New ebike build from scratch using a car starter motor!


New member
Local time
7:44 AM
Jun 21, 2018
i know this will sound crazy but i'm using a modified starter motor from a car. its a bosch starter motor 800 watts. with the solenoid taken off. the reason i have specific sprocket dimensions is because it will have to fit in the housing of the motor as i cannot take this off because it holds the front bearing in place. i'm not sure of the size of chain all i know is its a normal bicycle chain. i know there will be a chance the chain wont take the torque but i will try it this way first and I'll just be carefull not to apply too much torque. thanks for your reply. any ideas on my question.
Car starter motors aren't really designed for continuous running...... it probably won't last very long.

I'm sure bicycle chain will take the torque ok, no problem there.
i know they are'nt designed for running constantle but even if it does'nt last that long its a really popular motor from a cart scrapyard and costs £10 so i can live with that every now and again. and also the motor is only gonna assist my pedalling from time to time so it wont really be running constant. im hoping a motor will last me a month or two at least. im glad the chain should take the torque.
Exactly, their intended duty cycle is probably something like a maximum of 1 minute powered for every 10 mins unpowered to allow for cooling. This is because starter motors aren't designed to be very efficient and to run cool but for maximum power over a short period and with minimum weight/space occupied. Having seen the 2006 Presteigne bike rally entry with a starter motor, although it looked 'fun' it wasn't to be taken seriously and when it failed part way into it, I wasn't surprised.

I'm sure bicycle chain will take the torque ok, no problem there.

Although it wouldn't snap from the power/load it's duty life will be reduced, especially if the gears aren't built into the hub. In the case of hub gears though, longevity there may then become an issue. The small gears often made from nylon aren't designed for that sort of power over any length of time. I'm not sure if other components in the drive train would last either, I believe the Presteigne entry died from broken spokes due to the extra loads.

The OP is better off with a motor designed for an electric bike or at least continuous service through a controller. Although other designs such as that intended may be able to be cobbled together to work, it'd be unlikely to last or give decent battery consumption/range. Sorry to be a dampener on things but it's best to spend money wisely in the long run.
i know this idea is not the best its just a cheap option for the time being. the starter motor im using is easy and cheap to replace ( £10) so i could live with a couple of change overs. what causes the failure of the motor, is it the temparature of it and if so by any chance do you know a rough temperature it would fail at. because im going to monitor the temparature of the motor while its in use this way i can keep operating it at a suitable temparature. do you thing this will prolong the life of the motor for a while? as i said earlier the motor wont be running constantly anyway its just to assist me from time to time. and i certainly wont be pulling away from stop this way , this will be done with pedal power alone. im no expert with this but im thinking as long as i do all of the above i should get a bit of life out of the motor. please tell me what you think about this. thanks again for the info though its got me thinking.
Causes of failure would not only be overheating (you'd have to ensure it didn't get to hot to touch with your whole hand for a few seconds) but also the fact all the starter motors I've seen have only ever had plain journal (low lifespan) bearings. Admittedly my experience is quite dated so perhaps better bearings are being used and more heat resistant varnish over the windings these days.

What happens when you like the motor assistance so much you decide you want to rely on it more (eg. you decide to adopt a shorter different route which is more hilly)? Perhaps there's a user out there who's had their starter motor powered electric bike running regularly day in and day out for years with no problems who can counteract my reliability concerns but I wouldn't expect floods of posts along these lines. I'd also imagine these sorts of motors would be very difficult to efficiently and cheaply implement speed control.