How Disc Brakes Work
OK. When I see an article title like this one I have to ask myself one thing. What are they asking me.In simple terms? Or is it asking why do they work at all? Like, how does a balloon work?
In simple terms it works by applying a few simple hydraulic laws. Like a fluid can't be compressed, while a vapor can. So number one. Brake fluid must always be a fluid, never a gass or vapor. And on this one point alone, heat makes all the difference in the world. In fact! Heat energy is the one physical law that makes, or allows the whole system to work at all. At some points it's the enemy, and at others it's why the sytem can even stop the car at all.Any fluid can be changed to a vapor, or a gas, by applying enough heat. So heat is the number one enemy of the brake fluid. Too much and you get a vapor, and no brake pedal. Because remember.Vapors can be compressed, and compressing that gas makes the brake pedal spongy and very dangerous. So the brake fluid must have a very high boiling point, usually way above280 degrees, or more.But normal brake fluid is made from just about the same chemicals your anti-freeze is. So by nature it can and will combine with water. That makes it hdroscopic. And we all know that water boils at 212 degrees. Not good!In fact! Brake fluid will absorb water right out of the air, ruining it if left uncapped in it's bottle.So the fluid is the heart of the system and it must be checked oftem and changed regularly to avoid it failing on a hard stop.Another law is that applting any physical pressure to any point in a hydraulic system will transmit that pressure to all points in the system. Say like when you press on the brake pedal.The pedal is attached to a lever that multiplies the force your leg can apply. That lever is attached the the power booster of the vehicleby a link. The booster again multiplies your foot pressure many times over. That is linked to the master cylinder where all that physical, or mecahnical, energy is converted to hydraulic pressure inside and sent to the wheels in various brake lines made of steel and rubber.By now your foot pressure, not very strong at all, has been multiplied to 2000 psi or more. But we still haven't done the slightest thing to stop the vehicle yet!At the wheel are the real work horses. The brake lines attach to a thing called a caliper. The caliper has a large piston inside it. That's where the brake fluid pressure starts doing its real work. The caliper is firmly attached to the vehicle behind the wheel. So it can't rotate with the wheel. However there is whats called a rotor. A flat plate of metal that is firmly attached to the wheel itself, so it does rotate with the wheel.That rotor rotates within the caliper houseing. In the houseing are the brake pads. One pad is usually attached the the caliper solidly, while the other is attached the the caliper piston. So that one can move in and out with the piston as it respondes to the brake fluid prssure as you push your foot down. The pads pich against the rotor and the real work begins.
Now. I want you to think about something. It's called kinetic energy, or energy in motion.Ever try pushing a car by hand. Not easy! The darned things are heavy.They just don't like to start moving at all. And even after they do start rolling it still takes a lot of umph to keep them moving. In deed! Now think about how much energy it took to get that 3-5 thousand pound hunk of steel up to 50 or 60 mph. A WHOLE BUNCH! And now that you want to stop it? Well? You have to get rid of all that energy.
Yep. That's the way it works. You spent a lot of your hard erned cash getting that monster up to speed, in the gas.And now you have to pay the exact same to get it to stop. IT"S THE LAW. What law? The laws of the conservation of mass and energy. Newton wrote it and we have to obey it!NO CHEATERS ALLOWED!
The law simply says, you can neither create or destroy mass or energy. You can only change it's form.So every hard earned ounce of energy you spent accelerating that monster must now be changed into something else. AND THAT"S CALLED HEAT.
Let's see? 2tons of steel flying at 60mph = rots and rots of energy. And that much energy now must become an equal amount of heat. Oh boy!
So the pads hit the rotors, and all hell cuts loose, every time you stop the car. Hundreds of degrees of heat energy, actually more like calories,develope due to the friction between the rotors and the pads. All that heat must be given up to the air or everything will self destruct the very first time you use the brakes. But don't worry. We've got you covered.The rotors are thick and most have cooling fins. So they start doing there jobs, as committed heat radiators.That's there sole role in life, and they do a pretty good job. They quickly reach massive temperatures and start to disipate it. But still a lot get's into the other parts, like the calipers. And what's in the calipers? That's right. Our old friend the brake fluid.Now you can see why that fluid has to have such a high boiling point. It it didn't, it would boil and become a vapor. And you remember about vapors, don't you?
And why was water in the brake fluid so bad? Well. It drops the boiling point of the fluid drastically. So much so that even just average stopping could cause a brake fade, and failure of the SYSTEM.
