With this Quick Question series we will present fast fixes and collect comments from seasoned riders around specific D.I.Y. mountain bike repairs. While much of this trailside triage is covered in our repair articles and videos, this is a space for longtime riders and readers in the Singletracks community to share their knowledge. Please type your related experiences and advice in the comments below. Do you have a quick question? 🤔 Email [email protected]
Is that a bird or a cricket chirping? Nope, it’s my brake rotor. This week’s Quick Question comes from Singletracks reader Alan Ryan. “My mountain bike has hydraulic disc brakes (Tektro Draco). After riding for 30 minutes to an hour my front brake starts to chirp as if the disc is warped. This happens whether the disc is wet or dry. Once the brakes are applied the chirp disappears. When they are disengaged the chirp comes back. When spinning the wheel and looking closely, neither the rim nor the disc appears to wobble. Also, the gap between the disc and the pads does not appear to vary, at least to the naked eye.”
If this were an episode of the ever-entertaining Car Talk show, Tom and Ray would immediately dig into the caller’s mention of when the noise occurs and when it doesn’t. Then, one of them would likely relate the sound to their lunch somehow while the other one thought up a less likely response so that they could come together at the end of the call with a pair of disparate answers. Thank goodness for public radio!
Since I recently experienced this exact issue with a disc rotor, I can skip the guesswork and go directly to the punchline. It’s bent. The space between rotors and pads is hyper-thin, making it very difficult to see small wobbles in the rotor. Since rotors are made of steel, the braking surface is kept as thin as it can be in order to maintain a lower weight, which results in a chunk of metal close to the ground that’s easily bent. Fortunately, things that are easily bent are easily straightened, within reason.
Not all bent rotors will make chirping noises when the brake is disengaged. Sometimes you can feel the bend at the lever, or in the fact that the wheel doesn’t spin freely throughout its rotation. In extreme cases, the bent rotor can cause the brake to fade and fail, and sometimes the bend won’t be able to pass through the caliper at all.
To true a rotor out on the trail you will need to be able to spin the wheels in order to reproduce the noise and find the wobble. You can hang the bike by the saddle from a sturdy tree branch, or flip the bike upside down to rest on the handlebar and saddle.
Next, make sure not to touch the rotor with your bare hands. It may be hot if you have been descending, and the sharp edges can easily slice skin. Also, you don’t want to transfer skin oils onto the rotor and cause it to squeal once you’ve sorted out the bend. Lastly, you’ll want to be exceptionally careful working with the rotor when the wheel is spinning, as the thin steel can easily take off a finger, leaving you to type Singletracks.com into your browser every day with fewer digits.
With the wheel spinning, look closely at where the rotor passes the brake pads. There should be about 1mm of daylight between them on either side, and this is where you will notice the wobble. If the rotor is dead straight, and still making some noise, you likely just need to realign the caliper and be on your way.
If the rotor is in fact bent, you’ll need to decide if it has to be fixed on the trail, or if it can wait until you’re home, with good lighting a hopefully more tools. Regardless of where you fix the rotor, the next step is to make sure that it is tightly secured to the wheel. If the Centerlock ring or Torx bolts holding it in place are loose, you’ll need to tighten them first. Then, with the wheel tight in the bike you can use the brake caliper to locate exactly where the chirping wobble is, and in which direction it needs to be bent back.
For a smaller wobble, I like to try and bend it back with gloved hands first, after the rotor has had plenty of time to cool off. Most of the time rotors bend toward the hub, and you can brace the rotor with the palm of both gloved hands while pulling with your fingers against the bent portion to pull it back. Again, remember that the rotor edges are wicked sharp. This is another good reason to always wear full-fingered gloves.
If you’re not able to pull the rotor back with hand force, you’ll need a rotor truing tool. A large adjustable wrench, or crescent wrench, with a smooth inner surface, also works well, provided it’s clean and free of oils. Gently place the truing tool at the center of the bend and apply a small amount of pressure to send it back the other way. It’s notably easy to overcorrect and end up with a rotor that’s full of wavy wobbles that has to be recycled, so it’s best to be as gentle and patient as possible. Try not to twist the tool while applying pressure, as the sharp edges can create bite points in the braking surface.
There are also wheel-truing stands that have rotor truing attachments, and a host of shop tools for perfect rotor truing and caliper alignment. Since you won’t have any of those out on the trail, where rotors typically get bent, there are a few other things to consider. If you have a truing tool on your multi-tool, you’re set. If not, you can try using a stick, lodged between the hub flange and the disc, to pry the rotor back in the right direction. If that’s not doing the trick, a full Flintstone method of hitting the stick with a rock might do the trick.
If your rotor is fully kaput, two solutions are to loosen the caliper bolts slightly, allowing the caliper to float with the bent rotor, or remove the disc entirely and ride home with one brake. While this isn’t the safest option, it might be better than carrying your bike out with a wheel that won’t move, or sleeping in the forest. I guess it depends on the season. This past summer I had to remove the rear rotor on a Canyon Sender DH bike I was testing. I launched the bike into a rock garden, and the rear rotor landed directly on a jagged rock. The rotor was fully taco’d, and wouldn’t move through the brake caliper, so off it came. Fortunately, it was the rear, and I was able to carefully descend the rest of the trail with plenty of front brake power.
That’s enough from me. What are your methods for finding and repairing bent disc rotors? Do you have a harrowing story of rotors gone wrong? Please share it in the comments below.
Think I can bend this one back into shape?
Brian, that rotor in the opening pic is below minimum thickness…
Frankly, I have a dial indicator on my truing stand to get the best of my brakes. Runout will continue to throw the brake out of adjustment which is most annoying.
Tolerance is +/- .002″ for my bikes. My trials bike is +/- .0005″ since trials can be a hospital bill happening vs. trying to happen.