For a couple of years I had a very niche bike frame, a Haro Sonix. This hybrid URT design was… odd. To compound the oddness I fitted it with a Rohloff Speedhub. Assuming the presence of a derailleur, the frame had no chain tensioning device. Time to add one.
At great expense, I bought a Phil Wood Eccentric Bottom Bracket. The idea of these things is that kind of like a cam, they can be rotated to increase the effective distance between the BB and the rear axle. Unfortunately, it didn’t fit the frame! No worries, it nearly fits, I can adjust it.
The old triple ring crankset I was using also needed some “adjustment”.