calculating speed

[xsupermario]

Hello est forum!
I am looking for some help on what gear ratio to use with this motor http://www.unitedhobbies.com/UNITEDHOBBIES/store/uh_viewItem.asp?idProduct=5139

Also is there a program or someway to figure out how fast i could go with certain gear ratios?

 

[Link]

I've seen that one before, and it's really big for an R/C motor.

Easiest way is to figure out what RPM the motor will spin at with however many volts you'll be using, calculate the circumference of your wheel, and work out how fast it will spin with a given gear ratio. Then you can just multiply that together and get a figure.

Like this:

Motor spins at 1,000RPM and has a 10:1 gear ratio. That's 1,000 / (10 / 1) = 1,000 / 10 = 100RPM at the wheel.

Say it's a typical 26" wheel. 26 * π = ~82" circumference.

100RPM * 82 = 8,200 inches per minute. That's 8,200 * 12 to get ~683 feet per minute. Times that by 60 (the number of minutes in an hour) and you get 40,980 feet per hour. Divide by 5,280 (feet in a mile) and you get ~7.8MPH.

Or, the equation could be written as

((RPM / (teeth on driven gear / teeth on drive gear)) * (wheel diameter * π / 12) * 60) / 5280 = MPH

Substitute in my figures and it looks like this:

((1,000 / (10 / 1)) * (26 * π / 12) * 60) / 5280 = 7.735MPH

Alternately, if you don't like math and want something to do the hard parts for you, play with this thing. (Set the axle ratio to 1.) Sorry, I can't seem to find one that simply lets you enter a motor RPM, ratio, and tire size and have it spit out a number.

 

[Miles]

johnrobholmes posted this handy geardown calculator
[I couldn't find the original post]

 

[xsupermario]

Thanks alot to the both of you that was exactly what i needed!

 

[swbluto]

There's my program that takes into account the physics involved with determining speed depending on gear ratio. But, I won't recommend it as being extremely accurate until I program the no load current input.

edit: Ok, I just did. It's now available in the latest post at the program's thread.

Now the only thing's that not accurate, efficiency wise, is the hub motors... hehe. If I knew the no-load current of the hub motors, I could calculate the efficiency accurately past the "peak efficiency" point. But before the point of peak efficiency, it's generally accurate.