Physics In
Figure 1. Volvo crashes an XC90 SUV into an S60 sedan to test crash compatibility (SwedeSpeed.com)
Car Safety Design
Introduction
Since the invention of the automobile, technological advances have made radical progress in occupant safety. To deal with the immense forces acting upon human bodies in a car crash, automotive engineers have been making changes to the way they design vehicles. Cars have gone from motorized carriages of the 1910's, to the huge muscle cars of the 50's, to tin cans of the 80's, and now to generally midsize, soft and increasingly safe cars of today. Advances in ways of thinking, as well as technology, have made driving safer than ever—and physics is the key.
History of Car Safety
Since its formal introduction into society, the automobile, and the physics behind the automobile have made leaps and bounds in terms of safety. From a once simple, light and slow vehicle, the automobile has metamorphosed over time, changing its size and power, thus its inertia and momentum.
In the early days of the car, the body was light and weak, and the engine didn't have much output. However, due to the complete oversight of safety, a minor accident could easily be deadly. The physics principle of conservation of momentum showed it's ugly face as the energy of the moving car was transferred to the passenger who was almost always thrown from the car. The realization that the passenger must stay in the car led slowly to the invention of the seat belt.
Timeline of Car Safety
(From School Transportation News)
1930's -Several U.S. physicians equip their own cars with lap belts and begin urging manufacturers to provide them in all new cars
1953 -Colorado State Medical Society publishes a policy supporting installation of lap belts in all automobiles due to scientific evidence showing the slowing of bodies in accidents
1954 -The Sports Car Club of America requires competing drivers to wear lap belts
American Medical Association House of Delegates votes to support installation of lap belts in all automobiles
1956 -Volvo markets 2-point cross-chest diagonal belt as accessory. Ford and Chrysler offer lap belts in front as option on some models
1957 - Volvo provides anchors for 2-point diagonal belts in front
1958 - Nils Bohlin, a design engineer with Volvo in Sweden, patents the "Basics of Proper Restraint Systems for Car Occupants," better known as a three-point safey belt. The device comprises two straps, a lap strap and shoulder strap. Volvo provides anchors for 2-point diagonal belts in rear
1967 - Society of Automotive Engineers study at UCLA leads to calls for two-point seat belts, highback seats and other occupant protection strategies for school buses.
1968 - Volvo provides emergency locking retractors (ELRs) as standard in front, in Sweden
1974 - First production tension relief device on U.S. vehicle.
1980 -Mercedes-Benz provides driver side airbag and knee bolster, and pre-tensioner an all 3-point belts
1985 -Mercedes-Benz introduces driver side air bag with knee bolster (in addition to pre-tensioned 3-point belts)
Interior Safety Features
Modern cars have many safety considerations built right into the interior. Chrysler once advertised the padded dashboard by dropping an egg onto it. Multiple safety considerations in car interiors are designed to absorb energy in the event of an impact. Padding in the panels around pillars and headliners, knee bolsters, are all designed to dissipate energy. Seatbelts are perhaps the largest interior safety advance in the history of cars. Besides their obvious advantage of keeping occupants inside the car—protecting them from being ejected and colliding or being run over by other cars—they both help to reduce the forces acting upon the occupant as well as spread the forces that must act on the occupant to the strongest parts of their body. Seatbelts work on a few simple physics principals. First, they prevent occupants from being "ejected" from the vehicle. Newton's first law states the objects in motion remain in motion unless a force is exerted to stop them. Thus, if a car stops suddenly, the occupant will continue moving at nearly the original speed the car was going. Seatbelt locks can be activated one of two ways. A weight in the mechanism will slide forward when the car stops suddenly and activate a pin that locks the seatbelt. A second mechanism locks the seatbelt when it begins to reel out too quickly.
Modern seatbelts have devices called "load limiters", special mechanisms that allow the seatbelt to reel out under a certain amount of force. Load limiters prevent too much force from being exerted on the occupant. How?
An occupant that must come to a stop during a collision undergoes a change in momentum. A change in momentum requires a force multiplied by a time. Load limiters are usually clutches that slowly reel the seatbelt out at a threshold force. By extending the time, less force is exerted. Some automakers are fitting their seatbelts with pretensioners. Pretensioners are explosive devices that are activated in conjunction with airbags that reel in the slack from the seatbelt. When the airbag sensor determines a severe impact has taken place, an impulse is sent to the seatbelt pretensioner.
The impulse causes a spark to ignite a sodium azide pellet. The high nitrogen output from the azide pellet is channeled into a turbine, which spins around rapidly to reel the seatbelt in. Too much slack in a collision defeats the purpose of load limiters by extending the distance they travel before the seatbelt catches them—allowing less time for them to be more gently slowed down—and can allow the occupant to strike hard objects within the interior, or come in contact with an inflating airbag.
Airbags
Airbags, first fitted on the driver's side in 1980 are now standard equipment in US-bound cars for the driver and front passenger and come in multiple forms. Airbags inflate at an extremely high rate of speed so they may be fully inflated before the occupant hits them. They are supplemental restraints, meaning they do not replace seatbelts, but rather work in conjunction with them. Airbags only deploy under certain circumstances, and sometimes don't deploy even when the damage done to the car is great.
Airbag sensors are decelerometers—they measure the negative acceleration of the car. Some decelerometers use high-powered magnets that are forced apart in a severe collision and cause a change in the electrical fields. At a threshold deceleration, a signal is sent from the sensor, which travels to the airbag computer. If the airbag computer detects that deployment is necessary, an electrical impulse is sent to the airbag modules. The airbag modules send a spark into an extremely reactive substance related to gunpowder (sodium azide) that causes a combustion reaction, which rapidly fills the airbags with harmless nitrogen with an ear-splitting popping sound. The airbags break through their covers and are fully inflated within a fraction of second. Lubricant dust, usually talcum powder, reduces the friction to help the bag unfold, but many people think that their car is on fire because of all the dust.
In conjunction with load limiters, the front seatbelts reel out under the force and let the occupant plant their head and torso into the inflated airbag. While seatbelts are designed to wrap around the strong bones on an occupant's body, the airbag helps channel the force across more of the occupant's body. Furthermore, they prevent occupants from hitting hard objects during the collision (an impulse requires a force multiplied by a time—when you strike a hard object, the collision is more elastic and thus there is less time for the momentum to be divided by, resulting in a larger force), and allow load limiters to work safely. With more contact points for the force to be distributed, the chance of breaking a bone protecting vital organs—which can result in fatal injury—is less. For every contact point, the force is divided. Common airbag related injuries include temporary tinnitus (loss of hearing), triggered asthma attacks, broken hands, arms (from not holding the steering wheel at 3 and 9 o'clock), and broken noses; yet, the good they do far outweighs their problems. Children, less able to support themselves and more likely to be out-of-position, however, are often thrown into the hazardous path of a deploying airbag during emergency braking or because they did not trigger the seatbelt locking mechanism soon enough. The airbag fires so fast that children in the front seat can be killed by it.
Side-impact airbags work on a similar principle. When a protruding vehicle hits the side of a car, the door panels and pillars crush in and strike the occupant. The occupant undergoes a tremendous, often fatal acceleration. Side impact airbags distribute force over more time, "cushioning" the impact and keeping the occupant away from hard, protruding surfaces and debris
Head restraints and seatbacks
Padded head restraints are vital to protect occupants in rear-end collisions. A common misconception is that "head rests" are for comfort, when in reality, they are a safety device designed to minimize whiplash injury. A well-designed head restraint is high to at least the ears and as close horizontally to the occupant's head as reasonable.
When a person is rear-ended, the car accelerates forward and their seatback pushes into their torso. The car and the occupant's torso accelerate faster than their head. A good head restraint minimizes the time between when their torso begins to accelerate and their head also accelerates. The longer the time between these two events, the more the head is maligned with the body and the higher the differences in speed are. A high difference in speed eventually means that the head must play "catch-up" to the torso—usually resulting a sharp, sudden acceleration for the head, which can cause hyperextension and cause whiplash injuries.
Some companies, notably Saab and Volvo, have developed unique systems to prevent whiplash injuries. Saab's SAHR (Saab Active Head Restraint) system works using a lever in the lumbar support area of the seat. When the seatback pushes into the occupant, it exerts a force on the occupant's torso. Since Newton stated forces are equal and opposite, the occupant applies the same amount of force on the seatback. This force is used to push in a lever that raises and moves the head restraint closer to the occupant's head. This reduces the time between when the head and torso begin to accelerate and helps prevent whiplash.Volvo's WHIPS system, on the other hand, uses good fixed head restraint geometry and a special deformable hinge installed in the seatback. In the event of a rear-end collision, the force of the occupant pushing into the seatback deforms a hinge so that the seatback moves rearward, slowing the acceleration of the torso and reducing the forces acting upon the occupant rather than raising the acceleration of the head to match that of the torso (based on the principles of the Impulse-Momentum Theorem). When the hinge reaches the end of the rearward path, it begins to tilt rearward so that the seat is at a greater incline. This prevents injuries from "rebound", or damage to the neck's tendons when the occupant's head snaps back after contacting the head restraint, by placing them at a more horizontal position. Unlike SAHR, however, the deformed hinge must be replaced after the impact.
Both SAAB and Volvo's systems have been proven to work against a particularly painful injury. Still, many people are ignorant of the point of head restraints and neglect to adjust them properly just as they should put on their seatbelt.
Crumple Zones
One technique that has been proven to be successful involves the use of crumple zones positioned in specific areas of an automobile. Crumple zones are built using the integration of steel and fiberglass into the front and rear-end assemblies of the automobile. Crumple zones are sometimes used in the frame of the automobile, creating a point for the frame to buckle when subjected to extreme stress. These crumple zones yield during impact, redirecting the energy of the collision---often reducing the chance of injury to the driver. In an accident at the moment of impact, a car without crumple zones immediately rebounds in an elastic manner off a solid object as seen in various older test videos. The car regains nearly all of its kinetic energy, and consequently experiences a large force. However, a car with crumple zones uses the physics behind the Impulse Momentum Theorem to its advantage. As the car with crumple zones collides with a solid object, the crumpling action of the car's body slows the actual impact down. The car does not regain all of its initial kinetic energy. Instead, some of the kinetic energy is transferred into heat and sound energy, resulting in a smaller force experienced by the car.
Looking Forward
Automotive safety companies are developing all sorts of things for to improve vehicle safety. Four-point seatbelts, which cross twice over the chest and are more effective at distributing forces especially in rollover collisions, may someday soon be equipped in cars. Inflatable seatbelts further help the issue of force distribution and cushion the impact. External airbags that "predict" a collision can form a new layer of protection by increasing the energy-absorbing zone—and thus the time for the vehicle to change its momentum. BMW recently introduced knee-level airbags in the new 7-series for front seat occupants to prevent leg injuries and to provide a stable contact point for the occupant's knees. Rear seat occupants are getting more attention with a prototype under-the-seat airbag that raises the front of the bench to prevent them from "submarining", or sliding under their seatbelt. As automotive safety continues to improve, researchers and engineers turn continually to the laws of physics for advice. Forces, momentum, impulse, and energy are all factors that can and must be controlled—people's lives depend upon it every day.
Bibliography
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Insurance Institute for Highway Safety, <http://www.iihs.org> (20 February, 2002).
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National Highway and Traffic Safety Administration <http://www.nhtsa.dot.gov> (20 February, 2002)
School Transportation News, "Occupant Restraint — History" <http://www.stnonline.com/stn/occupantrestraint/seatbelthistory/> (1 March 2002).
SwedeSpeed, <http://www.swedespeed.com/> (3 March 2002).
