Electric Bicycle

e bike

My Electric Bicycle
I describe how I put together an electric assist bike...

Why convert a bike?

After my disappointing experience with lead batteries (plenty of current flow, but HEAVY) while I was converting my Electric GPz , I wanted to convert a mountain bike to run on electricity stored in NiMH (Nickle Metal Hydride) batteries, using off-the-peg kit if possible.
I had heard mention that the six tapped holes designed to enable a brake disc to be mounted on an otherwise stock (rear) mountain bike wheel, could easily have a sprocket mounted on it - and this is exactly what I did.
e bike
Electric Mountain Bike.

I bought a currie kit (450 watt motor fitted with a freewheeling sprocket and a 40 Amp Ananda controller) for $70 on-line, a used mountain bike collected locally (GBP 15 on eBay - receipt obtained!) and several NiMH 1.2v cells.
The 15 tooth sprocket fitted to the Currie motor was #25 (1/4") chain size - a tad on the small side, but should be OK - so I needed a largish #25 sprocket to drive, say 70 or 80 teeth.
e bike - Motor mounted
Electric Mountain Bike - Motor mounted.

Click for a larger picture showing a clearer view.

e bike - Sprocket
Electric Mountain Bike - Sprocket detail.

The sprocket on a stock Mini-Moto has 68 teeth and is #25 chain size. I bought one on eBay (GBP 1.50) bored out the central hole a few mm and drilled the required 6 holes (Hub dimensions and CAD file here).
I already had a suitable throttle. So the remaining problems to solve now became 1. How to mount the motor and 2. How to arrange my batteries.

1. Motor Mount

I wanted to keep things simple if possible, so instead of trying to braze motor mounts directly onto the frame, I fitted a strong aluminum rear rack and made a housing for the motor, and clamped the motor housing to the rack. (this means the kit is bike-to-bike swappable)
I had some stainless steel floating around in my garage, and after making a card template, formed a 'U' shaped 'motor housing'. I slotted two of the motor mounting holes (the third being a pivot) and had a fairly unsatisfactory, but 'chain tensionable' housing. I was much happier with the overall rigidity after I fitted a (carbon fibre!) lid to the thing.

e bike - Motor
Electric Mountain Bike - Motor detail.

2. Battery Mounting

As the batteries were 1.2v and the motor was 24v, I thought I'd slide 10 down one piece of plastic pipe, another 10 down another similar piece of pipe, connect the two in series - job done!
It almost was a simple as that. Standard UK plastic waste pipe is 40mm diameter (I erroneously thought I might need to run a wire inside the length of the pipe - incorrectly as it turned out) and grey in colour. If I did this again I'd go for the white 1.5 inch pipe also commonly available in the UK. The batteries would then be a snug fit - no rattling.
I fitted a pair of endcaps, used nuts and bolts for terminals in those endcaps, pressed tightly together with a compressed spring - a'la torch style - and that was my power supply sorted.
e bike - Battery Tubes
Electric Mountain Bike - Battery Tubes detail.

After charging the batteries (home brew charger, entirely capable of cooking my new batts!) I went for a spin or two. Works fine, except I either need a much more powerful motor (and I'm already more than twice the permitted UK wattage!) or I need to adjust the gearing.
I had of course done the calcs (doesn't everyone?) and knew that the motor would try and push me along at just under 40 MPH - given infinite power!
I intend to improve matters by:
1. Fitting a 90 final drive sprocket (instead of the 68 tooth MiniMoto one) and
2. Will consider fitting a 600 watt currie motor - expecting 24v - which just might find itself receiving 36v...

With mod number 1 above, the maxed out motor revs (2300 RPM) will try to push me along at 29 MPH. Not brilliant but better than the existing (39 MPH). Ideally I'd also change to the larger #35 chain, but the 15 tooth freewheel sprocket (means with the motor switched off and me pedalling, I'm not trying to spin the motor) is a nice feature.
If I could swap out the 15 tooth driving sprocket for a 12 tooth sprocket (driving a 90 tooth), that would top out at a theoretical 23.7 MPH - much more realistic.

(Oh - and before anyone asks - no, a dynamo on the wheel would not allow me to perpetually remain in motion - but you knew that anyway, right? )

E Bike Detail