Motors, gearing, and speed.

I have done some very preliminary research into what I should use for the electric motors.   Thanks to the wonderful popularity of those silly motorized scooters, there actually a decent number of affordable electric motors.  I will need 2 motors of course, one for each side.  This way each track can go forward or reverse, independently of the other.

The motors I’m considering at this point are 24v, around 250W and run up to like 2500 rpm.  They even come with small pulleys, around 1″, already on them  (I will have a future post about belts and pulleys versus chains and sprockets).

So, I wanted to get an idea of the top speed the tank would have, if the motors were running at 2500 rpm.   I am figuring a gear ratio of 6:1 (1″ pulley on the motors, to a 6″ pulley on the drive wheels).  I will also have approximately 6″ drive wheels, which, when driving a track, you can pretend they are actually just wheels on the ground, and the numbers will work out close enough (there is a slight difference due to the thickness of the track, but that’s not a huge deal for now).

Anyways, here’s the equation to figure out top speed:

(RPM/gear ratio)  x  60  x  WheelDiameterInches x Pi
5,280 x 12
= 7.4 mph with a 1:6 gear ratio
=5.4 mph with a 1:8 gear ratio

I am very happy with those speed estimates.  If they came out to like 30 mph, I would know I’d need to gear it way down or get lower revolution motors… likewise if it came out to 1 mph I would need to gear it up and so on.   Also, when finally selecting the motors, if it’s a high torque motor, but turns way less revolutions, I will know I can use more even gearing, etc etc.

Installing the motors is obviously far off at this point, but these things need to be considered.  For instance, I am leaving myself enough room for a 6″ pulley on the drive wheels, when I construct the frame, axles, and the other associated parts.    That’s about it for today, sorry, no cool pictures, but at least we did some handy math you can use when making your own tank.