DIY Electric Bicycle, Entry #7: Power And Control

My original plan for controlling the speed of the bike was to use a chip to produce a PWM signal, which in turn would drive a stack of MOSFETS or a relay, and that would control the motor. However, when I was doing some bench-testing of the starter motor I bought at the Volkswagen show, I decided PWM would be pretty useless on something like that. I realized that turning the motor on and off over and over again would waste a lot of power due to the motor slowing, then speeding up, then slowing back down again several times per second. I decided it would probably drain the battery even faster than I had expected. Because of this, I decided not to use PWM. Instead, I plan to have a momentary push-button switch as my "throttle," mounted on the handlebars where my thumb would normally rest. I could easily pedal up to speed and simply tap the button to give the bike a bit of a boost. This idea also prevents the motor from staying on too long and reduces the risk of it burning out.
I still have not yet decided what I will use for a battery. Right now I'm thinking of having two motorcycle batteries, which could easily fit on the rack behind the seat, next to the motor. I plan to have the bike frame itself grounded, so I only need one piece of wire to go from each "component" to the battery. This will not only be easier to do, but it will also prevent a tangled mess of wires that--let's face it--looks just terrible on an electric bicycle. :p