RacerX20

iTrader: (9)

I often see people debating over the proper KV value of brushless motors for certain cars based on their intended use... racing or bashing... and so on. My question is, does changing the pinion/spur gear ratio yield the same affect as swapping the motor for one of a different KV value?

In my opinion, I think yes it does.

Here is an example.

A 4600 kv BL motor on 7.4 volts will spin at approximately 34,040 RPM
With a pinion/spur gearing of 12/32 the resulting RPM is 12,765 RPM

Another BL motor, this time a 5700kv on 7.4 volts, will spin at approximately 42,180 RPM
If this motor was geared at 23/76 the resulting RPM will also be 12,765 RPM

See my point?

Don't you think that selling and buying different motors is unnecessary being that changing gearing is so much easier and cheaper? Now I know that in the real world not all gearing ratios are available (I made the numbers to work out perfectly). Also the argument cannot be made for two brushless motors who's KV values are vastly different. But thats not the point. What I'm saying is that since torque and RPM are inversely proportional for a given wattage those two motors and gearing combinations are essentially the same, and will perform the same. Or is there a difference?

Other than different efficiency levels between the two motors, which isn't really the case, I cant think of any legitimate reason to switch KV levels over simply changing gearing.

Here is my situation:
I have a 5700kv Mamba Max and a 2s Hyperion Lipo that I'm planning on putting in a Hyper 12 Mini STe. And I've been reading that the ideal motor, to avoid over-heating, is a 3500-4600kv motor. Is there a benefit to selling mine and buying a 4600kv motor over simply changing the gearing?

-Tony

cbr74

iTrader: (14)

For any given vehicle/motor/esc/battery combination there is a finite number of gear ratios available. Within those available gear ratios is a narrow band of options that will give you acceptable acceleration, top speed, and run time and not cause over-heating and/or component failure.

Two motors of different power ratings will not exhibit the same characteristics regardless of gearing. They may reach the same top speed (theoretically) but the amount of time each takes to get there, how much power each takes, and how much heat each generates in the process will not be the same.

Say you have 2 1:1 cars both geared to reach the same speed at engine redline.
One has a small block 305, the other has a big block 454. Think they're gonna drive the same?

Shimmy

iTrader: (19)

Which one is supposed to be which?

A higher kv (454?) would probably be working harder, drawing more amps and running hotter (characters of the smaller 305?).

A lower kv (305?) would most likely not have the same startup/pickup as the higher kv/bigger engine (character of the 305?) but would probably be worked less, lower amp draw and run cooler (most likely anyway) (characters of the 454?).


This is just the way I understand it (and from my experience) anyway, correct me if I am wrong.


I think a lower kv motor is the way to go, there are more gearing options, they run cooler and run longer.
If you want more power, put in a 3s lipo.

The argument of a higher kv motor being more useful if you only need one battery for the power is not a good one if you have to put up with the traits of them.

I am not trying to say that a lower kv motor will be more powerful, or atleast on equal power of a far higher kv motor (3500kv compared to 6900 for an example), but the difference I see there is that the 3500kv would be a little slower in top speed but you could keep on driving it.
The 6900, I don't think you could keep on driving it, it would get quite hot and need to be cooled down.
Unless it was geared down and driven gently but it would still be likely to run hotter.

An example I can say from my experience is my Losi Desert Truck with a 3500kv on 2s compared to my brothers E-Firestorm Flux with the 5700kv.
My DT is about 2.2-2.3kg while the E-FS would probably be about 1.9 (?), maybe pushing 2kg at the max.
Top speed of these 2 are about the same, acceleration isn't that different, the 5700kv would probably just win mine over (would be quicker for sure if the front end would stay down and there was enough grip).
Mine is obviously geared higher but I can go for a full battery and come off with about 60C-62C (average), admittedly that is a bit hotter than I would like.
On the other hand, the E-FS goes about half a pack (getting treated similar) and is quickly reaching 90C+.

cbr74

iTrader: (14)

It was a loose demonstrative analogy, it wasn't intended to draw a direct corollary.

jmcvicker

iTrader: (2)

Different KV motors may have different wattage numbers for the electric circuit. If a motor of higher KV also is rated at a higher wattage then it may not just be a KV * voltage multiplier.

Two motors of different KV but same wattage (as Neu motor charts seem to say) may be correct in those calculations. Just change motor and gearing to get the same "result". Maybe present your statement to Steve Neu or other electrical engineers at the motor factories to see if they can give feedback.

A higher spinning motor will most likely lead to more heat and other things like EMF feedback or other electrical properties. Lower KV with the right power yields better results for racing conditions and longer runtime, it seems.

The 4600 is a great motor - gear it a little higher than you would a 5700 and should offer thermal-free operation.

RacerX20

iTrader: (9)

I get what you guys are saying, and I'd expect that realistically that there is some electromechanical difference between the two. Probably something to do with the mechanical resistance of the motor itself seeing as drag increases proportional to the suqare of the velocity. But to what degree, and whether it justifys buying another motor, I'm still not sure. I know that for those of you have experienced this situation first hand in one of your RC cars seem to agree that a lower kv is ideal over a lower gearing. However, if you haven't compared the two setups in the same car I don't think we can conclude much. A weight difference isn't the only thing that affects the strain on the motor.

Where did you read that lower KV motors are a higher wattage? That may be the case with the Monster Mamba Max motors but I don't think that is the case between the 4600kv and the 5700kv at least. The both can run on a 3s lipo. A BL motor will draw the most current under the acceleration of a load. Max acceleration occurs when the ESC supplies the highest duty cycle to the motor via PWM. Essentially it's getting the most voltage possible from the battery. The two motors have the same max voltage, max current draw, and efficiency. So it fallows that they'll have the same wattage. Torque and RPM together are derived from the wattage. They're inversely proportional. So that means that the difference in torque of the two is proportional to the difference in max RPM of the two.

All of that simply implies that even though the 5700kv motor is spinning faster the torque it has to resist is lower than that of the 4600kv. So I do not see how the 5700kv motor will run hotter. I'm not saying it will run cooler either I'm just trying to understand the physics conceptually to understand whats going on.
I hope I'm not coming across as a smart a$$ that's not what I'm getting at.

Anyways, do you think it'll be worth selling the 5700kv for a 4600kv? Another option to consider is to lower the timing but then you are affecting the maximum wattage the motor will consume.

-Tony

Dave H

In general, the 5700 will draw a lot more current than the 4600, thus more power, at the same voltage.

In your original question, the acceleration will be substantially different. The 5700 will accelerate much stronger.

Dave H

One example, except it's the other way around:

http://teamnovak.com/products/brushl...spec_chart.htm

Dave H

Correct, need to consider current draw.

The Neu charts I've seen that suggest equal wattage, also indicate different voltages. Rather an important point that you appear to be overlooking, you seem to only mention changing motor and gear.

Dave H

It's mostly the electrical reactance (complex resistance) of the different motors, causes them to draw different amounts of current at the same voltage.

The 5700 has less winds (turns) of wire, thus less reactance (inductance and resistance), thus draws more current and power for a given voltage. The increased current leads to more heat.

RacerX20

iTrader: (9)

Interesting. OK yeah, I couldn't find anything on Castle's site about current draw and wattage about the Mamba Max Motors. Then I suppose I could decrease the timing on my 5700kv and that would emulate a lower wattage if overheating was a problem. But if the 5700kv has more power what's the advantage of going to a lower kv? Like I said you can change the gearing to get the car down to a reasonable speed and thus the operating temperature should be comparable to the lower kv motor.

-Tony

Dave H

Yeah, you generally won't find much data on current draw, there are too many variables. My earlier comments are only trends, need to consider the rather complex system, sadly it's just not as simple as we would like at times.

Reducing the timing will help lower temps, but only to a point.

You can change gearing to impact speed, but you are not considering acceleration. We are blessed (& cursed) these days with virtually unlimited power, it becomes a matter of driving the darn thing. It simply becomes undriveable at some point. Even the best drivers do not use the fastest motors for racing. Especially for offroad with limited traction.

RacerX20

iTrader: (9)

Oh I definitely agree. That's part of the reason I want a Hyper mini st over a mini Revo. I just think that the mini revo has to much power for its weight. Going fast is more of a skill than simply buying the fastest setup.

-Tony

ta_man

iTrader: (168)

That Novak chart is the way it is (Lower KV showing Lower wattage) simply because they rate the motors at a constant voltage. If you look at the Neu chart, the max voltage goes up as the Kv goes down and wattage is essentially constant. But Novak doesn't have ESCs that operate on arbitrarily high voltages, so they rate all the 540 size motors at 7.2V. I assure you you can get more than 85 watts out of a Novak 21.5 motor if you run it on 4S LiPo. And that is well within the RPM limit of the motor.