Is this motor useful for a DIY ebike?

Kawana

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May 14, 2022
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I was given a DC motor/gearbox from work for free and wanted to put it to use. It's a 12v, 900w, 54rpm (73:1 gear reduction). So it's pretty high torque (190ft/lb) which would be good with all the hills we have here, but the low RPM wouldn't make for a very fast ride. That's not my biggest concern because of our dumb 20mph limit here (for non-insured electric bike). Id need a 4.5:1 sprocket setup to reach the required SFPM for 20mph.

It could allow me to use my 18v milwaukee batteries for a power source possibly.

Does this sound like a contender or should I sell it to get something else?
 
Kawana said:
I was given a DC motor/gearbox from work for free and wanted to put it to use. It's a 12v, 900w, 54rpm (73:1 gear reduction). So it's pretty high torque (190ft/lb) which would be good with all the hills we have here, but the low RPM wouldn't make for a very fast ride. That's not my biggest concern because of our dumb 20mph limit here (for non-insured electric bike). Id need a 4.5:1 sprocket setup to reach the required SFPM for 20mph.

Put it thru your pedal drivetrain using a 1:2 ratio chaindrive into the pedal chainrings, and then you'll have about normal human cadence range out of it, and then your pedal drivetrain will do the rest of the gearing changes for you.

If you drive the left side, Stokemonkey style, then it leaves you all the rightside gears like always. Use a freewheel on the motor output shaft so you aren't backdriving the motor when just pedalling. (unfortunately the motor will always drive the pedals this way, so you need to keep your feet on them thoroughly even when not pedalling, or get whacked in the hamstrings just like you would if a human stoker on a tandem kept going when the captain stopped pedalling).

I did basically this type of setup to use an internally-geared powerchair motor on an early version of CrazyBike2; you can see pics of the various versions of that in the CrazyBike2 thread
https://endless-sphere.com/forums/viewtopic.php?f=2&t=12500
(and on my old http://electricle.blogspot.com blog about it before I found this forum). It's more complex than you would probably need it to be for your setup; mine was this way because the bike is very long....
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It could allow me to use my 18v milwaukee batteries for a power source possibly.
Note that using the 18v batteries on the "12v" motor (if that's automotive 12v it's more like 13-15v) means it will run proportionally faster at full voltage (full throttle), meaning 54rpm * 18v / 12v = 81rpm. (so already close to human cadence, if you are a slow pedaller you could even use 1:1 gearing from motor to cranks).


FWIW, I have a hefty 12v unknown power motor with gearbox, which looks basically like a giant powerchair motor (about the size of 5 or 6 of them, though) out of a wheelchair lift from the back of a van, that I have ideas for powering a new version of the SB Cruiser heavy cargo trike via a differential; I'd be running it at 36v via an old Curtis golfcart controller, and I could probably tow cars with it. :lol:
 
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