Motor sizing ?
- Thread starter Peeeg
- Start date
You have to understand the implications for efficiency and current. When you put a a 20" (328 rpm) motor in a 700C wheel, it will draw maximum current nearly all the time it's below 20 mph, which will overheat your controller. If you fitted the motor to a road bike and were able to cruise at 20mph plus, it'll be no problem. That will only work if you wear tight clothing, are relatively fit and light, and you pedal hard. I'm assuming it's a small motor.
Wheel size is like gearing, it has a direct relation to torque. If you need a lot of torque to get your weight up hills, you need to think carefully about your choice of motor.
Wheel size is like gearing, it has a direct relation to torque. If you need a lot of torque to get your weight up hills, you need to think carefully about your choice of motor.
Yes thanks for that I was assuming when you say 36v 250w that's that but clearly that is not the case
36v 250w doesn't mean anything. there are some 250w motors that are heavy brutes when it comes to power, like the Bafang BPM, which is a real workhouse. A 328 rpm version wouldn't have any problem powering a hybrid type bike, but you'd need at least a 20 amp controller for it and a 25 amp battery with enough capacity to get the range you need.
If you had a specific motor in mind, it would be easier to advise.
If you had a specific motor in mind, it would be easier to advise.
I went the other way & built a 201 rpm Q100 into a 24'' rear wheel, keeping the frame & front wheel as a basic 26'' ATB style. The motor draws 15A max @ nominal 36V . This gives a decent hill assist without too much sacrifice of speed. The saddle is only 1'' nearer the ground, I weigh 200Lb the bike without a motor or batt 35 Lb. I would rate this as adequate & better than the Tongxin in a 26'' wheel / or a c'lyte 408 in a 20''. All I am saying is a small motor would do very little in a 700 unless you live somewhere extremely flat. The stock controller was outing about 12.5A - I did hack it to 15A with advice from D8veh - he said I could have gone to 18A - but I did not because of batt limitations. The controllers could come as standard anywhere between 11.5A & 15A I was told as quality control is not rigorous on them. A disc brake does not care what the rim size is. I had to make a plate to carry a rim brake caliper as this was my set up
I remembered that Bafang used to offer a range of windings & gearings to suit different rim dia.s - the slowest - aimed at 26'' off road MTB users may be an idea in a 700 wheel. Do not buy this from BMSbatt - as it is pot luck which version they send.
A smaller option is OutRider - was Tongxin - they had likewise in a smaller lower amp motor - aimed at different wheel sizes - I bought one off 'Brett White' - he stocked a 13.7 mph top speed one to give a bit more torque in a 26'' rim - the factory did have a lower speed more torque one than this. Mine still works - Max draw about 11A - so good for smaller battpacks
A smaller option is OutRider - was Tongxin - they had likewise in a smaller lower amp motor - aimed at different wheel sizes - I bought one off 'Brett White' - he stocked a 13.7 mph top speed one to give a bit more torque in a 26'' rim - the factory did have a lower speed more torque one than this. Mine still works - Max draw about 11A - so good for smaller battpacks