If you’ve ever wondered how your weak little foot could possibly squeeze brake pads hard enough to bring your enormous two-ton car to a stop, the answer is in the latest episode of David Dissects. It all comes down to something called a “mechanical advantage.”
A mechanical advantage is simply a mechanism that can take an input force (from a human foot, for example), and multiply it to create a much higher output force (the clamping force on brake pads, for example). Mechanical advantages come in different forms (one of the most widely recognized “mechanical advantage mechanisms” is a pulley), but in the case of a brake system, it all comes down lever arms and fluid incompressibility.
For one, your brake pedal is hinged on a very large lever, and pushes a rod that’s also connected to that same lever, but closer to the axis of rotation (which is signified by the dotted line above). This means that your foot will travel in a longer arc than the end of that rod will, but the force into that pushrod (which ultimately helps squeeze your brakes) will be higher than the force your foot puts into the pedal.
Put more simply, recall that torque is equal to force times a distance. Your foot exerts a small force, but at a large distance, meaning it makes a large torque. With your foot steady on the brake pedal, the brake rod must impart an opposing torque to keep the pedal from rotating. To do so, since it has such a small lever arm, the rod must exert a higher force than that created by your foot.
The force your foot sends to that pushrod is equal to the ratio of the distance your foot is from the axis of rotation to the distance the pushrod is from the axis of rotation (in the picture above: brake pedal lever divided by brake rod lever).
That pushrod from the pedal goes into your brake booster (also called power brakes), which has a little one-way valve on the front with a hose going to it. That hose connects to your intake manifold (or an auxiliary pump), to allow a vacuum to suck on the front face of a diaphragm in the booster. When you push the pushrod in via the brake pedal, you open another valve on the back of the diaphragm.
What you end up with is a low pressure at the front of the diaphragm (from engine vacuum), and higher atmospheric pressure at the back. This “delta P” moves the diaphragm forward and creates a higher output force that makes it easier to stop your car.
That diaphragm has its own pushrod on the front, which goes into a brake master cylinder to push a piston (or two in the case of this Jeep). That piston’s job is to move fluid through brake hard lines, which route towards the corners of your car along the body or frame. Once the hard lines get to the corners, their fittings thread into rubber brake hoses, which take fluid from the fixed hard lines and transfer it to the brake calipers (which move up and down with bumps in the road and side to side when steering).
Once the fluid is at the calipers, it pushes on the back side of a piston, which then pushes on the back side of the inner brake pad.
That inner brake pad then makes contact with a spinning rotor, and the caliper slides relative to the knuckle to allow both pads to squeeze the metal rotor, thus stopping your car.
The size of the piston in the brake caliper is significantly larger than that in the brake master cylinder, and since we’re dealing with an incompressible fluid in a closed system, the pressure throughout the hydraulic system is constant.
Since pressure is equal to force divided by area, the force at the larger-area caliper piston must be larger than that at the master cylinder. So what ends up happening is the brake booster’s pushrod plunges the master cylinder piston a long distance to get fluid to push (with a higher force) the caliper piston a short distance to squeeze the pads against the rotor.
So there it is: the question of “How can my foot make so much clamping force?” comes down to three main features: the brake pedal lever arm, vacuum assist from the brake booster, and the difference in the sizes of the master cylinder piston and the caliper piston. These features are a godsend, because not everyone has the leg strength of Fred Flintstone.