Originally Posted by
JonnyHK
...most MTB put them up on the top of the seat stay. The stays are thicker and heavier (and shorter) than a road bike.
Many of the CX and commuter bikes I've seen photos of seem to have them inside the triangle on a bracket that connects the chain stay and seat stay, presumably acting as a significant brace as well as putting the mount in the right place.
Mounting the caliper inside the rear triangle does make things tidy and adds a measure of protection in a fall. It also is easier to mount racks & fenders with the caliper out of the way. The downside is awkward access for initial set up and pad removal later. The seat stay gets in the way of ratchets & torque wrench when tightening the rear mount bolt, and reaching the inboard pad adjuster on BB7s with short fingers and wide hands is a challenge at that angle. This location can also limit the size of the rear rotor; a slotted bracket mount and arcing stay can fix that issue.
Mounting the caliper on the rear of the stay places the caliper in easy reach. Note the rear drop-out on the white bike incorporates the ISO mount.
There are options for steel forks and disc brakes. I doesn't have to be an unyielding metal monolith to provide adequate support.