Davey,
Looking at how Surly implemented a chainstay-mounted disc brake on their Long Haul Trucker (
http://surlybikes.com/bikes/disc_trucker ) I would imagine it would require a new left-side dropout, and that could be costly, given Thorn's proprietary dropout for the Rohloff reaction tab. I'm not sure a disc mount there could replace the brace between the chain- and seatstays that appears on Sterling frames I've seen in photos.
I'm guessing such a change might have to wait until Thorn introduce an entirely new frame designed around this very feature, as it doesn't look easy to implement in the existing design.
Might it be easier to modify the BOB Yak trailer mounts? I think that's the route I'd pursue.
As a hobbyist framebuilder, I like the looks of a chainstay-mounted caliper, and the chainstay certainly presents more "beef" in absorbing brake torque than does a thinner seatstay, but it would be considerably harder to mount one there without a bit of work and there could well be clearance issues with the Rohloff shifting mechanism, depending on the design.. I fabricated a new under-seatstay mount for my tandem's Arai torque-reaction arm, successfully brazed it in place, and it works fine and looks factory original after a repaint. I believe relocating the caliper could be done as a retrofit if you didn't already have a brace there on your Sterling. It would, of course, mean the end of your Thorn frame warranty and might not even be possible on an existing frame. It would take a pretty thorough redesign to accommodate.
You have a wonderful bicycle in the Sterling.
Best,
Dan.
