Basic motor selection question
 

Can you all help me out with some basic motor selection questions?

Let's say that I have a car, and with a GPS I've measured it to go a certain speed (call it the Target Speed). To reach the Target Speed, I had to give it full throttle for a few seconds, and it was using a 3000kv motor with a 15 tooth pinion.

In that exact same car, I could also use a 5000kv motor with a 9 tooth pinion.

Everything else is identical (same tires, same spur, same chassis, etc).

So the questions comes down whether a low KV motor with a large pinion, is better than a high KV motor with a smaller pinion.

In my example above, the car hits the same top speed with either set-up.

In both cases I'd be running a 7.2 volt Lipo.

The major difference is obviously RPM. With the lower KV motor, full throttle corresponds to about 25k RPM. With the higher KV motor, full throttle corresponds to about 42,000 RPM.

What do those two motor options give me in terms of performance? How will performance differ? Which set-up is better? It is better to have your Target Speed correspond to a lower RPM (like 25,000) or some value closer to most motors' max RPM (50,000).

Thanks!

You've only mentioned top speed but there's a lot more to motor performance than that. Higher KV motors usually put out more power, so it'll probably accelerate faster to get to that same top speed. There's also heat/efficiency/runtime to consider too.

with motors and lipo(2 cell weird voltage) mentioned you will have the same exact speed in to end..

I would recommend using this speed calculator here to give you a general idea on where to start with your gearing:

https://scriptasylum.com/rc_speed/top_speed.html

thanks for all the comments.

I am still not getting a clear answer to the basic question, which is this: how should I evaluate the following two choices?
-3000kv motor with a 15 tooth pinion
-5000kv motor with a 9 tooth pinion
(both motors running on 2s)

Both setups will get to the same top speed (at least on paper - and that's good enough for our discussion here).

The main thing I want to optimize for is heat - which set-up will run cooler? The one in which the motor spends most of its time at 45k RPM, or at 25k rpm?

Too many variables to get an honest/correct answer.

For speed runs on flat ground, the higher KV (more RPM) will probably do better
For off-road jumps, etc the lower KV (more torque) will probably do better

I personally have found low KV and high voltage to work better for me overall

For example:

2000KV @ 4S
vs
4000KV @ 2S

both systems will offer almost identical acceleration and top speed, however the 4S setup will be far more efficient and offer cooler temps as well as longer run time, plus the battery will not be stressed as much which will more than double the lifespan of the battery longevity.

The higher KV motor has less turns to make more rpms at the same voltage as the other motor. That also means it has less resistance and can draw more amps.

Like mod vs stock on a track. Stock 17.5 can't make a triple coming out of a tight corner, but in mod with a 7.5 the triple is no problem. You still gear down the mod motor, but the speed is higher and the acceleration is a lot higher as well.

With your example of regearing the faster motor the acceleration should be MUCH higher to the same top speed. It comes down to less turns = more amp draw and more rpms at the same time.

Try them out, measure the speed and the heat. This is the only way to really know.

Most of us are not shooting for a target theoretical speed, but instead gearing and adjusting motor timing to get the best combination of performance without cooking the motor. The gearing will make a huge difference to heat and speed, and totally depends on the internal gearbox ratio of the car, whether it is 4wd or 2wd, tire size, outdoor temp, surface you'll be running on, how long you plan to run, whether you'll be using a fan etc.

What are you trying to achieve? Which vehicle are you planning to use?

Yep, assuming the same general motor types the higher kv (or lower turn) motor will have more power and performance everywhere. Likely even a bit more top end in the example, as the more powerful motor will overcome secondary effects like air drag better, get closer to the no load speed rating.

Tradeoff though, more amps and power drains the battery quicker, reducing runtime, and can lead to heat issues if gearing is not right.

Thanks Bill. I agree with you on the "gear down / volt UP" philosophy. I tend to run higher voltages in general. I just meant that even within a given voltage (let's take 4s) you could run a 2000 kv motor or even a 3000kv motor and still have max RPMs below say 50,000 (which usually the max for those larger motors).


Great points. I'm a general-purpose basher, not a racer, so all I'm trying to achieve is a balance of good runtime, good top speed, and reasonable motor temps. The vehicle I'm working on tuning right now is a Tekno EB48 2.0.

Thanks! Good points.

So basically, you want to go with a lower KV to get the optimal efficiency, then you can gear up to get desired speed, just make sure you check your temps to make sure you've geared properly.


Most motor manufacturers will provide recommended gear ratios for each motor they sell, you can plug those gear ratios into the calculator I linked above so you can figure out which KV motor will hit your desired speed range.


You won't find out for sure if you hit your target until after you actually run/test the electronics.


For an EB48, I would consider the following 1900KV motor:

https://www.rcjuice.com/motor-esc/br...1-8-buggy.html


4S would net you speeds in the 45 mph range

5S should get you in the 55 mph range

6S should get you close to 70 mph


Lots of variables for gearing... speed runs vs off-road vs jumps


While you can extend run times with larger capacity batteries, you will risk overheating your electronics for extended sessions.


I would limit to 4S with a 16T pinion and 6000mAh pack for run times between 15-20 minutes depending on how hard you hit the throttle.

• What gearing for upgraded electronics?
  

Perhaps some data can be helpful in understanding the motor differences. Some old Novak data, details will vary but the trends should be similar.
Note within each group, as the turns go up the kv rating goes down. Importantly look at the watts column, the power generated, significant differences. Power drops off in rough proportion with the kv rating.

https://cimg0.ibsrv.net/gimg/www.rct...404e534e42.png

If you are suggesting similar performance can be achieved with a lower kv motor geared higher, that is not realistic. Efficiency is a wonderful thing, but performance requires power.

i recognize that RC uses the term KV in a way that is useful to RC but I dont use it to select a better motor. So I use power, torque, rpm, efficiency. nothing wrong with people using the term KV to learn and share and it seems to work for everyone. But I use simple terms

a motor creates torque and rpm. Power.
torque looks like this

10, 9, 8, 7, 6 5, 4 3, 1

that torque creates rpm

1, 2, 3, 4, 5, 6, 7, 7.1, 7.5,...

torque and rpm is power

1, 2, 3, 4, 5, 6, 6.5, 7, 7.2, 6, 4, 2,

power in versus power out efficiency looks like this

1, 2, 2.5, 3, 3.5, 5, 7, 4, 3

if your track is mud (less stop and start) or very long and you pick a motor with power at the rpm you want the tires to spin you will be faster and cooler
if your track is high traction and very short you need torque. torque at the rpm you want the car to accelerate. you will be faster and cooler.

if you start with the RPM you want and determine if its an application for torque or an application for work then you obviously pick the motor that is most efficient at those points
the one efficient at churning mud is a power motor
the one efficient at accelerating is a torque motor

a torque motor can beat a power motor (even with less overall power) in stop and go

so lets say you have 2 motors with the same power. 1 has it at 10000 rpm and the other 20000 rpm. remember same power.
you can gear them both to do the same thing. true
lets assume peak power is at 50% rpm for each motor and peak efficiency is at 70% rpm for each motor.
all this power is transmitted through the wheels. wheels go from say 63mm to 61mm over a race day.
in order to keep the motor at peak efficiency or rpm or torque or power the low rpm motor will need between 2 to 4 teeth while the tires are shrinking to maintain the (power, efficiency, torque...) over that period. the rpm motor will only need 1 maybe 2 teeth to do the same.

a torque motor has a narrow power band. use it when you need to stop and start allot.
an rpm motor has a wide power band. use it when you need to do continuous work.....allot

when two motors are close and you are not a motor designer (dont understand the difference between work and torque) pick the motor with the wider power curve (not the torque motor) so you suffer less as traction goes down, tires shrink, ... its easier to stay consistent and requires less work. when you are alien fast you’ll know if its time to scrape tenths out of the difference in one motor versus another. Unless you just gotta know. then thats different.