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Master cylinders often create varying answers on bulletin boards when the question “Should I get a radial pump master cylinder for my bike?”. The problem with those varying answers is that only a small handful of responses are correct. Those along the lines of changing lever feel, volume, and force requirements are on the right track. However, that’s about it. With modern motorcycles, an aftermarket will not greatly help fade, stiffness, or effectiveness of the brake system. The purpose of this article is to unpack all that I know about master cylinders (as a brake system engineer) in plain English and apply it to the motorcycle arena. I’ll cover the following topics:

Purpose: The purpose of a hydraulic master cylinder (m/c) is to provide a source for pressure to be applied to the brakes. In other words, the function is to translate the rider’s input lever force into brake line pressure in order to apply the brakes. Without getting into the valving too much, you have a brake lever connected to a pushrod connected to a piston inside the m/c bore. The pressure generated is captured and maintained by a rubber seal between the bore and the piston. That’s it. OK, there’s a little more to it than that, but thats the basics.

Master Cylinder Sizing: This is where many people stray from reality when explaining master cylinders. An OE master cylinder is sized to be able to have enough fluid capacity for the combined total caliper piston volume consumed at maximum ‘calculated’ running clearance (the gap between the pads and the rotors - usually less than 1mm). Its not the total piston volume as that would be huge, but the following simple equation [pad gap * piston area of all pistons in circuit]. Most caliper piston diameters are in the 30mm range while most m/c’s are on the 13-20mm range.

OE m/c’s are sized to give the rider the best blend of lever force and travel characteristics. Since the m/c’s purpose is to translate a lever force into fluid pressure (force per unit area = psi), the bore diameter determines how the lever feels. To understand the impact of bore diameter, think of the equation force/area with the force being lever force and the area being the area of the m/c bore. To display the math a little, I’ll use nice round numbers. Say you can generate 1000lbs of force and have a 1 in² m/c bore area. That equals 1000lbs/1 in² or 1000psi. By increasing the m/c bore area to 1.25 in², using the same equation, you can only generate 800 psi. Likewise, by decreasing the m/c bore area to 0.75 in² you generate 1333 psi.

M/C bore area summary: The bigger the area of the bore, the less pressure is generated for a given input lever force. The smaller the bore area, the more pressure can be generated from that same input force
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