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1、外文原文：THE BRAKE BIBLEBrakes - what do they do?The simple answer: they slow you down.The complex answer: brakes are designed to slow down your vehicle but probably not by the means that you think. The common misconception is that brakes squeeze against a drum or disc, and the pressure of the squeezing
2、 action is what slows you down. This in fact is only part of the equation. Brakes are essentially a mechanism to change energy types. When youre travelling at speed, your vehicle has kinetic energy. When you apply the brakes, the pads or shoes that press against the brake drum or rotor convert that
3、energy into thermal energy via friction. The cooling of the brakes dissipates the heat and the vehicle slows down. Its the First Law of Thermodynamics, sometimes known as the law of conservation of energy. This states that energy cannot be created nor destroyed, it can only be converted from one for
4、m to another. In the case of brakes, it is converted from kinetic energy to thermal energy.Angular force. Because of the configuration of the brake pads and rotor in a disc brake, the location of the point of contact where the friction is generated also provides a mechanical moment to resist the tur
5、ning motion of the rotor. Thermodynamics, brake fade and drilled rotors.If you ride a motorbike or drive a race car, youre probably familiar with the term brake fade, used to describe what happens to brakes when they get too hot. A good example is coming down a mountain pass using your brakes rather
6、 than your engine to slow you down. As you start to come down the pass, the brakes on your vehicle heat up, slowing you down. But if you keep using them, the rotors or drums stay hot and get no chance to cool off. At some point they cant absorb any more heat so the brake pads heat up instead. In eve
7、ry brake pad there is the friction material that is held together with some sort of resin and once this starts to get too hot, the resin starts to vapourise, forming a gas. Because the gas cant stay between the pad and the rotor, it forms a thin layer between the two whilst trying to escape. The pad
8、s lose contact with the rotor, reducing the amount of friction and voila. Complete brake fade.The typical remedy for this would be to get the vehicle to a stop and wait for a few minutes. As the brake components cool down, their ability to absorb heat returns and the next time you use the brakes, th
9、ey seem to work just fine. This type of brake fade was more common in older vehicles. Newer vehicles tend to have less outgassing from the brake pad compounds but they still suffer brake fade. So why? Its still to do with the pads getting too hot. With newer brake pad compounds, the pads transfer he
10、at into the calipers once the rotors are too hot, and the brake fluid starts to boil forming bubbles in it. Because air is compressible (brake fluid isnt) when you step on the brakes, the air bubbles compress instead of the fluid transferring the motion to the brake calipers. Voila. Modern brake fad
11、e.So how do the engineers design brakes to reduce or eliminate brake fade? For older vehicles, you give that vapourised gas somewhere to go. For newer vehicles, you find some way to cool the rotors off more effectively. Either way you end up with cross-drilled or grooved brake rotors. While grooving
12、 the surface may reduce the specific heat capacity of the rotor, its effect is negligible in the grand scheme of things. However, under heavy braking once everything is hot and the resin is vapourising, the grooves give the gas somewhere to go, so the pad can continue to contact the rotor, allowing
13、you to stop.The whole understanding of the conversion of energy is critical in understanding how and why brakes do what they do, and why they are designed the way they are. If youve ever watched Formula 1 racing, youll see the front wheels have huge scoops inside the wheel pointing to the front (see
14、 the picture above). This is to duct air to the brake components to help them cool off because in F1 racing, the brakes are used viciously every few seconds and spend a lot of their time trying to stay hot. Without some form of cooling assistance, the brakes would be fine for the first few corners b
15、ut then would fade and become near useless by half way around the track. Rotor technology.If a brake rotor was a single cast chunk of steel, it would have terrible heat dissipation properties and leave nowhere for the vapourised gas to go. Because of this, brake rotors are typically modified with al
16、l manner of extra design features to help them cool down as quickly as possible as well as dissapate any gas from between the pads and rotors. The diagram here shows some examples of rotor types with the various modification that can be done to them to help them create more friction, disperse more heat more quickly, and ventilate gas. From left to right. 1: Basic brake rotor. 2: Grooved rotor - the grooves give more bite and thus more friction as they p
