How do brakes work step by step?
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 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.
Advertisement
Simple hydraulic system
Two pistons are fit into two glass cylinders filled with oil and connected to one another with an oil-filled pipe. If you apply a downward force to one piston, 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.
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.
Hydraulic multiplication
To determine the multiplication factor, 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).
Next, we'll look at the role that friction plays in brake systems.
Advertisement
Today’s Wonder of the Day was inspired by Nya. Nya Wonders, “ How do car brakes work ” Thanks for WONDERing with us, Nya!
What do you think about when you're riding in the car with friends or family members? On your way to school, you might be thinking about the test you have that day or what's for lunch. On the way home from soccer practice, you might be WONDERing what's for dinner or how you're going to get all that homework finished before tomorrow.
You probably don't give much thought to the car itself. Does it need an oil change? How does the engine work? Do the tires need to be rotated? Unless something breaks down, we tend to take cars and how they work for granted. They simply get you from one place to another quickly.
If a deer runs out in front of your car, though, you might start thinking about how one part of the car works: the brakes. When the driver slams on the brakes, you'll be glad that they quickly bring the car to a stop, preventing you from a nasty collision.
If you give it some thought, brakes are an amazing invention. If you're riding a scooter and you need to slow down, you can put out your feet and drag them along the ground. But what about whizzing down the highway at 55 miles per hour? Putting your feet out on the highway wouldn't do much good, would it?
So how can a light push on a car's brake pedal slow a speeding car to an abrupt stop? Is it magic? Of course not! It's science.
A car in motion has a lot of kinetic energy, which is energy of motion. To stop a car, the brakes have to get rid of that kinetic energy. They do so by using the force of friction to convert that kinetic energy into heat.
When you press your foot down on the brake pedal, a connected lever pushes a piston into the master cylinder, which is filled with hydraulic fluid. That hydraulic fluid gets squirted along a system of pipes into other, wider cylinders positioned next to the brakes on each wheel.
This hydraulic system multiplies the force of your foot on the brake pedal into enough force to apply the brakes and make the car stop. The brakes themselves are usually one of two types: disc brakes or drum brakes.
Many modern cars have disc brakes on the front wheels and drum brakes on the rear wheels. More expensive models may have disc brakes on all four wheels. Only very old or very small cars tend to have drum brakes on all four wheels.
Disc brakes consist of a brake disc, a brake caliper, and a brake pad. When the brake pedal is depressed, the hydraulic fluid causes the brake caliper to press the brake pad against the brake disc. The rubbing of the brake pad against the brake disc generates friction, which converts kinetic energy into heat in the brake pad.
How much heat? A lot! Stopping a speeding car can heat the brakes to 950º F or more! To withstand such heat, brake pads must be made of special materials that won't melt at such high temperatures. Some of those special materials include composites, alloys, and ceramics.
Drum brakes also use friction but in a slightly different way. Drum brakes consist of a brake drum and brake shoes. The hollow drum turns with the wheel. When the brake pedal is depressed, a hydraulic cylinder pushes brake shoes with friction linings against the inner surface of the brake drum, creating friction and thereby slowing the wheel.
Comments