badgineer said:
Hi TorontoBuilder
TorontoBuilder said:
The rear 55 tooth cog has the same 190 mm diameter as the cheap system from aliexpress while the drive cog is ~80 mm versus the 39 mm of the aliexpress system. The gates drive belts are far superior and cogs are available up to 243.2 mm in diameter.
Pair that drive system with an alien power 30KV 2800 watt APS 6374S motor and you've got a sound ratio to deliver a lot of torque and do about 50 mph using 29" wheel. If you want more power then there is the 50KV 4000 watt APS 6384.
The belt drive you are describing has a gearing ratio of about 1:2.4...
I think that at this low reduction, the 6374s will not be spinning nearly fast enough to produce the 2.8 kW it's supposed to. (You have to spin a motor it in its optimum RPM range to get the nominal power, regardless of kV.) I am aiming for gear ratio in the vicinity of 1:8 for a 20 inch driven wheel (or ~1:10 for a 26 inch wheel, but this is utopia). 1:2.4 is far far too low to get decent power from small motors.
I can't concur with your thinking because I've actually done calculations rather than made an assumption. I am also sure that you're thinking of the typical performance of RC motors with high velocity constants used in quad copters and planes. Not all motors are alike, hence my specifically saying a 30KV motor.
The maximum rpm of the motor I mentioned is only ~1200, unlike the typical RC motors of this range that have maximum rpm ratings between 7,000 to 12,000 rpm.
The 30 KV APS 6374S is designed to operate at a much lower rpm so that the corresponding power, torque and efficiency curves will also be lower and suit my usage very well. I know this with confidence from the manufacturer's specifications even without looking at performance curves. I know this for another reason as well.
There is a calculation to assure that ideal operating rpm of the bike drive system will fall within the peak power range of a motor.
Tom Stanton defines the output of his version of this type of calculation the 'golden ratio" and quantifies it as a numeric value of approximately 10. Nice, round and convenient eh.
Check out his video response to everyone asking how to select the correct RC motor for use on an e-bike. His comment starts at the 11:47 mark of the following video:
[youtube]https://youtu.be/1pm1RtCuE3A?t=707[/youtube]
The gist of Tom's calculation is to multiply motor KV by the drive system ratio.
Regardless of how the power is transferred to the wheel, direct drive, friction drive roller, pulleys, or sprockets you calculate the ratio, and then multiply that by the motor velocity constant KV.
My result is well within the parameters: 30 KV x (22 tooth / 55 tooth) = 12 or;
30 x (75.3 OD / 109.7 OD) = 11.84
The calculation is at the 12:49 mark of the video.
Hub motor typical KV is 8.7:
8.7KV x 1 (direct drive) = 8.7
IF I was designing a friction drive using the exterior can of a 63 series motor as the drive roller I'd choose a 130 KV motor:
130 x (197.92 mm drive wheel circumference / 2268 mm driven wheel circumference) = 11.3 or;
130 x (63 mm diameter drive roller / 736 mm wheel diameter) 11.12
This is a whole numbers game... 9, 10, 11, 12 all acceptable outcomes. The higher the value the lower the low end torque.
However, with decent motor controller and a high peak current a motor can achieve higher torque/power output at lower rpm. My selection will have ample low speed power and torque.