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Brake Fade
Brake fade typically happens for one of two reasons; cooked brake fluid or cooked pads.
Cooking the fluid: Brakes get hot. On track days, brakes get REALLY HOT. The heat travels from the rotor and pad, to the caliper. Since fluid is in the caliper, the fluid gets hot, and then it boils. How hot does it get before it boils? How about 500°F? You can read additional information on brake fluid here. When brake fluid boils, it turns from a non-compressible liquid, to a compressible gas. That compressible gas is what causes the brake pedal to have a spongy feel. If the fluid continues to boil, the pressure required to squeeze the calipers will be greater than the brake fluid can transfer. This results in the brake pedal going all the way to the floorboard. After the fluid cools, the gas will change back to a liquid, and pedal feel will return to normal.
Unfortunately, after the fluid has boiled once, it’s never the same. A single heat cycle is enough to begin breaking the fluid down, reducing the boiling point. That’s why many people flush their brake fluid after every track event. Brake fluid also degrades with the simple passing of time. That’s because it’s hygroscopic. Since it absorbs moisture from the surrounding air, brake fluid should only be replaced from a brand new sealed container.
Over the course of a few short years, moisture can reduce the boiling point of brake fluid by 25%. Manufacturers commonly recommend flushing the system every other year. When flushing brake fluid, it’s important to replace all the old fluid. This is done by bleeding the brakes starting at the right rear caliper and working your way to the caliper nearest to the master cylinder. Always flush until the fluid comes out clear. Two liters is usually enough to flush the hydraulic system in most vehicles.
Cooking the pads: The friction material in a brake pad is composed of two main components; the organic/Kevlar/metallic/ceramic powder, and the glue that’s mixed with the powder and used to compress the concoction into the shape of the friction pad. The formula used in the compounds is what determines the operating temperature range of the brake pad. Street pads will work well from a ambient temperatures, up to a specific temperature. Race pads are ineffective at colder temperatures with fantastic stopping power when heated. Regardless as to what ad you may have read, pads have not been invented that work well in all conditions.
With street pads, after the temperature range has been eclipsed, the binding material (glue) will melt and vaporize. This vapor will become trapped between the pad and the rotor. The vapor allows the pad to float on a layer of vapor. Once the pad starts gliding on the layer of gas, additional pedal pressure is needed to slow down. This increases braking temperatures, which has the dual disadvantage of helping to kill what’s left of the pad and boiling the brake fluid. This is why most street pads won’t last more than 1 or 2 days at a track event. Although unknown to most poseurs, this is the purpose of slotted or cross-drilled rotors - to allow the gas to escape. Or, that was the purpose. It’s true that during actual race conditions, street pad life is measured in laps. However, that’s on race cars. For street cars, pad formulation has improved to the point where high performance street pads cannot be cooked at anything near legal speeds.
Race pads are designed to operate at a much higher temperatures. They have an aggressive friction material, which causes more rotor wear. The benefit of wearing both the rotor and the pad is, this stops the vehicle with less pedal and pad pressure in a shorter time frame. This is how Mercedes-Benz street cars can carry the weight AND stop in short distances. This lower pad pressure reduces the amount of heat generated. In addition, the glue in race pads has a much higher melting point. Since the glue doesn’t melt, there less gassing of the pads. The down side to race pads is poor cold braking, more brake dust, and they are loud.