Has either one of these scenarios happened to you?
You line up to race through a section of tight trail. Your buddy gets the hole shot and beats you to the first corner, so you begin riding a little more aggressively in an effort to catch him. Because you are accelerating harder you are braking harder as you enter each corner and after a few corners you notice your brakes don't seem to be as effective and you can't slow down as quickly.
While riding in the mountains, you have climbed towards the top of a long steep hill. As you turn back down you are surprised to see how high you are. When gravity takes control you begin applying the brake to keep your speed in check. About midway down you begin to gain speed rapidly while applying the same amount of brake. Naturally you try to apply more brake but it doesn't help- instead you continue to gain speed until reaching the bottom of the hill.
If you have experienced either one of these scenarios you understand how quickly braking power can be lost. The cause of this is due to extreme heat build up in the brake pads, caliper, and rotor. This loss in braking power is called brake fade. Brake fade occurs in a series of stages that all have warning signs, which include the smell of hot brake pad material, a mushy feeling in the brake lever and reduced stopping power.
First the basics. When braking, the pad pinches against the rotor. This causes friction, which in turn slows the sled down. Friction generates heat and this is where the problems are initiated. In regular conditions of on and off breaking or short hard braking intervals, the brake assembly usually will cool without major problems. Cooling comes from underhood air, the jackshaft and chaincase absorbing heat from the rotor caliper assembly and engine coolant on some models.
In cases such the scenerios described above, if the brake system is not allowed sufficient time to cool, a chain reaction of events can occur.
1. The excessive heat build up between rotor and brake pads begins to transfer to the brake piston and caliper assembly. As the piston and caliper assembly become hotter, the temperature of the brake fluid rises, causing brake fluid to expand and even boil.
2. As fluid expands and/or boils it begins to compress rather than transmitting equal brake pressure from the master cylinder to caliper, thus reducing mechanical or hydraulic advantage.
3. When the brake pad surface temperature gets hot enough, the pads begin to burn and glaze over, causing reduced ability to grip the rotor.
4. If the brake rotor warps and glazes over from extreme heat, it further reduces friction; which causes braking power to be near zero.
WAYS TO AVOID THE PROBLEM
1. Replace worn pads and brake fluid at regular intervals and check rotor for signs of excessive heat, wear and warpage. Old break fluid absorbs moisture, which lowers the boiling point and increases expansion due to heat. Once brake fluid has been boiled, it must be changed to eliminate a mushy brake feeling.
2. Learn to recognize situations that may require long periods of excessive brake use and brake intermittently, allowing brakes to cool. Apply enough brake pressure to allow maximum braking power but avoid locking the track which could cause loss of control.
3. If signs of brake fade begin to occur then stop immediately or at least slow and let brakes cool.
4. Better braking efficiency can be obtained by using higher quality pads, brake fluid, brake lines and rotors designed for extreme conditions.
There are two important factors affecting brake performance. First is the owner's willingness to ensure peak brake system efficiency through proper maintenance and setup. Second is the rider's ability to predict extreme situations before they occur and know how not to abuse the brakes.