You spin your wheel and you hear a soft “shhhhhhh” or a “sheeng sheeng sheeng” (pardon the onomatopoeia) coming from the caliper. Your wheel doesn’t spin ad finitum like it used to - instead it comes to an abrupt-ish stop as the disc contacts one of your pads.
You Google “fix brake disc rub” and all you see is “loosen the caliper mounting bolts, compress the lever and re-tighten the bolts with the lever still compressed”. Well, duh. You do that like 20 times, but the rub keeps coming back in one form or another after you re-tighten the mounting bolts. Your back is now sore, you’ve put a crick in your neck, and your brake is even worse than when you started. Biking is the worst.
This is common. As with my tubeless tire post, this is not a tutorial or a product technical manual - there are plenty of those out there that you should read first. This is to succinctly make you aware of some tricks that often aren’t covered in the tutorials.
First, a safety moment
Any time your hands are near your rotors, you need to be careful of 3 things: 1) If the wheel is spinning and your finger gets caught in some channel of the rotor, it’s a trip to the hospital. Keep fingers clear while the disc is spinning. 2) It’s possible to contaminate your rotors with the oils on your hands. Keep your fingers away from braking surfaces and/or wear nitrile gloves. If you contaminate your rotors on accident, you can clean them with disposable alcohol wipes. 3) Your brakes are safety critical (especially if you’re only running 1 brake!). When you’re finished adjusting, it’s a good idea to torque the caliper mounting bolts to spec and check them periodically, especially before nasty descents. If ever in doubt, take your bike into the shop. I will say that it is safe to ride a bike short distances with one or both brakes rubbing slightly (i.e. several miles to the shop). It just sounds ugly and robs you of some power.
Always try the easy fix first
With the caliper bolts loose, spin the wheel at a pretty good clip before stopping it by clamping the brake lever down and retightening the bolts. Try in both directions of wheel spin. Don’t ask me why, but this sometimes shocks the caliper out of its rut and into good alignment.
Backlighting (optional)
You’re still reading, so I guess spinning the wheel didn’t work. This whole process is easier if you can put a white sheet of paper behind your caliper and shine some light so you can see the disc between the pads. If the disc is warped, you’ll be able to see the direction of the wobble. If the disc isn’t warped, you’ll see which brake pad (left or right) is being contacted. Handy, and much cheaper than a rotor alignment tool.
Tighten the bolts very gradually and brace by hand
Reminders: Don’t let your hand touch the braking surface and don’t spin the wheel fast with your fingers near the rotor.
Many times, the rotor is centered when you clamp the lever, but when you tighten the bolts, the caliper twists slightly as a result of the tightening force and rotation the bolt exerts upon it. This is especially likely if you torque down one bolt while the other bolt remains loose. Instead, while holding the brake lever, alternate tightening each bolt 1/8th (or even 1/16th) of a turn at a time. One bolt will prevent the twisting forces of the other bolt from rotating the caliper out of alignment. You can also use a finger to brace the caliper from rotating (pictured), or at least detect if the caliper is rotating in a particular direction as bolts are tightened, and pre-adjust for it.
The business card trick
This is a classic. I have a lot of leftover business cards after finishing business school and going to a new job. Loosen the caliper mounting bolts. Fold a business card (preferably an old one, as it’s going to get marked up) around the rotor. Rotate the wheel so the business card gets wedged between the brake pads and the rotor. Adjust as you tried to before: Squeeze the brake lever, tighten the bolts (while still holding the lever), then release the lever.
Throw money at the problem: Brake alignment tools
These are basically metal reusable business cards that are shaped to sit atop rotors. I tried them on 160mm Shimano wavy discs and found they were kinda fiddly and not really worth the trouble, but other people find them to be a godsend.
Fixing a warped disc
For what it’s worth, it’s not a big deal if your rotor is ever so slightly warped. It’s often possible to eliminate the “sheeng sheeng sheeng” sound with decent alignment. But it’s also possible to fix warped discs with or without the proper tools.
For one thing, the backlighting trick discussed above will tell you where and in what direction your rotor is out of true - without using a professional-quality gauge.
Note that this is a little sketchy and it shouldn’t be your first option. Bending rotors back and forth could eventually cause failure, although a single adjustment shouldn’t be anything to worry about. You can do it with your (gloved) fingers, applying as little force as possible to straighten out the bend.
Or you could just, y’know, buy another rotor. I’m all about lazy solutions.
Bonus tip 1: Preventing disc warping
Shortly before smoking my brakes on a 5,000ft descent.
As an overweight cyclist who rides a lot of hills: Don’t drag brakes - stop with relatively short stabs to prevent heat buildup. Alternating front-back brakes can help too, if traction is not an issue. Don’t ride like I do sometimes, and certainly don’t dunk your brakes in cold water (e.g. a creek crossing) shortly after heating up your brakes on a descent. When installing rotors, always tighten rotor bolts in a star pattern, and tighten them all at once (rather than going to full torque on 1 and then moving to the others). Before the bolts tighten down into the hub, rotate the disc clockwise on the hub to eliminate play (so braking forces don’t cause it to rotate against the bolts).
Bonus tip 2: Perfect disc alignment across multiple wheelsets
Having multiple wheelsets can be like having multiple bikes in one, because they can be mounted with different tires for different surfaces. But the manufacturing tolerances for rotor mounting points on hubs are not super precise, especially between different wheel makers. You can use disc shims to place the rotors in the exact same place across multiple wheelsets, so you don’t have to adjust the brakes every time you swap wheels. (You might still need to adjust the derailleur when you swap wheelsets)