Big Brake Kits

Technical information on pedal effort

If you fit a Big Brake Kit, the larger discs  will increase the braking mechanical advantage. This will reduce pedal effort. If you find this results in brakes that are too sensitive for track use you can increase pedal effort by changing the master cylinder or removing the brake servo. A larger master cylinder bore size will increase pedal effort but reduce pedal travel. Master cylinders are usually available in inch sizes, pedal effort in lbs per sq inch and piston sizes in mm (great!).

Some formulas will demonstrate what you might end up with:

If you increase disc size from 284mm to 300mm you would end up with a 5% decrease in the pedal force required to get the same braking force (assuming the piston area in the caliper remains the same). Therefore a 100lb force required before would become 95lb

New pedal force = Pedal force x (Old brake disc radius/New brake disc radius)

The rolling radius of the tyre has the opposite effect - increase the diameter of the tyre/wheel combination and you increase the pedal force required.

New pedal force = Pedal force x (Old tyre radius/New tyre radius)

In this case if you went from a 14” Wheel with 195/60 up to a 16” wheel with 205/55 you would increase the pedal effort required by 10% (bear in mind though legally for road use rolling radius must be no more than +/- 3% from Original specification. So when people say they will fit bigger wheels to fill the arches you know they aren't aware of this!)

When looking at master cylinders the formula is as follows:

New pedal force = Pedal force x (new master cylinder bore diameter²/ old master cylinder bore diameter)²

Because of the square in the above formula the effect of a change is more pronounced. E.g a move from a 1” master cylinder to a 1.25” master cylinder will change the pedal force required from 100lbs to 156lbs.

Next thing to remember is bigger pistons in the calipers mean more force and, therefore, less pedal effort. Bigger pistons in the calipers mean more pedal movement. For comparing one brake to another on the same car, you can simply multiply the total caliper piston area times the effective radius. If you have a sliding calliper although it has piston(s) on one side only the area should be doubled. A single piston sliding calliper with the same piston size as a twin fixed calliper will have the same clamping force, this is due to the fact that the pressure from piston operates on both sides via the sliding mechanism.