Brake systems
We all know that pushing down on the brake pedal slows a car to a stop. But how does this happen？ How does your car transmit the force from your leg to its wheels？ How does it multiply the force so that it is enough to stop something as big as a car?
Brake Image Gallery

Layout of typical brake system.  See more brake images。
When you depress your brake pedal， your car transmits the force from your foot to its brakes through a fluid. Since the actual brakes require a much greater force than you could apply with your leg， your car must also multiply the force of your foot. It does this in two ways：
∙Mechanical advantage (leverage)
∙Hydraulic force multiplication
The brakes transmit the force to the tires using friction， and the tires transmit that force to the road using friction also。 Before we begin our discussion on the components of the brake system, we’ll cover these three principles:
∙Leverage
∙Hydraulics
∙Friction
Leverage and Hydraulics
In the figure below, a force F is being applied to the left end of the lever. The left end of the lever is twice as long (2X) as the right end （X）。 Therefore， on the right end of the lever a force of 2F is available, but it acts through half of the distance （Y） that the left end moves (2Y）. Changing the relative lengths of the left and right ends of the lever changes the multipliers.

The pedal is designed in such a way that it can multiply the force from your leg several times before any force is even transmitted to the brake fluid。
The basic idea behind any hydraulic system is very simple: Force applied at one point is transmitted to another point using an incompressible fluid, almost always an oil of some sort。 Most brake systems also multiply the force in the process。 Here you can see the simplest possible hydraulic system：
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Simple hydraulic system
In the figure above, two pistons (shown in red） are fit into two glass cylinders filled with oil (shown in light blue） and connected to one another with an oil—filled pipe。 If you apply a downward force to one piston （the left one, in this drawing）, then the force is transmitted to the second piston through the oil in the pipe。 Since oil is incompressible， the efficiency is very good -- almost all of the applied force appears at the second piston. The great thing about hydraulic systems is that the pipe connecting the two cylinders can be any length and shape, allowing it to snake through all sorts of things separating the two pistons. The pipe can also fork， so that one master cylinder can drive more than one slave cylinder if desired, as shown in here：
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Master cylinder with two slaves
The other neat thing about a hydraulic system is that it makes force multiplication （or division) fairly easy。 If you have read How a Block and Tackle Works or How Gear Ratios Work， then you know that trading force for distance is very common in mechanical systems. In a hydraulic system， all you have to do is change the size of one piston and cylinder relative to the other, as shown here:
汽车制动系统
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Hydraulic multiplication
To determine the multiplication factor in the figure above， start by looking at the size of the pistons。 Assume that the piston on the left is 2 inches （5.08 cm) in diameter (1-inch / 2.54 cm radius）, while the piston on the right is 6 inches （15。24 cm) in diameter (3—inch / 7。62 cm radius)。 The area of the two pistons is Pi ＊ r2。 The area of the left piston is therefore 3.14， while the area of the piston on the right is 28。26. The piston on the right is nine times larger than the piston on the left. This means that any force applied to the left-hand piston will come out nine times greater on the right—hand piston. So, if you apply a 100-pound downward force to the left piston, a 900-pound upward force will appear on the right。 The only catch is that you will have to depress the left piston 9 inches (22。86 cm) to raise the right piston 1 inch （2.54 cm）。
A Simple Brake System
Before we get into all the parts of an actual car brake system, let's look at a simplified syst
em：
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A simple brake system
You can see that the distance from the pedal to the pivot is four times the distance from the cylinder to the pivot， so the force at the pedal will be increased by a factor of four before it is transmitted to the cylinder。
You can also see that the diameter of the brake cylinder is three times the diameter of the pedal cylinder。 This further multiplies the force by nine。 All together， this system increases the force of your foot by a factor of 36。 If you put 10 pounds of force on the pedal， 360 pounds (162 kg) will be generated at the wheel squeezing the brake pads。
There are a couple of problems with this simple system. What if we have a leak? If it is a slow leak， eventually there will not be enough fluid left to fill the brake cylinder， and the brakes will not function. If it is a major leak， then the first time you apply the brakes all of the fluid will squirt out the leak and you will have complete brake failure。