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smirk-racing's Avatar
"Experienced user"
smirk-racing - 8.08.17, 11:36 pm Post #1: | Reply With Quote
In my p4de, I've been running a 3660 motor and a 4070 motor (both tenshock).

Another member mentioned that the *smaller* of his two brushless motors felt 'zippier'... so I decided to run my two motors back to back with the same gearing (but different lipos to compensate for their respective KV difference - 3s and 4s).

Much to my surprise, I found that the smaller motor felt "zippier" also. It is hard to explain, but it felt lighter, faster, and seemed to get up to speed even more quickly than the larger motor. Both motors are more than strong enough to flip the truck over if you stab the throttle, so in both cases I had to be careful to avoid that, and even with that in mind, the smaller motor felt like it was somehow faster.

After similar long runs, the smaller motor was too hot (close to 180) while the larger motor was about 150, so that part of the experiment was in line with expectations.

Is this just in my head? I don't have a good way to quantify or measure "zippiness" but it sure did feel like the smaller motor was more responsive even though the larger one is obviously more powerful and has more torque.

One hypothesis is that the larger motor is much heavier, and at 50k rpms, that might make a difference.
stevenator128's Avatar
"Inexperienced User"
stevenator128 - 8.09.17, 12:32 am Post #2: | Reply With Quote
How did you calculate your gearing?
One can lose things and one can loosen things. "Loose" is very rarely a verb; it is an adjective, and it describes the grasp some have on this concept.
smirk-racing's Avatar
"Experienced user"
smirk-racing - 8.09.17, 7:38 am Post #3: | Reply With Quote
The small motor is 3400kv and the larger is 1800kv. I used a 3s pack with the smaller motor and 4s as well as 6s with the larger motor. Same pinion for both motors.

With the 6s pack the larger motor was of course a beast and went incredibly fast but the smaller motor just felt lighter and nimbler.
stevenator128's Avatar
"Inexperienced User"
stevenator128 - 8.09.17, 8:44 am Post #4: | Reply With Quote
Small motor on 3S: 3400kV x 11.1V = 37,740RPM.
Big motor on 4S: 1800kV x 14.8V = 26,640RPM.
Big motor on 6S: 1800kV x 22.2V = 39,960RPM.

That's why it felt quicker with the smaller motor. You'd need to adjust your gearing to get the most out of the bigger motor. The larger rotor and magnets should result in more torque and therefore a faster spin-up, despite being heavier.
One can lose things and one can loosen things. "Loose" is very rarely a verb; it is an adjective, and it describes the grasp some have on this concept.
fyrstormer's Avatar
"Precision Basher"
fyrstormer - 8.09.17, 9:16 am Post #5: | Reply With Quote
A smaller motor will have a lower rotational moment of inertia, but honestly I'm surprised there would be any noticeable difference when you make the motor drag around an entire vehicle with it. The rotating parts inside smaller motors also have less leverage (which translates to lower torque per watt) and heat-up more quickly, so the benefits should disappear over the course of an entire run.
Check out "Fyrstormer's Garage" in the General RC forum.
thegrafxguy's Avatar
"Experienced user"
thegrafxguy - 8.09.17, 9:22 am Post #6: | Reply With Quote
You already mentioned the main issue though, and that's heat. As mentioned above, gear the bigger motor up
Arrma Talion - ACE Ripper 2000kv, HW Quicrun 150a, Brushless 25kg servo

Sakura D4 AWD - Tacon 3930kv, HW EZRun 60a, Blue Bird Servo

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smirk-racing's Avatar
"Experienced user"
smirk-racing - 8.10.17, 1:14 pm Post #7: | Reply With Quote
Fyrestormer, grafx, steve, thanks a lot! appreciate your advice.
KristofferR's Avatar
"Experienced user"
KristofferR - 8.11.17, 6:58 am Post #8: | Reply With Quote
One way to test the theory of inertial influence would be to somehow measure the moment of inertia and compare the two rotors' energy used for achieving a certain rotational speed

My gut feeling is that the difference between the motors is negligible compared to the collected inertia of the tires, despite their slower speed. An unloaded motor accelerates to max revs in a fraction of a second.

Another way to look at the phenomenon is in my aircraft. Without a prop, the inertial effect is almost non-existent. Even with a prop attached and in full 3D hover mode, slamming the throttle hardly makes the plane flinch from inertia, it is there but not significant. For big and powerful machines such as IMAC maybe its different.
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