How to True a Bicycle Disc Brake Rotor
Why Bicycles Stack Exchange recommends it
Bicycles Stack Exchange answerers point people to it when a disc brake rub or wobble might come from a slightly bent rotor and they need a step-by-step demonstration of how to straighten it.
Recommended by 3 people across 5 different months, 2018 to 2020.
What it is
A Park Tool shop tutorial on straightening a bent disc brake rotor to stop brake rub. It covers diagnosis, truing on the bike with a rescue tool or adjustable wrench, truing in a wheel truing stand with a rotor fork, and using a dial indicator gauge for precise results.
What people said
Recommends Argenti Apparatus · 2018-12 · Bicycles Stack Exchange · on “Is there way to fix a disc brake rotor if slightly bent?”
Yes. Disc brake rotors can warp a little in use, or get bent from impacts and straightening them is routine maintenance. The basic procedure is to find spots where the rotor is bent inboard or outboard, and carefully incrementally bend it back. There are tools made for this purpose (e.g. Park tool DT-2) but the job can be done with a large adjustable wrench. There are many tutorial and how-to videos that show you how to perform the procedure. Here's one from Park Tool.
Recommends Argenti Apparatus · 2019-05 · Bicycles Stack Exchange · on “Disc brake rub problem”
Check that the rotor is true. If the rub occurs at a specific point of wheel rotation the rotor may be slightly bent. You won't be able to adjust the caliper position to eliminate rub if this is the case. Rub is not caused by the pistons being advanced too much, so there is no point pushing the pistons in. As soon as you use the brakes they advance again. However, a piston might be sticking, so that it is not retracting from the rotor when the lever is released. You can try to unstick a piston by carefully advancing an cleaning it. These Park Tool videos may help you:
Recommends Argenti Apparatus · 2019-07 · Bicycles Stack Exchange · on “What tool to adjust brakes”
Shimano BR-MT520 [LINK] brakes are hydraulic discs. The alignment can be adjusted by loosening the two mounting bolts. The bolts typically have Torx T25 size heads, so you need a basic T25 wrench at least. If your rear caliper is mounted between the seat and chain stays the bolts can be difficult to get to, so a L-shaped rather than a T-shaped wrench is necessary. The basic short-cut technique for aligning the calipers to loosen the bolts so the caliper can move but is not loose, pull the brake lever hard, tighten the bolts. See this Park Tool page [LINK] for more details on caliper alignment. If your pads are rubbing periodically as the wheel spins, your rotors are probably warped. They can
Recommends mattnz · 2019-10 · Bicycles Stack Exchange · on “Front disc brake wobbling / grinding on pads after full braking and minor plunge”
The amount of wobble I see is probably not enough to cause a correctly aligned caliper to rub, so if the caliper has moved, it might have been like that before the prang. The only way to know is to (try to) align the caliper. If that cannot be done with rubbing, the disc may need truing (see park tool "How to true a disc rotor - [this video] )
Recommends Poquontchn · 2020-04 · Bicycles Stack Exchange · on “Disc brake rotor scraping against mounting bracket”
As all of the comments and the answer above address the potential causes of your issue, I'll offer my 2 cents for potential solutions, mostly of the MacGyver variety. But first: to easily diagnose whether the problem is with the fork or with the rotor/hub/wheel, try to borrow another 26" (I'm guessing) quick-release disc front wheel from a friend. Have them drop it off on your porch and then grab it the next day once the surface is maybe virus-free. If the borrowed wheel has the same issue, you have a problem with your fork, probably that it got bent in a crash at some point. If the borrowed wheel spins on your fork without rubbing, you have an issue with your hub or rotor, and I would try t
Cites Maarten -Monica for president · 2020-01 · Bicycles Stack Exchange · on “Chain falling off front sprocket”
You might also want to check if your front chainring or the crank spider (the crank arms) is/are bent. Check this by rotating the cranks and measuring the distance from a fixed point (eg the frame) to the teeth of the chainring. If this distance changes significantly (more than 1mm) whilst rotating the cranks this might be causing the issue. If so you might need to replace your chainring or right side crank (depending on the severity). In some cases (if it's not too bad) the chainring can be straightened/trued (similar to how disc brake rotors can be trued). Similar to this: