We converted a "foot-forward" Sun Brickell7 tandem beachcomber to a mid-drive Bafang motor for hill assist. Later commissioned a custom, coupled steel frame** with many desired changes for handicapped adaptation.
Lessons learned on both builds:
1. Move entire drive train to right, driven by the Bafang.
2. Bafang chain ‘spider’ is threaded for four-hole chainrings only; we finally found a five-ring adapter made by California Cycle I think.
3. Bafang is not centered left to right; chainline is WAY out 1-1/2” to right of normal. This mandates a customized drive train, loads of spacers and new/machined parts, and may hit frames, feet, etc. Changes how you ride- no longer centered left to right in saddle.
4. This precludes any derailleurs. Used Alfine 8-speed which skipped, and Rohloff 11-speed disc hub which is wonderful but costly.
5. #4 and #5 may not clear your frame, period.
6. Bafang has a freewheel; we quickly found a freewheeling chainwheel (Sick Bike Parts is the brand) for the other person so we can pedal and stop independently. This is important on startup and goosing motor on hill or hazard.
7. Second build moved motor and battery to the front under captain for better balance and handling. First build had them in back; front wheel had nearly no weight on it on hills and handled terribly.
8. Enormously heavy bike due to 38 pounds of motor and battery, heavy Rohloff, steel wheels and 18 pound frame: loaded we run about 92# with tools and spares. 400# GWV with riders and medical gear means braking is horrible. We retrofit with 203mm (I think, maybe larger) manual disc brakes but it still takes too long to stop. We retrofit with third caliper rim brake in rear for downhill drag brake- required welding and grinding of our new frame. (It drags but absolutely does not stop us no matter how hard it is squeezed.)
9. 26” steel wheels for strength, 2” Schwalbe Marathon tyres. We bike max 15mph on rail trails and some gravel to explore. Almost never road bike except during Covid. Handles like a tank, cannot U turn in a road have to dismount.
10. Motor: day one I disabled the PAS pedal assist- scary for tandem or handicapped rider. Nudging the pedal resulted in lunging. Reprogrammed the Bafang for throttle-only 100% and have to finger the heads-up display from 0 to 9 before any power to motor, for safety. Also used password protection so someone else cannot start motor once shut off.
11. Motor use: We always pedal and use it for hill assist and soft ground assist. The 1500watt 48-volt motor (52v battery) in low gear can easily pull 400# GVW up a steep hill for ¼ mile without pedaling, without overheating. Typical usage is low throttle 30watt to 325 watts on nonpaved surfaces or windy conditions; 400w to 900w assist on hills. Never had an overheating issue. 20 to 38 mile rail trail trips have only exhausted the battery once, at the very end. Usually draw battery from 58v fully charged down to 50, occasionally 48. Programmed to cut out at 42.
**frame fabrication by Roulez Cycle, Lynn MA.
Eleven builders said it could not be done! Kudos to Jay for a monumental task and loads of custom machining. He is a master of innovation.