Your Go-To Guide To Disc Brakes For Bikes
It’s no wonder: With their excellent stopping power, superior speed modulation, and better all-weather performance than traditional rim brakes, disc brakes make huge impacts on your ride. But they can be heavier and sometimes challenging to maintain. Don’t let that stop you from investing in them. Here are the parts of disc brakes you should know, and some quick fixes to their most common mechanical problems.
What’s even better is that great braking power is now available at almost every price point. And while disc brakes make huge impacts on your ride, they can be heavier and sometimes more challenging to maintain. So whether you’re a holdout or already rocking them, here’s a basic guide on disc brakes and how to make the most of them.
Types of Disc Brakes
There are two main styles of disc brake: mechanical (or cable-activated), and hydraulic.
Cable-activated brakes are what they sound like: The brake levers are attached to cables, which activate braking. The hydraulic version replaces the cables with hydraulic fluid in a fully sealed line. When you brake, the pressure forces the fluid to move into the caliper, pressing the pads against the disc.
Hydraulic Line Maintenance
If your disc brakes have a spongy and indistinct feel or fade on long descents, it’s likely due to bubbles in the line or contaminated fluid.
The fix is a brake line ‘bleed,’ where the old hydraulic fluid is flushed and replaced with fresh fluid. This is a skilled repair, best left to a shop mechanic. You need to use the right fluid, which is matched to your brake for proper heat management.
SRAM recommends bleeding hydraulic disc brakes every six months. Shimano’s official user manuals do not specify a service interval but does say to replace the fluid when it becomes discolored.
Disc Brake Calipers
Brake levers are attached by the brake lines to calipers located on both the front and rear discs. Calipers contain opposed pistons that sit on either side of the rotor; pressure from the brake line engages these pistons, which push the brake pads inward to contact the disc. The resulting friction slows the bike.
Disc Brake Caliper Maintenance
If your rotor doesn’t spin freely (resulting in rubbing, grinding, or squealing), the caliper might be misaligned.
Fix this by loosening the two bolts attaching the caliper to the frame or fork just enough so the caliper can move slightly side to side. Wiggle the caliper to make sure it moves freely, then pull the corresponding brake lever, hard. This will clamp the caliper to the rotor. Hold the brake lever down to keep the caliper in place, meanwhile tightening the top bolt until snug. Tighten the bottom bolt until snug and then re-tighten the top bolt to torque spec, then the bottom one.
Rotors come as small as 140 millimetres in diameter for road and cyclocross applications, all the way up to 205mm for downhill mountain biking. Generally, road and cyclocross use 140-160mm, XCO uses 160mm, trail riding uses 160-180mm (sometimes a mix, with the larger rotor up front), enduro uses 180mm, and DH uses 200-205mm.
Larger rotors are able to dissipate heat over a larger surface area, but are heavier, so you’ll want the smallest rotor you can get away with for the type of riding you generally do.
Disc Brake Rotor Maintenance
If you’ve adjusted your caliper but still hear rubbing, check your rotor: Sometimes, they warp from a hit or even just excess heat.
To find out if your rotor is warped, spin the wheel and sight along the rotor path. (Looking through the caliper provides a good reference.) If the rotor moves in a wavy fashion, like a wheel that’s out of true, it’s warped.
Often, but not always, warped rotors can simply be bent back.
Disc Brake Pads
Brake pads are found inside the calipers. They’re designed to clamp down on the rotor at high speeds, which means their main job (besides stopping your bike) is to hold up under heat and friction.
Resin Vs. Sintered Brake Pads
There are two main types of brake pads. Resin brake pads (also called organic) are composed of organic materials like glass, rubber, and fibrous binders bonded together with resin. Sintered brake pads (also known as metallic) are made of metallic grains that are bonded together at high pressure.
In terms of feel, resin pads are quieter and have a stronger sense of bite. They’re better at managing heat, but they can fade as heat builds up. They also wear more quickly, particularly in muddy conditions. Sintered pads are the choice for riders who do mostly downhill mountain biking. They produce more heat but are less susceptible to its effects; and they last longer under heavy use and in wet conditions.
Resin pads are what most brakes come with, but you should consider switching to metallic if you’re bigger, ride downhill terrain, or ride mostly in wet weather.
Disc Brake Pad Maintenance:
If you’ve checked your caliper and rotor but still hear squealing when braking, it can mean that the braking surface (including pads and/or rotors) is contaminated and needs cleaning.
Detach the wheel and use isopropyl alcohol on a clean rag to wipe off the rotor, then remove the pads and clean them as well. It’s also a good idea to wear disposable latex gloves; you’re trying to get dirt and oil off the braking surface, not add more.
Make sure to let the pads dry completely before reinstalling.
SRAM recommends replacing pads when the total thickness of the backing plate and the pad material is under 3mm. Shimano says that when the pad material alone is less than half a millimetre thick, it needs to be replaced. If you switch pad type (resin to metallic, say), swap your rotors as well; the pads and rotors pair by “bedding” small amounts of the pad material on the rotor, which means an old rotor won’t get optimal performance with a new pad type because it’s already bedded to the other kind of pad material.