Want to better understand gearing
 

Just wanted to better understand whats going on when i change spur gears and pinion gears. I am currently using a 48t/15t. if i go up or down in T in the spur what is that doing? If i go down up or down in T on the pinion what is happening? Thanks

Increase spur tooth count=lower gearing, and vice-versa
Increase pinion tooth count=higher gearing, and vice-versa.

Easier to just divide the spur by the pinion to get a number. The lower the number, the taller or higher the gearing is. And, the higher the number, the lower the gearing is. In your setup 48t/15t=3.2:1, which means the pinion has to turn 3.2 times for every spur revolution.

And, for a typical setup, going up or down by one pinion tooth creates about the same ratio as three spur teeth. Example:
Your combo: 48T/15T=3.2
Original pinion, but increase spur by 3T: 51T/15T=3.4
Original spur, but decrease pinion by 1T: 48T/16T=3.428

Don't mean to jack thread, but this is related.

Why don't RC guys just go by overall gear ratio? For instance my Tranny is 2.727:1 and If I run a 58 spur and a 15 tooth pinion my overall is 10.54:1

I see guys with buggys and all they list is the spur and pinion and seem to neglect to mention the ring and pinion in the Diff. which is around 3:1 in most of the ones I have been able to find info on. Or a monster truck with a transmission, ring and pinion in the diff and a spur and pinion, Is it you guys just know or what? I always see people say you are geared too high (numerically low), but all that is given is spur and pinion.

Jeff

Probably to make things more simple. Generally, buggies all use the same diff ratio, so why bother to figure that in all the time? But, some do use different ratios which would throw things out of whack. So, to cover all the possibilities, the overall ratio is broken down to diff and spur/pinion.

When someone says "you are geared too high", they probably are familiar with the rest of the setup and know the general ballpark for that vehicle. Or, they see the resulting speed for that vehicle, vehicle weight/size, and tire size, can infer that the gearing is wrong by experience.

Ok, I can see that.

Jeff

Also keep in mind that having more teeth on the pinion will help with how well the spur and pinion mesh.

Pretend you have the desire to run a 3:1 ratio, you can accomplish that with a 10t pinion and a 30t spur, but a 20t pinion and a 60t spur will run smoother, as the teeth angles are better on the 20t and more teeth are in contact with each other at any one point.

So a larger pinion with the same spur will increase your top speed, but place more load on the motor, esc, and battery.

A smaller pinion will provide faster acceleration, but less top speed. You can gear a system too low, and the motor temps will be high as the motor is revving too fast with not enough load. Not usually a problem, but say you are trying to run an 1/8 buggy with a mmm/2200 combo on 6s lipo and it is geared for 20mph... that will cause motor heat issues.

The opposite applies with spurs, smaller spur = higher top speed, larger spur = lower top speed.