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Gearing Goals

Gearing Goals

Old 06-27-2011, 08:03 AM
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Hi. Regarding gearing, I have been going by an old forum post on rcuniverse.com (M_3255405) called "Understanding gear selections for 1/10 Electric RC!" by SkrapIron. (I apologize, I don't have enough posts to put the link here).

I threw this in a spreadsheet to try the various combinations. The key seems to be the "efficiency" or rollout value. Where the goal is to get the wheel to turn one revolution for every motor revolution.

So, in my cases:

SC10FT (Tekin RS/17.5T, blinky mode) - 75/19 get's me 0.99 efficiency and a rollout of -0.06
RC10B4.1FT (Viper/17.5T, blinky mode) - 81/20 get's me 1.00 efficiency and a rollout of -0.01

My current gearing for my SC10 is 75/25, which has resulted in motor temps ~ 150 degrees. I just built my B4.1FT, so I don't have any real life examples.

My concern is that if I gear for efficiency I will be too slow to compete. For instance, with the SC10 (using comeseethis (now rcgearhead) to compute top speed, I will go from 29.99 to 22.8mph


Am I way off base here? Should I just keep experimenting and monitor performance and temps?

Thanks for any help!

BTW - What max temps should I be shooting for on the motor and ESC?

Craig

Last edited by HeavyDuty; 06-27-2011 at 09:16 AM.
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Old 06-27-2011, 09:01 AM
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I think I remember that article and if it's the one I think it is, it is complete BS. The goal is NOT to get one tire rotation for one motor rotation. The goal is to get the BEST PERFORMANCE POSSIBLE.

Rollout is the DISTANCE a car will travel for one revolution of the motor, usually expressed in millimeters (like 70mm).

Lower gearing (higher numerical ratio/shorter rollouts) will provide better acceleration, lower temps, longer runtimes.

Higher gearing (lower numerical ratio/longer rollout) will give more speed, higher temps, shorter runtimes.

Testing is the only real way to determine what works best. There are just too many variables for anyone to be able to tell you exactly what is best. The local fast guy should be able to get you close but you will have to try different combinations to tweak it to perfection.
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Old 06-27-2011, 09:09 AM
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Originally Posted by wingracer
I think I remember that article and if it's the one I think it is, it is complete BS. The goal is NOT to get one tire rotation for one motor rotation. The goal is to get the BEST PERFORMANCE POSSIBLE.

Rollout is the DISTANCE a car will travel for one revolution of the motor, usually expressed in millimeters (like 70mm).

Lower gearing (higher numerical ratio/shorter rollouts) will provide better acceleration, lower temps, longer runtimes.

Higher gearing (lower numerical ratio/longer rollout) will give more speed, higher temps, shorter runtimes.

Testing is the only real way to determine what works best. There are just too many variables for anyone to be able to tell you exactly what is best. The local fast guy should be able to get you close but you will have to try different combinations to tweak it to perfection.
Ahh..ok. So go by temps and experiment. Thanks!
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Old 06-27-2011, 09:09 AM
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Found it, yep that article is BS. Ignore it. Never EVER go by its recommendations. It shows a complete lack of understanding of even the most basic of gearing principals.
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Old 06-27-2011, 09:15 AM
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Originally Posted by HeavyDuty
Ahh..ok. So go by temps and experiment. Thanks!
That and laptimes. Though that is easier to do in onroad than offroad. Typically for onroad, you will start out a bit conservative on the gearing (a smaller pinion than you think would be best). Then look at the laptimes. If they didn't fall of much during the run and temps aren't bad, you can go up a tooth or two on the pinion. If laptimes fell off bad and/or temps were high, you need to drop a tooth or two.

For offroad, laptimes often aren't consistent enough to really judge so you have to go more by feel, comparative speed and temp. For instance, if you are the fastest down the straights early in a run but are getting smoked late, you probably need to drop a tooth or two.
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Old 06-27-2011, 10:08 AM
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At least in off-road, my friend taught me to go by motor temp. We gear up until we're seeing 140 after 6 minutes in practice traffic and no more than 160 or so after a qualifier/race. Depending on how technical the course is, you may also end up dropping some teeth to get more punch to clear jumps.

Not sure on the ESC, though, as we don't check their temperatures.
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Old 06-27-2011, 10:14 AM
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Just a "one percent" chance you run in to this initially. If you gear "way to low" you may have increased heat as well since the motor is always running at top rpm. Again, just food for thought. If your in the "ballpark" of gearing the rule previously stated is true.

Lower gearing (higher numerical ratio/shorter rollouts) will provide better acceleration, lower temps, longer runtimes.

Higher gearing (lower numerical ratio/longer rollout) will give more speed, higher temps, shorter runtimes.

I pretty much gear to temperature unless I'm obviously lacking in performance.. As my friends that I race with can attest, it's not usually the "motor" that lacking performance as much as the, ahem.. driver
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Old 06-27-2011, 10:46 AM
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Originally Posted by eri3f0g
Just a "one percent" chance you run in to this initially. If you gear "way to low" you may have increased heat as well since the motor is always running at top rpm. Again, just food for thought. If your in the "ballpark" of gearing the rule previously stated is true.

Lower gearing (higher numerical ratio/shorter rollouts) will provide better acceleration, lower temps, longer runtimes.

Higher gearing (lower numerical ratio/longer rollout) will give more speed, higher temps, shorter runtimes.

I pretty much gear to temperature unless I'm obviously lacking in performance.. As my friends that I race with can attest, it's not usually the "motor" that lacking performance as much as the, ahem.. driver
Ha! I hear ya! That is still a problem...probably always will be, too!!
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