Technical FAQ: Fixing disc brake rub, wheel balance, and more

Swapping out disc wheels and fixing brake rub

Dear Lennard,
I am considering upgrading my ’cross bike with those new ENVE SES disc wheels for road riding, and I want to keep using my old Bontrager wheels for winter and gravel riding. Will I be able to switch between these two different disc mounting wheels (ENVE centerlock and Bontrager bolt-on) without major readjustment of my Avid brakes every time?
— Ted

Dear Ted,
Yes. Absolutely you’ll be able to do so.

That said, there is no guarantee that the two rotors will both initially line up in exactly the same plane so that there wouldn’t be any brake-pad rub with either one. But initial brake rub doesn’t mean that you would need to readjust your brake calipers each time. Rather, you readjust one or both rotors so that they both line up the same in the brake. I go into this in depth in my books.

Generally, you readjust the rotors with gentle bending, because the difference in lineup with various wheels is very small and rotors bend easily. If the alignment varies a lot between the two wheels, however, you may choose to readjust a bolt-on rotor by putting thin shims under the rotor at each bolt, coupled with fine tuning by bending.

You can bend the rotor into alignment with your thumbs; better yet, rotor-tuning forks, like Park’s DT-2, provide leverage to precisely bend the rotor. For small bends, one fork is plenty; for bigger bends (due to crashing or after jamming a bunch of wheels together in a car or bike case and yanking them apart), if you have three DT-2s, you can stabilize the rotor in position with two forks, one on either side of the bent spot, and with the third fork you bend the rotor to eliminate the warped spot.

First, adjust the brake caliper so it is centered over whichever rotor runs the most true. Then put the other wheel in, and notice which way that rotor needs to move in order to be centered in the brake the same as the other rotor was. Here are three ways to do this, ranging from the least accurate method (A) to the most accurate method (C).

A. Eyeballing the rotor in the caliper
By eyeballing, you can often do an adequate job to at least minimize brake-pad rub, but be forewarned that this approach requires patience because it can be hard to tell on which pad the rotor is rubbing as the pad gap is so small. Place a piece of white paper on the floor or the wall, below or level with the caliper, so that you can see the space between the rotor and the pads. Slowly turn the wheel, marking where the disc rubs on each pad with a felt-tipped pen.

Carefully bend the disc into alignment with your thumbs or the Park DT-2 fork at the points you marked, rechecking it constantly by spinning it again through the brake.

B. Using a gauge that grazes the rotor
A more accurate method is to attach a pointer to a truing stand or to the frame or fork in such a way that you can adjust it to graze the rotor. A great solution is the Park DT-3 Rotor Truing Gauge, which bolts to a Park truing stand. It has a micro-adjustable threaded rod that you can move to just graze the rotor and indicate where its alignment is off.

You can also rig up a pointer to graze the rotor while the wheel is on the bike. The pointer must be mounted securely for this method to work; otherwise, you can make mistakes, thinking the rotor is bent one way when it is actually bent the other.

Bend the rotor with your thumbs or the Park DT-2 fork away from where the pointer grazes it as you rotate the wheel.

C. Rotor truing with a dial indicator
If you straighten rotors often, the Park DT-3i.2 or other dial indicator will show the lateral position of the rotor within 0.001 inch (0.025mm), so you can get the rotor as straight as it was when it was brand new. The DT-3i.2 dial indicator clamps to a Park truing stand (it attaches to the DT-3 mentioned above in step B, which bolts to the truing stand). Set the dial indicator tip against the rotor.

Wherever the needle indicates the greatest deflection in either direction, bend the rotor back, continually rechecking it with the dial indicator.

This is the way to get multiple disc-brake wheels to be interchangeable on your bike without brake rub. And if you have multiple bikes, you adjust the calipers on the second bike to any pair of wheels that have gone through this process, and you should be able to mix and match wheels between the two bikes with no brake rub or readjustment.
― Lennard

Swapping out components

Dear Lennard,
I have a 2015 Trek Boone with Avid Shorty 6 brakes and a SRAM Force CX1 group with 40-and 46-tooth chain rings and an 11-36 cassette.

Having just moved to Tucson from central Wisconsin, where it’s very flat, I find my gearing choices very limited here and am considering a switch to a compact (50/34) Ultegra 6800 setup with an 11-32 cassette.

I use my Boone for road and gravel but never for actual ’cross.

1: Will Ultegra 6800 levers work with my Avid Shorty 6 brakes?

2: Will I need any type of BB adapter for a new Ultegra crank for my Boone, since I’ll be switching from SRAM to Shimano and Trek’s BB90 system is proprietary, or can I just use the same bearings that are already press-fit into my frame with no additional adapter(s)?

I would really appreciate your help with this. I have wrenched on bikes for a long time, but compatibility issues seem to have gotten a lot more complicated in the last 5-10 years.
— Charlie

Dear Charlie,
The Shimano levers will work OK with the Avid cantilever brakes. They have a bit more cable pull and lower leverage than SRAM levers, but cantilevers have a lot of leverage and often benefit from more cable pull opening them up further from the rim.

You will need to switch the bottom bracket, at least the non-drive-size bearing. The I.D. of the drive-side bearing on both Shimano and SRAM GXP is 24mm, but the GXP non-drive bearing I.D. is 22mm, so your Shimano spindle won’t fit into it. Often this is achieved with a step-down sleeve with a thick flange on the inboard side of the bearing; pulling this out is often not realistic, and, even if you could, you would still need a different dust shield, since the SRAM dust shield won’t fit on the Shimano spindle, either. The press fit on Trek BB90 is very light, so it is easy to pull out those bearings and push new ones in.
— Lennard

Feedback on balancing wheels

Dear Lennard,
It’s true that spinning up an unbalanced rear wheel on a bike in a workstand will cause an oscillation, often an alarmingly large one. But it only happens because the bike/stand combination has a natural frequency in the neighborhood of 14 Hertz (which is roughly how many revolutions per second a 700C wheel turns at 30 miles per hour).

In other words, if you “plucked” the bike/stand combination and let it vibrate like a tuning fork, it would vibrate at about 14 Hz. The wheel imbalance acts like a parent pushing a child on a swing: a push at the right time means that the oscillation increases in both cases.

On the road, the imbalance is hard to feel not because the rim mass is low compared to the rest of the bike/rider system, but because the natural frequency of that system is not near 14 Hz. Similarly, an unbalanced front car wheel may feel fine at 55 miles per hour but terrible at 73. The unbalanced wheel has no discernible effect at 55 miles per hour because the wheel/steering system’s natural frequency is higher than the forcing frequency produced by the 55 mph wheel. But at 73 mph, the wheel frequency matches the natural frequency of the system, and you feel an oscillation.

It’s important to remember that you can’t feel the vibration of an off-balance wheel on the road not because it’s masked by a weight difference but because it isn’t vibrating, at least not very much. The oscillation of a bike in a stand depends on the stand being part of the system. On the road there’s no stand, so the bike doesn’t vibrate at the same frequency as it does in the stand. A bike whipping back and forth in a stand with a spinning wheel is an arresting sight, to be sure. But that phenomenon doesn’t have much to do with what happens on the road.

At this point, some of your readers might object and mention speed wobble as an important consideration. I’ve encountered a few cases of speed wobble that seemed to be affected by a particular wheel, but as you know, speed wobble is a very complex phenomenon that’s often independent of forcing frequency.

In other words, speed wobble is so complex and has so many variables (which often do not include wheel balance) that one can’t say a balanced wheel generally prevents speed wobble.

Many riders feel intuitively that a balanced wheel is a good thing. In reality, it doesn’t do much. It may seem a little absurd to spend lots of money on light wheels and then spend a bit more money to add weight to them, but on balance, it’s harmless.
— Jason

P.S. I’m a mechanical engineer, for what it’s worth. I also worked as a bicycle mechanic off and on for about 15 years. I didn’t put these things in the body of my letter because my argument depends on physics, not my credentials.

Dear Jason,
Thank you very much for that. It makes perfect sense. The magnitude of the oscillation of the bike in the stand when cranking up the rear wheel always surprised me, as the relative mass involved in the imbalance of the rear wheel is so small. This explains it nicely.
― Lennard