We get a lot of press releases for e-bike conversion kits
in our inbox. Most are hub-drive motors, which aren't ideal for mountain biking. Recently we got an email from an Italian college student called Davide Zanetti who has made a DIY mid-drive motor that can be fitted to a regular MTB, and he says it only cost around $300. The advantage of a mid-drive motor is that it utilizes the bike's gearing, so it can provide more torque to the rear wheel when climbing in the lowest gears. It also improves the sprung-to-unsprung mass distribution for better suspension performance.
Davide's DIY unit uses a skateboard motor which he's limited to 300W of power. This isn't far off the output of commercial e-bikes, and Davide says it's enough to roughly double his climbing speed. It's paired with a DeWalt 5ah 21v
power tool battery. These only provide about 100 Wh of energy, which he says is enough for about 500 m (1600 feet) of elevation gain, but because these batteries are relatively cheap and light and commercially available, it's possible to carry a spare or two to extend the range. You could even pop into a hardware store if you run out of juice!
The motor uses a belt drive on the left-hand side to provide a 4.5:1 reduction gearing. This is connected to a 12-tooth sprocket on the right, which drives the crank via a chain and a larger sprocket welded to the chainring. This should provide an overall reduction gearing of about 12:1, meaning the motor's torque is multiplied by 12 times at the crank. The skateboard motor is only rated for 2 Nm of torque so that works out to about 24 Nm at the crank - about half the assistance of the lowest-torque commercial e-bike motor, the TQ HPR 50
. But still, if you use a high cadence that should still be a meaningful amount of assistance. Davide says his drive unit weighs 2.5 kg including the battery. That compares to 3.7 kg for the TQ (the lightest on the market) with its 360 Wh battery.
Best of all, it can be removed with a few bolts to return to a regular bike.
There doesn't seem to be a pedal sensor but rather a throttle on the handlebar to control the motor. This technically makes it different from pedelecs
(which we usually incorrectly call e-bikes), because the bike can move without the rider pedaling. This may have certain legal implications depending on the country.
But whatever the case, it's really cool to see what can be done on a budget with a bit of ingenuity. While this is just a hobby project, the idea of a lightweight, removable motor that can be fitted to regular bikes with interchangeable off-the-shelf batteries is something I think plenty of people would be interested in.