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Brake Bias Calculator
 
*For more comprehensive specs and variables use the Dual MC Bias calculator
 

What is BRAKE BIAS? Brake bias is a representation of torque values at each end of your vehicle expressed in a percentage form. For example; 65/35% means that the front brakes are expected to produce 65% of the braking duty. The calculator deals with static bias only. (Dynamic Bias is active bias under braking when pressures may be effected by proportioning valves, electronics or other outside inputs)

What is the ideal bias for my car? There is no one size fits all answer to this question. As much rear brake as you vehicle will allow before the rear end becomes unstable is a simple way to look at it. However that does not take into account suspension settings for weight shift, road surfaces- which effect adhesion, tire compounds and sizes and of course; driver preference.

What effects bias? Brake bias in this format is effected by three elements; rotor diameter (leverage), piston area (clamp load) and pad Coefficient of friction (bite). Changing any one or more of these can swing the % one way or the other.

Can you have too much bias? Certainly. Too much front brake can leave the front doing more work than necessary and lead to over heating. Too much rear can create an unstable and 'tail happy' feeling while braking and be quite dangerous.

How much of a change can I make? Generally most vehicles can exploit upwards of about 5% more rear brake if the vehicle has a reasonable wheelbase and can keep weight over the rear wheels. On FWD cars with shorter wheelbases this may not be as effective. Two extremes would be the Impala SS and Ford Focus; two very different applications. Boosting front bias by upwards of 5% doesn't express huge negatives and in some instances might well be the only way to gain braking performance. Remember that "performance" can come in the need for more rotor mass also and a front bias increase can by a by-product.
 
 
Using the calculator:
Using the bias calculator is simple; It's a 'plug and play' tool for your use. The values of a number of factors can be changed including rotor diameter, piston areas, pad Cf, input pressure and master cylinder bore size. We've included pressure in the calculator as it can also show the resultant pressure drops or gains with the changes in values. The primary goal of this calculator is to be used as a compartive tool. By starting with your current design and making changes you can evaluate those changes.

The calculator has a 'default' built into it and we suggest you use this to become familiar with how your inputs can change the results before you enter your own data. If you're comfortable with the values enter away. If not please take a moment to read the terms at the bottom of the page to understand the inputs required first.
*Note that all values are in inches and not MMs. To convert your data from MM to Inches multiply MM x .03937 to covert. 




Leg input in pounds:
 
Master cylinder diameter:
 
Front line PSI (un boosted):
 
Front caliper pist dia 1:
 
Front caliper pist dia 2:
 
Front caliper pist dia 3:
 
Front caliper pist dia 4:
 
Total front piston area *:
 
Front pad Cf:
 
Front rotor diameter:
 
Front pad radial hieght **:
 
Front Rotor Torque In.lbs.:
 
Rear line PSI (un boosted):
 
Rear caliper pist dia 1:
 
Rear caliper pist dia 2:
 
Rear caliper pist dia 3:
 
Total rear piston area*:
 
Rear pad Cf:
 
Rear rotor diameter:
 
Rear pad radial height:
 
Rear rotor torque In. lbs.:
 
Percentage of front Bias:
 


 

Terms and values explained:

Leg- the amount of pressure placed on the brake pedal.


Master Cylinder Diameter- The bore size of the mc. If unsure use 1.00" as the default.


Front/Rear Line Pressure- This is the hydraulic pressure (un boosted) generated by Leg input based on the MC bore and a common 6:66 pedal ratio. For the rear the pressure is un restricted or non-proportioned. Different vehicles may limit this pressure by various proportioning devices so final pressure can be less- which then reduces rear rotor torque.


Front/Rear Piston diameter 1- The diameter of the first (or only if single pot caliper) piston fitted to the caliper.
Piston 2- The second piston on one side of a caliper.
Piston 3- The third piston on one side of a caliper.

Piston 4- The fourth piston on one side of a caliper.


Note: multi piston calipers express their values using one HALF of the caliper body. This accounts for the floating aspect of the single piston caliper. A six pot caliper would be 1.625/1.125/1.125" for example. True clamping force would be double that but also double the single piston of a floating caliper taking into account the 'pull' of the outer pad to the rotor surface. Using total area (all six for example) would require you double that of the floating caliper also- thus the net result is the same whichever way you do it.

 

*Total Front/Rear Piston Area- the total area value of the piston(s) expressed in square inches. See note above.


Front/Rear Pad Cf- the coefficient value of the brake pad. If not sure; use .40 for street pads, .48 for street performance pads, and .60 for racing pads as a rough guide.


Front/Rear rotor diameter- Expressed in inches, the total OD of the rotor.


**Pad Radial Height- This is the height of the pad friction material. Viewed from operating position it may appear as width. Most FRONT production pads will be about 2.0-2.5" and aftermarket or race pads fit to alternate calipers about 1.75 to 1.875" depending upon caliper requirements . Use 1.875" if unsure. REAR production pads are generally in the 1.500" range. Aftermarket calipers may range from 1.625 to 1.875".


Front/Rear Rotor Torque- the value of torque expressed in inch pounds.


Percentage of Front Bias- this is the net front bias of your vehicle.


Additional thoughts:

While Bias is important, you can also see the cause and effect of other elements of your proposed changes. The calculator will demonstrate how a change in piston size can effect the required LEG input to get the Rotor Torque desired. Rotor Torque is the real value one must watch as it is the work being performed. Should you choose to run much less piston area you can calculate the required increase in Leg necessary to generate the same final value. This often provides a much 'firmer' pedal feel but will also need far more Leg to generate the same Rotor Torque. On the other hand you may wish to see what a smaller MC will do in the same instance to boost Rotor Toque. Rotor OD and pad Cf can also effect the amount of pressure required for the final torque value.

Why does my bias percentage shown not represent the rotor torque values to the same percent? The bias calculator expresses final bias as tire torque at the ground. By default the calculator assumes the same size tire is used front and rear. When rotor torque is expressed it is yet again a ratio of the tire in which it is functioning. The final bias is then calculated at the tire relative to those calculations. Changing the size of the tire from one end of the car to the other will effect tire torque just as changing rotor size does to rotor torque.

There are many variables which can be used to alter the final results. When shopping BBKs suppliers should be willing to give you the data here openly. If not you may want to ask why the information to make an educated decision is not afforded you. Don't be surprised if many of the sales people know nothing of this so you may have to ask for someone who does. To compare the options it helps to know the base information about your vehicle. This can often be found in the service manual and sometimes by asking on a forum if someone has compiled the data for you.

Some folks have questioned the results using this calculator. The format makes some basic assumptions; pedal ratio, pad radius etc and computes tire torque behind the scenes... The intent of the tool is to compare cause and effect more than establishing definative values. If you have questions about this calculator please feel free to contact me.

 

*For more comprehensive specs and variables use the Dual MC Bias calculator
 


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