Disc brake caliper won't line up using post-to-flat mount adapter
 

I'm currently working on a new bike build. Everything was completed yesterday and the only thing left was to attach the front disc brake caliper. The fork is flat mount and the disc brakes are post mount using a post-to-flat adapter. I attached the adapter to TRP HY/RD brakes I'm using, but the disc rotor ended up rubbing towards the outer pad and there is no adjustment space on the bolts to center it. I tried using washers underneath the post/flat adapter, but that didn't help and I tried multiple adjustments including trying to realign the wheel with no luck.

So I'm wondering what options I should try next? The post-to-flat mount adapter I was using is Shimano. Should I try another adapter? I was thinking maybe the Shimano adapter isn't playing well with the TRP brakes, so I was wondering if a matching TRP adapter would make a difference? Maybe it's the fork that's causing the issue? Since it's a flat mount fork, perhaps the tolerances aren't tight enough to compensate for the addition of the adapter? Last option would be to swap out the post disc brake for a flat mount version? Though I'd prefer not to do that, before exhausting all other options.

Any advice would be appreciated. Thanks!

Is the adapter symmetrical, and if not, is it in the correct orientation?

The Shimano adapter is brand new and looks straight. The frame itself is post mount and the rear post brake align just fine. I initially bought the flat mount fork thinking it would be more future proof if I wanted to go with a newer groupset later on. The TRP front brake caliper is practically brand new.

I notice when I have the adapter not fully screwed in I have some play to center the brake calipers to the rotor, but the moment I tighten the adapter to the fork, the alignment goes off by a few millimeters causing the pads to lean on the rotor with no room for adjustment. I tried 2 small washers under each bolt of the adapter, but that didn't seem to help. Also I don't think spacers would make a difference with the brake bolts. I'm ordering another adapter and see if that'll help on alignment, but I'm worried it could just be sloppy tolerances on the fork bolt mounts, since it was probably intended to be used with regular flat mount brakes.

Is it possible that some file work on the adapter would help the alignment. If it was my bike I would make the changes to the (relatively inexpensive) adapter rather than the mount on the fork.

Resetting your pistons may help to resolve this issue or you may have a sticky piston on one side that is causing this issue. Remove pads and use a piston press or a plastic coated cone spanner to push your pistons all of the way into the calliper, pull brake lever very lightly to see if both pistons are moving freely and equally (be very careful not to push pistons all of the way out). If not clean pistons with isopropyl using cotton buds, then with a cotton bud dipped in brake fluid smear around sides of piston. Hope this helps.

Does this mean you tried adjusting the dish or whatever it's called, to move the hub/rotor over? If not,I'd check that.

I'm wondering if this might be the solution? I notice as long as I don't tighten the bolts to the recommended spec, the calipers don't rub. If I file off a bit of the adapter bottom surface to be slightly angled it might actually align the brake calipers better without having to face the surface of the fork or do any mods to the brakes themselves.

I tried this last night and it didn't much. I might give it another try and see if it pushes pistons back in a bit more.

Not really, I just checked if there were any alignment issues with the thru axle and I also checked the disc rotor and both appear true.

That's what I would do at this point. I assume the adapter is aluminum, so it would't be that difficult to shave off a bit at an angle.

jonathanf2 I just remembered facing disc brake mounts is a thing:

it's very common for flat mounts to be out of spec. I can't imagine it would be your adapter. You need to find a bike shop with the facing tool shown in the previous post. My lbs faces every flat mount bike that they sell.

I'm wondering though, if I face my flat mount to bring the alignment better on my post mount adapter, will that affect alignment in the future if I plan on using flat mount brakes?

I was just thinking, if I face the post mount adapter, I could file it at a slight angle to compensate misalignment without affecting the fork.

I just called my LBS that does all my work, unfortunately they don't have that particular Park Tool facing tool. Any suggestions for doing the facing work myself?

Don't try to face the fork without the proper tool, it's too optimistic to eyeball the necessary tolerance for road disc.
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Find a shop with the tool.

Quick update. I found an LBS that will face the fork with the proper Park Tools facing tool and the procedure shouldn't cost more than $85 (or less). Though when I was talking to the mechanic he advised against the procedure recommending I just go with a flat mount disc caliper. So looking at my options, I could just buy a single mechanical TRP Spyre for $65, but for the cost I could spend a little more and get a pair of Shimano BR-RS305 flat disc brakes for $112 (no experience with those brakes). If I want to properly match my rear post TRP HY/RD caliper, I could get a pair of TRP HY/RD flat mount brakes with/rotors for $145. Unfortunately I cannot find those particular calipers sold individually.

I'm leaning towards just getting the TRP HY/RD pair, but I'd be open to other brake suggestions or just go with the flat mount facing. Thanks!

I'd get the TRP HY/RD pair and sell off your excess parts. You won't lose that much $ in the end.

Second follow-up update! When I posted this issue a few days ago, I had ordered a third party post-flat adapter from Amazon, but I didn't think it would have resolved my alignment issue. Anyways, after posting my first update the part arrived 30 minutes later and I decided to install it. Amazingly, the new adapter appears to put the caliper closer into alignment (just barely though). I'm not sure if the Shimano adapter was perfectly in-spec or this no-name adapter just had more leeway for sloppiness. It appears to have resolved my rotor rubbing for now. Thanks for all the help!

My experience as a working mechanic is that out of spec flat mounts are pretty common. You'll only make things better getting the mounts faced accurately. Had to do this to an entire run of bikes for the floor.