Quote:
Originally Posted by y8s
it is. but each piston only moves half the distance. same fluid volume.

Not exactly. The pistons are pulled back from the pads slightly by the pressure seal, which will be approximately the same for a slider with pistons on one side and for a fixed caliper with pistons on both sides. So the fluid displaced before a fixed caliper starts doing work may be slightly greater. Where a welldesigned fixed caliper gains is in stiffness, so less fluid is displaced as force builds, providing a firmer pedal with less travel and many other advantages.
The part that's confusing until you get your head around it is that the force applied by any caliper (assuming 100 percent efficiency) is equal to the circuit pressure times the pistons on one half of the caliper.
For a one or twopiston slider, you calculate the force using all the pistons. The equal and opposite reaction force that keeps the caliper from shooting sideways (after it shoots sideways a little on its pins and everything equalizes) is provided by the outboard portion of the clamp.
With a fixed caliper, you calculate the force using half the pistons. The equal and opposite reaction force that keeps the caliper from shooting sideways is provided by the opposing pistons pushing on the other side.