Before buying a new Master Cylinder from Performance Machine and plumbing it to the stock factory Caliper from that 1996 Sportster you have to consider the hydraulic ratio between the two components.
To explain hydraulic ratios we have to re-visit high school physics. It all comes down to one formula: Pressure = Force/Area. So if you put ten pounds of force on a master cylinder piston with one square inch of area, you have created a pressure of 10psi. If you change the master cylinder to a design with only 1/2 square inch of piston area, then you’ve created twice the pressure. The important part isn’t just the 10psi of pressure, it’s how that 10psi of pressure is utilized by the caliper.If the caliper in question uses one piston with one square inch of piston area the force on the brake pad will be 10 pounds (Force = Pressure X Area). If you double the piston area you also double the force on the brake pad. After only a moment’s consideration you realize that the way to achieve maximum braking force is with a small master cylinder piston working multiple caliper pistons with relatively high total area.Remember though that everything has a price (no free lunch, ever). Small master cylinder pistons don’t displace much liquid, and may not fully extend the caliper pistons. Matching up the best components is a matter of balance then, between pressure and volume.
If you’re buying a complete new set up for the front wheel of that chopper, ask the sales person to recommend which master cylinder (they come with various piston sizes) to use with that four-piston caliper. If instead, you’re mating a new master cylinder to an existing caliper, buy one with the same diameter piston as that used on the donor bike.
The bigger the master cylinder piston the more fluid is displaced. As the piston size grows however the amount of pressure (all other things being equal) goes down. Output is also affected by the lever ratio, which is why some metric masters have adjustable pivots.
The force created in the master cylinder is transferred fully to the caliper pistons. The more piston area the more net force is created by a certain pressure. The trade off: more pistons (or calipers) require more volume to move all those pistons. That’s why the diameter of the master cylinder piston is so important.
–Jacki Mitchell
Wolfgang Publications
217 Second Street North
Stillwater, MN 55082
651.275.9394
Fax: 651.275.9330
24 hr. order line: 651.275.9411
http://www.wolfpub.com
