I'd like to build a Tom Stanton-style rear wheel belt drive for my road bike. One of the requirements for this project is to keep it UK road legal which means 250W and no assist over 25kph, so I'm wondering how to pick gear ratio and voltage to get the peak efficiency out of an RC style small brushless motor to make the most of my limited watts.
I believe the highest efficiency comes near the unloaded RPM of the motor - if there's a formula to calculate it I'd be interested, or just use something like the Aerodrive SK3 as an example. I'll use the maximum motor assist speed as my target speed for peak efficiency, because it's quite flat where I live and 25kph isn't very fast
For some initial numbers, a 149kv motor (the lowest I can find most places) on a 6S battery would need a 15:1 reduction to hit 25kph at 80% of no-load speed. All else being equal, I get the idea higher voltages and lower currents give better efficiency, but is the gain in switching to a 12S battery at half the current enough to outweigh needing a second reduction stage to get to 30:1?
Secondly, while I'm sure I need to overrate the motor if I want to run 250W through a small brushless for long periods, is there a major advantage in eg. a 2000W rated motor over a 1000W? Are motors with larger rotor diameters likely to give better torque under load?
I believe the highest efficiency comes near the unloaded RPM of the motor - if there's a formula to calculate it I'd be interested, or just use something like the Aerodrive SK3 as an example. I'll use the maximum motor assist speed as my target speed for peak efficiency, because it's quite flat where I live and 25kph isn't very fast
For some initial numbers, a 149kv motor (the lowest I can find most places) on a 6S battery would need a 15:1 reduction to hit 25kph at 80% of no-load speed. All else being equal, I get the idea higher voltages and lower currents give better efficiency, but is the gain in switching to a 12S battery at half the current enough to outweigh needing a second reduction stage to get to 30:1?
Secondly, while I'm sure I need to overrate the motor if I want to run 250W through a small brushless for long periods, is there a major advantage in eg. a 2000W rated motor over a 1000W? Are motors with larger rotor diameters likely to give better torque under load?