Motor RPMs

[TwistedCreations]

Hey guys,Me and John was having a conversation this morning about gearing and such. but if you take 2 trucks with the EXACT same setup, but one is running a lathe and the other is running a MM motor, the truck with the MM motor will win the race.So it all kinda boils down to what the RPM is of the motor.If you knew that, you could figure out your final ratio better than just saying 80:1 or whatever.

I figured that i have 151:1 final ratio and run a 27t johnson motor( too slow for me). if i was to run a Lathe motor in there,the final ratio would be the same, but with the low RPMs, it would make it seem that it was at 225:1..

so how do you figure out the RPM of a motor?

I know a dyno, but i don't have one handy. If you guys know for sure what the RPM's are of a certain motor, please post it up.

[Ace]

Doesn't really answer your question but heres a formula I use to figure gearing:

Speed=(wheeldiameterinches*3.1415*60*motorrpm)/(12*5280*axleratio*transratio*pinionspurratio)


As an example, my clod uses lathes with 9 tooths and goes about 5mph top end with dirt hawg 40's (6" dia acording to tower)..

So

5=(6*3.1415*60*motorrpm)/(12*5280*7.822*1*(50/9))


5=1,130.4*motorrpm/2753344

or 13,766,720/1,130.4=motorrpm


motorrpm=12,178.6


So if my 5mph top end is a good estimate a lathe motor runs about 12,178 rpm under load max.

[TwistedCreations]

Damn Andy, ya have me more confused now

are your * marks degree marks or something else?

[Ace]

Sorry about that!

*=X or multiply.

(from excel or most computer languages)

[JohnRobHolmes]

the easiest way to find rpm of a motor is to use a tach. every motor has a constant rpm per volt, and as such you can adjust the voltage to suit your rpm needs.

in your lathe vs MM example, you could bump up the voltage on the lathe motor to make them match each other.

i figured my lathe motor to be right at 1000 rpm per volt, but every one will be different by (hopefully) no more than 10% if its in good condition. from there you can determine what voltage you need to hit 5mph with your final ratio and rollout.

OR you can fix your voltage and change your final ratio or rollout to hit your 5mph speed.

[JohnRobHolmes]

according to ace's calculations, the lathe motor is spinning about 1,666 rpm per volt assuming 7.2 volts nominal voltage and 5mph top speed.

[TwistedCreations]

Originally Posted by Ace

Sorry about that!

*=X or multiply.

(from excel or most computer languages)

OK, Thats what i thought at first, but wanted to make sure. I use * as degree also.. so just wanted to make sure