I’ve got a Genesis Datum frame on order and have a groupset from Merlin which has post mount calipers. I’m thinking of buying some flat mount calipers because it seems the adapters are nearly £20 each. Do flat mount calipers mount directly to the frame or do you still need some sort of adapter. Running 160mm rotors.

Bump for the afternoon crowd.

I’ve just been and had a look at a Datum. On the rear a 140mm rotor wouldn’t need an adaptor, a 160mm will do. The front will also need an adaptor.

Brilliant, so with 160 rotors I’ll need adapters regardless. May as well stick to post mount then.
So the idea of flat mount is that no adapters are needed as long as you use 140mm discs?

You can’t fit a post mount on the rear, it’s a flat mount frame with a +20mm flat mount to flat mount adaptor.

Brilliant, so with 160 rotors I’ll need adapters regardless. May as well stick to post mount then.
So the idea of flat mount is that no adapters are needed as long as you use 140mm discs?

It’s a bit more complicated than that: front and rear flat mounts are not the same. At the front, the brake sits on a flat adapter that you use one way up for 140mm and the other way up for 160mm. That adapter should be included with the brake. At the rear, the brake mounts directly to the frame so will need an adapter for anything other than 140mm – but a different one from that included with the brake.

I went for 160mm front, 140mm rear on my 4S disc, no need to buy any adapters.

Edit: there are some pictures here http://www.bikerumor.com/2015/03/31/first-look-shimano-flat-mount-hydraulic-road-disc-brakes-debut-new-11-speed-105-level-hydro-shifter-more/ that show the difference. The thing that the front brake is bolted to is the standard 140/160 adapter.

Oh bugger, I’ve already ordered the discs. Anyway will buy the flat mount calipers and turn the RS785 calipers into mtb brakes.

You can adapt a post caliper to a flat mount frame as long as you use 160mm rotor on the rear.

That’s what I thought. http://www.chainreactioncycles.com/mobile/shimano-mount-adaptor-rear-post-to-flat-road/rp-prod137893

But this says it’s for 140mm rotors in the description, but there’s an option for 160.

My mistake then, for some reason thought it was 160 only. Spec book not to hand, rtfm etc..