Calipers make the forces to push the pads onto the disc. Hydraulic
pressure from the master cylinder is converted to force on the pistons
and act on the pads to apply friction to the rotors.
Replacing a single piston caliper with a four small piston with the same
size pads will have only a small effect. However when fitting a larger
rotor, you can get the best from them by having a larger pad; but
applying a greater force via a single piston can cause the pad to bend
in the middle. Therefore the recent trend to twin piston/four piston
calipers which allow greater force and more even loads on the pad.
Master Cylinders Make Pressure
To apply a hydraulic fluid pressure to the brake calipers, we use a
master cylinder, in essence small pistons connected to your brake
pedal, which generates fluid “pressure” according to the force applied
to it and the area of the piston by the formula.
Pressure = Force
Area
Therefore a smaller piston generates a greater pressure. However there
is a lot of elasticity in a brake system and so master cylinder piston
diameters are sized according to pedal travel constraints as much as
pressure requirements.
A small piston may give nice high pressure and thus reduced efforts,
but at the expense of excessive pedal travel. Master cylinder bore sizes
are always a balancing game between fluid pressures and pedal
travel. Everything has to be balanced including front to rear.
Pedal Ratios and Brake Pedal Leverage
Brake Pedal leverage ratios affect pedal effort, the greater ratio giving
the greater force on the master cylinder, but once again the trade off
is increased pedal travel. Ratios range from 5-1 up to 7-1. Physical size
as well as ratio does come into it. The free play in every system means
that the push rod must move a small distance before the brakes begin
to apply, therefore a reasonable length from push rod to pivot must be
used. A very short pedal length with a good ratio of 5-1 will still move a
long way before taking up the slack. In the end, you have to balance
getting the pedal position comfortable, the ratios and sizes correct,
and still fit the assembly to the car. Car companies rarely make
mistakes so if not sure, see how they do it.
Brake Systems for Modified cars
Whenever fitting larger brakes, you have to balance master cylinder,
caliper and wheel cylinder hydraulic sizes. Small cars usually have