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How to convert a 10-speed 29er wheel to a ’cross disc hoop and align rotors

If you’re racing cyclocross or gravel races on disc brakes, you have almost certainly pondered the issue of building a collection of multiple wheelsets. There are three questions here: what to get, how to afford them, and how to make it so that wheel changes take a minimum amount of time and hassle.

In a story we published on Monday, Michael Robson provided a solution to the question of how to build affordable disc-brake wheels. I propose that another way to come up with disc-brake wheels on the cheap is to re-purpose old 29er wheels. I am using as training wheels a pair of Shimano 29er tubeless mountain bike wheels that I no longer have any use for, because my mountain bike now requires through axles rather than quick-release skewers. Thankfully, nobody tried to make the road/cyclocross rear hub axle overlock dimension different from the 135mm we have used on mountain bikes for years, so these wonderful wheels have the opportunity for a new life.

However, there are two problems with both of these solutions (mine and Robson’s, I mean). One problem is that the rotors on the 29er wheels will most likely not line up in the brake calipers the same way as new wheels, resulting in some rub on the pads. This issue will occur even if you buy brand new wheels. Another problem is that your new disc-brake bike most likely has an 11-speed rear derailleur, and the old wheel will have a 9/10-speed freehub body. Both of these problems are surmountable on the cheap, although some of you may cringe at some of my methods.

Aligning the rotors

To align the rotors across your wheelsets, start by setting your brakes up with one set so that you have no pad rub on the rotors. My experience is primarily with TRP Parabox and TRP HyRd cable-actuated hydraulic brakes, and pad retraction is minimal on them, so the rotors must be very true to avoid pad rub. The tweaking of the position of the caliper bit by bit until it is rub-free can be time consuming and frustrating, but that gives you motivation to set the rotors on all of your wheels up the same so that you don’t have to go through it again.

Properly truing the rotor with a minimum of time, hassle, and frustration requires a precision tool. I use the $80 Morningstar ROC tool, which mounts on my truing stand and has a dial indicator that allows me to quickly and repeatedly get rotors to a given truing tolerance. Park’s $40 DT-3 tool also mounts on a truing stand and has no dial indicator, but you can later add one to it when you get sick of eyeballing and listening for telltale rubbing. For straightening the little bends in the rotor, you can use a rotor-truing fork like this or like this, but for small adjustments your thumb will do the trick just fine.

Once the rotor is true and you’ve set the brake to match it, drop the other wheel in and see what happens. I’ll bet you it will rub. No problem; you can move the rotor. In my case, I have one wheelset with a six-bolt mounting system and one with Center Lock. Because Center Lock is much easier and faster for removing and replacing the rotors, I chose to set up my brakes to the six-bolt wheels and then adjust the Center Lock rotors to match.

If you need to move a Center Lock rotor outboard to match the position of the rotor on another wheelset, you can put a spacer behind it. I have stacks of bottom bracket and freehub spacers sitting around. They have slightly too small an inner diameter to slide over the splines on a Center Lock hub, but I found that I could simply snip them with wire cutters and they’d drop right over the splines. There’s a relief step machined behind the splines, so the spacer can flex back to its original shape when it gets there. If you pick the right thickness of spacer, when you tighten the rotor down against it, your rub will be gone (once you true out any wobbles in it as well).

If you need to move a six-bolt rotor outboard, you can put a thin washer at each bolt hole, or you can buy a six-bolt rotor spacer.

If you need to move the rotor inboard, spacers obviously will not help. In my case, one of my Center Lock rotors was further inboard than the corresponding six-bolt rotor, and the other was further outboard. I spaced out and trued the inboard rotor as described above. I could have dealt with the outboard rotor by spacing it on the other set of wheels and setting the brake to this new rotor position. However, I neither wanted to reset the position of my brake caliper, as that had taken more time and patience than I had wanted to devote to it in the first place, or remove all six of those bolts, put a spacer at every one, and bolt them down again.

Instead, I just bent the Center Lock rotor outboard, using the ROC dial indicator to guide me, until it was perfectly true and lined up exactly the same distance from the dropout face as the corresponding six-bolt rotor. It didn’t take much, as there was already some wobble in the rotor, and I just trued the whole thing out to even with the most outboard point of the wobble.

Now I can interchange these two sets of wheels quickly and easily. I have no brake rub, and readjustment of the brake caliper is unnecessary.

Converting a 10-speed freehub body to 11-speed

An 11-speed Shimano/SRAM freehub body is essentially 2mm wider than a 10-speed Shimano/SRAM freehub body (1.85mm, to be exact). If you try to put an 11-speed cassette on a 10-speed Shimano/SRAM freehub body, you will neither be able to engage the splines of the first cog, nor will the lockring clear the dropout, even if you can get its threads started.

If you have a DT Swiss disc-brake hub, either Center Lock or IS, you can interchange its 10-speed Shimano/SRAM freehub body with a Campagnolo one, since Campagnolo freehub bodies are unchanged between 9-, 10- and 11-speed. Then you can run a Campagnolo 11-speed cogset on it, and it will shift fine with your Shimano or SRAM 11-speed drivetrain. This is what I have done on one of my road disc wheelsets. However, if the wheel is already built, you will have to re-dish it, since the axle spacing must be changed as well. There are a few other hubs you can do this with as well.

Otherwise, you will be in the same situation I was in with my Shimano 29er wheels. As you might imagine, there is not a Campagnolo freehub option for a Shimano wheelset, and I didn’t want to give up on the wheels. Since they don’t work on my mountain bike and were otherwise doing me no good anymore, I had nothing to lose in attempting to convert them to 11-speed.

The splines on the freehub body terminate at little feet that stop the further inboard progress of the cogset, and these protruded outboard by about 2mm from the face of the hub shell. I decided I would grind these back until my cogset went on.

First, I had to check that I would have clearance for my rear derailleur, as the wheelset was not designed to have the cogs inboard so far. As I have a carbon Campy EPS electronic rear derailleur, I wanted to be very certain I would not catch it on the spokes. So, without any cogs on the wheel, I installed it into the bike and shifted the derailleur to its furthest inboard position. There was plenty of clearance between the jockey wheel cage and the spokes. I knew this would only get better with cogs on there, since the chain running on the largest cog would push the jockey wheels radially further outboard from the center of the hub where the spokes, which angle from the hub flange inward toward the rim, are even further inboard toward the central plane of the wheel.

Convinced that I would not ruin my derailleur, I ground and filed the little feet of the splines back. I kept checking as I filed by installing an 11-speed SRAM cogset. I found that I needed to file the feet until they were flush with the face of the hub shell in order to get the SRAM cogs to mount properly, particularly if I wanted them in the same position as the Campy 11-speed cogs on my other wheel so I wouldn’t have to readjust my rear derailleur each time I switched wheels.

I was concerned that the SRAM cassette might drag on the hub face, but it doesn’t. It works just fine, and I’ve got a sweet second wheelset that was just gathering dust in my attic. Obviously the usual disclaimers apply: don’t try this at home (even though I did), and don’t blame me if you screw anything up.