I ask this question as a unicyclist, manufacturer and dealer
On my mountain bike it was never a problem to replace a brake disc and then align it. This is quick and easy.
As a unicycle dealer/manufacturer, I pre-assemble many disc brakes and align them so that the end customer has an easy time of assembly.
Well, when I mount disc discs on unicycle hubs, I often have problems aligning them without rubbing. I used to blame myself, e.g. that I tightened the discs with the wrong torque, etc., or that the disc was manufactured inaccurately. A few discs ended up in the trash.
Well, somehow you can get the disc to run without dragging, at the latest after you have ridden the unicycle and braked several times. But it is absolutely not as perfect as you would like and it can take time and nerves. And it’s not like it’s just my hubs, I’ve ridden other products before and had similar problems. Of course, there is also the case that it can be installed quickly and easily.
There are of course many suspicions:
Hub manufactured inaccurately
Holes where the discs are screwed on may have small bumps
But also: Bearings (if they run inaccurately, this also affects the hub)
What are your experiences?
Finally, the most shocking statement comes from a manufacturer: “I have asked our factory which is especially assembling unicycle. Over 90% disc rotor of unicycle must be adjusted again before the shipment.”
I can’t really believe that and if I did, the problem would have to be looked into seriously.
I had the same problem on my KH26, I just can’t get the disc aligned properly. Like even if I did, it goes out of align easily. When I took it to the LBS, they say it’s because the frame isn’t rigid enough that the brake pads are kind of wobbling around with the frame.
This sounds dubious to me as I assume there must be some KH26 with their brake perfectly working without any rubbing issues.
Later I moved the brake system to a titanium 29er. It does not rub any more, but I can still feel the disc being a little bit tilted when braking, i.e. some part of the disc catches the pads earlier than the other parts. I then left it as-is and try not to think about it.
Unicycle frames have a lot more flex than bicycle forks generally, that is not because of material choice, but mostly because the way we apply forces to it. That leads to more brake rubbing. Up to a certain point it doesn’t really affect anything, but I understand the reason we want them well installed anyways. My brake will rub going uphill when I pull on the handle, but it doesn’t if I just spin the wheel freely without me riding on it.
Installing disks on painted surfaces would be my call for imperfection. Paint can’t always be uniform and it cannot really be controlled. As long as we use painted hubs, there will always be imperfection. I suspect this wouldn’t be an issue on the Qu-Axle and Boundary hubs since they are machined and anodized.
Some brakes also have more space between the pads than others. I’ve found Sram and Formula to be the 2 tightest brands out there. I’m happy with my Hope Tech 3, I get enough room to adjust even though I’ve seen brakes with more space. I think Magura has a pretty big gap.
Spirit cranks - with an externally mounted disc and ISIS hub interface - I think are more likely to exhibit signs of rubbing where the rotor appears to be bent when spinning when it is in fact not. The ISIS interface may not always align the same each time the crank is installed. I’ve seen this on my unis where I remove the disc crank then reinstall it and it is different than before relative to the caliper.
I primarily meant the initial installation of a brake. The fork has no influence at all, no matter how stiff or flexible it is, because I have it upside down on the assembly stand. There is no body weight on the fork. I didn’t think about painting, but I did think about the drilled holes for the disc attachment, for example. It could also be that under certain circumstances it is not 100% flat there. And one unknown in the equation are the bearings. These are all relatively cheap bearings. A small inaccuracy there would probably have a huge impact, e.g. just with a wobbling disc. I mostly use Shimanos MT200, 401 or 420. The manufacturer advertises that the distance between the brake pistons is +30%.
I don’t think bearings can be that far out of round and still work, but the bearing clamp interface inherently has a fair amount of play, which may be a part of the issue. If the disk mount is not parallel to the disk, you essentially reduce disk to pad clearance. (A disk going through the pads at an angle leaves less space than if it was perfectly parallel). That would make setup more tricky and there is even opportunity for movement there.
It would be fairly easy to investigate if the hubs are the cause with a dial indicator to check if they run true. However, the only reason for them being wobbly I can really see is paint, they should be machined on a lathe in the same setup as the bearing seats, it would be unlikely that there is any angle error between those introduced there.