The first cars had brakes only on the back wheels, which led to much skidding
and swerving. Like coaster brakes on a bicycle, rear-only brakes needed long
distances to bring the vehicle to a complete stop. (In 1902, for instance,
the Automobile Club of America found that to stop a car going 20 mph required
an average of 60 feet.) Brakes on all four wheels did not appear in the United
States until 1920, when they were introduced in high-priced cars. It wasn't
until the advent of the new Ford Model A in 1927 that four-wheel brakes
became standard fare in most cars.
Instead of mechanical rods and cylinders, hydraulic four-wheel brakes employ the use of fluid pressure to actuate the brake pads of each of the wheels. Modern brake systems split the hydraulics into two subsystems to safeguard the complete loss of brakes.
In the 1930s, hydraulic four-wheel brakes came into use. They were an improvement
over the mechanical variety, except that when hydraulics were lost, so were all
four brakes. Europe led the way to a new system: dual hydraulic brakes. These
systems consisted of two parts: If a hydraulic leak occurred in one part, the
other could still function, albeit with less efficiency. Despite their obvious
utility, dual hydraulic brakes weren't fully embraced in the United States until
1961, when the American Motors Corporation built the system into all of its cars.
The 1950s marked the advent of power brakes, which increase the amount of hydraulic
pressure pushed towards the master cylinder with the help of a booster. Most modern
brakes today consist of some combination of power and dual hydraulic brakes.
Power brakes involve the use of a master cylinder and booster to improve the effectiveness of a car's braking system. By creating an air vacuum within the booster, a power brake can magnify the force of a depressed brake pedal and apply greater fluid pressure to the individual wheels.
Until recently, however, braking on slippery surfaces was still a risky
proposition. Under such conditions, a braking wheel might "lock up" in its
position and begin to slide without steering control. To combat this,
automakers devised anti-lock brakes. Relying on a computer to monitor the speed
of each wheel, the car can tell when a wheel is beginning to slide and can
apply an automatic series of braking pulses, which are faster than those
created by a human driver. Anti-lock brakes can stop a car faster in such a
situation than a human driver can and also allow the driver to maintain
steering control. back to top
The use of plate glass in early windshields created the danger of the "glass necklace," a situation in whic passengers could fly headlong into and through the windshield during a head-on collision.
The earliest windshields, introduced by 1904, were folding affairs. When mud, rain,
or other substances blocked his or her view forward, a driver could simply tip the
top half down for an unobstructed view. (Goggles came in handy in those situations.)
While the usefulness of windshields was clear to everybody, they posed a serious
danger. Manufacturers quickly discovered that during an accident, their glass
windshields could shatter, sending a shower of sharp shards into the vehicle.
Glass windshields proved most hazardous during front-end collisions, when passengers
could end up smashing headlong through the glass. Not surprisingly, when the first cars
with glass on all four sides were introduced, many people were afraid to ride in them.
Early this century, two European scientists independently invented a solution
to deadly windshields. While working in his lab, French scientist Edouard
Benedictus accidentally knocked a flask to the floor. To his amazement, the
glass did not break. Looking closer, he discovered that the chemical that had
been inside the flask, nitrocellulose, had dried up, leaving an adhesive film
that kept the numerous bits of fragmented glass from separating. Benedictus
went on to develop a window consisting of two layers of plate glass held
together by layer of cellulose.
Laminated safety glass consisted of two layers of plate glass held together by a layer of cellulose.
Meanwhile, on the other side of the channel, British inventor John C. Wood had
also been working with cellulose and had come up with his own method for
cementing a layer of celluloid between two pieces of glass. Wood's
shatter-resistant glass came to be produced under the brand-name of Triplex.
Though it was first developed in 1905, Triplex was not brought to the U.S.
until 1926. A year later, Ford began incorporating laminated glass into each of
In the 1950s, cars came off the line with side and rear windows of tempered
glass. Tempered glass is made by placing one piece of glass into an atmospheric
oven, which heats and hardens the glass. This treated glass can withstand
forces of hundreds of pounds per square inch. When broken, it breaks into
smooth beads that do not cut the skin, and unlike safety glass, rescuers can
cut into it to reach victims trapped in a car. back to top
Early seat belts, sometimes known as lap belts because they crossed over a passenger's lap, improved the safety of the automobile passenger.
The seat belt was introduced in 1949 by Nash Motors, but it was not until the mid-1950s
that most automakers began offering seat belts as optional safety features. These
safety belts crossed over a passenger's lap, securing him or her to the seat in
order to prevent a collision with the dashboard, steering wheel, or windshield in
an accident. While the seat belt vastly improved a passenger's safety, it remained
far from perfect, and nasty head, spinal, and other injuries continued to occur.
Nils Bohlin, inventor of the three-point belt, demonstrates its ability to secure a passenger's torso and center of gravity through a combination of the principles behind lap and diagonal belts.
Recognizing this, Volvo designed a diagonal two-point belt. Rather than simply
securing your hips, the belt would hold your upper torso to the chair as well.
As they watched tapes of crash tests, however, Volvo engineers realized the
diagonal belt was flawed. It crossed the body well above the center of gravity.
As the force of a collision rippled through the car, the test dummy would slip
downward and under the safety belt, which became a kind of clothesline that
caught the occupant's neck and chin, resulting in neck injuries and possible
decapitation. To solve the problem, Volvo engineer Nils Bohlin created the
three-point seat belt. The lap belt held the body in place while the diagonal
shoulder belt kept the torso from colliding with the structure of the car.
First installed in the 1959 Volvo, three-point seatbelts would not be required
in American cars for another decade.
In 1981, Mercedes-Benz improved the design by adding pre-crash tensioners.
Within milliseconds, seat belt pre-tensioners tighten safety belts and help
prevent belted occupants from sliding and bouncing around during a crash. Six
years later, Mercedes-Benz, along with Audi, also added shoulder belt height
adjusters to provide a tighter fit.
Today, seat belts are recognized as a passenger's primary line of defense.
According to the U.S. National Highway Safety Transportation Agency (NHSTA),
lap/shoulder belts, when used properly, reduce the risk of fatal injury to
front-seat passengers by 45 percent, and the the risk of moderate-to-critical
injury by 50 percent. back to top
In 1952, after witnessing the speed at which a Navy torpedo canvas could be
filled by compressed air, John Hetrick was granted the first patent for what
would become the predecessor to the airbag. Despite the airbag's obvious
utility, it would take nearly two decades for automakers to become convinced of
Through a series of crash tests, engineers and safety experts determined what was needed to make the airbag a successful safety feature.
In 1970, the newly founded NHSTA, alarmed by the fact that fewer than 15
percent of all Americans wore seatbelts, began to look for new ways to protect
accident victims. Hetrick's airbag concept was seen by many as the ultimate
passive restraint, even though manufacturing safe airbags would clearly be an
enormous challenge. Engineers had to determine how big the bag should be, what
materials should be used in it, and how to inflate it within 30 milliseconds
after impact without blowing it apart. These challenges were met, however, and
by 1980, Mercedes began offering airbags as standard equipment. Eight years
later, all U.S. car manufacturers were required to follow suit.
By the late 1990s, a rising number of airbag-induced deaths had created a new
concern: the airbag itself could be dangerous. The explosive force of a filling
airbag was powerful enough to injure and possibly kill both adults and
children. In almost every case, airbag injuries involved women under 5'4" or
adults and children who were either unbelted or improperly restrained.
As a solution to the growing number of airbag-induced injuries, automakers devised a new design, which incorporated the use of multiple sensors to calculate the rate and speed of airbag deployment. Depending on circumstances, airbags would engage one, two, or no initiators.
In late-model autos, a computer decides how airbags should deploy. It uses an
electronic scale that directs the bag to deploy only if the rider is above a
certain weight, and it can deploy at two different speeds using one of two
initiators. In a minor accident, only one initiator fires, and the bag inflates
more softly and to a smaller size; in a high speed accident, both initiators
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The same year that Volvo engineer Nils Bohlin invented the three-point safety
belt, Bela Berenyi, an engineer at Mercedes, came up with a safety concept that
would completely change how cars were designed and built. Prior to 1959, people
believed the stronger the structure, the safer the car. But in actuality, such
construction proved deadly to passengers, because the force from impact went
straight inside the vehicle and onto the passenger. Berenyi knew that he would
have to find a way to dissipate the force of a crash before it reached the
passenger. In the end, he designed two "crumple zones"—one in the front of a
car and one in the back. Crumple zones relied on a skeletal frame of special
materials that would crumple in predictable ways, absorbing the energy of a
Crumple Zones, indicated here in red, were designed to absorb the energy of a collision by bending and breaking in a predictable manner. In doing so, the force of the collision would be diverted away from the passenger's compartment, thereby keeping him or her from serious injury.
First introduced in 1959, crumple zones with rigid cabs are now the standard in
every car made throughout the world. In the event of a severe frontal
collision, the frontal frames that support the engine are designed to slide
underneath the passenger's compartment. In combination with the crumple zones,
the passenger compartment is often built using high-tensile steel, which
creates a rigid passenger cell often referred to as a "safety cage." This is
enhanced by new side-impact beams, which are found within car doors.
Previously, in extreme side impacts, these beams could bend and protrude
dangerously into the interior. So companies developed a special side-impact
beam that breaks after absorbing impact energy.
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