JL's Brake Calculator

I used this calculator a few times over a decade ago and went looking for it but his site was no longer up.  I reached out to offer to host the calculator again and here it is.  I'll be making some stylistic changes for some cleanup and for mobile users while the content is remains unchanged.  Thank you Jake Latham.

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.

Front Weight (@ Rest): lbs  
Rear Weight (@ Rest): lbs  
Total Weight: lbs  
% Front Weight (Static)  
CG Height in Inches in. Help...
Wheelbase (inches): in.  
Max Braking Force (in G): g Help...
Dynamic Weight Transfer: lbs  
Dynamic Front Weight: lbs  
Dynamic Rear Weight: lbs  
% Front Weight (Dynamic) %  
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
Max Braking G (From Step 1): g  
Dynamic Front Weight (From Step 1): lbs  
Front Tire Diameter (inches): " = " radius
Dynamic Rear Weight (From Step 1): lbs  
Rear Tire Diameter (inches): = " radius
Required Brake Torque Front Axle lb-ft = lb-ft per corner
Required Brake Torque Rear Axle: lb-ft = lb-ft per corner
Total Brake Torque Required (all four corners):  
% Front Bias Required: %
% Rear Bias Required %  
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)

Leg Input force (pounds):  lbs Help...
Pedal Ratio: Help...
Front:
Front Master Cylinder Diameter (inches):  in.  
Front Line Pressure:  psi  
Front Caliper Piston Diameters: Help...
Total Front Piston Area (one side):  sq in.  
Front Brake Pad cf: Help...
Front Rotor Diameter:  in.  
Front Pad Radial Height:  in.  
Front Rotor Torque:  lb-ft  
Rear:
Rear Master Cylinder Diameter (inches):  
Rear Line Pressure:  psi  
Rear Tire Diameter (from step 2):  inches  
Rear Caliper Piston Diameters: Help...
Total Rear Piston Area: (one side)  
Rear Brake Pad cf: Help...
Rear Rotor Diameter:  in.  
Front Pad Radial Height:  in.  
Rear Rotor Torque:  lb-ft  
Percent Balance:
% Front Brake Torque: %  
% Rear Brake Torque %  
Total Brake System Torque:  
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:

Brake Pedal force on Balance Bar:  lbs  
Distance between Master Cylinder Rods:  in.  
Distance from front MC Rod to Pedal:  in.  
Distance from rear MC Rod to Pedal:  in.  
Adjusted Front Pressure:  in.  
Adjusted Rear Pressure:  psi  
Adjusted Front Rotor Torque: lb-ft  
Adjusted Rear Rotor Torque: lb-ft  
Adjusted Percent Balance:
Adjusted % Front Brake Torque: %  
Adjusted % Rear Brake Torque %  
Adjusted Total Brake Torque:  

5: Pedal Movement This may help give some idea of the pedal feel in your system:

Front MC Movement  in.  
Rear MC Movement:  in.  
Pedal Travel  
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 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:
  1. 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