Tim,
i have thought about this quite a bit over the last few days. and i would like to share my thoughts.
any type of chain drive would require many fabricated and custom designed parts. it would be a very involved process to make. it would involve a machinist/fabricator and would be beyond the average bike shop. the biggest problem would be the chainline changing everytime the gears are shifted. the next would be the motor mounts.
that leaves using a hub motor. i would use one that is designed for lots of torque and low RPM. you would be using a rear motor. this motor would have a thread on freewheel on one side and a disc brake on the other.
here is how i would do it.
Motor: Crystalyte 5306 or 9Continents model 2810. both of these are high turn count windings and are designed for high torque and lower RPM. you can use a 6speed freewheel with either. i don't think that the orignal Sachs freewheel will fit. but you should be able to adjust the existing Sachs deraileur and shifter to work with standard freewheels mounted to the motor. mount a large disc brake rotor to the motor. this motor can be laced into your existing rim by your bike shop. if your wheels are 26" you can get the motor already laced into the correct rim. the Crystalyte motor will be more expensive, heavier and a bit more powerfull.
Brakes: use an axle mounted disc brake adapter to attach the caliper. a simple bracket can be made attaching the torque arm of the adapter to where the torque arm of the existing brake is currently attached. i would not skimp on the disc calipers use hydraulics. the best that your budget allows. use the existing lever and cable arrangement to activate.
Controller: a 12 FET 40A 36 to 48V brushless speed controller. for the throttle use one of the available PAS sensors to match motor speed to pedal cadence. add a magnet and reed switch to the brake lever to sense when the brakes are activated to cut the motor. i would also add the Crystalyte Cruise control module.
Batteries 36V or 48V Lithium Manganese or LiFePO4 batteries to save weight over the ususal Sealed Lead Acid Batteries. they will save weight and space. mount these under the seat. brackets will have to be made for these. i would choose batteries that come sealed in a case. like those available from ebikes.ca or e-bikekit.com.
budjet about $1100 for the 9Continents motor, controller, 36V batteries. the Crystalite will be a little more at about $1400. add $150 for PAS sensor, e-brake sensor and Cruise control. budjet an additional $400 to $500 for the disc brake calipers, rotor and mounting adapter. remember to add the time that your local bike shop will charge for installation. total cost could easily exceed $2000.
hope this helps. maybe your local bike shop could use this as a start in designing your system.
rick
