I recently got a new toy, a Raptor 4.0 electric skateboard made by the Exkate folks in California. Let me tell you this is ~by far~ the best 'toy' I've bought for myself, and believe me, I've bought many toys over the years ;-)
Anyway, the reason this is relevant is because I've found this skateboard, powered by a 450W 24V motor, is able to climb some serious (bicycle => granny-gear) grades and has incredible range (I cruised for over an hour by the waterfront in downtown Portland last weekend with juice to spare)!
Experiencing this two facets (speed & range) of electric propulsion first hand has inspired me to want to extend my electric experience to my commute (~13mi, some hills), especially with summer now here and gas approaching $3.50. I figure that if the single-speed electric skateboard, with its single-speed drivetrain and tiny soft wheels, and an upright rider, can already do so much, then a well-designed electric road bike may actually be a viable transportation vehicle!
My primary design goal is to attain a 'safe' average speed of 20mph, with a useable range of 15mi (I can recharge at work for the return trip). I'm not a super athlete, but I reckon I should be able to put out 150-200W without breaking a sweat (which is a design goal for a commuting bike). To take a bike up a 6% slope at 10mph takes 300W so assuming a continous-duty derating factor of 0.8, a motor mechanical-efficiency factor of 0.6, I figure I'll need a motor of about (300-150)/(0.8*0.6) = 312W rating. At 36V, this would imply a peak current of 8.7A. http://www.electricscooterparts.com/motors.html has a very well built surplus 300W Kollmorgan (not the cheapo Chinese motors like those on my skateboard) for $40, which for me, is what inspired this whole train of thought!
Assuming a desired average combined output of 300W, I put in 150W, and the bike puts in 150W, and assuming a motor-controller efficiency of 0.9, a battery conversion efficiency (high-current derating factor) of 0.8, a discharge factor of 80%, for trip duration of 40mins, I figure I need a battery of (0.7*150)/(0.9*0.8*0.8) = 182Whr.
The batteries I have in mind are the new Dewalt 36V lithium nano-phospate packs that uses the bleeding-edge crem-de-la-creme cells made by A123 Systems. They are 2.4AH, I believe, or 86Whr. These go for ~$110 on Ebay these days, so I can only afford two, for 164Whr, which is shy of the 182Whr above calculated :-( Oh well, I guess I have to get off my lazy butt and up my leg share to 55% (165W).
Which brings me to why I'm posting this -- the drivetrain. This is no doubt the most challenging, costly, labor-intersive part of this whole project to build a custom e-bike. I would like to find a partner (or two) who is interested in building such a bike in the hopes that together we can drive cost, effort and risk down while banging our collective heads together and (hopefully) having fun as well.
My philosophy is to KISS as much as possible. I view the two foremost goals of the e-bike are to make hill-climbing a non-gasping/sweaty affair and upping average travelling speed -- all other considerations can be sacrificed. My current idea for the drivetrain is a very simple one, but one that I think can be made cheaply, is highly efficient, robust and lightweight. The only caveat (heh heh) -- clipless pedals are required to ride this bike (which should not be a problem for most experienced bicyclists).
I have a Trek aluminum (all round tubes) road-bike for this project. I'm thinking of taking a bicycle chain sprocket and attaching that to a jackshaft (i.e. an intermediate shaft). This shaft gets attaches to the bottom of the upwards-sloping bottom tube of the bike. A chain links the innermost (smallest) chainring on the pedal cranks to this sprocket. On the other side of the jackshaft is a large pulley, and the motor is mounted as low as possible on the inside V of the frame and drives the pulley via a timing belt and a small pulley. That's it! If you've followed me this far, you'll ask -- where's the freewheel? Er hmmm -- remember the clipless pedals requirement ... ?! Don't worry, to prevent embarrassing bloody shins when you walk in the office in that all-revealing lycra, a safety motor cutoff switch will get installed on the brake lever -- pull the brakes and the motor-controller shuts off immediately. A resume-button will resume the previously set throttle-level. Seems vaguely familiar? Well, you heard it here first -- the world's first cruise-control bicycle!
So if you're read this far, you must be interested, or critical -- in any case please reply and let me know :-)