View Full Version : Help with motor selection for lightweight motorcycle

04 March 2020, 1451
Hey folks, I’m pretty new to the electric vehicle game. Plenty of mechanical experience but would appreciate some input on motor choice for my little project bike.
I’ve been researching and it seams a hub motor is easiest but the whole unsprung weight/rotational mass thing doesn’t sound great.
Ideally I’m looking for a top speed of 80-100kmph but low speed control and acceleration are key.

Any help much appreciated.



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09 March 2020, 0952
Unsprung mass is unlikely to affect the handling of a machine like that all that adversely - I wouldn't be too concerned.
Take a look at this thread (http://elmoto.net/showthread.php?t=5091) for some relevant commentary.

10 March 2020, 0930
I agree with Spoonman … for this conversion a hub motor makes perfect sense.

16 March 2020, 1415
100kph on a postie bike is going to be a bit iffy handling wise. Single spine frame, very flexi forks, and insanely flexy pressed sheet swingarm with a tint swingarm axle and wheel axle , is going to be problematic. I would limit it to a true town commuter speed of 70 maybe 80kph ( if you improve the suspension). If I remember correctly ( and I may not), the swing arm from early cb/sl/cl 100/125's may be a straight fit or require just some spacers. At least they are tubular steel and the axle plate is possible to use for a bigger axle.
Having said that the single spine frame will almost certainly limit you to a hub motor because of room. That will make the suspension have to do even more work. Not a problem at low city speeds but not a good idea for a supercommuter( highway capable 100kph).
Add to that the very tiny and marginal drum brakes of the postie bike and you may need a better host bike if you need the higher speeds.
As much as I love this bike ( and I do) in it's original guise, my take is is possibly not the best choice for your target use.
One of the bikes I am considering is the late model CBr125R, though I would be aiming at a centre mounted motor.

17 June 2020, 0922
I'm planning the conversion of a CT90 Trail that I mainly use to ride on my property, mail box, etc. I started out planning to use a 3000 watt hub motor on 72 volts which should give me about 50 mph. I want to keep the bike looking as original as possible. I just bought new tires and want spoke wheels. It's very difficult to find a hub motor with the proper width 17" rim however there are a couple of companies that will lace up a rim to your specs. Due to the fender configuration on the CT90 and the swing arm cross brace, I can't use a wider tire with an increased outer diameter.

Since most hub motors require 200mm width on the swing arm, stock on the CT90 is 165mm, and bolt up differently, the swing will need to be widened and revamped at the motor mounting location. This will also effect the shock mounting. I even bought an extra swing arm to make the mods on. After studying this problem at length, I've decided it will be less work to mount a 3 to 5 kW mid mount motor in place of the original. I've made a cardboard template for 2 motors and hopefully this weekend I can make a decide how to fabricate a mounting bracket. I'll need to work on the sprocket sizes and chain however that shouldn't be to much of a problem.

Here is a video on swing arm mods. https://www.youtube.com/watch?v=pVtAolbLMhQ&t=121s