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Old 03-28-2007, 01:16 PM   #16
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Originally Posted by syndr0me
Thanks for the thoughtful replies, guys. I measured the packs after the race without load, so those numbers may not mean as much as I originally thought.

It's sounding like heat is indeed the issue. I'm going to try using a higher capacity pack next week (4800 LiPo rather than 3200) which seems to have more voltage at the end of a race. I suppose if it is heat, it's time to look into cooling the brushless motor, or (gasp) gearing down. Even though it's not getting hot enough to go into thermal shutdown, it is apparently getting hot enough to slow down significantly by the end of a 5 minute race. If so, I believe this would be a function of the stator getting hot and losing efficiency rather than the sintered magnet fading, since it's supposedly unfazed by normal brushless operating temperatures.
i would have guessed you were Driver 2 in your original post.
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Old 03-28-2007, 01:21 PM   #17
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Originally Posted by syndr0me
think I've this problem figured out, but I was hoping to get a sanity check here. Please keep in mind that even though this example includes LiPo and brushless, I believe this would apply to all batteries or motors.

We've got two guys running Orion Carbon batteries with the Novak 13.5 motor. The start and end voltage of the batteries for both racers are nearly identical, and the motors appear to be very similar. Keep in mind the 13.5 is all torque, which may make a difference.

Driver 1 gears his motor to the very edge of its limits, and is noticeably faster at the start of a race than Driver 2 who gears a bit more conservatively. As the race goes on, toward the middle, Driver 1 flattens out and has about the same speed as Driver 2. By the end of the race, Driver 2 is quite a bit faster than Driver 1. While this all seems to make pretty good sense, when you check the voltage of the packs by the end of the race, they're the same for both drivers.

The first theory is that Driver 1 is getting his motor hotter than 2, and it is therefore suffering from head-related fade, or stator fatigue. The sintered rotors are supposed to mitigate this somewhat, but it's still a reasonable assumption, and has indeed been pointed to as the likely cause of this problem.

The second theory is that when the race starts, and the batteries have more voltage, the taller gearing of Driver 1 works out well. However, as the voltage begins to fall, Driver 1's motor doesn't have the extra power available to work as hard as it did before, while Driver 2's motor is humming along happily since it's now more appropriately geared for the available voltage.

So, do motors work that way? Is it even a possibility that the motor that's geared higher will perform better at the beginning and worse at the end than the same motor/battery that's geared lower? And I mean completely exclusively of issues relating to heat, fatigue or anything else. Does a motor's ideal gearing change based on available voltage?

Hopefully that's a clear way of explaining it. You never really hear about motor gearing being targeted to be consistent throughout the race rather than fast early, slow later. Perhaps the problem is more pronounced with torquey motors, or brushless in general.
In your theory, with all things being equal except gearing which car should come over the finish line first? I'm sure some of you math geeks out there can figure this out...

Last edited by Apex; 03-28-2007 at 02:12 PM.
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Old 03-28-2007, 02:03 PM   #18
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Originally Posted by Scrubb
This is how I should have wrote it.

higher heat = higher resistance, higher resistance = less magnetic field being produced (per given wind) which produces a slower motor.
I knew what you meant... it just wouldn't have been the same if I didn't give you any crap.
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Old 03-28-2007, 03:46 PM   #19
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Originally Posted by RussB
i would have guessed you were Driver 2 in your original post.
Thanks I think? I was driver 1 in this case. The car is blazingly fast at the beginning, and then loses ground hard in the last 10 laps as it slows down significantly. I've observed this across two cars, and two different types of brushless motor (10.5 and 13.5). The only common factor is the batteries (3200 LiPo) and the fact that both motors are brushless, and geared to the edge.
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Old 03-28-2007, 03:47 PM   #20
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Originally Posted by or8ital
Orbital? I know syndr0me and I both post more then we should but we really are different people!
Your reputation is way better than mine. I'm still doing damage control. :-)
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Old 03-28-2007, 03:58 PM   #21
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Quote:
Originally Posted by syndr0me
Thanks I think? I was driver 1 in this case. The car is blazingly fast at the beginning, and then loses ground hard in the last 10 laps as it slows down significantly. I've observed this across two cars, and two different types of brushless motor (10.5 and 13.5). The only common factor is the batteries (3200 LiPo) and the fact that both motors are brushless, and geared to the edge.
does everyone run lipo at your track? could the fact that you loose time at the end of the run be due to competitors using 4200's that don't fall off as much towards the end of the run(compared to the lipo)?
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Old 03-28-2007, 04:27 PM   #22
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Quote:
Originally Posted by syndr0me
Thanks I think? I was driver 1 in this case. The car is blazingly fast at the beginning, and then loses ground hard in the last 10 laps as it slows down significantly. I've observed this across two cars, and two different types of brushless motor (10.5 and 13.5). The only common factor is the batteries (3200 LiPo) and the fact that both motors are brushless, and geared to the edge.
that's exactly how it would go down with two similarly equipped brushed cars. sounds like you're geared beyond the edge.
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Old 03-28-2007, 04:31 PM   #23
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Originally Posted by geeunit1014
does everyone run lipo at your track? could the fact that you loose time at the end of the run be due to competitors using 4200's that don't fall off as much towards the end of the run(compared to the lipo)?
Nope, it's about 60% LiPo I'd say. The person that I'm comparing to in particular is running the same motor and battery, is geared more conservatively, and is not having the same issues. I'm ultimately trying to determine if it's simply a matter of heat, or if it's something related to gearing vs. available voltage. I'm going to try two things, run the car after the motor has cooled with the same pack and see how it feels, and try a pack with larger capacity that doesn't seem to get as far into its discharge curve.
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Old 03-28-2007, 04:35 PM   #24
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Originally Posted by RussB
that's exactly how it would go down with two similarly equipped brushed cars. sounds like you're geared beyond the edge.
I agree, and it would be much easier to determine with brushed motors, since they continue to get hotter the higher you gear them. The brushless with sintered rotors are curious, though. The motor isn't really getting -that- hot, maybe 160 or so, but its performance falls off pretty dramatically (about 5 tenths per lap) by the last quarter of the race. That's why I'm trying to examine all avenues. I remember Charlie mentioning once that while the motors may not get that hot, they do get fatigued due to the load, and begin to slow down. I don't know enough about electronics to understand how that makes sense, but it does seem to be the case. The heat isn't really that high to begin with.
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Old 03-28-2007, 04:47 PM   #25
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I know the answer to your question,but I'm not going to tell you.I know what the results of your test are going to be,and the train of thought you're following will lead you to an erroneous conclusion.I'll save you the embarrassment of having to learn from your "inferiors",but I will give you a hint-temp everything,If you know enough that will lead you down the right path,and if you don't it won't confuse the issue.
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Old 03-28-2007, 04:52 PM   #26
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Originally Posted by DrOlds
I know the answer to your question,but I'm not going to tell you.I know what the results of your test are going to be,and the train of thought you're following will lead you to an erroneous conclusion.I'll save you the embarrassment of having to learn from your "inferiors",but I will give you a hint-temp everything,If you know enough that will lead you down the right path,and if you don't it won't confuse the issue.
Hahhaa, thanks. I like tinkering and trying to get my head around things as much as racing, so the discovery process doesn't bother me much. I guess I should temp the ESC, since I'm running it without a fan, but it's usually not even very warm to the touch after a race. I'll double check, and pay more attention to temp in general.
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Old 03-28-2007, 05:09 PM   #27
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yeah, i really don't know much about brushless other than i want a 13.5 and a 10.5

maybe it really is as simple as: it runs cooler than brushed, so it fades at a cooler temperature than brushed?
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Old 03-28-2007, 08:38 PM   #28
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Heres an observation. A 4.5 same track 2 or 3 laps faster, and it comes off 120 degrees. When I run 4300 140 degrees. When I run 13.5 130 to 140.
The hotter motor comes off cooler because I'm not pushing it.
The slower motors comes off hotter because I'm trying to push them to go as fast as the track will go, only the motor will not go that fast.
If you gear the the motor perfect to its power band you will need a smaller track to feel fast.
Slower motors will make you over drive, and a faster motor will make you under drive, and the motor will run cooler.
The slower the motor, the more the urge to over gear.
Over geared is a battery drain due to the stall of the taller gear coming out of the turn or just ripping the throttle in general. Lower gears are less resistance. More consistent over the run.
The taller gear draws more amps over the run. Even if the motors are the same temp after the run.
On a small or medium track with a sintered rotor low and over geared motors can come off at the same temp. Amp draw is the Villon here.
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Old 03-28-2007, 08:50 PM   #29
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OH and one more thing> Over geared is over geared, and the first sign you need a faster motor.
Altough from reading some of these posts It could be both.

http://rctech.net/forum/showthread.p...9&page=8&pp=30

Syndrome maybe you remember this one post 211 and 212 its been a while.

This one had some good stuff in it.

Last edited by UN4RACING; 03-28-2007 at 10:15 PM.
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