It’s estimated that seatbelts have collectively saved over one million lives, making them
one of the most cost-effective public health inventions ever made.
[1] These incredible devices can reduce fatalities in car accidents by about half.
Yet their development and implementation as standard equipment was surprisingly rocky.
For something that seems so straightforward, the humble seatbelt is more sophisticated
than you may think.
Even though wearing a restraint in a vehicle moving 100 kilometers per hour may seem like
common sense to us today.
It took many years to convince the driving public to buckle up, a problem that persists
in many countries around the world.
This ingenius device saves lives, and this is how it does it.
In 1903 race car driver Barney Oldfield became the first person to drive over 100 km/h.
He was all too aware of the dangers he was subjecting himself to and commissioned Leslie
Leroy Irvin, another dare devil of the 1920s, to develop a harness for his racing cars.
Irvin had developed his own static line parachute to jump out of airplanes, and 1919 became
the first American to jump out of a plane and manually deploy a parachute from a backpack
in midair.
A design he helped the US military to develop.[2]
The harness they came up with did not catch on however.
Despite its life saving potential, it had a number of problems.
First, they were simply uncomfortable, which discouraged drivers and passengers from wearing
them.
Then there was the very real problem of “seat belt syndrome,” injuries directly related
to wearing a seatbelt during a collision.
Early seatbelts only went across the lap.
In the event of a crash, this rigid single point restraint, laying in line with vital
organs and the lumbar spine, could cause severe injuries.
[3]
The prevailing thought at the time was that it would be better to be ejected from the
vehicle, which might catch on fire or go under water, than to remain inside of it during
a crash by being stopped forcefully by a thin piece of fabric.
Despite rising fatalities and injuries during the rise of the automobile from the 1920s
to 1950s, the general public showed little interest in wearing seatbelts. [4]
Even though Nash Motors installed seat belts in 40,000 cars, only about 1000 of them were
actually used by drivers.
[5] When Ford led a marketing campaign in 1955 to promote seat belts and other safety
options, they too faced public backlash.
However, in the 1950s resistance began to ebb away with a new innovation by Swedish
engineer Nils Bohlin.
Bohlin first began developing safety features for Saab, including an ejection seat, but
was then poached by the president of Volvo.
He experimented with several designs, including a seatbelt that fastened between the legs,
an idea which 95 % of my audience just gasped at, [6] He ultimately settled on the design
we’re familiar with today.
The three point harness.
This set up more evenly distributes the forces of a crash across the shoulders and hips.
[7]
Bohlin was able to prove the effectiveness of this design and received the U.S. patent
for it in 1962.
But rather than profit from it, the design was released for free, paving the way for
a new and incredibly important safety feature.
[8]
While certain elements of the seat belt have been modified and improved over time, the
main design remains the same.
We need a way for the strap itself to wind and unwind under normal circumstances.
Allowing passengers to easily move around, and take it on and off without adjusting its
size for different passengers and drivers.
We have all experienced that annoying phenomenon where you try to put on a seatbelt too quickly
causing it to jam.
This happens as a result of a false alarm in the seatbelt mechanism.
This mechanism.
It has a number of components, the simplest being this spiral torsion spring.
When the seatbelt is unwound it spins a cylinder at the center of this torsion spring, pulling
the spring into the center into a compressed state, storing energy that can retract the
seat belt tightly against your body once the the belt buckle is locked into the buckle
socket.
This mechanism is usually attached to one side of the webbing spool, while the locking
mechanism that will prevent the spool from being unwound at all during a crash is located
on the other side.
For this mechanism to work we need a way to detect a crash extremely quickly.
One kind of locking mechanism reacts to the momentum change of the vehicle.
Just as our bodies want to continue moving forward in a crash, a weighted mechanism inside
the seatbelt will too.
First we need a ratchet gear, like this, attached to the same cylinder at the center of our
torsion spring.
This gear will be engaged to prevent rotation.
Now we need a mechanism that can engage this gear when the car decelerates.
This can be done with a weight pendulum.
When the car decelerates this weight will swing forward, causing its opposite end to
jam into the ratchet, preventing it from rotating any further.
Like any good safety mechanism there is a redundancy in its design, being backed up
by another locking mechanism.
One you can test without crashing your car, by tugging on your seatbelt hard enough this
mechanism will activate.
As it depends on centrifugal force to activate.
Two weighted levers are attached to the central spool.
When the central spool unwinds at a normal speed, these levers do not create enough force
to overcome the force of a spring that pushes them towards the center.
But, if the webbing comes out in a sudden jerk, the centrifugal force exceeds the force
of the spring and the levers push outwards engaging the locking teeth and prevent further
rotation.
There is another final safety mechanism added to make seat belts even safer.
When driving we often slump forward slightly, leaving slack between our body and the seat.
In the event of a crash it’s best if we are tightly held against the seat, so there
is a mechanism within seat belts that actually forcefully pulls us against the seat during
a crash.
This mechanism is fired electronically by the same sensor that initiates the explosive
that expands airbags, and this mechanism is also fired by an explosive charge.
Animation 6 It’s called the pretensioner.
A chamber at the bottom of the device has two electrodes, an igniter material and a
piston.
When the car’s central processor detects a collision, it applies an electric current
that ignites the explosive and the sudden burst of pressure forces a mechanism to actuate.
To quickly retract the seat belt and pull the passenger tightly against the seat.
Finally, the seatbelt has several mechanisms built in to reduce the risk of seat belt syndrome–injuries.
Some webbing has a fold sewn into it, with stitches that will break when a set amount
of force is applied, allowing the person to move forward a bit more.
Animation 7 There is also the torsion bar, a length of
metal attached to the rotating spool.
This torsion bar is designed to twist and absorb some of the force from a crash, instead
of allowing that energy to be transferred to the passengers body..
[9]
Despite the clear benefits of this three-point belt system, it took decades before the use
of seatbelts really became widespread.
Although Wisconsin was the first state to require all cars to install seat belts in
the front in 1961, it wasn’t until 1984 that another state, New York, required passengers
to actually wear them.
I remember growing in Ireland in the 90s and seeing the change in opinions about seatbelts,
and I especially remember moving to Malaysia, a country with a terrible road safety record
and where a friend of mine died because he wasn’t wearing a seatbelt, and arguing with
local co-workers who refused to wear them.
The National Highway Traffic Safety Administration estimates 15,000 lives are saved by seat belts
every year in the U.S. alone, so please put your seatbelts on.
They do save lives.
Of course there are plenty of other technologies helping to make cars safer.
We have front and side airbags, crumple zones, driver assist, and automatic emergency braking.
And Volvo is now proposing a blanket-style seat belt for its autonomous cars that would
keep passengers safe and comfortable even when they’re napping. [10] But that’s
still something of a pipe-dream, since we’re still a ways away from self-driving cars.
For now we are stuck with the three-point harness, a simple and cheap device to keep
passengers safe.
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