How well do the hydraulic brakes in your vehicle perform? Does your pedal give immediate, hard effort, or take up a long travel before much happens? Have you ever considered the connection between the type of fluid power you see on this website, and the components on your car or truck? They’re really not that different.
A hydraulic braking system on a vehicle is actually quite simple; the “pump” is the master cylinder, which uses vacuum energy from the engine’s intake tract to assist the pressure applied by your right foot (or left foot if you’re a race car driver). The master cylinder is really just a hydraulic cylinder with some simple valving.
When you push on the pedal, you push fluid out of the master cylinder (let us omit ABS components or other electronic nannies for this discussion), through four channels to each of the four brakes. Each brake, whether disc or drum, has a slave cylinder to push the brake friction material out to the disc or drum, turning kinetic energy into heat energy, thereby slowing the vehicle.
I’m a car guy, and I’ve done some amateur racing in my day, so I pay attention to the little things related to vehicle dynamics, such as pedal effort during braking. Why do some cars have touchy brakes and others have a softer feel with a long travel pedal? It has to do with the ratio of the master cylinder working area to the combined areas of the slave cylinders.
A large master cylinder will send more fluid to the slave cylinders more quickly, giving the brakes an aggressive initial bite as higher levels of friction are achieved with little pedal movement. What you’ll notice if you drive a vehicle with this braking characteristic, is that more pedal effort results in not a lot shorter braking distance. It’s a technique some auto manufacturers use to give the impression a vehicle’s brakes are better than they are, just as a touchy throttle gives the complimentary impression a vehicle is faster than it really is. But I’m not fooled.
I prefer a vehicle with a smaller master cylinder. With a smaller master cylinder, the pedal has to travel deeper towards the floor to achieve the same braking power as the above example. The same amount of fluid has to enter the pistons at all four calipers to create an equal braking load, but the pump is essentially smaller. This causes the pedal to feel a little soft, perhaps squishy. I prefer this style of system because the brakes are more progressive and easier to modulate, which gives better control over advanced driving techniques such as trail braking, threshold braking and mid-corner corrections.
It should be noted with a smaller master cylinder, although pedal travel is higher, more force form your leg is translated into more pressure at the brakes. This is elementary hydraulic math using the simple Pressure = Force / Area. A smaller area on the piston results in more pressure created in the hydraulic system. More pressure can be created with higher vacuum assist, of course, but higher assist takes away from “pedal feel.”
A common complaint of first-time BMW owners is that the brakes have a squishy feel with long pedal travel, but as most know, the driving dynamics of a BMW are second-to-none. BMW leaves little for chance in the design of components related to driving performance and enjoyment, so it makes sense they intentionally engineer their cars with smaller master cylinders. And if it’s good enough for BMW, it’s good enough for me.
