The Dark Art of Rotor Shimming
If you have multiple sets of wheels for your bike with disc brakes, there’s a good chance you’ve decided to swap wheels at 10:00pm on the night before a ride. There’s also a chance that you were subsequently greeted by the gentle pinging of a rubbing disc brake, and stayed up later than you intended to adjust that brake. I’m here to tell you that you could get more sleep if you shimmed your rotors.
Tire technology has absolutely soared in the past 10 years. We’re fortunate enough to have tires that grip better in all conditions, roll faster than their 20mm-wide ancestors, and prevent punctures like nobody’s business.
However, in a world of uncertainty it’s also easy to accumulate a pile of tires to handle whatever conditions may lie ahead. With this pile of tires comes a burning desire to be able to swap them as quickly as possible (especially when said tires are tubeless), which leads to a pile of wheels with different tires mounted on them. And with that pile of wheels comes the frustration of having to adjust your disc brakes with every swap. That’s a problem that rotor shims can help you solve.
Why are my brakes rubbing when i swap my wheels?
To put it simply, your different wheels and rotors aren’t perfectly identical.
The front wheel of “Wheelset A” might have a rotor that’s 39.6mm from the center of the hub, while the front wheel of “Wheelset B” might have a rotor that’s 40.4mm from the center of the hub. I’ve fabricated these dimensions for the sake of explanation, but I’m hoping you get the picture.
This could be from variances in hub design, rotor design, or even just manufacturing tolerances. I currently have multiple DT hubs that place the Shimano rotors in slightly different locations, and I think that’s a pretty typical experience.
Regardless of how or why this variance among rotor spacing occurs, it’s probably why your disc brake rubs when you put a different set of wheels on your bike. This slight variance is just enough to mean that your rotor on “Wheelset B” is no longer centered between the brake pads on your brake caliper, which you’ve adjusted to be centered around the rotor on “Wheelset A.”
So How can I make this haunting, pinging sound stop?
It’s time to buy some rotor shims and take control of the situation.
Rotor shims are very thin pieces of metal that can be installed between your rotor and hub to adjust the spacing. The goal is to use these shims to get all of your rotors spaced identically on your different wheelsets so that you no longer need to adjust your brakes when you swap wheels.
Once you have identical rotor spacing on all of your wheelsets, your brakes will rub less often and you’ll get more of that precious sleep so you can hang onto Alvaro’s wheel for just a little bit longer.
Supplies
Your bicycle
All of your wheels, with rotors for each
Tools to remove/install rotors
If 6-bolt: A T25 torx wrench and some blue threadlocker (Loctite Blue 242, Loctite Blue 243, or Park TLR-1)
If centerlock with notches on the inside of the lockring: A cassette lockring tool like Park FR-5.2
If centerlock with notches on the outside of the lockring: A 16-notch bottom bracket tool like Park BBT-9 or BBT-69.4
Note: Fasteners on rotors have a torque spec, and I strongly recommend using a torque wrench.
Another note: Most manufacturers do not recommend using threadlocker or grease on centerlock rotor lockrings.
Rotor shims
If 6-bolt: I like the Syntace 6-bolt Disc Rotor Shim Set
If centerlock: I like the November Centerlock Disc Rotor Shims
Buy more than you think you need. If you end up with extras, give them to a friend and show them this blog post.
Strongly recommended: Fresh brake pads
Step-by-step: Let’s make with the shimming
Let’s start by shimming your front wheel(s) first.
If your brake pads are worn, start by replacing them.
Worn brake pads have rarely worn evenly. You’ll save yourself a future headache if you replace your worn pads with new ones and reset the pistons.
Identify which of your rotors sits furthest from the center of the hub.
Because you can only shim a rotor “out” and away from the center of the hub, you’ll need to identify which rotor sits the furthest out and shim all of your other rotors out to match.
I suppose you could do this with a set of calipers if you’re crafty, but I tend to take a foolishly-thorough approach: Install and uninstall all of your wheels until you’ve figured out which one rubs the non-drive-side pad the most (or rubs the drive-side pad the least).
Call this Wheel A. This rotor will not need to be shimmed. All other rotors on your front wheels will likely need shims.
Adjust your brakes to be perfectly centered around the rotor on Wheel A.
Make sure you do this with the thru-axle tightened properly, and take your bike for a spin around the block once your brake is adjusted. Stand out of the saddle and put some power to the pedals to make sure the rotor doesn’t rub when your frame and fork flex under load.
Put shims behind your other rotors so that they match this rotor position.
This is the tedious part.
Remove the rotor from Wheel B and install a shim between the rotor and the hub. This will space the rotor roughly 0.25mm away from the hub center.
Install Wheel B on your bike. Tighten the thru-axle and check for rubbing.
If the rotor rubs the drive-side pad, it needs another shim. If it rubs the non-drive-side pad, you’ve got an “In-Betweeny.” See below for troubleshooting.
Repeat for Front Wheel C, D, and beyond.
Take a breather and drink some water. Then, work steps 1 through 3 on your rear wheels.
Upon completion, bask in the glory of knowing that you’re safe from the sound of rubbing and pinging brakes. Enjoy your peaceful evenings and restful nights consisting of 8 hours of sleep. Feel better than all of your friends… until some other variable enters your meticulously-shimmed life and you plummet into a sleepless pit of despair.
Troubleshooting
The curse of the In-Betweeny
You’ve identified which rotor sits furthest from hub center and have adjusted your brake around it. When you install a rotor shim on Wheel B, the rotor rubs on the non-drive side pad. Without the shim, it rubs on the drive-side pad. You’ve got an In-Betweeny.
At this point you have two options: You could find a thinner shim, or you could fudge your brake adjustment a bit so that it works with Wheel A and your freshly-shimmed Wheel B.
I haven’t found any rotor shims that are thinner than the ones I’ve linked above, so you’ll likely have to fudge your brake adjustment a bit. This adds to the faff factor, but it’s still very possible to live a happy and noise-free life with an in-betweeny.
Your brakes only make noise after descents or long brake applications
Ah. The song of my people. You’re probably riding a Shimano bike and your brake pistons have a bit of crud on them. This isn’t related to rotor shimming, but it’s a common disc brake experience. Allow me to explain/wax poetic.
Your brake pistons have the lovely job of pushing your pads into your rotors when you squeeze the brake lever. When you stop squeezing that lever, the pistons return to their resting position and bring the pads away from your rotor. If those brake pistons are dirty, they don’t return to their resting position as easily. This can make your pads sit closer to the rotor, which causes to brake rub.
If this happens while you’re out on a ride, gently squeeze your lever a few times to move the pistons in and out of the caliper. This usually loosens up whatever crud is on them and helps them return to their resting position.
When you get home, remove your brake pads and gently clean those brake pistons with a q-tip and rubbing alcohol.
Celebrate in silence
If you’ve made it this far, I hope you’ve now spent a lovely evening cursing on the floor of your tiny apartment while you install shims and adjust disc brakes.
More importantly, I hope that this time was worthwhile and that you’re able to seamlessly swap wheels with minimal rubbing, pinging, and cursing.
I now recommend that you purchase a stylish sleep mask to celebrate your long and restful nights.
If you have money left over from purchasing that sleep mask, I recommend you use your rotor shimming skills to justify investing in more wheels. I find that the best pile of wheels is one that’s large enough to inconvenience your partner or roommates. Your mileage may vary.
