I think the laws about electric bicycles have changed a little since I built mine. But
as far as I'm aware, the changes are not retrospective, so any bike built like this one
is OK. Just be vague about the date of construction!
An electrically assisted bicycle was not considered to be a motor vehicle (No driving license required) and could be ridden by anyone over 14 years old, so long as it met certain criteria.
The first thing to do is to fit a "rear wheel" in to the front forks so that there is
a sprocket and free wheel unit attached to the front wheel, for the motor to drive. Rear wheel units are usually a bit wider than front wheels, so you will need to widen the forks a little. A car jack is a
good way of doing this CAREFULLY! Also the axle is a slightly bigger
diameter, so the slot in the front forks will probably need some attention from a
rat-tailed file.
I bought a 12volt, 200watt, motor and gearbox unit from "Parkside Electronics" Nelson, Lancashire. Its output shaft rotates at 180RPM . These are the sort of motors used by people building 5inch gauge model railways. Its output shaft is a hollow tube, so I made a clamp to grip it tightly, and welded a sprocket to the clamp, after making sure the sprocket was running true.
My push bike has 27x1.25 inch wheels. At 15MPH these will be rotating at 185 RPM. Some experimenting with sprocket sizes was necessary. I think I ended up with a 21tooth on the motor. If the bike goes faster than 15MPH (Down hill for instance) the the motor continues to rotate at its own speed while the free wheel allows the front wheel to rotate faster. The rider can then flick the switch to off
to save power, while the bike continues in ordinary push bike mode
I gave a lot of thought to re-generative braking., In simple terms that means you put electricity back into the battery when going down hill! At first glance this seems like a great idea. But....... after some more thought I realised, that on most hills you don't want to decend at about 5MPH, being held back by the motor braking. You want to go down as fast as possible, to get as far up the other side as you can before needing power assistance. Momentum is the best way of conserving energy.
| The motor unit is mounted on two legs, made of very thin wall steel tube flattened at the ends. The upper ends have a short Tee piece welded to them to bolt the motor/gearbox unit to them. As shown in Fig1. right. |
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 | The unit had a tendency to distort like this, so I welded a small bit of metal across the two legs, beneath the motor. This made the whole unit very rigid. |
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The whole unit is mounted on two legs, infront of the head stock. and it is stopped
from swinging forwards by a big pipe clip, right round the motor and the cycle lamp
bracket. These legs not only support the motor, but also act as radius arms, keeping the
unit the correct distance from wheel axle.
As you can see in the pictures, I decided to fit a selectable gear system on mine,I found this was not really necessary, except on very steep hills.
This page will be continued when I get a bit more time .............
There are two 12volt 7ampHr gelcell batteries in a ply wood box.
