Gear Ratio

[Kai0721]

I'm looking at the Pololu 25D motor and see their are a lot of gear ratios. How do I determine the one I need?

[sciolyperson1]

I'm looking at the Pololu 25D motor and see their are a lot of gear ratios. How do I determine the one I need?

This all depends on the type of transmission, 2wd/4wd, wheel diameter, and a lot of other factors.

Can you give some background info about your car? (About the things listed above)

[SPP SciO]

Those motors you mentioned are also suitable for direct drive - you can attach one directly to a wheel and not worry about configuring external gears to fit on an axle etc. How heavy your vehicle is will determine how much torque you need- the more torque, the fewer RPMs.

For what it’s worth my students last year used a similar motor from servocity that had 900+ rpm and more than enough torque to move a relatively heavy (about 1kg) vehicle pretty quickly. It wasn’t optimized, but it was effective, and the ease of use freed up more time for distance testing.

[Kai0721]

My wheel diameter is 2 in, I have 2wd, the vechicle weighs about half a pound. Is this enough info?

[SPP SciO]

My wheel diameter is 2 in, I have 2wd, the vechicle weighs about half a pound. Is this enough info?

Where are you at in the design process? If you have a vehicle with 2wd already, what benefit are you seeking in a new motor? Roughly, if your wheel diameter is about 2in, you’ll get just under 16cm distance per revolution - about 50 rotations for 8 meters. So, if your wheels can spin at 1,000 rpm, that would be 50 rotations every 3 seconds (which would definitely be a fast vehicle).

Most inexpensive hobby motors spin much faster than that, and if your vehicle is only 1/2 a pound (are you including a full battery pack?) - you could likely achieve those speeds with some plastic gears. The motors you mentioned already have internal gearing though, so I would recommend holding off on buying one until you know exactly why you need it (through testing what you have on hand)

[windu34]

My wheel diameter is 2 in, I have 2wd, the vechicle weighs about half a pound. Is this enough info?

Where are you at in the design process? If you have a vehicle with 2wd already, what benefit are you seeking in a new motor? Roughly, if your wheel diameter is about 2in, you’ll get just under 16cm distance per revolution - about 50 rotations for 8 meters. So, if your wheels can spin at 1,000 rpm, that would be 50 rotations every 3 seconds (which would definitely be a fast vehicle).

Most inexpensive hobby motors spin much faster than that, and if your vehicle is only 1/2 a pound (are you including a full battery pack?) - you could likely achieve those speeds with some plastic gears. The motors you mentioned already have internal gearing though, so I would recommend holding off on buying one until you know exactly why you need it (through testing what you have on hand)

While you have the right idea, its not quite this simple to translate advertised RPM to actual performance.
1.) RPM of a motor is measured at a specific voltage. Higher-quality motors give rpm in the form of a "kV" measurement, which essentially translates to RPM/Volt. For example, a 3600kV motor powered at 10V should have a "Measured" rpm of 36,000.
2.) RPM is measured using some form of a dynamometer (Dyno). These dynos for hobby motors typically do not place a significant load on the motor, and therefore the RPM value given will be higher than the actual RPM of your motor when attached to your device.
While its all good and fun to be able to perform these calculations, you have to keep in mind the numbers you are relying on are wrong and inflated

[retired1]

Straight DC motors will give a speed close to the rated speed times the fraction of applied voltage divided rated voltage.
Now comes the fun part, trying to find gears that will give you the gear ratio that you desire to convert motor rpm to wheel rpm which will convert to time.
A problem that I found was the motor shaft was different from the axle diameter. It was easy to find gears to give the desired ratio, but not ones with different bore diameters.
It would have been much simpler to have picked the gears and then the axle diameter to find available and affordable gears.