When riders think about going fast, there is a tendency to correlate horsepower to speed. To some degree this is true. Drag racing is a good example. From start to finish in a drag race, its mostly about horsepower and the ability to apply the available horsepower for maximum speed. In the case of dirt bike racing, such as motocross and enduro, going fast requires not only the ability to use the available horsepower but to also utilize the suspension and the brakes.
I am not saying that suspension and brakes are not important in drag racing, but the dynamics involved in dirt biking are vastly different from the drag racing example. On a motocross track or mountain trail, knowing when to brake, how hard to brake and how long to brake are equally as important as acceleration. For example, a faster rider may have better lap times by being able to brake later into corners and transition off of the brakes sooner on the exits. Of course, this article is not intended to be a riding techniques tutorial. Rather, I would like to point out how to maximize braking performance by focusing on the bike itself.
On a modern dirt bike, particularly motocross bikes, there are two master cylinders. One for the front and one for the rear. In a sense, a master cylinder is the engine of a braking system. When a lever is depressed, the piston of the master cylinder pushes on the brake fluid within. The resulting pressure acts directly on the remaining parts of the system, causing the caliper piston to move in concert. Internal damage to a master cylinder can cause either difficulty with piston movement or lack of movement all together. It depends on the mode of failure.
The most typical failure mode is seal failure. Seal failure on the internal piston will cause fluid to leak past the piston when the brakes are operated. When this happens, the force being transmitted to the brake calipers is greatly reduced if not lost altogether. Another possible symptom is a seal can roll over itself causing difficulty in piston movement. Realize the fluid leakage described here is strictly internal therefore there will be no visual clues.
External leaks can occur when outer seals fail. Depending on how severe the problem, it may take a little time for braking performance to suffer. The combination of insufficient brake fluid and the introduction of air into the system eventually (or quickly) losing braking function altogether. If you find that you have a leak, service your brakes immediately.
Check to make sure the calipers are in alignment with the rotors. The calipers are somewhat vulnerable to being hit with rocks and can also be misaligned from crashing. Whatever the case, a misaligned caliper will not be able to squeeze the pads onto the rotor evenly. This is often made evident by brake pads that are worn unevenly.
Another possible problem is sticking caliper pistons. Internal mechanical damage or dirt can make it more difficult for the pistons to move in and out. You can check for this by watching the caliper movement while operating the brake lever. The caliper should move to the rotor freely with no effort and should also relax simultaneously with the release of the lever.
For your brakes to work properly, it is important to have good brake fluid. This means a few things.
Brake pin wear can cause resistance to the applied braking force. Because the brake pads ride back and forth on a limited area of the brake pin, a divot or flat spot will eventually form in the pin. Once this divot forms into the pin, the brake pads are no longer able to move as freely as they once could. This means extra braking effort is required by the rider. Inspect brake pins regularly and replace them if needed.
In addition to a worn brake pin, a dirty brake pin can exhibit the same problem. Dirt and mud are abrasive and will inhibit the needed sliding motion of the pads across the brake pin. Keep the brake pins clean and smooth. In many cases, all that is need is regular cleaning. I occasionally polish my brake pins with a fine Scotch Brite pad and WD-40.
Building on what was said about brake pins, the brake pads follow suit in the same way. Here I am talking about the hole in the pads where the brake pin goes through. While the likelihood of the holes being a problem is rare, be aware that damage to a pad hole could inhibit sliding just as with a bad brake pin. For example, distortion at a hole could exist as a manufacturing defect or by impact from a rock.
Cleanliness of the pads is also crucial. If the pads become contaminated by chain lube, premixed gasoline, WD-40 or "who knows what?", braking power will be reduced thereby forcing the rider to exert more braking force. Arm pump anyone? Keep the brake pads clean using an appropriate brake parts cleaner or soap and water. After washing, some benefit can be gained by resurfacing the pads. This can be done by rubbing the pad faces back and forth on medium grit sand paper (on a flat surface, something in the 200 grit neighborhood). After doing so, the resulting dust should be cleaned from the pads.
The type of pad is another variable affecting braking power. In my opinion, full metallic pads are the way to go for aggressive braking performance.
Periodically inspect the rotors to insure neither are bent or warped. A straight edge can be used as an aid to see how flat they are. Also make sure the rotors are clean. Rotor cleanliness is as important as clean brake pads. Not only will a contaminated rotor reduce braking power, it will also contaminate the brake pads.
Brake rotors can become glazed over time. Once a rotor is glazed, braking power will be affected. You can deglaze your rotors by honing the surfaces with 180 grit (or similar) sand paper or emery cloth.
Rotor size, or more correctly, rotor diameter, can be increased to improve braking power. Obviously the ability to do so depends on availability of parts and limitations on the bike itself. An increase in rotor diameter provides for more resistive torque on the wheel. Typically this would be done on the front. The main advantage of this is the rider can achieve more powerful braking while using less effort at the controls. Most modern stock hydraulic braking systems are certainly great as is, but increasing rotor size can be of benefit. It depends on the rider's needs. A rider with a very strong grip may not need to do anything, while a rider with a weaker grip may find this to be a great option.
Lever length can possibly be increased, depending on the parts available and the design limitations of your specific bike model. With a longer lever up front, brake pull becomes easier, similar to increasing rotor size. Adding lever length can possibly have a negative consequence on reach though. Given the same master cylinder stroke, the end of a longer lever has a greater swing distance.
Without changing to a longer lever, it is possible to increase leverage by the proper positioning of the lever. This depends on the current lever placement and the manner in which a rider uses the lever. Moving the lever in toward the center of the handlebars places the fingers more toward the end providing more leverage. Of course, this may also mean that you are forced to use only one or two fingers. But, for performance dirt bike riding, this is exactly what you should be doing.
Your brakes get a lot of use, especially if you are a motocross or enduro racer. Perhaps you just ride a lot. Whatever the case, poorly maintained pivot points will eventually haunt you. Even before a part failure becomes evident, braking effectiveness may degrade slowly over time without you realizing it. Failure to clean and lubricate the pivot points on the lever and pedal can result in excessive friction in the pivot points which contributes to increased rider effort. Neglected pivots will also result in accelerated wear of pins and bushings.
The whole point of all this is to create awareness that the setup condition of your brake system can effect braking performance. Neglected or improperly maintained parts will wear out more quickly and can also cause decreased braking power which transmits to increased rider effort.
On the other hand, keeping the brake system in tip top condition will maximize power and effectiveness. Power and effectiveness can also be improved by proper riding technique, proper controls setup and by using more effective and better quality replacement parts.
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