Automotive Brake Calculator

 


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"Master Cylinder Rear " refers to the internal diameter of this cylinder.

The size of a M/C is inversely proportional to the force it can exert. In other words, a larger diameter requires a higher force on the brake pedal (to generate a certain hydraulic pressure) than a smaller diameter, everything else being equal.

The trade-off is that the smaller diameter requires a longer stroke of the brake pedal to displace the same amount of fluid (needed to generate that same hydraulic pressure).

When choosing a master cylinder, find the available size that is closest to the calculated size. When the calculated size ends up roughly in between two available sizes, go with the larger size. This is the safer choice, since it is always better to NOT run out of stroke on your brake pedal, than to have to push harder than is "comfortable", in order to achieve maximum possible deceleration.

 

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In case you are working with a tandem M/C, the calculated bore sizes refer to an imaginary situation in which the front and rear hydraulic circuits are independently pressurized.

However, in a tandem M/C, hydraulic pressure behind the primary piston and secondary piston are the same, regardless any differences in diameter between both pistons (because the secondary M/C piston functions as a slave cylinder piston).

Auto manufacturers get the bias right by careful selection of all other components that affect bias, and with help of a proportioning valve. Any difference between front and rear bore size (stepped master cylinders are rare but not unheard of) is meant to deal with differences in fluid volume requirements.

So if you want to make changes to your brake system and you are working with a tandem M/C, you could do the following:

  • Try to find components that result in (nearly) identical M/C bores front to rear, when entered in the brake calculator (effective rotor radius, total caliper piston area, friction of brake pads, oe even: changes in static weight distribution, CG-height and rolling radius of tires).

  • If the calculated (desired) rear M/C bore (usually the secondary one) turns out bigger than the front M/C bore, use an adjustable proportioning valve to compensate for the extra pressure in the rear circuit caused by the rear (slave) M/C piston replicating the pressure behind the smaller front (master) M/C piston.

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For single Master Cylinders (to install in a pedal box with balance bar), the available (current) sizes are:

    0.551" 14.0mm
    0.591 15.0mm
5/8" = 0.625" 15.9mm
    0.661 16.8mm
11/16" = 0.6875" 17.5mm
    0.700" 17.8mm
3/4" = 0.750" 19.1mm
13/16" = 0.8125" 20.6mm
7/8" = 0.875" 22.2mm
15/16" = 0.9375" 23.8mm
1" = 1.000" 25.4mm
1-1/32" = 1.031" 26.2mm
1/1/16" = 1.0625" 27.0mm
1-3/32" = 1.094" 27.8mm
1-1/8" = 1.125" 28.6mm
1-5/32" = 1.156" 29.4mm
1-3/16" = 1.1875" 30.2mm
1-1/4" = 1.250" 31.8mm

Single master cylinders are usually available in two stroke lengths: 1" and 1.25" (go with 1.25" if possible).

Tandem aftermarket master cylinders come in the following sizes:

7/8" = 0.875" 22.2mm
15/16" = 0.9375" 23.8mm
1" = 1.000" 25.4mm
1-1/32" = 1.031" 26.2mm
1/1/16" = 1.0625" 27.0mm
1-3/32" = 1.094" 27.8mm
1-1/8" = 1.125" 28.6mm

Although bigger sizes are available, you will only find those in the auto parts store as a Ford part, GM part, or Chrysler part, for example. And smaller sizes are available as OE parts for small European or Japanese cars.

 

Last Update: 10/13/2017
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