The bicycle brakes you're most familiar with are rim brakes. You pull the brake lever on the handlebar, and a cable transmits that pull to a pair of brakes pads that squeeze against the rim, slowing the bike. There are a couple of different versions of this technology (some terms you might hear include U-brakes, V-brakes, direct pull brakes or caliper brakes). They all do pretty much the same thing and have the same limitations.
There are two main problems with rim brakes. First, the brakes make contact at the outer edge of the rim. If conditions are wet or muddy, it's very easy for the rims to get slippery. Obviously this can seriously affect the performance of the brake. Second, braking power is inherently limited by the small surface area the brake pads have to grab on to. If you're taking a downhill trail at high speed, there's no way rim brakes would have the braking power to slow you effectively.
The more advanced option is a set of disc brakes. These function just like disc brakes on a car. The hub holds a metal disc that normally spins freely along with the wheel. The fork has a caliper that squeezes the disc when the brakes are applied, slowing the bike. Disc brakes work better than rim brakes for a few reasons:
- They're farther away from the trail than the outside edge of the rim, so they're less likely to pick up mud or water.
- Braking power at the center of the wheel translates to greater overall breaking power because of the difference in speeds between the center and outside edge of the wheel.
- You can build larger discs and calipers for more braking power.
Disc brakes sometimes add weight to a bike, but some companies have engineered advanced disc brake systems that weigh less than rim brakes.
You can also choose between mechanical and hydraulic disc brakes. Mechanical brakes squeeze the calipers with the same force that you squeeze the brake lever. Hydraulic brakes use hydraulics to amplify the force, resulting in even greater braking ability.
While the frame, suspension and brakes might be some of the most important parts of a mountain bike, they certainly aren't the only parts. What about the tires, gears and safety equipment? That's coming up next.