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.

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.

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.

Originally Posted by RussB

i would have guessed you were Driver 2 in your original post.

Originally Posted by or8ital

Orbital? I know syndr0me and I both post more then we should but we really are different people!

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.

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.

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)?

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.

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.