The concept of braking is, in all honesty, pretty straightforward. Simply put, it’s the art of making a moving vehicle stop or slow down. In this beginner’s guide, we will delve a little deeper, explaining how this is done and learning what separates good brakes from world-class brakes. So, without further ado, let’s get into it.
In the
series “Explain it like I’m 5,” we break down complex topics in a way that even
a 5-year-old would understand. To guide us on this journey, we have picked the
brain of Gareth Thomas, Senior Engineer at Polestar, who has countless years of
experience within the braking world.
Just like a
previous edition of this series, we start with a simple crash course in
physics. When a car is in motion, the wheels have energy in the form of
movement, also called kinetic energy. Should you press the brake pedal in your
car, the brakes will apply friction to the wheels, i.e., they squeeze the brake
disc, converting the kinetic energy into heat energy. The kinetic energy of the
moving car is converted into heat due to this friction, much like rubbing your
hands together when cold. This dissipation of heat allows your car to come to a
stop.
While
countless versions of braking systems have existed throughout automotive
history, they all share this need for friction. There is no braking without
friction. Some of the very first versions of brakes were just pieces of wood
that pushed against the wheel. While they were far from optimal, causing
significant wear and tear on the wheel, they still worked due to the friction
they managed to create.
So, how do
you apply friction to the wheel without damaging it? The brilliant
brake-building brains of the early 1900s came up with the ingenious solution of
attaching a drum to the wheel. The drum had two heat-resistant pads inside that
would push out against the drum. As the drum was attached to the wheel, slowing
the drum would slow the wheel. This solved the problem of wear on the wheel,
but there were still a few more chinks in their braking armour.
Early cars
relied on a complex system of cables and wires to get pressure from the pedal
to the wheels. Pushing your pedal would pull the brake wire attached to the
wheel. However, the problem with this system was twofold. Apart from the
tendency of the wires to snap, the precision required to achieve the same
braking pressure on all wheels was an enormous challenge. Incorrectly tensioned
wires would result in the car swivelling when you pushed the brake.
The
solution to this came with the invention of hydraulic brakes. Hydraulic brake
lines are pipes that connect the braking system with the drums located on the
wheel. These pipes are filled with brake fluid that cannot be compressed, which
means they transmit the force created by pressing the brake pedal without any
loss of power. By using pressurised brake fluid, pushing the brake sends
pressure throughout all brake lines simultaneously. In the 1950s, hydraulic
brakes were the only braking systems used in cars.
Now, we
have managed to convert the kinetic energy of the spinning wheel into heat
energy in a controlled way thanks to the drums and hydraulic brake lines. But
what about all the heat energy we are left with?
Discs and
Callipers
Brakes that
get too hot are disastrous for braking. If they overheat, they can no longer
convert the kinetic energy into heat and “brake fade” will occur (a loss of
braking power due to overheating). The solution to this is simple: throw out
the drums and replace them with discs and callipers.
The brake
disc sits on the wheel and rides inside the brake calliper. While drum brakes
push out, disc brakes work by squeezing. The calliper squeezes the disc, which
creates… yes, you guessed it, friction. The larger the disc and calliper
combination, the better the braking force.
Disc brakes
are also superior in dissipating heat. This is done in a number of ways. First,
the caliper is not enclosed inside a drum, which allows heat to escape. Second,
holes that are designed into the wheels allow hot air to escape. And third,
brake cooling ducts, which are little tunnels that run from the front of the
car to the back of the brakes, allow fresh air to cool the brakes. Much like
sitting in front of a fan to cool you down on a hot day.
Although
disc brakes offer superior stopping power, drum brakes are by no means bad or
unsafe. Disc brakes are much more expensive to make and replace compared to
drum brakes. They can also suffer from corrosion when not used frequently or if
the car has been parked for a long time. Because a drum brake is a fully
enclosed system, they do not corrode in the same way. Drum brakes can also
function as parking brakes, simplifying the braking system.
Wear and
Tear
A key
approach to ensuring your brakes last as long as possible is to avoid putting
too much energy into the system. This means that instead of braking harder and
later, try to apply the brakes sooner and with less force. This causes less
pressure to be sent through the brake system and reduces the friction between
the brake and the disc, meaning the energy is dissipated more gently.
In colder
months, some people can find their parking brake “freezes” if the car is parked
outside in very cold conditions. To avoid this, park the car inside a garage,
if at all possible, where the slightly warmer temperatures can prevent this
from happening.
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