Oops...Wrong Brake?

I wanted a disk brake on my KH36, and someone suggested the TRP Spyre mexchanical, so I snagged one and sent it over to my favorite bike shop for installation during my regular maintenance work (the dood’s a total guru and has done all the spoke and lube maintenance on my uni for the last couple of seasons…very happy with the results). Anyway, here are my mechanic’s comments and the image he sent me. Should I maybe pick a different brake?
Thanks for any tips on selecting the right disk hardware.

BTW: in the comments, it’s apparent that my mech has the impression that this was a KH-recommended solution–i didn’t mean to give him that impressions; another rider mentioned this might be a good brake.

“Hey John, I just completed the work on your unicycle and wanted to bring one thing to your attention before you came to pick it up. If you check out the attached photo, you’ll notice there’s barely any clearance between the crank and the disc caliper, a little less than 1mm. On an upright bike, more or less regardless of the frame and crank involved, this would be a no-go because there’s not any room left to accommodate flexing of the crank and frame members under heavy pedaling loads. The crank should never be allowed to rub cyclically on anything, even occasionally, due to the risk of rub/scraping on the crank creating a stress riser that can result in failure. On your unicycle I suspect the amount of flex the crank might experience is probably much less because you’re always sitting, and also the “power stroke” part of the crank’s rotation is far away from the part where rub might be an issue. So all told I think the chance of there being a problem with the low clearance on your uni is probably small, but on the other hand it’s an extremely tight clearance and we’re not unicycle experts, so in the end it’s a little too close for comfort for […] and I to warrant that this setup will work without harm to your cranks, brake caliper, or self. If I understand correctly, Kris Holm recommended this brake to you for use on this frame, so I might suggest contacting them to ask if they’ll warrant the setup having seen the attached picture. If they say there’s not any appreciable flex and it should be fine, I would believe that, we just don’t have enough big wheel uni experience ourselves to say for sure.”

I don’t think you need a new one. If there is any rubbing you will find out right away and can change it then, it won’t be too late. Lets say you didn’t notice, I think the brake would fail long before your crank. If your crank did end up failing it would be well past it’s warranty anyways.

What size of rotor it is? Maybe the guy that recommended it to you used a bigger one. Then the caliper should be bit further from the crank.

Hopefully you will be able to adjust the pads using a 3mm hex key. This can give you about 2mm extra play in either direction.

Otherwise it is possible to stand the rotor off from the crank. Either you can use a special adaptor or just 6 washers that will fit on the rotor bolts.

…it was definitely the bigger of the two options available. I think it was a 60-something? I checked the KH docs for recommended rotor size, and this was the size it suggests.

It’s probably a 160. Given how hard it is to find a cable brake narrow enough for a uni I would suggest a larger rotor (with the right adapter) as a better solution than a new caliper.

Or, you could do what I did and grind off metal from the interfering caliper. I remember the first generation of Cooks Brothers MTB cranks that had a groove ground into them by a bike shop to get enough clearance for a front derailleur. That mod was later added at the factory. Maybe that will happen with spirits?

Sorry I don’t have an answer but is this shop in Seattle? I’d like to find a shop that does a good job with unicycles. Do they build wheels?

I shimmed mine and everything is fine. Just shim the rotor over from the crank with disk shims or good washers. I have a little less than 1/8" clearance but if you went to a 180 rotor it would help. Its a fantastic brake!
Edit: Just don’t shim the rotor so far over that the rotor bolts rub the bearing housings…

Thanks for the TONS of great information! I’m going to forward this thread to the mech and maybe he can use it.
Yeah, this shop is in Georgetown, right at the south end of the main drag (Airport Way) close to the airport. :wink:

It’s an easy fix, just go to a 180mm rotor, that’ll add lots of space.

For those reading this thread and thining about getting a TRP brake, not that there are two different TRP mechanical disc brakes, they look similar, but differe in throw (how the lever pulls cable):

The Spyre is a road brake, it uses a road lever.
The Spyke is a mountain brake, it uses a mount lever.

You can use a mountain lever with the Spyre, but it will have a “spongier” feel. There are fewer good road levers than mountain levers. BMX levers are typically road throw.

I run both types, but the Spyke is my preferred because the pads and cable are enclosed by shields and it works better with the Avid Speed Dial levers.

Thanks again for the VERY useful input, thread posters! My mech just finished the brake install and handed my baby back to me. She never looked spankier in her new seat bumper, tire, seat cover, and of course, the shiny new disk brake! I just finished a quick 25-miler around the south end of Lake Washington and I’m UTTERLY BLOWN AWAY with the new setup! The ride, of course, is plush thanks to the new tire and spoke work, but O…M…G, I was completely unprepared for the disk! Initially, I was worried that adding a disk brake would introduce a potentially dangerous complication and a significant learning curve. I could not have been more wrong. Far from being grabby and twitchy, the action of the mechanical disk was smooth, easy to control, and an absolute pleasure from the very beginning. Example: there’s this ultra-steep little bridge on the bike path–not long, just really steep. Even though I have managed on one or two occasions in the past to actually descend the thing while mounted, my usual procedure at this point in the route is to get off and walk down. This afternoon, I just grabbed a bit of brake lever (ergonomically mounted right next to my left bar-end) and FLOATED down the little hill! I was elated. Not having to brake with my knees, I finished the ride fresh and ready for more. I never imagined that just slapping a brake on my uni would dramatically extend my range…can’t wait to do a long tour! :smiley:

BTW: the mechanic used the “shimming” strategy mentioned earlier in this thread.