Crumple Zones: Article by : Duane Bong
Introduction To Crumple Zones
Since crumple zones were first introduced by Volvo in 1966, they have become an indispensable safety feature in modern cars.
By absorbing part of the impact from a car crash, crumple zones have been credited with saving thousands of lives each year.
Nearly all cars sold today have front and rear crumple zones.
How Do Crumple Zones Work?
Crumple zones are deliberate weak spots that engineers have placed in the structure of a car. While this might appear contrary
to passenger safety, there are sound principles behind this approach. By placing the weak spots in strategic locations, the metal
work of a car can be made to collapse in a controlled manner. This creates 2 mechanisms by which the energy from an impact
can be managed:
1. In deforming the metal work of the car, energy from the impact gets "used up" or converted into heat. This reduces
the amount of energy left to damage the passenger area.
2. Since the collapse is controlled, energy from the impact can be directed away from the passenger area. In most
designs, force from the impact is channeled to areas such as the floor, bulkhead, sills, roof and bonnet.
Force On The Passenger
To understand how crumple zones affect passengers, consider a car crashing head-on into a stationary concrete wall. Before the
crash, the car and its passengers move together at the same speed. If the car has a rigid body, an impact will cause both the car
and passengers to come to a stop in a split second.
It is this rapid deceleration that causes injuries and fatalities in a car crash. The force acting on the passengers is given by
Newton's 2nd law:
As the stopping time is only a split second, the force on the passengers is very high. Cars with crumple zones, however, do not
have a rigid body. One can think of them as springs being compressed against a wall. Although the front bumper of the car
immediately becomes stationary, it takes some time for the metal work to collapse. This allows the middle and rear of the car to
continue in motion for a short time. Since the stopping time is increased, the force acting on the passengers is greatly reduced.
Of car crashes and crumple zones
By Charles E. Buban Inquirer News Service
What is a safe car?
NEWS of vehicle accidents always gets our attention, especially if celebrities or huge fatalities were involved. The untimely
demise of personalities like actor James Dean and England's Princess Diana sometimes leads us to wonder what vehicle they
were in and when the accident happened. Does this mean they rode in a not-so-safe vehicle? Is there really a safe car?
A safe vehicle is one that provides outstanding protection from injury in a vehicle crash for all occupants. Cars being built
today include safety measures proven to save lives, including improved crumple zones, three-point seatbelts, improved front
airbags and side airbags. Laws of physics would tell us that when a vehicle strikes an immobile object it immediately starts to
decelerate or slow down. The rigid structure of the vehicle will immediately absorb the impact crumpling the areas affected by
the collision. Sometimes people wonder why crashed vehicles nowadays do not look so majestic compared to older vehicles
that also figured in a similar accident. The answer lies with what auto manufacturers call crumple zones. With crumple zones in
vehicles, passengers nowadays are much safer. Crumple zones, which are usually the hood part and the rear compartment area
of a vehicle, absorb the force of a crash and provide some cushioning to the main body of the vehicle where passengers are
located. Just like a spring, the crumple zone will reduce the suddenness of the change in velocity for both vehicle and the
passengers, making it safer than any other vehicle built decades ago. So while vehicles involved in accidents today might end
up looking worse, passengers today stand a much greater chance of walking away from a crash. Auto safety experts maintain
that two key factors determine crash compatibility, especially between vehicles of dissimilar size. First and foremost, the main
structures of the two colliding vehicles must align and meet squarely, which means that the frame members of both vehicles
must be about the same height above the road. This not only reduces the possibility of one vehicle overriding the other
catastrophically, but also helps the protective crumple zones in both vehicles serve their purpose in absorbing crash energy.
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Soft Walls or Soft Cars? By Mark Cipolloni
Since Dale Earnhardt's fatal accident at Daytona, safety in racing has received a lot of attention. A lot has been
written and discussed concerning the HANS Device because Dale Earnhardt died of a Basal Skull Fracture. I firmly
support the idea that drivers wear the HANS Device, however, there is another area of safety that needs to be
addressed - Race Car Crumple Zones. More specifically, should the concrete walls be made soft and crumple, or
should sections of the cars be made soft and crumple. That is the issue this article will attempt to address.
Any race car designer will tell you that the rigidity (see sidebar) of the chassis is a key component of a good
handling race car. A good chassis is one that offers good torsional rigidity as well as transverse and lateral rigidity.
A rigid chassis allows weight to be transferred from one end of the car to the other, from one side to the other, or
diagonally from one corner to the other (Called cross-weight transfer).
The most important measure of torsional rigidity is how much stiffness there is from the center of gravity of the car
to the front axle and from the center of gravity to the rear axle. This is part of the front weight transfer equation. If
the distance from the center of gravity to the front axle is larger than to the rear axle you will have more front
weight transfer (before springs and tires etc.).
Unfortunately, in an accident, a very rigid structure does not crumple much and, therefore, much of its Kinetic
Energy is lost in a very short timeframe and the object decelerates very rapidly.
Crumple Zones
In recent years, the automotive industry has improved the crash worthiness of passenger cars by the use of Crumple
Zones in the front and rear of the cars. Crumple Zones are created by the integration of variable grades of steel and
composites into the front and rear-end assemblies of the automobile. These Crumple Zones yield during impact,
redirecting the energy of the collision, often reducing the chance of injury to the driver.
This is best illustrated in the two examples below. The first example shows a solid steel block hitting a cement wall.
The block does not crumple at all, stops almost instantly, and rebounds or bounces off with almost as much Kinetic
Energy as it had just before it hit the wall. In the second example a Coke can hits a wall. However it crumples and
deforms just as if you stepped on it, and much of its energy is dissipated over a period of time while the can
crumples. In an accident, the general goal is to keep the safety cell (where the driver sits) intact at the highest speed
possible without killing the occupant, this implies that the safety cell has to be as stiff as possible (to avoid the
collapse of the safety cell and the intrusion of the wheels, engine or steering wheel which can be deadly) and has to
be surrounded by a crumple zone that is not too stiff (the sudden deceleration would kill the driver instantly) nor
too soft (useless, the wall you'd hit would go right through the crumple zone and you'd be killed anyway).
A trade-off has, therefore, to be found for the strength of the crumple zone: at low speed (25mph for example), the
car (car #1) with a very soft crumple zone might inflict less damage to the driver than a car with a stronger crumple
zone: the driver would be shaken badly in the 2nd car, yet the driver "wouldn't feel anything" in the first car.
However, at 60 mph, the driver of the 1st car would probably be killed by the intrusion of the engine, the steering
wheel, etc. where in the 2nd car, because of the " extra margin" allowed by the stronger crumple zone, the driver
would probably be injured but still be alive.
It is, therefore, possible to find a car (# 1) that manages to protect the driver better than car # 2 at a speed of 25
mph, yet car #2 could be safer than car #1 at a higher speed than 60 mph because car #2 has a stronger crumple
zone than car #1, allowing it a " last resort" extra safety margin that the driver would not have with car #1.
That is why it is crucial to see how well the structure of a car performs in a crash test. Only the one whose structure
hasn't started to collapse will allow the driver that extra safety margin in case of a stronger impact. Note that the
size of the crumple zone is important : that is why, in general, minivans (with shorter "noses") don't perform as well
as cars in frontal crash tests. So crumple zones have to be "attached" to the stiffest cell possible and must not be too
soft nor too strong!
Crash tests results have nothing to do with the weight of the car tested. It has to do with how well the safety cellcrumple zone-restraint system combo have been designed. So if you drive a very light car with good crash tests
results into a wall, you will be better off in that car than in a truck, even 2 times heavier, that would have "bad"
crash tests results. However, if that truck hit the small car, even though that car has better crash test results than the
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truck, you'd be better off in the truck because of the weight difference. In general, you'd be safer in a heavy vehicle
hitting a lighter vehicle, regardless on how it performs in crash tests.
What happened to Dale Earnhardt?
Many people can't believe that Dale Earnhardt was killed at Daytona. The accident just didn't look that severe.
Unfortunately I don't have any crash data to analyze because Winston Cup cars are not equipped with any CRASH
boxes that are found on CART Champ Cars, F1 cars and IRL Indy cars. NASCAR is taking a lot of heat because of
this. Therefore, it's hard for us to determine exactly what happened. Yes, his seat belt may have broken, and yes, the
HANS Device very well may have saved him. However, the fact of the matter is that the vector component of speed
that Earnhardt's car hit the turn 4 wall at Daytona was no more than 55 mph. Stop for one moment and think about
driving your car into an immovable wall at 55 mph. I would not want to try it, even if it does have a crumple zone.
The one thing probably working against Earnhardt was the fact that the front of Winston Cup cars have become
stiffer over the years in the search of better handling. In the photo to the right (top), one can see that, when finished,
the front assembly of a modern Winston Cup car is relatively stiff compared to the back (bottom photo to right).
That is one reason why drivers typically walk away unscathed in rear-end accidents - the backend of the car acts as
a Crumple Zone, absorbing kinetic energy in the form of heat and sound as it folds up right to the back of the strong
drivers compartment cage. However, on front impacts, a modern Winston Cup car crumples up to a point near the
front wheels, after which the chassis becomes very stiff, upon which deceleration becomes very rapid.
A properly designed front Crumple Zone on a Winston Cup car would probably mean the end of cars going back
out on the race track after an accident just to collect points. The front assembly would need to be designed to
crumple in a more predictable manner than it does now. That would probably render the car unfixable at the race
track, or at least might require a nose change before re-entering the race. I think it's better to make the cars safer in
an attempt to save a drivers life, than to collect a few points trolling around the apron of the race track for threequarters of the race.
Soft Cars vs. Soft Walls
Typically, the most lethal accidents are those in which a race car hits an immovable concrete wall and the 'g' forces
felt by the human body are not survivable. The crashes you see when a car goes somersaulting down the race track
tend not to kill a driver because at no time does the driver decelerate too fast. While the car is somersaulting, it's
losing kinetic energy over a relatively long period of time (seconds vs. milliseconds when you hit a concrete wall).
Therefore, the race car industry has to take a serious look at the concrete wall impacts.
In order to lessen the blow to the driver, we are going to have to make a concerted effort to develop an acceptable
'soft' wall design, or we are going to have to design the race cars with Crumple Zones. Both ideas have their
problems:
Soft Wall Challenges on oval tracks
 Cost to the track owners to outfit the many existing tracks
 The variable resistance needed for light open wheel cars vs. heavy 'stock' cars
 The potential for snagging a car because the wall will tend to pocket when hit.
 The possible bounce back of a race car into traffic
 The walls ability to perform a 2nd or 3rd hit in the same race.
 After accident cleanup
Crumple Zone Challenges
 Design and testing that will be required to make the front of the cars just stiff enough
 Cost to the car owners to make changes to all their cars
 The possible tradeoff between a stiff, good handling car, and one that is a little more flexible but gets
around the race track a little slower..
 The possible need to introduce space-age composite, energy absorbing materials, into the front of what is
essentially a 1950's designed unsophisticated tube frame structure.
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Questions:
Name ___________________
1. Describe the basic principle behind the use of crumple zones in vehicles.
2. Compare the force acting on a 75 kg person traveling at 8 m/s when the stopping time
is 0.25 seconds compared to a stopping time of 1.5 seconds. Please show your
calculations.
3. Besides crumple zones, what are some other safety features car manufactures are using
to improve automobile safety?
4. Is the appearance of a the vehicle the best way to judge the extent of injuries suffered
by occupants in an accident? Explain your answer.
5. In addition to increased mass, what other factors may contribute to the severity of
injuries in a crash between a mid-sized car and a larger SUV (Sport Utility Vehicle)?
6. Why might a racecar driver be opposed to better crumple zones on their car?
7. After reading this article, what is your opinion of soft walls and crumple zones in
racing?
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