How to Fix a Rubbing Hydraulic Disc Brake

How to Fix a Rubbing Hydraulic Disc Brake

A rubbing disc brake is one of the most common things we get asked about. Even a tiny rub that isn’t really slowing you down is enough to drive you crazy on a long climb. The good news is most of the time it’s a simple fix once you know what you’re actually looking at.

This guide walks through diagnosing the cause first, then the alignment procedure itself. I’ll flag the gotchas that catch people out, so you don’t waste an hour centring the caliper when the real problem is somewhere else.

What you’ll need

  • 4 or 5mm hex wrench, or a T25 (depends on your brake)
  • A torque wrench in the 0 to 10 Nm range
  • A light to backlight the caliper (your phone flashlight works fine)
  • A stand or something to hold the bike up so the wheel can spin freely

Start with diagnosis, not adjustment

Most people reach for the hex key and start loosening caliper bolts before figuring out why the brake is rubbing. That’s how you end up chasing your tail. Lift the bike, spin the wheel, and look at the gap between the rotor and each pad with the light behind the caliper.

While you’re in there, give the pads a quick look for wear. If they’re down to the backing or close to it, replace them before going any further. (see: How to Check and Replace Your Brake Pads)

A few possibilities for what you’re seeing:

  1. No rub. You’re done. Some brakes will tick lightly with no real noise or drag and most riders just call that good.
  2. Constant, steady rub. Caliper is off centre, or the pistons weren’t reset after a recent pad or rotor change. Also happens if someone pulled the lever with the rotor or wheel out.
  3. Rub that comes and goes as the wheel spins. That’s a bent rotor, or a rotor that isn’t sitting flat on the hub.
  4. Rub that only appears after long, steep descents. Different problem entirely. That’s heat and too much fluid in the system, usually from a bleed done without resetting the pistons first. Don’t touch the alignment for this one. Reset your pistons and bleed it properly. (see: Do You Actually Need to Bleed Your Brakes?)

Things that look like alignment problems but aren’t

Before you loosen any caliper bolts, check these:

  1. Wheel not fully seated. Especially on open dropout bikes, but it happens on thru-axle setups too. Loosen the axle, push the wheel firmly into the dropouts, re-tighten. Sometimes that’s the whole fix.
  2. Bent rotor. Sight down at the rotor as it spins. If it’s waving side to side you’ll never get clean alignment until you true it or replace it. (see: How to True a Bent Disc Brake Rotor)
  3. Loose rotor on the hub. Grab the rotor and wiggle it. If it moves on its bolts or lockring, snug those up before going any further.
  4. Hub bearing play. Rock the wheel side to side at the rim. If you feel a click or knock, that’s bearing play. Fix that first.
  5. Sticky pistons. Dirt on the pistons stops them retracting evenly. Usually shows up as uneven pad wear too. Cleaning the pistons will often solve a rub that no amount of caliper centring will touch. (see: How to Clean Sticky Brake Pistons)

The alignment procedure

If you’ve ruled out everything above and it’s still rubbing, here’s the actual procedure. It works the same front or rear.

  1. Loosen the two caliper mounting bolts until the caliper can slide side to side freely. On some frames these bolts sit under the chainstay or on a bracket, so look at where yours actually live before reaching for the wrench.
  2. Squeeze the brake lever firmly and hold it. The hydraulic pressure pushes both pads against the rotor and self-centres the caliper body.
  3. While still holding the lever, snug both mounting bolts.
  4. Release the lever. Spin the wheel.

If it’s clean, torque the bolts to spec (usually 6 to 8 Nm, check your brake’s manual) and you’re done.

If it’s still rubbing, that’s normal. Loosen one bolt at a time and nudge the caliper a hair until both sides of the rotor are clear. Keep your fingers and tools well away from the spokes while the wheel is spinning. Once it’s clean, torque the bolts to spec.

A couple of last things worth knowing

Some calipers come with conical washers between the caliper and the mount, or under the bolt head. They’re there for a reason. Don’t remove them, and don’t add them if your bike didn’t come with them. The pads will sit at an angle to the rotor and wear funny.

If you’ve worked through all of this and the alignment still won’t hold, your frame or fork mounts may not be machined square. That’s a shop job. A brake mount facing tool will dress the mounts true to the hub, and you’ll get a clean alignment afterwards.

Still stuck?

If your brake only drags after long descents, or you’ve got uneven pad wear, or one piston is clearly doing most of the work, the issue is upstream of alignment. Reach out and tell us what brake you’re running and what you’re seeing. Easier to point you the right way once we know what’s actually going on.

And if your pads are toast and you’re not sure which set fits your caliper, find your pads here. Every common caliper shape, in stock.

This article is based on Park Tool’s video Hydraulic Disc Brake Alignment.

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