What's the most powerful part of your bike? The engine, right? It makes 160 HP and slingshots you towards the horizon like a rocket. Zero-to-60 mph in under three seconds, wheelies into fifth gear and makes million-dollar supercars look like lumbering SUVs.
But there's another part that's almost as powerful-the brakes. Slam on the twin Brembos aboard a BMW S1000RR and you'll come to a stop from 60 mph in just a few seconds, meaning the brakes are nearly as powerful at stopping your bike as your engine is at propelling it. And all they have to work with is the power in your right hand.
The most remarkable thing is how simple a set of brakes is. The discs are dumb bits of steel, and while calipers look zingy and exciting, they're little more than cast lumps of aluminum with precisely machined holes carrying close-fitting pistons.
So where's the clever stuff in brakes? Partly it's just harnessing the classic power of mechanical hydraulics. This technology goes back to the ancient Greeks, but the basic principles are simple. Make a sealed system, fill it with hydraulic fluid and then use pistons and cylinders to alter the pressure inside. Physics means that the pressure will be equally distributed through the system and you can use this to transfer work.
Master cylinder design is...
Master cylinder design is all about leverage. Just know when to say when.
Imagine a really simple setup, with one brake caliper that has one piston inside it, connected by a single hydraulic line to a master cylinder. Imagine also that the master cylinder piston is exactly half the size of the caliper piston. Now, when you squeeze the lever on this master cylinder you push the piston in. As you squeeze harder you're increasing the pressure inside the system (like pushing a syringe with your thumb over the nozzle). This pressure is transferred through the hydraulic fluid and pushes against the piston inside the caliper, pushing the brake pads against the disc and slowing you down.
Now, remember the two pistons in our system (the master cylinder piston at the lever and the slave cylinder piston in the caliper) are different sizes. The master cylinder piston is half the size of the slave cylinder piston, and this causes a sort of 'lever' effect. Force is defined as pressure times area. That makes sense-it's easier to make a lot of pressure on a small area than a big area. Try pushing a thumbtack into the wall-the pressure at the point is massive because it's a tiny area, so it's able to penetrate into the plaster. Try and push your thumb into the wall and it won't work: the bigger area of your thumb means you need much more force to make enough pressure to push it into the plaster.
So if we generate a pressure of 100psi at our small master cylinder piston, the same 100psi appears at the caliper slave cylinder piston. But because that slave cylinder is twice the size of the master cylinder piston, we have double the force generated.
How come we have this 'free' extra force? Well, like on a lever, the force is applied over a shorter distance. Like using a three-foot crowbar to lever out a nail, we're using our hand's limited force over a longer distance to move a nail a short distance-maybe moving your arm a foot to lever a nail out by an inch. Similarly, when you squeeze your brake lever and it moves an inch or so toward the bars, the pistons inside the calipers may only move an eighth of an inch or less. In addition, the mechanical design of the brake lever will also be multiplying your hand's force by a classical lever action. Like a pair of pliers, you squeeze on a long lever that pushes onto the master piston a small distance from the pivot, again multiplying the force from your hand.