Master Cylinders
Friday, March 25th, 2005 at 9:45 pm by angrybob
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
…continued from teaser here
Lever stroke is also impacted by m/c bore area. The total volume of fluid generated in the m/c is the equation [m/c bore area * m/c piston travel]. Again, using the 1 in2 m/c bore area from above and keeping the m/c piston travel constant at 0.5 in, the original case 0.5 in3of fluid volume. Increase the bore area to 1.25 in2 = 0.625 in3. Likewise, decreasing the m/c bore area to 0.75 in2 = 0.375 in3.
M/C volume summary: The bigger the area of the bore, the less stroke required to move a given volume. The smaller the bore area, the more stroke is required to move that same volume.
Combining the two summaries is the key to understanding master cylinder sizing:
The bigger the bore, less stroke but more force will be required to reach a given pressure. The smaller the bore, more stroke but less force will be required to reach that same given pressure.
The balance between force and stroke is what the tuners at the OE call “feel”. From my experience, I have seen smaller m/c’s on OE machines which leads me to believe that the goal is to give a system with less effort and more stroke than the other way around. Many stock units are around 13mm in diameter.
Types of Master Cylinders: There are two basic types of master cylinders available on OE bikes and in the aftermarket sector. Most bikes still get the standard type m/c where the piston bore is parallel to the clip-on or handlebar, where a few of the new(er) sport bikes are starting to get the radial pump as stock equipment.
Aftermarket Units: The standard design is still the most common m/c on stock bikes. My guess for the reasoning is cost and sizing. Most stock brake systems do not require the volume that the bigger radial pumps offer and are usually cast aluminum. The disadvantages are in the space taken up on the bars and the lack of lever adjustment (either none or 4 adjustments).
While the radial pump has been around a while in the aftermarket, its only been stock on
upper end sport bikes for the last year or two Brembo is still the household name among radial pumps although many other brands exist (AP, Speigler, Magura to name a few) in the aftermarket sector. The ‘base’ models are forged (pictured) and the ‘bling’ models are machined from billet.
The main advantages are they offer larger diameter bores, a fine adjustment for the lever position, and a selection of pivot lengths. Brembo sells a 19mm bore x 16,18, or 20mm pivot lengths. The smaller pivot length has the same affect as a smaller m/c bore size - the 16 requires the least effort to generate a given pressure, but has the longest stroke. The 20 requires the highest force to generate a given pressure but has the shortest stroke (this is the one req’d for 6-piston calipers). AP has a design that has a variable pivot length controlled by a dial & set screw - very cool.
The main disadvantages in the aftermarket radial pumps is the fact that they do not come with a brake switch. No big deal for a track bike, but for a street bike that needs a brake light, more $$$ needs to be spent. You can dump from $20 for a banjo bolt / pressure switch type to $80 or so for the bling Brembo switch setup. I’ve only owned the cheap kind and the pressure response to illuminate the brake light was variable. The other disadvantage is that if you break a lever, they are usually pricey.
NOTE: A common misconception is that the bigger bore m/c’s make the brakes stiffer. They do not. The increase in required force and more importantly shorter lever stroke is mistakenly called increased stiffness because the brakes engage sooner.
Real World Benefits - WWABD?: I have owned both Brembo and AP (non variable pivot length) radial pumps for street and track bikes. My opinion is that unless you are racing, save your money. For the street and even canyon carving, its really not necessary or at least not the best first step(s) for a street bike to improve the feel or fade. Braided lines and brake pad selection are better first steps IMO. The next step if you are concerned about or are getting brake fade would be the CRG Roll-A-Click Adjustable Levers. The lever can be adjusted out (to address fade) on the fly with your thumb. I have these on my Mille R and they work great. Heh - I also have the stock m/c! If you race, that may be a different story…especially for endurance racing where fade and shorter stroke are more important.
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I also am an engineer with experience designing braking systems. My experience and knowledge of the subject leads me to question several of your conclusions. The one statement I disagree with most is the assumption that squeezing the lever farther (”If you race, that may be a different story…especially for endurance racing where fade and shorter stroke are more important.”)
Increasing lever travel by adjusting the lever farther out “on the fly” will not diminish fade. All it will do (possibly, in some cases) is slightly prolong the point where fade is too severe to deal with and braking just goes away. You start with a short travel, and as fade starts setting in you extend the travel. That’s what you seem to be saying. If so, then the advantage of the radial cylinder boils down to adjustable stroke. And added stroke will not help when the brakes start to fade.
It is true that you have some measure of “feel” control with a radial master cylinder. However, that kind of “feel” difference can be mimicked by selecting different cylinder bores. As you stated, changing the pivot point length is the same as changing the cylinder bore.
Also, as you point out, increasing cylinder bore increases lever effort and decreases lever travel per effective braking effect. Some racers (and street riders) are more comfortable gauging braking by travel rather than effort.
I don’t know what the current pricing schemes are, but when aftermarket radial masters first came out they were more expensive than the standard layout. At that time (and if this is still the case, at this time) it was smarter to opt for a standard layout with the bore you wanted.
My suspicion is that there’s a lot more marketing than science behind aftermarket radial masters. Similar to “radial calipers”, where the only difference is the orientation of the attachment bolts. This was originally done to decrease weight, but suddenly they were touted as some trick solution to braking troubles. On a motorcycle true radial calipers (where the mounting of the caliper is in the same plane as the rotor, eliminating twisting and bending moments along the axle’s axis) do not exist. Note that several production bikes equipped with “radial” calipers wound up with calipers and brackets heavier than the ones they replaced. That smells like advertising.
There’s nothing wrong with radial master cylinders, but they are being oversold and badly explained to the non-technical public. “Radial” calipers are pure advertising.