Gear question

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.

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

Whoops sorry to hear that. Oh well, you'll keep coming up with new ideas that we all can use - jd