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One of our concerns when we began designing the transmission is the large amount of tension on the derailleur cable that is needed to change gears. The Maxon motors we had available in the lab would not be suitable because they could not provide sufficient torque to the spindle. Additionally, similar motors could provide enough torque to move the derailleur cable, but the derailleur cable could also move the motor shafts. The only way to keep those motors stationary would be to provide continuous power. This would have been very inefficient and led us to consider motors with larger gearboxes.
Another DC motor available in the lab fit our requirements (of being non-backdriveable and providing necessary torque) due to its extremely high gear ratio (728:1). While it was not the ideal motor for this project, we were able to make it work with an alternate circuit design (see electrical design page for details). The motor required a high current to turn under load, and the gearbox on this motor is specified to be 53% efficient. If a smaller, less expensive motor were used, our design could easily be applied for commercial uses.
Three steel mounting plates hold fasten the motor to the bike. Two of the plates are flat and are brazed on the top and bottom (respectively) of the rear wheel supports. The third plate contains two 90-degree flanges; one on the left side and one on the right side (of the bike). Each flange has a semicircle corresponding to the motor diameter on its side. Finally a #6 threaded rod wraps half way around the motor in two places. The rod passes through matching holes on all three plates, and is secured by nuts on the top plate. This holds the motor firmly underneath the rear wheel supports in a sheet-metal cradle.
The spindle is a turned-aluminum part which connects the motor shaft and the derailleur cable. There is a center through hole (axial), for the motor shaft, and a threaded set screw hole, to secure the motor shaft. There is a small groove that serves as a cable guide. In the bottom of the grove is another hole (radial) for the end of the derailleur cable, with another threaded hole for a set screw.
In examining the manual bicycle transmission, we noticed a sheath on the derailleur cable. There is also a small bracket on the bike’s frame which holds the sheath in place and allows the cable to move without affecting the sheath. Since we redirected the derailleur cable to run up the frame toward the seat, we needed a similar bracket. By brazing a small, two-diameter cylinder near the bike’s gears, we were able to successfully mimic the manufactured bracket. Steve Jacobson makes similar brackets for modifications to his own bike. He offered helpful comparisons and advice on this part.