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Thanks a lot.

So a 9:1 is safer / cooler running than 8:1, in theory? I will dig out my pinions and do some calculations at some point...

Correct, a numerically higher gear ratio means the load on the motor is less, which should give cooler temps and maybe [very slightly] longer run-times.

I just checked one of my JRX-2 runners which uses a 19-turn Trinity motor,
I am running a Final Drive Ratio closer to 10.5.

calculation as follows (using JRX-2 transmission ratio of 2.18):
(81/17) x 2.18 = 10.39

That particular buggy pulls wheelies fairly easily and the motor stays cool.
I'm sure I could push the FDR down to 9:1 or so.

dldiaz wrote:Correct, a numerically higher gear ratio means the load on the motor is less, which should give cooler temps and maybe [very slightly] longer run-times.

I just checked one of my JRX-2 runners which uses a 19-turn Trinity motor,
I am running a Final Drive Ratio closer to 10.5.

calculation as follows (using JRX-2 transmission ratio of 2.18):
(81/17) x 2.18 = 10.39

That particular buggy pulls wheelies fairly easily and the motor stays cool.
I'm sure I could push the FDR down to 9:1 or so.

dldiaz, can you do me a favor and measure the diameter of your rear tire? Thanks!

They are 2.2" buggy wheels/tires (RC10B3 wheels, actually).
The outside diameter of the tires is about 3 1/4".

dldiaz wrote:They are 2.2" buggy wheels/tires (RC10B3 wheels, actually).
The outside diameter of the tires is about 3 1/4".

Thank you. With that info, you have a rollout of very close to 1:1; it's 0.983 with those tires & that pinion gear. According to a rollout calculator I've been working on, that's about as close to optimal as you can get. The next pinion gear size up (18T) would also be good but further from 1:1 than the 17T pinion (1.041), and therefore (according to theoretical math anyway) you're running your drivetrain close to max efficiency.

Coelacanth wrote:...you have a rollout of very close to 1:1; it's 0.983 with those tires & that pinion gear... you're running your drivetrain close to max efficiency.

Thanks for that info.

I'm curious though, the gearing would have to be changed for a different motor
(hotter, milder - whatever).
How can there be an optimal ratio, that does not account for the wind of the motor???

Maybe this means a 19-turn motor is sort of an "optimal" wind?
I actually love the 19-turn brushed motors -
I find them an excellent combination of torque and horsepower,
strong motors for a variety of driving situations.

dldiaz wrote:

Coelacanth wrote:...you have a rollout of very close to 1:1; it's 0.983 with those tires & that pinion gear... you're running your drivetrain close to max efficiency.

Thanks for that info.

I'm curious though, the gearing would have to be changed for a different motor
(hotter, milder - whatever).
How can there be an optimal ratio, that does not account for the wind of the motor???

Maybe this means a 19-turn motor is sort of an "optimal" wind?
I actually love the 19-turn brushed motors -
I find them an excellent combination of torque and horsepower,
strong motors for a variety of driving situations.

I wondered that myself until I realized that nearly all the motors in our cars are the same approximate diameter, all are 540-sized motors. That might play a role in this consistency of the 1" rollout. The important thing more often relates to changing sizes of tires, as that plays as much a part in the rollout equation as spur & pinion gearing.

Anyway, I'm not sure WHY 1" is the so-called target to aim for, but I've read that on many websites about RC car rollout ratios. The only common denominator seems to me, is the circumference of the comm inside the motor can. Regardless of how hot or wimpy your motor is, that comm still makes the same circumference when it rotates one turn. That's all I can come up with.

Once you've found the motor's optimal efficiency with the rollout equation, you can always under-gear for more torque or over-gear for more top end, while checking your motor & ESC temperatures.