Flat-mount to post-mount converters

So…the lure of a bargain is strong - but I have both hydro and cable/hydro brakes in post mount (105 and TRP Hy/Rd, respectively)
Questions:

  1. Modern frames are flat-mount - do I just buy some converters and use existing calipers?
  2. My winter bike has the hydro discs, but this would be a new gravel bike build. Do I use my nice 105 bits off the winter bike? To be honest, it’s a b@ll-ache what with bleeding, etc. and also, the boggo bits off the existing gravel bike are okay, and I wouldn’t be upset about wrecking them.

Thoughts?

You want to reuse existin post mount groupsets on a flat mount frame?

I believe their are adapters but result might end up ugly. You’d better swap the calipers as you will need to rebleed the brakes anyway. You should be able to buy a pair of shimano 105 calipers for ~130€.

I would be careful with flat-to-post-mount adapters. The adapter and post-mount caliper are much bigger and may interfere with seatstays on the frame. It will depend on the bike and your bike’s size (larger bikes have more clearance).

A hydraulic caliper swap is a lot easier than you think. It’s a good opportunity to get some hands-on experience:

Just make sure you move your brake rotor and pads far away from the oil. Preferably in a different state.

Sure…works fine. Note - you can’t go the opposite way though (post-mount adapter to flat-mount caliper)

Adaptors do exist but they’re a lot harder to find:

Post mount adaptors will work fine but as mentioned above, the aesthetics are, challenging…

Interesting…I searched high and low a few years ago for those and they simply did not exist.

Ended up just getting some cheap post-mount calipers (Deore, IIRC).

Yeah, but that would mean disassembling the winter bike rather than just carrying over existing TRP Hy-Rds from the CX frame.
The calipers aren’t the issue - new Hydro calipers aren’t that expensive - it’s the bl00dy STIs that are the pain