Why 65?

[Sedri]

Excuse me if this question has been asked before, but I searched the forum and didn't find the same question, so...

If brushless motors are most efficient at 40,000 rpms and under and so should be battery-matched and geared to have them spend most of their time in that range,

what's the fun - er, purpose of having a 60 or 65,000 ceiling brushless motor if this is true?

Is is okay to have your brushless motor occasionally run up to max rpm, but not necessarily spend extended time there?

[Serum]

40K is quite hot already. It's better to use a lower KV motor with more torque and gear it a tad taller. The setup will run cooler and you will gain runtime.

Here, please read this;
http://www.rc-monster.com/forum/showthread.php?t=5115

[Sedri]

Well yes (and thank you for the link; I read thru it again) - I have read all of the stickies and infomation threads in the forum that I could find (including that one) and I fully understand that UNDER 40,000 is the place to stay. Mike firmly holds to that very rule of thumb.

ergo, the question - what's all the hype and advertising and ratings for about "high rpm's" from brushless motors, when the informed brushless hobbyist understands that torque is maximum at stall, horsepower peaks at 50% of maximum rpm's, and going over 40,000 only brings bad results?

rating a brushless motor for 60,000, for example then, for all practical purposes, doesn't mean anything useful?

Well, a few applications will benifit from the high RPM motors, but not practical for typical use. Another reason is...people love big #s. Alot of people go with the crazy high kv thinking its much faster, as serum pointed out...its not the best route. I wouldnt pay much attention to it.

[Sylvester]

Im just pointing this out but ; even though a motor can be rated for up to 60k rpm, it doesnt have to be pushed up to that, it will just run hotter on the higher rpms.

[BrianG]

Is the 65k limit maybe a mechanical limit for the bearings, rotor, etc? You know there has gotta be LOTS of centrifugal force spinning at that speed...

[Serum]

Yeah, that 65K number is just a number to show what the motor is capable of.

It's like a fast 1:1 car. It's got a maximum speed, but it doesn't mean you drive it the entire day. (most cars would damage if you did)

[hyperasus]

I have also been confused about RPM ratings on these brushless motors. You say 40,000 rpm is the rule of thumb. Does this mean it is best to find a gearing/kv/voltage combo that puts the motor at 40,000rmp with max throttle? Or is running the motor at less then 40,000 rpm just as good(or better). If running at less rmp then how much less is acceptable before the motor is running with too little rpm to be efficient.

Another thing. Most brushless motor talk and "rules of thumb" have been for motors used for larger scale trucks. What about S can sized motors. In general it seems that the smaller the motors, the higher the kv. So does this mean that a smaller S can motor would be more at home at say 50,000rmp? I've looked threw the forums trying to find info on this and didn't come up with much. It would be nice to have better "rules of thumb" when selecting a motor for a vehicle. Maybe some kind of chart or calculator that takes your trucks weight, wheel weight, voltage, speed you want it to run at comfortable, or whatever information is needed and tell you the best motor size and kv that will best work for that particular vehicle.

Right now if someone comes onto our forum and asks, "which motor for my stampede?". He would probably get several people advising him to get a 4200~5300kv motor. Or at least....thats the advice I got when I started out. Yet threw trial and error I've come to find out that is WAY too damn high of a kv rating and will run smoking hot in my pede. There has GOT TO BE A BETTER WAY to determine which motors will run good in a specific vehicle and not have a meltdown.

[bdebde]

Here's a calculator

calc

by BrianG

[Sedri]

My unfortunate problem, I guess is to just let go of what I've learned about nitro motors and accept that brushless power apparently has no relationship to the way a nitro produces power.

The words I'm hearing (i.e, reading here in the forum and elsewhere) is that a brushless motor would

1. be most efficient
2. put out the most power
3. AND give the longest run times

if it were both matched with a battery/voltage and gearing that let it turn between 30,000 - 35,000 rpms, under load, under "full throttle".

True?

So that means that a 7T 540XL (2144 kV) on 4S (16v) would in all applications be a better choice than a 6T 540Xl (2500 kV) on 4S simply because the 6T would operate over 40,000 rpms, while the 7T would operate below 40,000 rpms, even though the 7T would be geared higher (more strain on the motor) to give the same speed as the 6T?

I would almost be open to believing this (even though it feels contrary to common sense) simply because, when I ran my inferno with a 14T pinion, the motor ran hot, the battery and esc always ran cool. But, when I upped the gearing to 18T, the battery warmed up (a lot) and the controller heated up as well, but the motor seemed to cool off a little, even though the speed of the car jumped up by over ten miles an hour!

Yes, no, or just my imagination?

It's funny to think that undergearing/overrevving the brushless motor actually was harder on it than running taller gearing!!