force distribution properly places this science firmly in the technology camp. So if you have ever wondered why a car looks the way it does after an accident, read on, and we will explain the basics of what makes impact force distribution such an effective technology.

What is impact force redistribution?

Impact force redistribution can be described as the science of redistributing, redirecting, or absorbing the impact forces that arise when a car collides with a solid object such as another car, a tree, or a solid concrete wall. On most passenger vehicles (most SUV’s excluded), this is achieved with crumple zones, which are areas of a car’s overall construction that are designed to literally crumple during impacts, to reduce the effects of inertia (sudden accelerations) on the occupants of a vehicle.

By absorbing the impact forces of a crash, the destructive forces are in effect redistributed to, and thus confined to specific areas of the body shell, leaving the passenger compartment relatively undamaged, unless of course, something protrudes into the compartment to cause injuries.

So how does impact force redistribution work exactly?

Although crumple zones are typically located in front of, and behind the passenger compartment, they can also be placed along the sides of a vehicle. According to a recent study by the British Motor Insurance Repair Research Center to define the areas on cars that are the most frequently damaged during crashes, 65% of impacts are from the front, 25% are from the rear, and 10% of impacts are from the side.

So to mitigate the effects of impact forces during crashes, crumple zones are engineered to manage impact forces either by absorbing most of it, or to dissipate the impact forces over as wide an area as possible before they can reach the passenger compartment, which in auto engineering jargon is referred as the “safety cell”.

The absorption of impact forces is achieved mainly by designing a vehicle in such a way that during an impact, some parts of the vehicle are forced to deform in a controlled manner. The specifics of how this is done varies from car to car, but generally, these areas are not as rigid as the rest of the body shell.

A rough analogy would be the drums that contain water at toll booths. Should a vehicle run into these drums, they will absorb or dissipate the vehicle’s kinetic energy by exploding, so by the time the vehicle reaches the toll booth itself, it will have been slowed down enough not to injure the toll attendant in the booth.

Newton and impact force redistribution

Newton’s First Law of Motion states that a moving body will continue moving in the same direction, and at the same speed, unless, and until an equal or greater force acts on the moving body to stop its motion. Since this law pertains to a moving car and its occupants as well, it means that should a car be in a collision, its occupants will continue to travel in the direction the car was moving in. In practice, this means that unless the occupants are restrained in some way, they will collide with the front of the passenger compartment at the speed the car was traveling at when it came to a sudden stop.

The energies and forces that are at work during car crashes are greater than you might think. Take the following example of a car that weighs 4 409 lbs ( 2 000 kg), and travels at 37 m/ph (60 km/ph). This car moves at the rate of 16.7 metres per second, and should it run into a solid object, it undergoes the same impact energy as if it were hoisted into the air, and dropped on its nose from a height of 47 ft (14.2 meters).

However, if the speed of the car were to be increased to only 56 m/ph ( 90 km/ph), the impact forces during a collision with a solid object would be the same as if it were dropped onto its nose from a height of 105 ft (32 meters). Thus, increasing the speed by only 50%, the impact forces increase by a whopping 125%, since the car’s kinetic energy (E), increases by the square of its speed.

From the above, it should be clear that properly engineered crumple zones play a crucially important role in the prevention of injuries during car crashes, but there is more. For instance, what happens to the occupants of a vehicle during a crash?

How crumple zones save lives

To fully explain what happens to the occupants, we must make a another short detour into basic physics, which usually measures acceleration in multiples of a value that is equal to one gravity (g), which at sea level, is equal to 32.2 feet per second squared. Thus, during the first second of acceleration, the occupants of the vehicle will have moved 32.2 feet, but to keep things simple, we can say that for every second after the first, the occupants’ speed will change (increase) by 32.2 feet per second.

However, since the car and its occupants were traveling at the same speed prior to the crash, there was no acceleration acting on the occupants. Now imagine the car traveling at 80 mph coming to a full stop in less than a second during a crash; in this scenario, the occupants will be accelerated relative to the car from a standstill to 80 mph in less than a second- the same time it took the car to stop.

There is more to this however: when an occupant that weighs say, 150 lbs is accelerated at twice the value of g, (an acceleration at 32.2 ft/sec), that occupant’s mass will double to 300 lbs. Thus, an opposite force of greater than 2 g’s is required to stop the motion of the accelerated occupant but since there is no such force available, the occupant’s motion is stopped by an impact with the front of the passenger compartment instead, which is when most fatal injuries occur.

To prevent the occupants of a vehicle slamming into the front of the passenger compartment, cars deform in a controlled manner during a crash, which effectively lengthens the time it takes the car to come to a stop. This in turn, reduces the acceleration (and g forces) that acts on the occupants, so where a car without crumple zones can stop in less than one tenth of a second during a crash, a car with crumple zones traveling at the same speed might take as long as one full second to stop, significantly reducing the chances of the occupants sustaining serious injuries.

Advantages of impact force redistribution

Of course, the time it takes a car to stop during a crash depends on many factors, such as the mass of the car, its speed, the nature of the crash and many others, but assuming that the occupants are wearing seat belts and that the air bags deploy successfully, it is highly unlikely that the occupants of a modern car will sustain serious injuries during crashes at legal speeds.

However, crumple zones are only one part in a “package” of energy-dissipating and speed reduction technologies, so it would be a mistake to rely on crumple zones alone to prevent injuries or fatalities during car crashes. Crumple zones may be effective, but they are so only up to a point.

Other measures such as seat belts, padded interiors, and airbags complement crumple zones to reduce the effects of impact forces on vehicle occupants. During a crash, any reduction in the rate at which occupants are accelerated can drastically reduce the effects of impact forces. Force is calculated as mass × acceleration, so by reducing acceleration by 50%, force is also reduced by the same amount.

Thus, by increasing the time it takes a vehicle to stop in a crash from 0.2 seconds to say, 0.8 seconds by using crumple zones, the final impact force is reduced by 75%- which we are sure you will agree, can make the difference between dying in a crash, and walking away from it.

To read more articles from the “Automotive Technology Explained” series please visit our Auto Body Shop Blog.

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