eV Challengers: Drivetrain

Sit back and relax, coz this is a long one.

From the very start of this project, we knew this years drivetrain was going to be a beast to complete. After many, many designs and layouts we finally decided on a motor > gearbox > rear wheel arrangement, all chain driven. Sounds simple? Unfortunately, far from it.

Here are the specs for our setup.
Brushless Motor with inbuilt ESC - 400w, 24v, 2500rpm
Sturmey Archer Gearbox - 3 speed (1st - 33% reduction, 2nd - 1:1, 3rd - 33% increase)

The main difficulty was that the motor revs at a nominal speed of about 2500rpm and we want the rear wheel to max at around 450rpm (40km/h). This means a huge amount of down gearing is needed. So we needed a small motor sprocket to large gearbox-in sprocket then a small gearbox-out sprocket to large rear wheel sprocket. Unforutatley no one really makes the extreme parts we needed, like a 53T rear freewheel sprocket! We had to make one out of a regular bicycle freewheel and 2 large chainring sprockets. In the end (after a bit more bodging) we had a 9T motor sprocket, a 28T gearbox-in sprocket, a 21T gearbox-out sprocket, and a 53T rearwheel sprocket. This gave us roughly the ratios we were after.

The other difficulty was that we now had 2 chains to align, whilst keeping them taught and all inside the rear triangle of the bike frame. Chain alignment is very important in terms of efficiency. Not only can a badly aligned chain fall off, but it will also grind on the sprockets causing friction that will slow the chain down and make the motor do more work. It will also wear away the chain and sprockets faster. Its preferable to have the chains nice and tight so there are no losses to vibration and slipping.
A lot of patience was involved in this process of adjusting the positioning of the gearbox and motor into alignment and then making sure the chains were tight. It actually took 2 goes...