Step 1: Dynamic Weight Transfer The first step to calculating what you need in your brake system is to know how much dynamic weight transfer your car has. The harder you can brake, the more relative brake torque that is required at the front wheels.
Step 2: How much brake torque do you need? Now that we know how much weight is on each axle during a maximum braking event (From Step 1), we can calculate how much torque the brakes need to be able to put out to achieve that stop. These are per-axle calculations, not per-corner. To get per-corner, divide the dynamic weight by two.
Required Brake Torque (lb-ft) = DynamicWeight * (Tire Radius / 12) * Decel G's
Step 3: How much brake torque do you have? Now enter what your system currently has. You can also enter sizes that you are considering changing to, and you can match these with the required values from Steps 1 and 2. These are also PER AXLE calculations. The calculation looks like this:
Axle Torque (lb-ft) = (2 calipers per axle) * (Piston Area) * (Effective Radius) * (Pad Coefficient) * (2 sides of rotor & pad) * (Line Pressure) / (12 inches per foot)
Step 4: Balancing the system After setting the basic components of a 2 master system, you can then adjust slightly with the balance bar. Use this to see if your component choices will get you into the rough range of braking force that you require:
Step 5: Pedal Movement This may help give some idea of the pedal feel in your system:
Front MC Movement:
0.00 in.
Rear MC Movement:
0.00 in.
Pedal Travel:
0.00
Additional Information Determining CG Height
The height of your car's center of gravity can be measured using a set of cornerweight scales. So, hopefully if you know your car's cornerweights, you can also measure the CG height. Longacre has an article on their site about how to do this.
Determining Max G Braking Forces
In the best case, this number is generated through datalogging in your system. If not, then some estimates may have to come into play:
A race car with significant downforce can generate 3.0g under braking, from 150mph+
Racecars with somewhat less downforce will be more in the 2.25-2.5g range
A sedan on reasonable race tires will be able to generate 1.25 to 1.5 g braking
If you have lateral g data, but not longitudinal, you can approximate your lateral g by adding 10 to 15% to your lateral g numbers.
Leg Input Pressures
This is how hard you are pushing on the pedal. For unboosted racing systems, typically about 80lbs of brake pressure is the goal. A driver can usually apply up to around 120 pounds if required, but while this might be possible on stands in the garage, over the course of a race distance, it will be very tiring. Street cars are typically around 40 lbs. So, about 80 is a reasonable target input force.
Pedal Ratio
This ratio is the amount of mechanical advantage the pedal has over the rod of the master cylinder. Stoptec says that one should aim for 6.2:1, with a working minimum of about 5:1 for unboosted racing systems. Typical street car ratios are around 3.5-4.0:1, with their vacuum assist. Purpose-built race cars are usually in the 3.0:1 range, with some (such as my Radical) as high as 4.5 or so.
Piston Diameters
Diameter of the pistons on one side of the caliper. If you have sliding calipers, then the diameters of the piston(s).
Pad CF (Coefficient of Friction)
This is how grippy your pad is. If not sure, use .40 for street pads, .48 for street performance pads, and .60 for racing pads as a rough guide. You can typically look these up on manufacturer's websites.
Links + References Stoptech Dual Cylinder Setup Guide (link from saved copy) TCE Performance Products Outlaw Brakes FAQ Sports.Racer.Net Brake Bias page Tilton page on bias bar setup SAE Paper on Brake Torque (alternate eqn) Some miscellaneous threads with good discussion:
DSR Forum 1
Thanks to Philip Erickson for finding the missing factor of 2 in Step 3...
Notes / Guidelines A lot of brake system design comes from experience, so this page is certainly NOT a substitute for advice from a professional. There are small guidelines to be found from time to time, for example, this bit from Tilton's website:
Rear locks up first, pedal firm-reduce front master cylinder size.
Rear locks up first, pedal feels soft-increase rear master cylinder size.
Fronts lock up first, pedal very firm-reduce rear master cylinder size.
Fronts lock up first, pedal feels soft-increase front master cylinder size
