Aaron, I would define "feasible" as 15-20 miles for a motorcycle, depending on how many batteries you pile on. If you do a light scooter conversion and limit the speed to 30 mph tops than you may get more range if you find a way to mount the motor so it doesn't take up battery space.
If you ride a motorcycle slow at 30-35 mph tops than you might get a bit more range than listed above. Might consider a wheel motor: http://www.electricbikessys.com/Scooter_Motor.htm and converting a small scooter. If you were clever enough and found ways to really pack the batteries in than you could get close to your goal depending on top speed. To give you an idea of range: EVT-168 gets about 20+ miles real world (google it for specs) E-max sport gets 20-30 mils real world without too much use of the "turbo" both of the above are actually quite slow and have not anywhere near the power of the etek. The lectra EMB (out of production) was a bit more powerful and got 12-16 miles real world range http://www.electricmotorbike.org Scooters and other EVs on the market usually get about half the advertised range in real world use. Resources: http://visforvoltage.net http://www.austinev.org/evalbum/type/MTCY Regards, Andrew
