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.
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
Electric Mountain Bike - Motor mounted.
Click for a larger picture showing a clearer view.
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.
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.
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.
- 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?