Motoring corner: “Brake Fading”

What are brakes? Simply put, they are the devices that help to slow you down. 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 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 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 form to another. In the case of brakes, it is converted from kinetic energy to thermal or heat energy. 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 turning motion of the rotor. If you ride a motorcycle or drive a high speed 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 steep road using your brakes rather than your engine to slow you down. As you start to come down the, 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 will not absorb any more heat so the brake pads heat up instead. In every 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 vaporise, 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 pads lose contact with the rotor, reducing the amount of friction leading to break fade. The typical remedy 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, they seem to work just fine. Talk of brake fade, this phenomenon was or is more common in older vehicles. Newer vehicles tend to have less out gassing from the brake pad compounds but they still suffer brake fade. Why is this so? Its still to do with the pads getting too hot. With newer brake pad compounds, the pads transfer heat into the callipers 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 callipers. So how is the brake fade problem addressed? For older vehicles, you give that vaporised 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 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 vaporising, the grooves give the gas somewhere to go, so the pad can continue to contact the rotor, allowing 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. This is to channel 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 but then would fade and become near useless by half way around the race track. (to be continued) motoringcorner@live.co.uk

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Motoring corner: “Brake Fading”