Hydraulic disc brake. photo: Wikimedia Commons
Read this article in connection with:
Why Disc Brakes?
Mountain bikes first used cantilever brakes; next, direct-pull brakes, which avoided the risk of snagging a transverse cable on a knobby tire. But mountain-bike rims are often wet, muddy and warped, making for problems with any rim brakes.
Disc brakes have become increasingly popular on mountain bikes and are gaining some popularity for other bicycles..
John Olsen, expert mountain bike rider and engineer (and who supplied the photo at the left here), reports:
"When I got my first mountain bike disc brake that worked, the advantages off road were so overwhelming that I changed every bike I had over to them as rapidly as possible. Hub and rim and spoke design essentially didn't change, except for disc mount provisions on the hubs."
For bicycles used on-road, the advantages of disc brakes aren't as compelling. To some extent, they are a fashion statement, imitating motor-vehicle practice.
But be aware: disc brakes can't be retrofitted without frame modification. A front disc brake stresses the fork heavily and can tear the front wheel out of the dropouts unless special measures are taken.
Advantages and Disadvantages of Disc Brakes
- They are strong.
- They are little affected by wet conditions.
- They don't get clogged with mud and snow.
- They aren't affected by rim damage or out-of-true.
- They don't risk brake shoes' damaging the tire or diving under the rim and locking the wheel.
- Being external to the hub, they don't impose special lubrication requirements like a coaster brake, or risk contamination by lubricants like an integral drum brake, or overheat the hub on long, steep downhill runs.
- They also dissipate heat without overheating the tire -- of special importance when used as a downhill drag brake on a tandem or cargo bike.
- They don't wear rims -- especially an issue in sand and mud, or with carbon-fiber composite rims. They don't leave black dust (wear particles) on aluminum-alloy rims, to get all over your hands when you remove or replace a wheel..
- They allow switching out wheels with different-sized rims on the same frame, so you can switch from skinny road tires to fat off-road tires while maintaining the same bottom-bracket height.
- Hydraulically-actuated disc brakes avoid "stickiness" due to cable friction.
- Some disc brakes with hydraulic actuation are self-adjusting to compensate for pad and rotor wear.
- The rotor is easily replaced if worn or damaged. Adapters make it possible to replace most calipers, even of types no longer manufactured.
A front disc brake stresses one blade of the front fork very heavily, requiring a stronger, heavier fork, resulting in a bumpier ride with a non-suspension fork, and if a fork isn't quite rigid enough, producing 'brake steer".
A front disc brake caliper behind the fork blade generates a powerful force tending to loosen a quick release and pull the wheel out of the fork. Special hub and fork designs are needed to surmount this problem.
- Disc brakes are generally heavier than rim brakes.
- Disc brakes are more complicated, expensive and difficult to maintain than rim brakes or drum brakes.
- Some disc brakes are grabby. This problem is likely if dirt get trapped between the calipers.
- Disc brakes require special fittings on the frame and fork, and special hubs.
They can interfere with baggage racks and fender stays.
- The disc is vulnerable and easily bent. Other hub brakes do not have this weakness.
- Hub flange spacing is often reduced, resulting in a weaker wheel.
- Adjustment may be difficult to avoid calipers' rubbing on the rotor when the brake is not in use.
- The disc gets extremely hot and can cause injury if touched, and melt nearby plastic or cloth items. Care must be taken to use the correct brake fluid with a hydraulically-actuated disc brake, so it does not boil, resulting in loss of braking. (see additional comments below).
All in all, disc brakes are advantageous on bicycles which have front suspension and are ridden in mud and snow. Disc brakes are less suitable for bicycles where light weight is most important and the rim can do double duty as a brake disc. A large rear disk brake can serve well as a downhill drag brake on a tandem or cargo bike.
Disc brake safety
The disc-brake rotor wears, and becomes thinner. A thin rotor is weak and prone to warping or breakage. With most brands, if it is less than 1.5 mm thick, or seriously concave, it should be replaced. Moment Industries specifies 1.7 mm and Hayes, 1.52 mm. See Aaron Goss's Web page, linked below, for details. He also sells thicker rotors for cyclists who are not weight weenies.
Pads also wear and must be replaced from time to time. Wear limits are different for different pads -- check manufacturer's specifications.
The thickness of a rotor cannot be measured directly with a vernier caliper, due to the concavity.
Vernier caliper (image by Lucasbosch, from Wikimedia Commons)
You need to use a micrometer caliper or you could make a simple accessory tool by cutting and bending a spoke so a length of it lies flat along either side of the rotor. Then a vernier caliper or digital caliper could be used. Subtract twice the thickness of the spoke, and you have your measurement.
Micrometer caliper (image by Lucasbosch, from Wikimedia Commons)
Disc brakes are traditionally mounted on the left side of the bicycle. Some motorcycles have dual disc brakes, one on each side of the front wheel, but the added weight would be undesirable on a bicycle. One-sided mounting results in an unbalanced load on the fork, potentially leading to handling and safety issues. The load on the fork is heavy because the torque from braking is taken up at a relatively short distance from the hub.
As already noted, a front disc brake with the caliper in the usual position behind the left fork blade exerts a strong force tending to pull the hub axle out of the dropout. It has been conclusively shown that the alternation of force upward from weight and downward from the brake can loosen the quick-release of a front hub.
A suspension front fork and a disc brake work well together because the fork must have large-diameter sliders (lower part of each fork blade) and the left slider alone can easily bear the load from the disc brake. However, a through-axle front hub is still necessary to avoid the possibility of front-wheel loss. A disc brake caliper placed ahead of the fork blade would tend to pull the hub axle into rather than out of the fork, but is not usual.
There has been a recall of over 1 million Trek bicycles where the quick-release lever can open more than 180 degrees and catch on the disc rotor, resulting in ejection of the wheel. This problem is probably not limited only to Trek bicycles. It might also be possible for the lever to catch on the spokes of a wheel without a disc brake.
A front drum brake or Rollerbrake, by way of comparison, also exerts an unbalanced load on the fork, but does not tend to pull the wheel out of the dropout.
Long descents generate significant heat, so much so that first-generation disc brakes had a problem with failures due to the glue on the pads melting and brake fluid boiling. Shimano has gone to great lengths to overcome these problems, including heat sinks on the pads and rotors with aluminum cores and cooling fins. Shimano explains that disks are the future for carbon rims, as no one has found a good solution for rim brakes on carbon rims with a carbon braking surface. Shimano was able to convince the UCI, which had banned disks on road bikes, to change its position after providing evidence of the large number of race crashes caused by the shortcomings of caliper brakes on carbon rims.
Installation and maintenance
Disc brake rotors are available in several sizes. While it is possible to mix and match brands of rotors and calipers, the size of the rotor and caliper must match. Larger rotors are a bit heavier, but dissipate heat more readily and make for stronger braking. Smaller rotors, generally for "road" disc brakes, are lighter.
Most mechanical disc brakes use the same long-cable-pull brake levers as direct-pull brakes. Other brake levers do not pull enough cable, resulting in weak braking. "Road" disc brakes, on the other hand, work with ordinary brake levers and brake-shift combination levers.
There are several common ways to attach the rotor to the hub. Common ones are the 6-bolt ISO, Shimano Centerlock splined, Hope 3-bolt and Rohloff 4-bolt. Several older patterns are no longer in production, making it hard to find a replacement rotor for an older hub. Note that the Shimano Inter-M fitting, used on Shimano Nexus and Nexave hubs, is for the Shimano Rollerbrake, and not for a disk-brake rotor. Shimano Alfine internal-gear hubs do have a Centerlock disc-brake fitting.
There are also different ways to attach the caliper. A large variety of adapters is available, making many combinations possible.
Rotor-caliper alignment is critical so the rotor turns freely except when braking. Though some calipers are adjustable, there is little flexibility in adding or removing spacers at the left side of the hub to address wheel dishing, dropout spacing or chainline.
All in all, there is way too much variety among disc brakes to cover here, and it is most prudent to stay with manufacturers' suggested combinations. Chapter 11 of the 7th Edition of Sutherland's Handbook for Bicycle Mechanics has very extensive coverage of disc brakes, and especially of the many types of fittings and adapters. Any good bicycle shop will have a cop, or you might buy one for yourself. John Olsen has good technical information in the book High Tech Cycling, and that part of the book is online as a preview. (But, buy the book -- there's more good reading there too!). Park Tool has good information online about installing and adjusting disc brakes. See links below.
Thanks to John Olsen and Justin Haber for help with this article!
If you would like to make a link or bookmark to this page, the URL is:
Last Updated: by John Allen