KV / Gearing choices

[glassdoctor]

Seems that a good rule is to mimic a nitro which is good news when converting to electric, because it makes the gearing no problem... we can basicly run the stock ratios.

I used to think we would have to modify for lower gearing but now I'm actually gearing higher.

30-35K is typical for high-revving nitros I believe, and many don't get a sniff at 30K in reality.

So... 30K give or take seems a good choice.

As a side note, part of the reason I chose the low kv, is that it leaves some room to "volt up"... I expect to be trying 6s lipo at least in the future... so even my 1700kv may be to much eventually.

[BrianG]

Interesting thread. I too am trying to figure out the relationship between high V/low Kv/tall FGR vs low V/high kv/low FGR.

I agree with GD's remarks about being able to "volt up" later on by using a relatively low kv motor with the voltage to get the rpms to the mid 20k's.

Other than experience with various motors, it's very difficult to compare them on paper. Most motors specify the KV and can size and that's it. Maybe the "max amps" is listed too, but it means very little. Is that the max amperage the motor can physically handle constantly without melting? What voltage did it take to produce those "max amps"?

I'd imagine finding the ideal voltage/gearing range for a specific motor would be a bit complex, but this is how I would imagine it could be done:

1. Run a motor on some type of dyno with a fixed gearing. Record the mechanical output power graph as the motor voltage is increased.
2. At the same time, graph the electrical power by measuring battery voltage and current draw at various points.
3. Then compare the mechanical power and electrical power. The closer these are to each other, the more efficient the motor is running.
4. Repeat the above steps for different gearing until you find the max voltage and gearing combination that has the highest efficiency (where the mech and elec power are closest).