*** Disc brakes (Not recommended):
1. Disk brakes are expensive to maintenance and, because of how they work, put a lot of stress on the spokes w/o motorizing. Braking stress combined with engine torque, puts stress on spokes going and coming.
2. Some disc brakes sit within bike frame, only requiring a washer or nut to space lower strap out, for clearance. Some find the need to grind or file an 1/8" sliver (crescent) from lower strap for brake clearance, others protrude so far beyond the frame of the bike that installation becomes tedious, if not impossible. If possible, we recommend you steer clear of disc brakes.
To check to see if the disk brake would obstruct clearance of Lower Mount Strap - a yardstick is helpful (or something similar).
Place yard stick on top of axle - pivot toward seat then back toward rear of wheel - does any part of the disk brake protrude so far beyond bike frame that it would interfere with the lower strap?
We encourage engine placement as close to the seat as possible for better balance. Installation details may be helpful as well, see Fig E (halfway down the page).