Facing flat mount fork

Question

Front brake on one of my bikes has always been near impossible to adjust. Since it is my first bike with flatmount calipers I thought that this is just specific to flat mount brakes.

However recently I learned about brake mount facing. I have looked at the brake again to find that the caliper is sitting at an angle to the rotor and in fact this is the reason why it is difficult to adjust.

While it is difficult to adjut it is also difficult to take pictures of, but I did my best:

I also included side view with caliper/adapter detached.

So the questions I have:

• Does the angle between caliper and rotor automatically mean that the fork needs to be faced or are there other variables at play here? Could it be bad adapter / bad caliper?
• Is facing flat-mount forks even a thing?
• Which part would get machined / faced? The fork or the adapter?

Thanks in advance!

Update 2024-01-14

I had to call several bike shops before I found the one that does facing of flatmount forks. The mechanic at the shop told me they used a DT-5.2

After picking the bike up from the workshop the caliper is noticeably straighter. Adjusting and eliminating the brake rub now took me under 60 seconds as opposed to ~30 minutes that it used to take.

One thing that I did not realize before and is very obvious now is that I had to discard a set of pads that were half-worn due to irregular wear pattern. Pads were worn at an angle and after fork mounts have been faced, these pads became unusable (more precisely impossible to adjust). I have now fitted a set of brand new pads:

Big thanks to Nathan Knutson. I wish I knew about fork facing earlier...

Answer 1

Yes, flat mount brake mounts can absolutely be faced and sometimes need to be.

The angled pad gap you show in the picture is the primary indication that facing is needed.

In facing operations it's usually the frame/fork that gets faced. It's possible to face an IS-to-post or flat-mount-to-post adaptor, but there's usually not a good reason to, since it's almost always the frame/fork that has the root problem.

Not every facing tool can do flat mounts. The Park DT-5.2 can.

Facing flat mounts on carbon frames/forks can feel a little dicey because it's often the case, as it appears to be in your pictures, that the cutter will be up against carbon and aluminum at the same time. Using the normal steel cutter on the tool isn't an absolute no-no here, but it carries some risk of splintering/tearing the carbon material even if you're careful. Park has a diamond abrasive cutter for the DT-5.2 to address this. You have the kind of fork where that's the right tool for the job.

It's likely you could improve the alignment, and possibly fix it completely, without going all the way to facing. Run a razor blade across the surface of the mount to shave off any paint that's keeping it from being flat, particularly in the areas shown. It looks like there could be some there in the spot that would cause the misalignment you have. You can use fine grit sandpaper to get to a neat final surface.