3000kv x 4:1 reduction = 3750kv x 5:1 reduction = 4500kv x 6:1 reduction…. or not?

[wouterb]

3000kv x 4:1 reduction = 3750kv x 5:1 reduction = 4500kv x 6:1 reduction…. or not?

So after digging through the ‘beginner’-, ‘motor/ESC’- and ‘bombproof’-stickies (and a lot more) I can’t seem to proof this wrong.
Throw this tread in the mix and you have something to think about:
Motor theory... and practice.
(it's partly over my head, so I hope I interpret the conclusions correct)

Will the one set up put more strain on the rig compared to others, either electrical or mechanical?

1. Theoretical top speed is the same (assuming same battery cell count and suitable ESC): given kv’s x resp.ratio are the same.
2. Typically a high kv motor produces more Watts then a lower kv motor of the same series.
3. More Watts means more mechanical stress throughout the transmission.
4. More Watts, same voltage means more amps means more stress on electrical components.
(Correct me if I’m wrong in these statements)

So what’s the catch?
Where does the claim “+4000kv is too much” come from?
Why is 3000 -3500 kv set as benchmark for the wraith?

Only thing I can think of is running half throttle. The higher amp draw of the high kv motor can be a drawback compared to lower kv. But if you upgrade the C-rating on the batteries, wires and ESC accordingly (not a very big increase in $$ if you were to buy a new set) that should be covered.

Any claims here can (and therefore: should) be challenged? Please do, I learning!

thnx.
Wouter B

[maslin]

I was running 4550kv geared 64/11. Lots of heat issues, 45 minutes run time on a 3200mah 3s, poor torque, lots of top end zing.

Now running 3100kv geared 60/13. Over an hour on the same battery, lots more low end, lots less top end.

The wraith isn't a top end power kind of rig. Stuff breaks, quickly.

I am much happier with the lower kv motor, the guys that have had these things for years say 3000-3500kv for a reason.

[Jtroutt19]

I just read that post and wow great post with lots of wonderful information. I run a 4 pole 4850KV sensored motor in my wraith with the stock gearing of 56/12 and I absolutely love the thing. I have not had any issues out of it.
And while the wraith wasn't made for super high speed stuff it can handle it just fine with a few mods. Every body that owns a wraith already builds them up for what they "want" to do. So build it for what you want it to do.

[wouterb]

I subbed your tread where you compare it with the Tekin. Very curious what the outcome will be.

[Eric0424]

Those guides are not crawler laws that need to be followed, they're just opinions based on the wants and needs of the author(s). They should be regarded as loose guidelines, not a map to the perfect Wraith. If you are targeting a specific maximum top speed or wheel speed then the motor kV to spur/pinion gear ratio formulas are of some use, but still just a loose guideline.

Your assumptions are correct in that theoretically more motor is typically more current draw from the battery through the ESC and more power is almost certainly more stress on the drivetrain. However not all 3000kV or 3500kv 540 motors are the same. If you include the high output 4-pole 540's (Tekin Pro4, Reedy 544) and the short (55-56mm long) 550's you can get a lot of torque without having to drop down to the ridiculously low kV rates of the typical or crawler specific 540. With the high output 540's and short 550's even though the motor can draw more current they may not be as stressful on the battery and ESC since they're not having to work as hard to reach the same RPM and torque produced by the low kV 540's. Overall weight of the Wraith will also be a factor in actual current draw, motor output and stress/abuse to the drivetrain.

[Billy D]

I am a firm believer that more is better. So I run the 2300 kv roc 412 with a MMP on 4s and I think it's perfect. 56/13 and it doesn't get hot and tons of power and runs forever on 3300 like 2 hrs plus, and I like to be on the throttle. Everyone says volt up and gear down. So I took it to the next level and went to 4s instead of the typical 3s. My wraith is about 8-9 lbs and is solid in the drive train but has control like no tomorrow. I wouldn't change a thing!

[JohnRobHolmes]

4000kv + is typically too much as of "today" because the ESCs have a hard time with the low inductance and high phase amp requirements. Not to say it can't work well when used right, but it's not ideal for the average situation where there is a chance the motor will get bound down at low speed.


Turn your question around and it becomes more applicable. @ 5:1- 4000kv on 2s or 2666kv on 3s or 2000kv on 4s. Then the higher voltage does get a clear edge from copper efficiency in the FETs and wires running between FETs and coils inside motor.



The reason that 2700kv to 3500kv is so popular, is because its just the sweet range for equipment and utility on 3s or 4s. Most people have a ton of 3s packs. Folks running 2s can choose a higher KV and still work well too, especially with brushless.