Brushless KV and gearing

[kiohio]

I've tried to read everything I can find on brushless motors.
We have between me and my 2 step sons 5 brushless motors. Varying from 3100kv to 5700kv.
The middle son is very interested in RC cars and is always asking me things.
We talk alot about motors and such while wrenching on the cars.
One question that comes up is about the different KV's of BL motors.
If the lower the KV means it has more torque, couldn't everything be basically equalized with gearing with similar motors?
Like say a 3100 with a 26/81 combo and a 5700 with a 15/81 (I don't know actual numbers just wild guess).
Wouldn't they be very similar and what is the better set up?
Running a lower KV and higher gearing or a higher KV and lower gearing?

I spend 2 hrs a day commuting, maybe that why I think about this stuff LOL

Thanks,
KO

[aconsola]

I believe that you are correct that different Kv motors can be geared to give the same wheelspeed, however this is only under no-load conditions.

Once the model is on the ground and having to propel its own weight then the power output of the motor comes into play. Just like with brushed motors, the lower the turns the higher the power output. For brushless lower turns equates to higher kv.

If you think of it as unloaded kv being a side effect of power, the higher the horsepower the higher the kv, then it all makes more sense.

now why a 3100kv can't put out the same power as a 5700 that I do not quite understand.

[Tamlok]

The lower KV motor will take longer to reach the top speed, but it will have smoother power band, and produce less heat. The higher KV will accelerate faster (have more "punch"), but produce more heat. The type of vehicle will make a big difference on selecting the right motor. Heavier vehicles will want the lower KV due to the increased torque. What vehicles are you running them in? Voltage plays a big part too. Do you use 2S, 3S, etc?

[kiohio]

What vehicles are you running them in? Voltage plays a big part too. Do you use 2S, 3S, etc?

At this point I'm just playing with RC10's. RC10T's and a T5M, but you just never know! LOL
When you say the higher KV will run hotter, would that be both the motor and ESC? Or just the motor?
I only run 2s lipo

Thank you

[Tamlok]

Both the motor and ESC will physically get hotter. 140-160 deg is a safe range. 180 would be the max and anything over that you will be sending smoke signals. The higher KV motors draw more amps, which means more heat. Gearing is everything to find that sweet spot. If the motor is running cool, below 140, it means you can gear up, you are not using the full potential of the motor, and leaving speed on the table. As an example, I have a 5700 Castle set up in my 10T and run 15/87 gearing, while my B3 had a 17.5 and I was running 30/69.

I bought a cheap temperature "gun" from Harbor Freight to monitor the motor and ESC.

[Coelacanth]

All else being equal, the theoretical optimal pinion gear to spur gear ratio doesn't change when you change motors, because one revolution of a 1700 kV motor, or a 6000 kV motor, or even a vintage brushed motor, moves the tires the exact same distance. The trick is in finding your best pinion gear to spur gear ratio, and to calculate that, you need the internal gear ratio and the tire diameter.

Once you know the optimal size of pinion gear, you can gear down for more torque and faster acceleration, but you'll have a lower top speed...and you can gear up for a higher top speed, at the expense of torque.

If you overgear (too big a pinion gear), you'll overheat the motor. If you undergear, you'll overheat the ESC. That's how I read the theory on it, anyway.

So in a nutshell, if you already know that a 15T pinion gear was your best choice, for a particular car with a given spur gear and tire size, it'll still be your best choice, regardless of motor.

Of course, a 6000 kV motor is probably WAY too powerful for most vintage RC cars that haven't been bulletproofed, but it's not because of the pinion gear. Lowering the pinion to compensate for an overpowered motor isn't really the proper way to go about it.

[kiohio]

Thank you very much, this info sheds alot of light on the subject.
It will help me make some decisions on my motor choices in the near future.
It is much appreciated.

Thanks again,
KO