Are there adapters that allow Flat mount callipers on Post mount frames with larger rotors? I do understand that this is impossible with the rotors of the same size, but what prevents adapting a 160mm Flat mount calliper to post mount frame with 180/200mm rotors?

Just curious.

Interesting you should ask. BikeRumor.com just had an article about Wolf Tooth introducing exactly those adapters. Look here: https://bikerumor.com/wolf-tooth-pos...-frames-forks/

Edit: Here is Wolf Tooth's web site description: https://www.wolftoothcomponents.com/...-brake-adapter

I saw that, seems to be a rebranded solution from https://www.assolutions.ca/flat-mount-caliper-to-post-mount-adapter-guide/. Still, I don't understand why it's not widely available/requested.

You could probably increase the rotor size by 20mm by using a SRAM '20 S' spacer kit with the flat mount to post mount adapter.

* The more common type of post mount adapters wouldn't work with the flat mount to post mount adapter because the caliper position on them.

If you already knew about these adapters, why did you pose the question? I expect their lack of popularity is that post mount calipers are still readily available. Why buy flat mount calipers and then an auxiliary adapter? The only need would be if you got a good deal on a post mount frame/fork and already had flat mount calipers. That's probably a relatively rare scenario.

1. Fork: Many (non-mountain) forks are rated for 160mm.

2. Rear triangle: I recently learned the hard way that there just wasn't enough room for my post-mount caliper, adapter, and 160mm discs.

So I coughed up for some new Ultegra calipers.

Now I have two flat to post 160mm adapters if anyone wants them.