I'm working on a bicycle assist motor project. I thought about using a hub motor or chain drive but given the economy, I'm going with a friction drive. Friction drive is cheap, has few little moving parts to go wrong, and is cheap. I think I can make it using parts almost entirely found around the shop -- with the exception of the friction roller.
Now what I like about friction drive is if you use a roller with a one-way bearing and take advantage of some physics, the motor can release from the wheel entirely when freewheeling, so the bicycle remains entirely pedal-able on its own.
I decided to go with rollers from the defunct EV Warrior project. They're available on the surplus market, have one way bearings, and are nicely knurled. Other people are making their own friction rollers from BMX wheel hub extensions, but they don't have one-way bearings.
Here are some dimensions almost entirely for my own benefit.
| Part | Inch | mm |
|---|---|---|
| Shaft OD | 0.500" | 12.70mm |
| Shaft ID | 0.315" (a tad over 5/16") | 8.00 |
| Shaft Width | 3.016" | 76.61 |
| Roller Width | 2.375" 2 3/8" | 60.35 |
| Roller OD | 1.275" ~1 1/4" | 32.38 |
| Key Notch Width | 0.130" | 3.32 |
| Key Notch Depth | 0.411" | 10.4 |
Now, if I want to go 20 kph with motor assist, then how many RPM do I need. The math is easy since the bike wheel circumference is 1:1 with the road surface.
The roller OD is 32mm. That means its circumference is 2*pi*r = ~200 mm or around 20 cm or 0.2 meters. To go 20 kilometers, it has to spin 100,000 times an hour or 1,667 times a minute. Ummm... I do know the difference between radius and diameter, I think.....
Take 2: That means its circumference is pi*d = ~100 mm or around 10 cm or 0.1 meters. To go 20 kilometers per hour, it has to spin 200,000 times an hour or 3,333 times a minute.
I just hooked up the motor to a 16v power supply and measured its
rpm with my optical tachometer as 6,000 rpm. That means that I'm really limited in my power source to 12 volts or maybe 24. In any case it's not good for my motor and I really should try to find one with a lower Kv but that would actually involve money which I am trying to avoid.
I just looked up the specs on my motor (MY-1016B) and it looks rated at 2900 rpm @ 24 volts @ 15 amps. So I must have either measured wrongly or maybe my jury rigged power supply was off spec or the no-load speed is quite high or the moon alignment was off with Venus. It's late so I will retry on a later date.
| Motor: | Unite MY-1016 |
|---|---|
| Voltage: | 24VDC |
| Rated Speed: | 2650-2900 RPM |
| Rated Current: | 16.0-16.4A |
| Output: | 300 watt |
Update: With my math corrected and the real motor specs, it looks like I'm good to go. 24 volts should get me going to 20 kph and 36 volts should be good to 30 kph (or around 20 mph) which is plenty fast for New Haven streets.
Update 2010.02.22: Real world performance at 36 volts is around 20 kph at full assist. Working backwards, at 20 kph the motor is spinning at 3,300 rpm.
I'd really like to go 30 kph, but I don't think the current motor will get me there since that would require it to go at 5000 rpm. Even now, it's only a 24 volt motor and running it at 36 volts won't be good for it in the long run.
Of course I could go brushless, in which case this would be a good choice (over 1500 watts peak!): http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=5204&Product_Name=TURNIGY_50-65D_270kv_Outrunner_(eq:_4030AXi)&aff=54444
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