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Old 12-01-2008, 01:37 PM   #1
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Default Electric RC gearing

So i'm sure you all know me by now, i ask a lot of questions! This time it is about proper gearing for electric RCs. So i've been reading quite a bit about this but i'm still a bit confused mostly due to terminology and conventions. Because i am an engineering student i learned it this way: A gear ratio is output over input. Therefore a higher gear ratio would mean there is higher torque output / lower required torque from input / higher output speed. A lower gear ratio would have lower torque output / higher required torque from input / lower output speed. I noticed that on Traxxas' website, specifically in the Rustler VXL page, they show a table for proper gearing for certain voltages used. What i noticed is that they lower the gear ratio as the voltage goes up. Why is this? Voltage ONLY affects the motor's speed and not torque. So to stop the motor from over working shouldn't you increase the gear ratio to make the required torque from input less? My understanding is that since the motor's speed is increased due to an increase in voltage, it will have a harder time reaching peak RPM because the torque stays the same. Therefore, you would need to lower the required torque input by increasing the gear ratio no?

Oh, also on a sidenote... i'm planning on using the Mamba Max SideWinder ESC with the VXL BL motor. Are there any required modifications to the Traxxas Slash, ESC, or VXL BL motor so that they will fit / operate correctly with each other? (ie. correct wires / parts fitting)

Thank you all!
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Old 12-01-2008, 01:56 PM   #2
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http://www.electricmotors.machinedes.../bdeee1_1.aspx

Mainly look at it in terms of acceleration vs. top-speed. You will have a lower starting torque requirement with a higher gear ratio but lower top-speed. So, that's quicker. So, in a truck - maybe gearing of 20 pinion, 80 spur gear. For top speed, a lower gear ratio would allow for slower acceleration but higher top speed (24/80). The motor spins fewer times per mph, for example. Since constant speed is fighting wind resistance and other forces - you are always accelerating to maintain a constant speed. Power is lost due to friction and a little but due to sound and even light (sparks in a brushed motor).

I think that the paper in the URL above could help give answers to what you're after. Everything would break down to simple fulcrum forces (lifting weight by a lever) - just keep the motor's wattage in mind when considering power. I think electric motors have a rated wattage which gives you a starting point for power-output.
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Old 12-01-2008, 06:36 PM   #3
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Wow i just went on that website and i must say... i was really confused lol. Thanks for the info though.
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Old 12-01-2008, 06:47 PM   #4
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I suspect you are confused by the pure theory, not considering the application. You are correct that voltage itself does not relate to torque. Electric motor torque is most closely related to current.

But in the real world, with a given motor, more voltage drives more current, thus more torque. Common mistake, theory tends to be taught one parameter at a time to simplify it. But often leads to wrong conclusions as the total system is not considered.

Also I suspect the chart is for top speed only. May not be the best gearing for all uses. I would agree with you in general, I would gear down (more gear reduction, higher ratio) with higher voltage myself in most any case, using the higher torque for more acceleration, using the voltage to offset the speed lost to the gearing and then some. Those big speeds require lots of power, quick battery drains and lots of heat.

Last edited by Dave H; 12-02-2008 at 03:46 AM.
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Old 12-01-2008, 07:08 PM   #5
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Cheezburger,

Higher voltage will generate a higher current at a given rpm. (I=V/R)

torque cross rpm = power

therefore

if you increase voltage, you will also increase power at a given rpm.
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Old 12-01-2008, 11:42 PM   #6
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I think Dave H explained it the best. I understand it better now. Thanks! I'm just worried about this because i want to get this right the first time. The price for brushless slash and lipo is already pretty hefty so yea.
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Old 12-02-2008, 02:37 AM   #7
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But pinions are fairly cheap, get a few and experiment, just keep tabs on the motor & esc temps. A big gear blast down the street is fun, but better to have lower gearing too for general bashing and such, not to mention track running. I oversimplified my answer some, it's not really current, but total current flux, the number of motor winds comes into play as well.

Last edited by Dave H; 12-02-2008 at 02:58 AM.
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