Cant figure it out, brushless motor numbers?

[Warn Off Road]

Ive researched this, but still not sure. I know with brushed motors the lower the turns the higher RPM. I see numbers like 4600kv, 5700kv, 6900kv, and 7700kv; yet I have NO idea what they stand for as far as torque to speed ratio. Whats the KV stand for?

[Culetto]

The KV rating the RPM of the motor per volt. So a 7700Kv motor would turn at 85470 RPM's on 11.1v, it might be a little fast for a crawler. Most crawler brushless motors run from 850kv - 1500kv that i have seen.

In regards to torque I think Castle Creations website has the best answer for this here:

High power capability brushless systems can act very differently than brushed systems and even differently than other brushless car systems. Where you used to approach gearing from a sense of managing the torque of the motor (smaller pinion gives the motor more torque), in this system you’ll need to look at gearing only as a simple matter of top speed at full throttle. For example: If you are not able to get up to full throttle on the longest straight at the track (car is too fast) you should gear down to the point at which you are actually using the whole range of the throttle trigger including full throttle at times. If you never get to full throttle, or have to dial down your throttle EPA, you are running the system hotter and with less runtime than you could have if you geared down. So again, think of gearing only in terms of what speed you can actually use at full throttle.
A high power capable brushless motor in electrical engineering and physics terms, has unlimited torque. We live in “the real world” so technically for us that’s not totally true, but – a brushed motor has a torque level that due to its design has an upper limit, regardless of how much power is being applied to it. That limit is low enough that you can see it clearly on an average track On the other hand, a high power brushless motor’s limit to torque in an RC vehicle is not within the bounds of the motor itself so much, but rather falls on the ability of the battery to deliver current to it. We generally don’t describe these motors in terms of “one has more torque than the other”, but rather “the 7700Kv motor is faster and draws more current than a 5700Kv motor in the same vehicle”. It draws more current, because it’s making the car go faster and doing more work than the 5700 motor is. As long as the batteries used are very good at supplying current without an excess of voltage depression (low internal resistance is good) both motors will appear to have the same torque, even though one is much faster than the other. Battery technology is constantly improving, and the first thing you’ll notice when you use a very good battery pack (or perhaps trying a Lipo pack for the first time) with these systems is a more “punchy” feel when you accelerate. The faster you set up the car to go at full throttle, the more reliant you are on good batteries to flow that current into the motor and maintain acceleration performance. So think of torque as a function of battery capability only.
You may have noticed we didn’t introduce many “don’ts” in the manual for these systems. The power capability for the ESC and motors during our development tended to be well beyond what most users would ever dream up for realistically driveable gearing options. The only real upper limit to gearing is the temperature of the motor. As long as the motor is less than 200 degrees F (would burn your finger almost instantly) at the end of the run, you’re OK.

[cuzican]

Wow...thats a really good thing to read. Most of the guys (myself included) I run with are just getting into brushless stuff.
That explained alot thanks for posting it.

[Culetto]

no problem. you can also contact castle directly or contact holmes hobbies www.holmeshobbies.com for great info. John Rob Holmes is a longstanding member here and knows more about crawler electronics than most everyone. He has a forum in the vendor section for questions also. good luck