Gear question

deacon

deacon

minor bike philosopher
here is the deal...

motor spins at 4500 rpm more or less
gear on the motor is 15 tooth'
I want to use the hub off a coaster brake wheel as the drive wheel. I think the diameter is about 2inches or so. I'm gonna add some jb weld and sand to make it a little thicker.

about the smallest sprocket I can get is 44 tooth.
top speed of 15mph is fine. torque is more important than speed. So what size sprocket should I put on the drive.

I know that on a 12 wheel the sprocket is good at about 65 teeth but it could run a bigger sprocket as well since it slips some when it goes up a big hill.
 
OK Deacon, I have something that should help with this. Unfortunately it involves some math, but it also involves common sense. The thing is, by knowing the number of inches we want to travel in a minute, we can convert drive wheel rpm into mph. Because it is friction drive, we don’t have to worry about tire or wheel size because the surface of the tire is moving the same number of inches per minute as the surface of the drive wheel (when there’s no slippage). The magic number to use is 5 mph equals 5280 inches per minute*. The thing that DOES matter is drive wheel size. If we know the outer circumference of the drive wheel in inches we can figure the rpms needed for every 5 mph of speed. You can wrap a tape measure around the drive wheel or measure the diameter in inches and multiply by 3.14 (remember pi?). So a 2 inch wheel has a circumference of 6.28 inches. That means every revolution of the 2 inch drive wheel turns the tire 6.28 inches. 5 mph is 5280 inches, so 5280 divided by 6.28 is 829, which means 829 rpm = 5 mph. So 15 mph would require the 2” drive wheel to turn at 3x829, or 2488 rpm. If we round that to 2500 and your motor turns 4000 rpm, that means for every turn of the drive wheel, we want the motor to turn 1.6 turns (4000/2500=1.6). A 15 tooth motor sprocket times 1.6 means you should put a 24 tooth sprocket on the 2” drive wheel. More teeth on the drive wheel sprocket would mean higher torque and lower mph. - jd

* There are lots of ways to figure this. One way: 5mph = 1 mile every 12 minutes. That’s 1/12 mile per minute. 5280 ft per mile divided by 12 = 440 feet per minute times 12 inches per foot = 5280 inches/minute. So 5280 inches per minute = 5 mph.
 
well that is out but try this one.

I weld a rear sprocket from a coaster to the motor sprocket that way I get a one to one ratio... I'm running 3900 rpm on the motor I'm willing to test it on. the rear hub is 5 inches in circumference. should give me about 22 mph top speed. A little more than I was looking for but if it will pull a hill that's fine.

How does that sound
 
Hate to say it but this is one of those things that has to go on the back burner. I ruined the motor trying to get the sprocket off. Somehow it got jammed on to the drive shaft. Motor is trash now but that's okay I never used it anyway. I have one more 600 watt with freewheel as a back up motor. The big drives I use work well enough so I will just soldier on with them.
 
Hi Deacon - my calculations say 3900 rpm should give a 5" circumference wheel about 18.5 mph.* I'll be surprised if you can get it up to 3900 under load, but you are a great experimenter. Let us know what you find out.

*Different circumferences will give different rpms for 5 mph. 5280 divided by 5 (inches) = 1056, so 1056 rpms = 5 mph for a 5 inch circumference wheel. 3900 divided by 1056 = 18.5 mph. The 12 inch scooter wheel on your rhino should have a circumference of about 37.7 inches, so that would take only 140 rpms for 5 mph (5280/37.7=140). That works out to 420 rpms for 15 mph. That's why you need such a big sprocket on it compared to the smaller drive wheels. - jd
 
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