Front and rear disc brakes consist of a caliper that attaches to the bike frame and a metallic disc rotor that attaches to the wheel hub. When you squeeze the hand lever, pistons in the caliper force brake pads against the sides of the rotor. Disc brakes are stronger than rim brakes and perform in wet conditions. But the basic design of the system and the type of power used to apply the brakes pose certain problems.
Design
The problem with disc brake design is the position of the disk caliper in relation to the wheel axle. When you apply the brakes, the force of the brake pads against the disc rotor pushes the wheel downward toward the ends of the forks or rear frame arms of the bike. An inexperienced rider may find himself lunging over the handlebars or sliding the rear wheel out if he applies one brake with full force without applying the other brake at the same time. For this reason, it's important that you become familiar with the type of disc brakes on your bike and the level of braking each type provides.
Types
Disc brakes come in three types: open hydraulic disc, closed hydraulic disc and cable disc. Open and closed hydraulic disc brakes use brake fluid and hydraulic lines to transmit energy from the brake lever to the disc brake caliper. Cable disc brakes work in the same way as rim brakes that transmit energy mechanically to the caliper when you apply the brake. Hydraulic brakes are stronger than cable brakes and you have to learn how much force you need to slow the bike without spilling or skidding the rear wheel. In most cases, it takes less time to become accustomed to using cable disc brakes.
Rotors and Pads
The majority of disc brake rotors have drilled vents that reduce weight and allow air to cool the rotors and brake pads. Vented rotors are best in conditions that call for using the brakes frequently, such as on courses or trails that have numerous tight turns in succession. The downside is that the vents can become clogged with dirt or mud, which may cause the brakes to work less efficiently or fail completely. Some disc brake systems have solid rotors that provide more braking surface but run hotter, causing the brake pads to glaze and become ineffective when applied frequently in a short period of time.
Vulnerability
Similar to the gear system on your bike, the components of a disc brake system are vulnerable to damage. Hand levers can be damaged in a number of ways, such as spills or impacts. Hydraulic lines and cables are not designed to withstand a direct impact or cuts from sharp objects. Certain types of spills or incidents that include an impact can warp a disc rotor or knock the caliper out of alignment. Disc brake pads are designed to work in wet conditions, but lose efficiency when subjected to oil or other fluids common on roadways. If you find it takes more force to slow or stop your bike, it's possible the brake pads have accumulated oil or other fluid and need to be replaced.
Repairs and Replacements
Replacing damaged hydraulic lines or cables is not particularly complicated for the average DIY mechanic. You can fill hydraulic brake systems with fresh fluid and bleed the brakes to eliminate air bubbles. Remove and replace damaged rotors, calipers and brake pads using metric tools. Many competition riders carry spare parts and the tools needed to remove and replace a damaged component. You can expect your disc brakes to perform reliably with normal use on the street or off-road. Becoming familiar with your disc brake system and keeping the appropriate tools on hand can save the cost of a repair shop.
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