This saga began over a year ago when I thought "hey, it would be trivial to add an adapter to allow plugging in an extra battery pack to my charger bicycle and get more range" .. unfortunately I didn't count on the Trickster smiling upon me in the form of toasting the controller board. That led to a complete reconfiguration of the control system and replacing the bottom bracket which required surgery to the chainring etc.
A bit about the Charger bicycle ... it was designed by Aerovironment, the same company that designed the EV-1 for GM, and I think the same engineers might have designed (part of) both. It's a "pedelec" design meaning the controller senses when you pedal and fires the motor to match your pedaling. There's no throttle. There are four power setting buttons controlling how much assist is added. The controller is embedded in a battery box that fits between your legs. The motor is a REVCOR motor, the same model (but a beefier size) as the ones ZAP used on the Zappy and DX/SX kits.
The charger bicycle was purpose-built as an electric bike. It's got a good frame and good parts. It originally sold for (I think) $1500. The company went out of business and Sam Wonderly has all the remaining ones in storage. See him at http://www.electroportal.com
So with a toasted controller board ... I could have found a replacement board, and I contacted Sam. He was gonna charge me a big pile of money because there's no source of controllers other than the ones on the bikes he has in storage. Or he could have installed a different controller and he mentioned having to do something with the bottom bracket but I had no idea what that phrase meant, nor how much trouble it would cause me to not take him up on the deal he offered. I suppose by now he's worked out the steps for what I ended up doing, but at the time he wasn't quite ready and we didn't manage to make a deal for him to do the conversion for me.
For the controller I chose a 4QD Uni-8, 24 volt. They're a well-made unit that comes from the U.K. and I've also used one on the Vego I'm working on rebuilding. I worked out a way to mount the controller inside the battery box ... the battery box has a huge heat sink which the original controller was mounted on, so I drilled some holes, mounted the controller and an on-off switch.
I thought that would be it. Just a matter of attaching the controller to the heat sink and wiring it to the batteries and motor.
But ... The motor and the pedals each have their own chain which goes to sprockets on the rear wheel. The rear hub is a Shimano Nexus internally geared hub, so there's no external deurailer etc. The two sprockets are on a common freewheel meaning that when one of the chains has power it ends up spinning the other chain. That means when the motor has power, it spins the chain coming from the pedals, and the pedals spin. This makes it real hard to ride the bicycle properly.
I posted about this on the VV.com site (hmm, I should try to find those postings and repost them here) ... and coincidentally someone else had faced the same problem and was able to tell me the solution.
There are some cranksets where the crankset does freewheeling. One version is the Shimano Front Freewheeling System (FFS) which was built for Schwinn and used in the Schwinn World Ladies bicycle of the 60's and 70's. The FFS may have also been used for tandem bicycles. Another possibility which I just recently learned of is the Vision, Independent Peddeling System which was also built for tandem bicycles. On a tandem bicycle it's important to allow either rider to pedal independently of the other, so the cranks need to freewheel.
I also read about rear wheels which have threading on both sides and you could install a freewheel sprocket on both sides of a rear wheel. Thus the sprocket on one side could be for regular pedaling and the one on the other side could be for the motor. But the Charger bicycle is designed a certain way, with both chains going to the same side of the wheel, and there wasn't any sprocket I could find that had two independent freewheeling sprockets. I did find a dual freewheeling sprocket, that could screw onto a rear wheel, made by White, but the shop that carries them says that the sprockets do not independently freewheel.
Anyway, back to my bike.
I eventually found the FFS crankset from a bicycle shop in Kansas City that specializes in used parts.
Unfortunately the chainring on the FFS is too large for the Charger bicycle frame. This really became like that childrens book .. Fortunately/Unfortunately .. fortunately I found the right crankset (FFS) but unfortunately it didn't fit the bicycle frame. By "too large" I mean the chainring intersected with some frame tubes and hence the crankset wouldn't be able to be installed all the way.
Sigh. That was last September or so and I've been puzzling over what to do ever since.
What I finally did was pure hackery. The chainring on the FFS is not removable so the option to replace it with a smaller diameter sprocket wasn't possible. However the one on the FFS has some holes which I thought might fit the holes of regular chainrings. I finally got up the nerve to do this ... it required cutting the FFS chainring so that only the minimum that provides a place to bolt on a regular chainring was there. I found a suitable chainring at an online shop, of the correct diameter, and it arrived a few days ago. Today I got out my grinder and cut up the FFS chainring grinding it down quite aways, and then bolting on this other chainring. I used some locktite in the bolts so that hopefully the chainring will stay in place.
It works. And suffice to say that I now know what a bottom bracket is, as the above involved replacing that thing.