low power system
 

I don't currently have an ebike system, but I'm toying with the idea of designing a hybrid bicycle and playing around with it. Is there a system on the market with a 100W motor and weighs ~5 lbs including batteries? I figure 100W is roughly half of the power I am putting into the pedals at 20mph. The battery would be sized like a hybrid car battery, IOW, it'd provide a boost on acceleration and hills, and drag slightly on the flats and especially downhills to charge the battery, so the battery pack would be significantly smaller than the normal (from what I can see) ebike system.

The way I figure, it'd be plugable, but it could also work as a closed system that never requires charging, like a hybrid car. Is there a system like that on the market or in someone's garage?

100 watts would be useless to even try, the weight penalty and drag alone would offset any power gain. 250 watt motor is probably absolute minimum to try if you don't weigh too much. with e bikes it comes down to how much is too little and how much is too much.

Actually, 100 watts is the maximum rating permitted to import electric vehicles into Canada as *toys*, last I heard.

tks

Lock
Alive, and Kickin' w/350w in the T-Dot

Check this youtube video about a really light electrical system on a bike: CLICK HERE

If you're creative enough you can probably take the next step and make it useable in real world situations.

SPARC by SRAM and the discontinued Sinclair Zeta III are probably the closest systems to what you describe.

Here is a pic of the 172 W Zeta III which was still available about a year ago at this site:
http://www.zapworld.com/ZAPWorld.aspx?id=2242 a pic is still visible although they seem to have cleared them. They pop up on ebay occaisionally. About 9 mph, 10 lbs, you also have a headlight out of the package. It uses a friction belt drive which is vastly superior to the roller friction drives.

SRAM SPARC has 2 100W motors embedded within a S5 internal gear hub, system with batteries adds about 10 lbs. also. Pricey, close to $1000 dollars. Like the Zap DX, runs the current through the motors in series for low speed and in parrallel for high speed.

You could probably get a 100 W scooter and make that conversion for a lighter system.

If you are doing 20 mph, you are probably putting out much more than 100 W though.

I think in terms of the efficiencies and the physics of it all, you're overestimating what you can get out of a "charge the battery on the flats" system. The thing about cars like the Toyota Prius is that if you compare them to a motorless bicycle, the gasoline motor alone makes the car ridiculously overpowered. (in my experience a steep uphill that slows a strong cyclist to 10mph will allow a hybrid car to go about 35mph on gas power alone.)

With that kind of power, a hybrid car can accept that the gas motor has to put out triple the effort to get forward motion by way of storing and discharging energy* than just putting power directly to the wheels. With a bicycle, that kind of loss is probably a deal-breaker-- make it a plug-in hybrid bike.

*using the batteries and electric motor

One poster mentioned outrunner motors, which if you're good at do-it-yourself stuff sound like a good option for a low-power system. They're definitely available around the 100watt size and they're sold for use with model airplanes. The ~100watt ones weigh about 1.5 ounces.

With the recharging aspect you desire, the developed Bionx system might be the best way even though it is heavier.

Alternatively, an even heavier Zap DX system still has the regenerative braking (it is no longer marketted but is still present on the newer models as an undocumented feature-it was of limitted benefit so they simply stopped marketting the feature as they did before 2003). Zap might be under 15 lbs with small 2 AH NiMH in place of the SLA battery pack.