Automotive Brake Calculator

 


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"Tandem Master or Dual Master Cylinders" refers to the type of M/C you (intend to) use in your vehicle.

If you are working with dual master cylinders, enter a "2".
This calculator assumes that the balance bar is in the center position!

If you are working with a tandem master cylinder, or one single M/C, enter a "1".

Note: One single M/C or tandem M/C requires half the force but twice the stroke to generate a certain hydraulic pressure as a dual master cylinder set-up, everything else being equal. So if you enter the wrong number in this box, your calculated results will be way off!

 

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For 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, or 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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Help: What are dual master cylinders? What is a tandem master cylinder and a single master cylinder?

Race cars are usually equipped with dual master cylinders (one cylinder for the front brakes and one for the rear brakes), since this set-up allows for easy adjustment of the balance between the two hydraulic circuits by adjusting the balance bar that links the two cylinders, or changing out a cylinder for one with a different diameter.

Road cars always have a tandem master cylinder, except for older cars up to '65 or '66, when cars were still equipped with single master cylinders. The tandem M/C divides the hydraulic system into two separate systems, so when a leak occurs in one of these systems, the other system is still functional.

A tandem M/C usually operates a system divided in front and rear brakes, but on some vehicles the division is "diagonally", which means the left front and right rear brakes share one hydraulic circuit, and the right front and left rear brakes share the other circuit.

"Diagonal" systems have to be re-plumbed into a "front/rear" system, before the tandem M/C can be replaced with a dual master cylinder/balance bar assembly.

"Diagonal" systems also require to be re-plumbed when you want to install an (adjustable) proportioning valve in the hydraulic line to the rear brakes.

 

A Balance Bar is a mechanical linkage (usually adjustable) that connects the brake pedal with the two master cylinders as used on most race cars. By adjusting the balance bar, the force from the brake pedal is proportioned between the front M/C and the rear M/C. This is useful when setting up a car for a dry track (lots of grip, a lot of weight transfer to the front wheels), or a wet track (less grip, less weight transfer to the front wheels). Balance bars are available in several designs: adjustable from under the hood, or from inside the interior.

Properly chosen master cylinders will have the balance bar positioned almost exactly in the center position. For a track that provides more grip than usual, the bar can then be slightly adjusted to one side, and for a wet track slightly to the other side. The balance bar is not meant to compensate for a master cylinder that is too small or too big!

Keep in mind that a balance bar is not allowed to bind when one master cylinder bottoms out due to a leak or vapor lock in that hydraulic circuit. And the brake pedal should not run out of travel when both master cylinders bottom out.

 

A Proportioning Valve reduces the rate of increase in hydraulic pressure to the rear brakes above a preset value, when pressure on the brake pedal is still increasing. An adjustable proportioning valve allows for different preset values, so the hydraulic pressure to the rear brakes can be adjusted to how much grip is available. More grip means more weight transfer during hard braking, so less weight will remain on the rear wheels. To prevent rear wheel lock up, you want a smaller percentage of the hydraulic pressure at the rear brakes, compared to a situation that provides less grip and thus less weight transfer.

 

 

Last Update: 03/30/2017
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