claim that better brakes make faster lap times, but how can something
that slows you down make you faster?
Simple; the better the brakes the longer you can stay on the power before
braking for the next corner.
Brakes are all about heat, and ditching as much of it as quickly as possible,
they work by converting the cars speed energy into heat energy which is
then taken swiftly away in the air streaming through them, in theory.
But a big car at high speed has an awful lot of energy; for instance getting
a Bugatti Veyron to do an emergency stop from high speed might put the
equivalent of several thousand bhp through the brakes make the discs glow
There are two basic types of brake, drum brakes get their name from the
drum of steel with curved shoes inside that are pushed outwards against
the inside of the drum when the brake pedal is pushed. These can be found
on the back axles of cheaper cars and are quite frankly a bit pants; the
braking force is limited, the pistons are small and the pressure pushing
the shoes out into the drum is similarly small. By comparison disc brakes
can exert a much higher force onto the disc without risk of it failing.
It uses a disc with a set of pads held in a calliper that are forced against
both sides of the disc when the pedal is pressed, generating much more
force. In both cases the disc or drum part is attached to the wheel hub
so it rotates with the wheel and the pads or shoes are held stationary
on the axle, or strut, or what ever dangly bits are attached to the suspension.
All brakes work by friction, pressing a pretty darn tough pad of friction
material against the spinning metal, the harder the friction material
is pressed against the metal the more friction is produced and the greater
the braking force. New pads will need to be bedded in before any serious
racing, the exact procedure varies for different materials so read the
manufacturers instructions before driving the car. Race pads often only
work when they have a bit of heat in, some are unsuitable for road use
and should be avoided if you have to drive the racer to events, check
with the manufacturer for recommendations. This also makes the warm up
lap quite important, and when starting a race caution and early braking
may be needed for the first few corners.
Brake systems use a special type of hydraulic oil to drive the pistons
which push the friction material into the disc or drum. At the pedal end
there is another piston in the master cylinder which is connected by hydraulic
brake pipe to the slave cylinders at each wheel. In some cars the brake
force is artificially increased by a servo directly connected between
the brake pedal and the master cylinder, this uses vacuum from the intake
manifold to move a large diaphragm when the brake pedal was pressed, as
the pedal moved down small holes in the servo control section are progressively
uncovered which applies more vacuum to the diaphragm which in turn applies
a greater force to the master piston of up to four times the force at
Some cars with Anti-lock Brake Systems (ABS) use a powerful electric pump
to do this instead. The ABS system measures wheel speeds and if it detects
that a wheel is slowing down faster than a safe limit then it knows that
that wheel is about to start locking up, so it lets the brake pressure
off the individual wheel by opening a solenoid valve in the ABS valve
block, just for a tiny fraction of a second until the wheel frees up just
enough to know it wont lock. You can feel this when it happens as a sort
of buzzing or vibration under the brake pedal. ABS allows maximum braking
force without the risk of skidding. But if you are going to fast then
you are still going to crash no matter what the brakes do. Some people
remove ABS systems for race cars but if the regulations allow then keep
it, it will hold the brakes at peak braking efficiency and control each
wheel individually which is something no driver could do, the system rarely
weighs more then a couple of kilos so removing it will make nor real difference
to the cars weight.
The fierce heat generated from heavy braking has to be dissipated into
the air which is why race cars have ducts taking fresh air from the front
of the car to the disc centre, the hot air then has to go somewhere and
the design of the wheel should allow it to escape readily through widely
spaced wheel spokes. To get more heat into the air some discs are vented
with radial channels cast into the disc to draw air from the centre outwards,
vented discs are more or less essential on a racer and if your car doesn't
have them it may be possible to swap with the brakes from a higher specc'd
model. Some performance discs also have small holes drilled through for
even more ventilation but these can lead to cracks starting unless they
are made very well with radius edges, usually better to avoid them. Groves
on performance discs can help remove the tiny gas layer that build up
between the pad and disc sometimes and increase pad bite, the down side
is that they can increase pad wear when used aggressively, the groves
should have a radius at the bottom of the cut, if they are cut squarely
into the disc then the corners may start cracks..
The brake size needed on a car depends on its weight and how fast it is
likely to go, more powerful cars can more readily get up to higher speeds
they need bigger brakes. Bigger pistons and a larger diameter disc make
better brakes. Also if the brakes are going to be used for long durations,
such as when racing, there is less time between brake applications for
them to cool down adequately, this is where vented disks can be a real
All that heat soaks through the system into the brake fluid and although
it is engineered to work at these very high temperatures in extreme cases
the temperature can get high enough for the oil to boil, this generates
gasses which compress easily and make the brake pedal feel very soft.
This is brake fade and in really bad cases the brake pedal can sink to
the floor with very little braking force generated, pumping the pedal
up and down a few times can sometimes help but basically if the brakes
fade on a race track then the car generally crashes. And it's always a
surprise when it happens because heat builds during the last application
then soaks into the fluid whilst the car is hammering down the straight,
so when the pedal is pressed again there is nothing, even though it worked
fine last time!
Old brake fluid absorbs water which boils and fades much more easily which
is why it must be changed every few years to stay safe. Silicon based
fluid is different and doesn't absorb water but moisture still pools inside
the system and needs flushing through every few years, it's also a bit
more squashy than mineral fluid making it unsuitable for fast acting ABS.
The DOT number can cause some confusion. Many people think a higher number
is 'better' but this isn't necessarily so. The number refers to a test
schedule with many features such as shelf life, water absorption, compressibility,
this also includes fade resistance both new and when it has aged a certain
amount, but race fluids age quickly and so only pass older DOT tests.
Some DOT tests are for specific types of fluid, for instance DOT5 is only
for silicone fluids which have to be tested slightly differently as you
cant force them to absorb water, but DOT5.1 is for ordinary fluids, so
you have to be careful. Most good race fluids are DOT3.
Another problem that can burden budget race cars is sticking brakes. If
using standard callipers this may be because of the outer dust seals burning
up and jamming the pistons, this simple solution is to remove them but
remember that dirt will now get on the exposed part of the piston and
will have to be cleaned off when fitting new pads. This seal must not
be confused with the main fluid seal.
Brakes are often overlooked and any wear only becomes apparent at the
mot or in an emergency stop. The trouble is that they have a hard life
and can disintegrate with the friction material splitting off the steel
backing or wearing down to nothing unnoticed, and they usually seem to
work fine right up to the point were they don't work at all and you crash.
Maintenance and regular inspection is vital.
One of the other problems
on race cars is the handbrake seizing up. Never apply the handbrake after
a hot lap as it will warp. Some people remove the handbrake completely
and use a hydraulic one instead, this is particularly popular with rally
cars where the handbrake is used when turning. This is a relatively simple
mod where the hydraulic line running to the back brakes has another master
cylinder fitted, the line from the foot brake goes into the reservoir
connection and the back brakes connected to the normal outlet from this
secondary master. A custom leaver is used to operate the secondary master.
In normal operation the pressure from the foot brake goes straight through
the secondary master and works as normal, but when the hand brake lever
is applied the piston in the secondary master moves forward, isolating
the front brakes and applying pressure to only the back brakes.