A disc brake stops you by squeezing two pads against a steel rotor bolted to the hub. The thing to watch most is pad thickness: Shimano says not to use pads with 0.5 mm of braking material or less, and SRAM replaces them when pad and backing plate add up to 3 mm or less (about 0.5-1 mm of friction material). If the rotor rubs, recentering the caliper almost always fixes it in a minute. And every time you change pads or rotors, you need to bed them in: about 20 controlled stops before you trust the brake.
How a disc brake works
The lever pushes one or more pistons inside the caliper (the part that straddles the rotor). The pistons push the pads against the rotor, and friction turns speed into heat. That’s why the rotor and caliper are scorching after a descent: don’t touch them.
Compared with rim brakes, disc brakes stop just as well with a wet or dirty rim, don’t wear out the rim and allow wider tires. In exchange, they weigh a bit more, need a frame and fork built for them and are more sensitive to contamination: one drop of oil on the pads is enough to stop them working.
Mechanical or hydraulic
| Mechanical (cable) | Hydraulic | |
|---|---|---|
| How the pad is pushed | A steel cable moves the caliper | Fluid (mineral oil or DOT) pushes the pistons |
| Feel and power | Firmer; takes more hand force | More power with less effort and better modulation |
| Pad wear adjustment | You have to move the pads closer by hand or with the barrel adjuster | Self-adjusting: the pistons advance as the pads wear |
| Maintenance | Tension and replace cable and housing | Bleed the system every one or two years |
| Roadside repair | Simple, with a hex key | If a hose breaks, there’s no fixing it on the road |
| Where you’ll find them | Entry-level, city and some touring bikes | Mid- and high-end road, gravel and mountain bikes |
Hydraulic self-adjustment has one practical consequence: when you replace the pads, you have to push the pistons all the way back so the new ones fit. Bleeding (removing air and renewing the fluid) is covered in the guide on how to bleed hydraulic brakes.
Organic or metallic pads
| Organic (resin) | Metallic (sintered) | |
|---|---|---|
| Feel | Better modulation, smoother | More bite when cold, less progressive |
| Noise | The quietest | Noisier, especially in the wet |
| Mud, sand and rain | Wear quickly | Hold up much better |
| Heat on long descents | Worse | Better |
| Who they’re for | Road and gravel in the dry, city riding | Mountain biking, winter, loaded touring, e-bikes |
SRAM fits organic pads as standard on its Force AXS brakes because they give “the best modulation and quietest braking,” and recommends sintered pads for sandy and wet conditions. Two warnings:
- Some rotors are resin-only. Shimano states that its SM-RT56, SM-RT54, SM-RT30, SM-RT26 and SM-RT10 rotors, among others, only accept resin pads: metallic pads damage them quickly. Check what’s printed on your rotor.
- If you switch pad compounds, replace the rotors too. SRAM recommends it because the old rotor is impregnated with the previous material.
When to replace pads and rotors
| Part | Shimano | SRAM |
|---|---|---|
| Pads | At 0.5 mm of material or less | Pad + backing plate at 3 mm or less (about 0.5-1 mm of friction material) |
| Rotor | At 1.5 mm or less, or if the aluminum shows through | 1.55 mm on 1.85 mm rotors; 1.7 mm on 2 mm rotors |
| When to check | At every service | Every month; every few rides in mud or sand |
To check, remove the wheel and look at the pads from above the caliper with a flashlight, or take them out. A metallic squeal when braking means the backing plate is already scraping the rotor: stop right away, because you’re scoring the rotor. Measure the rotor with calipers; the disc brake rotor size chart lists diameters, adapters and minimum thicknesses. Shimano also says to replace pads that have been contaminated with oil or grease.
Replacing hydraulic pads is simple: wheel off, remove the pad pin, pull out the old pads, clean the area, push the pistons straight back with a flat tool (never anything sharp), install the new pads with their spring and squeeze the lever several times until it feels firm. Then bed them in.
Centering the caliper to stop rubbing
This is the procedure Park Tool and Shimano recommend:
- Seat the wheel. Loosen the axle, push the bike down so the axle drops fully into place and retighten. A wheel sitting crooked in the frame causes a lot of rubbing.
- Loosen both caliper bolts just enough for it to slide side to side.
- Squeeze the lever and hold it: the caliper centers itself on the rotor.
- Tighten the bolts with the lever still held, alternating between the two.
- Release and spin the wheel. If it doesn’t rub, torque to spec: 6-8 N·m (53-71 in-lb) on Shimano road calipers; SRAM specifies 5 N·m (44 in-lb) on road and flat-mount mountain calipers, and 9.5 N·m (84 in-lb) on post-mount mountain calipers.
- If it still rubs, loosen one bolt, nudge the caliper toward the side that rubs while you spin the wheel and retighten. Repeat with the other if needed.
If nothing works, look for another cause: a bent rotor (you can see it when you spin the wheel and watch the gap), a loose rotor on the hub, play in the bearings or a sticky piston from dirt. Cleaning the pistons with isopropyl alcohol is usually enough. Torque specs for the rotor and the rest of the bike are in the bike torque chart.
Bedding in new pads and rotors
New pads brake with less power at first: Shimano warns that braking power builds gradually and that you can lose control if you don’t account for it. Bedding in lays down an even layer of pad material on the rotor.
- Find a flat, safe stretch with no traffic.
- Get up to a moderate speed and brake with one brake until you’re at walking pace. Don’t lock the wheel.
- Repeat at least 20 times with each brake (Shimano and SRAM agree on that number).
- SRAM adds a second round: about 10 stops from a higher speed and braking harder, always seated and without locking up.
- Let the brakes cool before riding on.
SRAM warns that skipping the bed-in reduces power and increases brake noise. Take particular care with the front brake.
When to see a mechanic
See a mechanic if the lever pulls to the bar or feels spongy, if there’s a fluid leak, if the brake loses power on a long descent and doesn’t come back, or if the rotor is cracked or badly warped. Shimano says never to use a brake that leaks. And on an e-bike or cargo bike, where the brakes work harder, don’t stretch pad life.
Common mistakes
- Touching the rotor with your fingers or cleaning the bike with an oily product near the brakes. The oil from your skin is enough to make them squeal.
- Lubing the chain with the wheel on and letting it drip onto the rotor. Hold a rag behind it.
- Squeezing the lever with the wheel out. The pistons advance and the pads close up. Use the shipping spacer or a plastic wedge.
- Putting metallic pads on a resin-only rotor.
- Skipping the bed-in and heading down a descent on new pads.
Check pad thickness every 300 miles (500 km) as part of your maintenance schedule by mileage. The terms for caliper, piston and hose in six languages are in the brakes glossary.
Frequently asked questions
When should I replace disc brake pads?
Shimano says not to use them with 0.5 mm of braking material or less; SRAM replaces them when pad and metal backing plate together measure 3 mm or less, which is about 0.5-1 mm of friction material. Also replace them if they get contaminated with oil or grease: they don't recover.
Are organic or metallic disc brake pads better?
Organic (resin) pads have better modulation and are quieter; metallic (sintered) pads hold up better in mud, sand and rain, where organic pads wear quickly. Some Shimano rotors only accept resin pads.
Why is my disc brake rubbing?
Almost always because the caliper is off-center after removing the wheel, the wheel isn't seated properly in the frame, or the rotor is bent. Loosen both caliper bolts, squeeze the lever, retighten them with the lever held and check.
How do you bed in new disc brake pads?
On a flat, safe stretch, get up to a moderate speed and brake with one brake until you're at walking pace. Shimano says to repeat it at least 20 times per brake; SRAM adds about 10 more stops from a higher speed and harder, without locking the wheel.
When should I replace a disc brake rotor?
Shimano replaces it at 1.5 mm thick or if the inner aluminum shows through. SRAM, at 1.55 mm on its 1.85 mm rotors and 1.7 mm on its 2 mm rotors. Any cracked or warped rotor gets replaced.
Sources
- Shimano, dealer's manual DM-RADBR10-05 for Dura-Ace, Ultegra and 105 hydraulic disc brakes (PDF)
- SRAM, "Brake Maintenance Guide" (pad and rotor wear, bleed intervals)
- SRAM, "Force AXS: easy maintenance" (organic and sintered pads)
- SRAM, "What are the torque specifications on my SRAM brake calipers?"
- Park Tool, "Hydraulic Disc Brake Alignment"
- Shimano, "Compatibility" version 2.7, October 2026 (resin-only rotors, PDF)