Understanding Brake Master Cylinders and Leverage Offsets

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Brake Piston Diameter and Leverage Offset and How It Really Affects Braking Power and Performance.

When brake manufacturers design a brake master cylinder there are two basic components to the design that greatly affect how the master cylinders perform: leverage offset and piston diameter.

Most stock brake master cylinders have a 16mm piston diameter, including Brembo master cylinders as stock on high performance motorcycles. Most aftermarket master cylinders use a 19mm piston. The larger the piston diameter, the greater the amount of fluid that is forced out of the master cylinder when the lever is pulled over an equal distance. However, the extra fluid requires extra force to move it. Every rider feels this extra force directly as the stock master cylinder will feel soft when the lever is pulled, while a larger piston with the same leverage offset (explained later) will actually be very difficult to pull over the same amount of distance. As is evident in the examples below, a 19mm piston will be approximately 30% harder to pull than a 16mm piston with the same leverage offset. A 20.5mm piston will be approximately 40% harder to pull with the same leverage offset.

Piston Volume Examples

16mm piston with 10mm of piston travel moves approximately 2009.6 cubic mm of fluid

17.5mm piston over the same distance moves approximately 2404 cubic mm of fluid

19mm piston over the same distance moves approximately 2833.85 cubic mm of fluid

20.5mm piston over the same distance moves approximately 3298.96 cubic mm of fluid

 

Leverage Offset

To compensate for the extra effort required to move the larger pistons, manufacturers use different leverage offsets that change the geometry of the master cylinders, making it possible to move larger amounts of fluid with less force.

 

As can be seen in the picture above, the leverage offset (2) is the distance between the master cylinder pivot point (1) and the point at where the lever applies pressure to the piston (3).

A common misunderstanding is that a greater leverage offset distance equates to more leverage; in reality, it is actually the contrary. The closer you can get the point of piston pressure to the pivot point, the more leverage that will be provided. See leverage offset examples below.

Leverage Offset to Piston Diameter Ratio Examples

Brembo 16x18 Ratio 0.88

Brembo 19x20 Ratio .095

Brembo 19x18 Ratio 1.05

Beringer BR14 20.5x14 Ratio 1.46

Beringer BR12 17.5x12 Ratio 1.46

Beringer BR10 14.5x10 Ratio 1.46

 

Leverage Offset Effects on Lever Travel

Using the above examples, we can see that a 16x18 master cylinder will require approximately 8.8mm of lever travel to achieve 2009 cubic mm of fluid displacement. The Brembo 19x18 will require 7.43mm of travel to achieve the same fluid displacement. The Brembo 19x20 will require 6.72mm of travel, while the Beringer BR14 20.5x14mm will require 8.9mm of lever travel and the Beringer BR12 17.5x12mm will require 12.2mm of lever travel to equal the same amount of fluid displacement.

It’s important to note that the higher the leverage offset ratio, the easier the lever pull will be to perform the same amount of work. The difference between the Brembo 16x18 and the Beringer master cylinders is 40%. That's 40% easier to achieve the same amount of fluid displacement. It’s one thing to displace fluid and another to do it with any amount of sustained pressure. That is where the leverage ratio comes into play: moving the fluid with great force.

The increased distance a lever moves to achieve the fluid displacement also greatly increases braking control. Imagine that we were able to move 3000 cubic millimeters of fluid in just 5 mm of lever travel. That would mean that just a slight, sudden movement of the lever would be enough to lock up the front wheel while leaned over, which obviously is not good for racing applications since slight adjustments are necessary from time to time. Now take that 3000 cubic mm and spread it over 15mm of lever travel and we can see that precise control becomes very easy to achieve. Which would you prefer a brain surgeon to use, a precision instrument or a chainsaw?

 

 

So which is better?

Isn't it obvious? Racers the world over prefer the higher leverage offset ratios. Most of us just didn’t understand the reason why, but now we do.

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