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Piston Travel

We have now seen that in a closed hydraulic braking system we can state that:

  • You can increase the FORCE at the caliper by increasing the size of the caliper piston surface area.
  • You can decrease the FORCE at the caliper by decreasing the size of the caliper piston surface area.
  • PRESSURE is constant within the system. If you put 200psi into the system you will get 200psi out of the system - only the FORCE can be altered within the system.

The only way therefore to increase the performance of your braking system is by instigating one or more of the options outlined below:

  • Change the rubber hoses for stainless steel braided hoses. This will mean that minimal swell in the hose will translate into maximising the travel of the slave cylinder ( caliper ) and also constant FORCE being exerted at the caliper.
  • Decrease the surface area of the master cylinder piston to increase FORCE at the caliper.
  • Increase the PRESSURE going into the system by replacing the master cylinder.

The only trouble is the effect the last two items will have on the distance the caliper will move within the system. If you can imagine that the master cylinder has a surface area of 1" and we can move it forward 1" by squeezing the lever and produce 1000 psi then the volume of brake fluid which will move will therefore be 1 cubic inch.

If we then imagine that the caliper has a surface area of 2 square inches we can see that:

  • The PRESSURE on the master cylinder will be 1000 psi.
  • The PRESSURE on the caliper will be 1000 psi.
  • The FORCE at the master cylinder will be 1000 lb.
  • The FORCE at the caliper will be 2000 lb.
  • The DISTANCE the master cylinder piston will travel is 1".
  • The DISTANCE the caliper piston will move will be 1/2".

This principle is basically seen when you have a small cup and a large bowl in a bath of water. If you push the cup down a smaller amount of fluid is displaced meaning the bowl will travel only a small distance upwards. However if you push the bowl down into the water a larger amount of water is displaced and the cup moves a great deal further upwards. A brake system is the same as this with the exception that the bath is an open system (the fluid within it can move upwards as well) where as the brake system is a closed system in which only the two pistons are able to move.

This is why some manufacturers offer a master cylinder with adjustable ratio (piston surface area) and adjustable length (distance lever travels and distance piston travels) for example AP Racing. These superior master cylinders allow fine adjustments to be made so that the rider can quickly change:

  • The quantity of fluid moved at the master cylinder to directly increase or decrease the amount of fluid movement at the caliper.
  • The surface area of the piston to directly increase or decrease the psi produced by the master cylinder.

Brake Doctor Index

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