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Braking Systems


Daniel Smith

on 10 September 2009

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Transcript of Braking Systems

Hydraulic Disc Brakes A Brief History of The first caliper-type disc brake was patented by
Frederick William Lanchester in England in 1902.
It was used on Lanchester cars, but due to a lack of materials available copper had to be used as the braking medium, causing them to be unviable for the roads at the time. The first modern style disc brakes were created by Dunlop and were used in the 1952 Jaguar C-type Use in production cars, such as the Citroen DS and the Triumph TR3 followed in 1955 Disc brakes are now the most common form of braking system. Most cars now have four disc brakes,
with some cheaper, smaller cars having two at the front and two drum brakes at the rear. How do Hydraulic Disc Brakes
Work? Disc Brakes use a piston in a caliper to force two
pads against a rotating disc, or rotor. Hydraulic Disc Brakes use hydraulics to push the piston, and hence the pads against the rotor As the pads are pushed onto the rotor, the kinetic energy of the rotating disc is turned into heat energy by the friction of the pad on the rotor. Due to the heat created, brake pads often create gas that builds up
between the pads and rotors, decreasing braking performance. To counter this, performance brake discs are
often either cross-drilled or slotted. Instead of being a solid disc, most brake
discs are vented. These discs are hollow with fins
joining the two sides. This helps dissipate the heat
created by the friction between the pads and rotor. Cross-drilled brakes have holes drilled through the rotor to reduce gas
buildups. These have recently fallen out of favour in racing due to the
tendency for the metal around the holes to fatigue under heavy use,
however they are still common on performance road cars. Slotted disc brakes have shallow channels
cut into the rotor to remove gas buildup.
They are common in racing but are not used often in road cars due as they cause greater wear on brake pads. When the brake pedal is pushed, a piston in the master cylinder increases the pressure in the hydraulic system. This forces the piston (or pistons) in the caliper to push outwards, pushing the brake pads against the rotor. Mechanical Advantage Mechanical advantage is acheived in hydraulic disc brake sytems by the use
of hydraulics to increase the force applied on the brakes to a force large enough
to stop a car. As pressure is applied on the brake pedal (the 10N force in this diagram),
it causes a piston in the master cylinder to move a certain distance. The
resulting increase in pressure of the system causes the much larger piston
in the brake caliper to move a smaller distance, but with a much greater
force (in this instance, 30N.) This is due to Pascal's principle, which shows
that F1(the force applied on the first piston)/A1(the area of the first piston
head) is equal to F2(force on second piston)/A2(area of second piston head) Materials Most brake rotors are made out grey cast iron. This is used as it is cheap
and easy to machine. While lacking in tensile strength, grey cast iron has
good compressive strength, making it ideal for the majority of cars. Some performance cars now use carbon reinforced ceramic brake rotors. These can withstand higher temperatures than cast iron and weigh less, however due to their cost they are not commonly used. Racing cars use reinforced carbon, or carbon-carbon, discs. They are used for their low weight and strength at high temperatures, they are not used elsewhere due to their need for a very high temperature to be effective. Brake pads used to use asbestos as a friction material. This was used as it was
cheap and could performed adequatly for consumer cars. After the health risks
of asbestos were found, it was phased out. Now brake pads use materials such
as ceramics, kevlar and other plastics dependant on the type of brake and their
application. Coefficient of Friction The coefficient of friction is the ratio of the frictional force and the normal reaction force of the mating surfaces. This impacts braking systems as it shows how the braking force generated by different materials will vary. materials with a higher coefficient of friction will be more effective in a braking system. The coefficient of friction for the pads on a disc brake to a metal rotor could be expected to be about 0.4 F =μR
=200 F N F F 200N 500N R N F 500N 200N x 200 +500 = x
x=539N 2 2 2 A force greater than 539N would be required to overcome braking
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