Bicycle Disc Brakes
Advantages of Disc Brakes
Disc brakes excel in 2 areas:
They stop better in wet or muddy conditions.
They dissipate heat in a better place--the disc. Too much brake-induced heat on the rim can cause real trouble.
Therefore, good quality disc brakes are best in lousy weather and when you have to brake a lot--like going downhill.
Disc brake disadvantages, relative to V brakes, are:
Disc brakes are much more expensive. At the same price, a bike with disc brakes will have cheaper components than a V brake bike.
Disc brakes weigh more.
Disc brakes are more complex, especially hydraulic disc brakes.
Disc brakes have more problems that are harder to cure, especially if you're a long way from a bike shop.
Cheap disc brakes work worse than good V brakes (but cost more). Only a few cable-actuated disc brakes work satisfactorily, relative to V brakes, and hydraulic disc brakes cost a lot and are more finicky.
Many frames, forks, and hubs do not have attachments for disc brakes.
The competition--V brakes--work great and are cheaper, simpler, lighter, and more durable.
So, think long and hard before you opt for disc brakes. Unfortunately, the trend among mountain bike manufacturers is to offer upper end bikes with disc brakes and with frames and forks that are not compatible with V brakes.
Spacing and Compatibility Issues
Disc brake calipers bolt onto the frame and fork. The rotors bolt onto the hub. Prior to 2000, there was little uniformity in bolt spacing. Unless you know exactly what you are doing, do not buy old, used disc brakes, or attempt to retrofit an old bike with new disc brakes.
Almost all current disc brakes conform to the "International Standard" (IS).
On forks and frames, IS caliper mounts are unthreaded and are drilled perpendicular to the plane of the bike. (You can look into the hole and see the spokes.)
Some forks, particularly Manitou (Answer) forks, do not use IS mounts. On "post mount" forks, the caliper mount is threaded and drilled parallel to the plane of the bike.
IS mount caliper holes on both front and rear are 51 mm apart, center to center.
Post mount caliper holes are 74 mm apart, center to center.
Before buying after-market disc brakes, study and measure your mounts so you will know what you need. Most after-market disc brakes come with adapters to fit both IS and post mount standards, but you should ask before making the purchase.
Current disc rotor mounts (where the rotor fits on the wheel hub) come in 2 standards:
IS rotors connect to the hub with 6 bolts (which usually require a torx wrench).
Shimano Center Lock splined rotors connect to the hub with a lockring that is very similar to a cassette lockring and which accepts a cassette lockring tool (Park FR-1).
You may replace an old rotor with one with a different diameter. Some feel that the larger diameter rotors stop better. But you will need to be very careful to buy the right spacer to fit your frame, the caliper, and the new rotor.
Disc brake calipers will work on rotors mounted with either standard. However, IS rotors will work only on IS hubs, and Shimano Center Lock rotors work only on Center Lock hubs.
Rotors--Truing, Removing, and Installing
A bent rotor is a very common problem that can sometimes be fixed by bending the rotor back into true, but often replacement is necessary.
Truing
Tools needed:
preferably a rotor truing attachment to a wheel truing stand
preferably a rotor truing tool
alternatively, an adjustable wrench
Procedure to true a rotor:
1. Test all your rotor attachment bolts or lockring to make sure they are snug.
2. Test your hub to make sure the ball bearing/race sets are not loose.
3. If you have a rotor truing attachment to your wheel truing stand, remove the wheel and put it in the truing stand. Position the truing caliper, rotate your rotor, to discover where to bend the rotor.
4. If you don't have a rotor truing attachment, leave the wheel on the bike, and use the disc brake pads to judge where you need to bend. Rotate your rotor, and use a felt pen to mark where you need to bend.
5. Very gently bend the rotor--a tiny amount--with a rotor truing tool or adjustable wrench. Retest and bend some more until the rotor is true or you give up.
6. Install the wheel and check whether you need to reset the pads.
Truing with Rotor Truing Attachment to Wheel Truing Stand
Remove and Replace Rotors
IS Rotors--6 Bolts
Tool needed:
torx wrench, or sometimes a hex wrench (usually 5 mm)
The procedure to remove and replace an IS rotor is:
1. Remove the wheel from the bike.
2. Rotate each bolt counterclockwise half a turn (usually with a torx wrench). Repeat the process--rotate each bolt only half a turn--until all the bolts are real loose. Then completely remove each bolt one at a time. Note: Do not completely loosen one bolt at a time. Braking action may push the rotor against the bolts' threads, and if you don't loosen the bolts gradually you risk damaging the threads.
3. Clean everything you can reach.
4. Place the new rotor over the 6 holes and attach each bolt a few turns. Tighten each bolt until each one is almost starting to be tight. Alternate between the bolts, tightening them gradually. Don't tighten one bolt all the way at once. If you have a torque wrench that fits your tool, tighten to the manufacturer's specs. Otherwise, make them real tight, just short of stripping the threads.
New bolts probably have a drop of Loctite on the threads. If not, lightly grease the threads.
5. Install the wheel in the bike and check whether you need to reset the pads.
Shimano Center Lock Rotors
Tools needed:
cassette lockring tool (Park FR-1)
adjustable wrench
The procedure for removing and replacing a Shimano Center Lock rotor is the same as for an IS rotor, except steps 2 and 4.
2. Place the cassette lockring tool in the lockring and rotate it counterclockwise to loosen it. Remove it all the way.
4. Put the new rotor on the hub, lightly grease the threads of the lockring, and attach the lockring, tightening it clockwise. If you have a torque wrench that will fit the tool, tighten to the manufacturer's specs. If not, tighten it good and snug.
Types of Disc Brake Calipers
There are 2 types of disc brake systems:
Cable-actuated brakes, which are less expensive and often don't work as well as V brakes. The brakes are activated by pulling cable.
Hydraulic brakes. The brakes are activated by hydraulic pressure applied to a fluid in a closed system, much like car brakes. Since hydraulics can apply tremendous force, these are superior brakes.
Installing and Adjusting IS Disc Brake Calipers
Note: Never press the brake pads directly together. Make sure something is between the pads before pressing them together--a rotor, a piece of cardboard--anything.
Tool needed:
hex wrench (usually 5 mm)
The procedure to install IS disc brake calipers (with 2 moving pads) is:
1. Place the wheel on the bike, with a true rotor attached.
2. Slip the caliper on the rotor, with the rotor between the pads, and position the caliper against the inside of the fork or frame attachment holes.
3. Insert the the mounting bolts from the outside and thread them clockwise into the caliper until they are barely attached. Be sure to include all washers suggested in the manufacturer's installation manual. Spacers may be necessary between the fork or frame mount and the caliper to get correct pad clearance.
IS Mount Caliper
4. Install the brake cable or hydraulic lines between the lever and the caliper. Make sure the hydraulic lines are full of fluid. Never compress a hydraulic brake lever unless the system is full of fluid.
5. Squeeze the brake lever shut, and hold it shut with a rubber band wrapped around the lever and handlebar. The brake pads should firmly clamp on the rotor. There is probably a gap between the fork or frame mount and the brake caliper. Measure this gap and collect spacer-washers (included in the brake kit) that will exactly fill the gap.
6. Remove the rubber band from the brake lever, remove the brake caliper, and insert the spacer-washers on the bolts between the fork or frame mount and the caliper, then reinstall the caliper. Now put the rubber band back on the lever and handlebar.
7. Tighten the bolts, alternating while you gradually snug-up. If you have a torque wrench that fits your tool, tighten to manufacturer's specs. Otherwise, tighten snuggly, but be careful not to strip the threads--they will strip.
8. Remove the rubber band. Rotate the wheel. Hopefully the pads are centered. If not, go back to the spacer-washer step and try again. If only one of your pads move, see the notes below.
Installing and Adjusting Post Mount Disc Brake Calipers
Post mounts are usually found on Manitou (Answer) forks or on old stuff.
Tools needed:
hex wrench (usually 5 mm)
The procedure to install a caliper (with 2 moving pads) on a post mount fork is:
1. Place the wheel on the fork, with a true rotor installed.
2. Slip the caliper over the rotor, with the rotor between the pads. Position the caliper over the holes on the fork.
3. Thread the bolts through the caliper and into the fork mount, being very careful to keep the concave and convex washers arranged as they came from the manufacturer. Tighten only a few turns.
Post Mount Caliper
4. Attach the cable or hydraulics between the lever and the caliper.
5. Squeeze the brake lever shut, and hold it shut with a rubber band wrapped around the lever and handlebar. The brake pads should firmly clamp on the rotor. The concave and convex washers should allow the caliper to position itself so the pads are centered.
6. Tighten the bolts. If you have a torque wrench that fits your tool, tighten to manufacturer's specs. If not, tighten snuggly, but well short of stripping the threads--stripping threads is an expensive mistake.
7. Remove the rubber band and rotate the wheel. Hopefully the pads are centered. If not, go back to step 6.
Calipers with One Moving Pad
Some disc calipers have only one moving pad. The stationary pad should be adjusted to barely clear the rotor. On most brakes, the stationary pad can be moved by a thumb screw, but some require a hex wrench. The best procedure on these brakes is to position the calipers as suggested above, and then adjust the stationary pad to barely clear the rotor. It may be necessary to loosen and retighten the bolts again after setting the stationary pad.
Lever Pull Adjustment
The amount of lever pull, pad adjustment, and rotor true are directly related. For tight lever pull, you have to have a true rotor and close pad adjustment.
On cable-actuated disc brakes, lever pull can be adjusted with the barrel adjuster on the lever. This may change your brake pad distance from the rotor, so you will need to check that and perhaps readjust. Pad spacing can be adjusted with knobs on the calipers. The closer the pads, the tighter the lever pull.
Hydraulic brake levers often have knobs on the levers that adjust pad spacing and lever pull.
Cutting Hydraulic Cable
Cut hydraulic cable with a very sharp knife, not a cable or housing cutter.
Bleeding Hydraulic Systems
Poor or mushy hydraulic braking is often caused by air in the system. Brakes vary a lot, so refer to the manufacturer's operation manual or web site for bleeding procedures.
Frame or Fork Mount Faults
There is extremely little tolerance for frame or fork mount faults for disc brakes. If you have problems with new brakes, or a new bike, it may be the mount surfaces. A bubble in the paint is enough to throw the system off. We often cure disc brake problems by filing the mount area smooth.