Adjusting gearing for voltage

[MrCrash]

I was wondering what kind of change in gearing is needed when you increase the voltage to a brushless system. I'm going from 6 to 10 cells in my mamba-powered Pede.

Thanks,
MrCrash

[GSMnow]

I was wondering what kind of change in gearing is needed when you increase the voltage to a brushless system. I'm going from 6 to 10 cells in my mamba-powered Pede.

Thanks,
MrCrash

You have a few options. Going from 6 to 10 cells is a 66% increase in voltage. 7.2 to 12 volts nominal.

1) You can go for 66% faster top speed and a bit less run time due to the greater current draw by running the same gearing. If you are already geared fast, this may be TOO fast and cause an over heat condition.

2) You can gear for the same top speed. If you were running a 20 tooth pinoin, you could change to a 12 tooth. Assuming a 5700 KV motor for example, it would turn 41040 on 7.2 volts and 68400 on 12 volts. 41040 x 20 = 820,800 teeth per minute and 68400 x 12 = 820,800 teeth per minute. This will result is much faster acceleration and greater pulling power while drawing quite a bit less current for a longer run time.

3) Split the difference to get a longer run time and more acceleration along with a bit more top end speed.

Assume you were doing 30 mph with the 6 cell battery and the previously mentioned 20 tooth pinion (easy math for me)
You didn't give any info, so I am plugging in numbers for comparison.

The first option ups your top speed to 50mph, not bad at all.

Option 2 keeps the top speed at just 30 mph, but pulls like a freight train with longer run time.

Option 3, you gear it for 40 mph with a 16 tooth pinion in this example and have a nice increase in top speed, accelleration, and run time.

[JDT]

with mashers and the 5700 system we usually go from 84/15 on 2s to 84/19 for 3s so maybe start there.

[GSMnow]

I think you mean the other way around, going from a 19 pinion on 2S to a 15 pinion on 3S. More voltage = smaller pinion.

[JDT]

no we actually go the way I indicated, up with higher voltage, I guess we were thinking if you have(example for easy math) 7.4 volts and 100 amps you got 740 watts, with 11.1 volts we only need 67 amps so by gearing up we could draw back those extra amps and get more power(hoping to get to the claimed 1000 watts the mamba can make lol) of course running 3s on the 5700 gets hotter than heck but maybe because I only tried 15-22 and should have tried lower but I will tell you at 84/15 on 3s it spent more time on its lid than on its wheels. Sorry for the bad info, I guess its good that I babysit my mm with a temp gauge or I would have smoked it by now.

[GSMnow]

The Mamba Max (and many other brushless systems) can make so much torque that many people end up over gearing the crap out of them because you don't really see a fall off in acceleration like you do with a brush motor. "It still pulls like crazy, I guess I can gear it higher still!" In a longer run it WILL overheat. Just do a little math working for the road speed you expect back to the motor rpm and you get a pretty good idea where to gear. And I ALWAYS recomend gearing a little slower at first and then working your way up. For an example. I run a XXX-T MF2 with a 2.55 internal trans ratio and 4 inch tall tires. On 3S LiPo with a 5700 motor it should go a solid 60 mph. 60 mph = 88 feet per second or 63,384 inches per minute. With 4 inch tall tires, it has to spin them at 5044 rpm. The motor will be turning about 63000 rpm. That works out to a 12.5 fdr, or 4.9 pinion to spur. With my 86 tooth spur I would need a 17 to 18 tooth pinion. I have run as high as an 18 tooth with just 6 cell NiMh and it topped over 40 mph and still ran over 12 minutes on 4200 mah. So it should perform as good or better than that on the 3S LiPo while pulling well over 60 mph. And don't forget the tires will balloon out a bit and go even faster.