How do you choose which brushless motor to run?

[Frankentruck]

When picking a brushless motor, I seem to always go for a 17.5, 13.5, or 10. But I see that 21.5 and 25.5 are both used for some racing classes. Is a larger number rating chosen so you can run really low gear ratios to make good speed instead of high rpms and high gear ratios? Or is it for running on 3S or more? So far on LiPo I've just been running 2S. The 25.5 motors seem like they were used with some VTA racing, which I know nothing about, so I'm not clear why they would be chosen. I understand brushed motor selection getter than brushless motor selection.

[RogueIV]

When picking a brushless motor, I seem to always go for a 17.5, 13.5, or 10. But I see that 21.5 and 25.5 are both used for some racing classes. Is a larger number rating chosen so you can run really low gear ratios to make good speed instead of high rpms and high gear ratios? Or is it for running on 3S or more? So far on LiPo I've just been running 2S. The 25.5 motors seem like they were used with some VTA racing, which I know nothing about, so I'm not clear why they would be chosen. I understand brushed motor selection getter than brushless motor selection.

The higher numbers, 21.5 and 25.5 are just slower motors and often more on par with the old 27turn stock motors. A lot of classes run these to keep the speeds lower with older cars.

[Charlie don't surf]

When picking a brushless motor, I seem to always go for a 17.5, 13.5, or 10. But I see that 21.5 and 25.5 are both used for some racing classes. Is a larger number rating chosen so you can run really low gear ratios to make good speed instead of high rpms and high gear ratios? Or is it for running on 3S or more? So far on LiPo I've just been running 2S. The 25.5 motors seem like they were used with some VTA racing, which I know nothing about, so I'm not clear why they would be chosen. I understand brushed motor selection getter than brushless motor selection.

25.5 and 21.5 are also very often 1S classes, especially on carpet pan car/oval and such classes. 17.5 is a very popular 2WD off road buggy class, also 17.5 Stadium truck occasionally, but typically I see more 13.5 or open Stadium Truck classes. The higher the number the more windings in the stator portion of the motor, and the as a result more resistance and less RPM (all brushless motors are very high torque) the lower the number, the more RPM and torque- 17.5 is like the old 19 turn super stocks IMO and 21.5 is akin to 24 deg stock motors from BITD.

The higher the number, the more expensive it is to be competitive, since little things mean big gains or losses. Very much a "motor of the week" thing i feel.

[RC10th]

Depends what you want...

For spec racing you have to pick what turn motor the class requires.

For general fun something in the 8.0-8.5T is a good choice, and with dynamic timing you don't really need any other motor.

In my opinion 3s+ is only for larger bashers/traxxas

[icanfixyourford]

So, to revisit this question;

How does kV rating of a brushless equate to the turn numbers of a brushed?
I'm new to brushless, but understand brushed well. I've wound and balanced my own motors previously.

I'd have run 17.5 (or so) in a buggy, but not sure what kV would be similar.

[Elkcycles]

It doesn't translate directly per se... Decimal based motors tend to be sensored for low speed resolution without cogging, Kv numbered motors tend to be sensorless. Generally, sensored motors give greater throttle finesse.

You can measure RPM's for a variety of brushless motors through an analyzer (which detects pole timing instead of using a dyno) to measure RPM. You would find a broad range of RPM's and efficiencies, even among stated motors. In mass produced motors, shimming stators can make a noticeable performance difference

[icanfixyourford]

Awesome, thanks. This is my first foray into brushless, so I'm still learning. I do understand the purpose and function of the rotary encoder, and definitely understand the relationship between airgap and field effect. These are all things I get to teach in my hybrid/battery electric vehicle courses.

Looking at Amain now, browsing the sensored motors. Initially, I wasn't worried about a sensored motor, as this thing will be a blaster, very littel time spent at low throttle positions. But, looking at the offerings, I see there's not much price gap, so I'll go sensored.

Any recommendations as to turn# ?

[Brendan2904]

If I am going for kv then I go Castle mostly but I like Hobbywing too an have a couple. If I am looking for turns I usually go Hobbywing. Most of my stuff is brushed though.

[icanfixyourford]

This is one option I'm considering;

https://www.amainhobbies.com/hobbywing-quicrun-10bl60-sd-g2-sensored-brushless-esc-qr3650-g2-motor-combo-hwa30803201/p1598530

[JosephS]

When picking a brushless motor, I seem to always go for a 17.5, 13.5, or 10. But I see that 21.5 and 25.5 are both used for some racing classes. Is a larger number rating chosen so you can run really low gear ratios to make good speed instead of high rpms and high gear ratios? Or is it for running on 3S or more? So far on LiPo I've just been running 2S. The 25.5 motors seem like they were used with some VTA racing, which I know nothing about, so I'm not clear why they would be chosen. I understand brushed motor selection getter than brushless motor selection.

It's going to depend a lot on what I'm putting it in and what I'm running that on.

In my 'racing' stuff I run 17.5 because I can't keep anything faster on a track.

For my bashing truck I switched out to a 550 motor because I seem to be able to run them longer than 540 motors without generating much heat. This really works well in the summer when it's above 100°F and I'm running in grass.

For onroad it's possible to calculate the the motor RPM to speed calculation. You can pick the top speed you want, the tire runout, the gears you think would be idea and size and pick the motor from that.

I just run the fastest motor I have and use throttle endpoint adjustments to make it manageable.

[Elkcycles]

You'll get an order of magnitude in terms of longer run times over brushed using modern LiPos with brushless motors. I find that the Hobbywing BLS120 with a 13.5 Surpass Hobby Rocket motor is an excellent combo for fun, manageable.power. I have used Castle systems in the past, typically in Traxxas platforms. I think that with older cars, a balance of stock (17.5) turns, or mod (13.5) is the sweet spot. You can go faster, but parts wear and break, especially in the back yard. You'll also keep the motors cool running within the original factory gearing range. Brushless motors don't like heat (nor any electric motor as you know all too well).

Consider an older Novak brushless system. They are long out of business, but can be found on the cheap, and if fits the vintage feel.