How to True a Bent Disc Brake Rotor

How to True a Bent Disc Brake Rotor

Rotors bend easier than most people think. Stacking your bike against another one in a parking lot will do it. So will a hard rock strike, or even just getting too hot on a long descent. The good news is a bent rotor is often fixable in five minutes if you know what you’re doing.

This guide covers truing a rotor on the bike with a basic tool, plus when the rotor is too far gone to save.

Is it actually the rotor?

Before you start bending things, make sure the rotor is the actual problem. A few other things cause the same symptoms:

  1. Caliper isn’t centred. Most “rotor rub” is actually a caliper alignment issue. (see: How to Fix a Rubbing Hydraulic Disc Brake)
  2. Wheel isn’t fully seated. Especially on open dropouts. Loosen the axle, push the wheel home, retighten.
  3. Hub bearing play. Rock the wheel side to side. If you feel a knock, fix that first.
  4. Loose rotor on the hub. Wiggle the rotor by hand. If it moves on its bolts or lockring, snug them down before going any further.
  5. Sticky pistons. Crud on the pistons stops them from retracting evenly and looks just like a rotor problem. (see: How to Clean Sticky Brake Pistons)

If you’ve ruled those out and the rotor is clearly waving side to side as it spins, then it’s truing time.

When to true vs. when to replace

If the rotor has taken a real hit and looks visibly bent, just replace it. You can try truing first, but think of it like a warped cookie sheet. You can push it around, but it’s never going perfectly flat again, and you’ll spend more time fighting it than the new rotor would have cost.

If you work at it for a few minutes and you’re not making progress, that’s your answer. Replace it.

Safety

Rotor edges can be sharp, and a spinning rotor will happily slice your finger open if you get careless. Watch where your hands are. Don’t rest your fingers against the inside of the caliper while the wheel is moving.

What you’ll need

  • A rotor truing tool. The Loam Goat 5 In 1 Disc Brake Tool has a dedicated truing slot built in, plus four other brake-shop functions in the same tool. Park Tool’s MTB-3.2 is a single-purpose alternative. In a real pinch, a clean adjustable wrench with the jaws set right will also work.
  • A flashlight, or a sheet of white paper held behind the caliper (makes it much easier to see the gap)
  • A bike stand, or some way to hold the bike off the ground so the wheel spins freely

On-bike truing

This is the method most riders will use. You’re using the brake pads themselves as your reference for where the rotor is bent.

  1. Get the bike in a stand and spin the wheel. Look through the caliper with the flashlight behind it (or hold a piece of paper there). You’re watching for the gap between rotor and pad to open and close as the rotor spins.
  2. Find the spot on the rotor that’s rubbing or running close. Note which direction it needs to move. If the rotor is pushing against the inboard pad, you need to bend it outboard, and vice versa.
  3. Rotate that section away from the caliper so you can get your tool on it.
  4. Engage the truing tool on the rotor and flex it gently in the direction it needs to go. Start with a small amount of force. You can always add more, but you can’t un-bend an over-correction without bending it back the other way.
  5. Spin the wheel again. Check that same spot. If it improved, repeat with a bit more force. If it got worse, you bent it the wrong way. Reverse direction.
  6. Work your way around. Most rotors have a couple of problem spots, not just one.

When the rotor spins through the caliper without rubbing, you’re done.

How accurate is “good enough”?

A business card works as a quick reference. The gap between rotor and pad should be smaller than a business card across the full rotation. If a business card slides through anywhere, that spot needs more work.

For most riders on most brakes, you’re aiming for “no audible rub and no visible deviation.” Don’t chase perfection. A rotor with 0.1mm of runout that you can’t hear or feel is fine to ride.

A note on two-piece rotors

Some rotors are made of two pieces, with an aluminum spider riveted to a steel braking surface. On those, do not try to bend at the spider arms. Bend only at the outer ring where the pads contact. Bending the spider can crack the rivets and ruin the rotor.

If you can’t tell whether yours is one-piece or two-piece, take a closer look at the centre. If it’s all the same colour and material the whole way through, it’s one piece. If the centre is a different colour (usually silver or black aluminum) and the outer ring is shiny steel, it’s two-piece.

For the perfectionists

If you want to go beyond eyeball-and-business-card accuracy, pull the wheel out and put it in a truing stand with a rotor gauge. For shop-level accuracy a dial indicator will let you dial in runout to within tenths of a millimetre. Park Tool’s full video walks through that process in detail if you want to go deep.

For 95% of riders, the on-bike method gets you where you need to be.

When the rotor is done

If you’ve worked on it for a few minutes and it’s not improving, the rotor has reached the end of its life. That happens. Rotors are a wear item, just slower than pads. While you’ve got the wheel out and the pads off, take a minute to check the pads too. The Loam Goat 5 In 1 has built-in wear gauges for Shimano, SRAM, Magura, and Hope rotors, plus a Shimano pad gauge. If they’re getting thin, swap them while you’re already deep in the brake. (see: How to Replace Your Disc Brake Pads)

If you need fresh pads, find your set here. Every common shape, in stock.

This article is based on Park Tool’s video How to True a Disc Brake Rotor.

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