Neu motor rpm speed question for BrianG

[VintageMA]

I think we are getting a little bit away from my original point. I don't think there is a question here about LV compared with HV. It's a proven point by Ohm's law that if voltage goes up across any resistive load then current goes down (it is the resistive part of the motor which generates the heat - aside from the spinning bearings which do generate some mechanical heat).

I've easily proven to myself that running a 6S setup is much cooler than running a 4S setup due to amps pulled.

You can't really make a comparison between a 1512 and a 1515 at different kvs - the 1515 is bigger and will draw more amps.

The question is more about that happy point where you get the most efficiency out of the motor. Compare a 1515 2.5D (1650kv) and a 1515 1Y (2200kv) both running at 6S. That 36600rpm vs. 48800 rpm say geared for the same speed - which would run better.

With the Faiegos and other cheaper motors you couldn't run over 35K without major heat issues. So that is where I have been shooting for even when buying Neus. Now I am more curious what happens when you aim your peak rpms to be closer to 50k.

Both motors will have gobs of torque and the question of efficiency comes into play asking the question - which will draw more power and produces more heat:
- The slower motor with a bigger pinion, or
- the faster motor with a smaller pinion

When do you hit the point that the motor is spinning so fast with too small a pinion that it isn't loaded enough, or conversely when do you hit the point that the pinion is too large and you are over loading the motor.

[TexasSP]

Originally Posted by BrianG

Define "better".

If better means "more efficient"; to me it doesn't matter if temps during my informal testing are any guide.

If better means "smoother"; then yes. I daresay almost as smooth as a sensored setup at low speed. Smoother running would also lessen the chance of cogging at really low speeds - and we know that a cogging (effectively stalled) motor pulls LOTS of current.

You don't want to go crazy with this idea though, like putting a 1527/1d (kv=2300) on 6s lipo on a 8th scale buggy, but gear down for 20mph. The motor wouldn't be loaded enough.

What I am trying to say is that I don't like getting a low wind and applying low voltage for low rpm and then gearing up to compensate for lack of speed.

Understood, thank you. Unlike some people on the "other boards" I appreciate your long explanations.

[BrianG]

Which boards would you be talking about I wonder

[TexasSP]

You know, those "other boards" with the people and the mods and what not......

So down to the real question, do you then recommend running the 1515/1y at 6s in an e-maxx geared for 45 or so versus a 1515/2.5 all in an e-maxx?

[lutach]

So what is the average (Or best) final drive ratio that we can have to determine the right motor for our application? Most of my vehicles are geared for 15+ final ratio now and everything is working good. Speed is around 39-40mph.

[BrianG]

I dunno.

I kinda work backwards. First, I estimate the motor series (1515, 1521, etc) I'll need based on vehicle size/weight. Then, work backwards with my desired speed and gearing to calculate the motor rpm I'll need. Then, the kv is determined by the motor rpm and voltage I plan to use. This is the method my calc program uses for the motor estimator.

If I know I'm looking for an unusual application, like super high speed where tall gearing is necessary, I'll go up on the motor series to give extra torque to compensate.

[rabosi]

Originally Posted by BrianG

Without gearing down for equal speed, I'm surprised it didn't get hotter actually. If you can, try the test on 6s vs 4s again, but gear down for the same speed. Note temps and smoothness.

I was thinking about this and you may be right. The field I was running on had a lot of irregularities so I was wot only for short burst since I was geared for about 50mph on 6s. On 4s it was mid 30's so I could wot almost the entire time. Probably explains why it was cooler on 6s. I will try what you said however to verify. Incidentally, the last two times out on 6s I blew the front center shaft on my e-maxx. Never had any issues before on 5s or 4s.

[VintageMA]

Originally Posted by lutach

So what is the average (Or best) final drive ratio that we can have to determine the right motor for our application? Most of my vehicles are geared for 15+ final ratio now and everything is working good. Speed is around 39-40mph.

Lutach - I read a thread on another "reliable" forum (can't remember which) about a year ago by a really hardcore 1/10 on-road guy who was describing how the final drive ratio should be 1:1 with the rollout of the tires for highest efficiency of the setup.

I blew this off originally and didn't think much of the logic behind it - probably at the time because we were all setting up our systems for 30-35k rpm to "match" the equivalent speed of the nitro that would have been in the system originally and that was what most motors could handle before overheating too much.

Using the RC-Calculations page you can find this ratio easily by looking at the fields for "Total Reduction Ratio" and "Tire Circumference". If the two numbers match then you ratio is 1:1. If the rollout is 15" and the total ratio is 10, then the number is 15/10=1.5.

This makes sense if you think about it for a while because if you increase the size of the diameter of your tires, you have to reduce the size of the pinion to make up for it. And MTs with larger tires have a higher total reduction than buggies with smaller wheels.

I also came across an article within the past week or two that eluded to this same concept that was talking about the way people are setting up their vehicles lately. It is hard to get a true 1:1 ratio and usually it is closer to 1.25, but lately people are gearing closer to 1.5 and even approaching 2 at times - and this is leading to much more heat being generated in the setups.

Another way I came up with looking at this is that you should setup your system for 1" of travel for every 1 revolution of the motor.

Here's how I am going to test this and Brian's theory on faster motor speeds on the Neu on my CRT.5.

I just got a 1509/2Y (1820kv) for my CRT.5 and have been running it at 15/46 on 5S with ProLine 30 series wheels and tires. The specs for this setup are:
Tire diameter = 4.1"
Motor RPM = 33670 (@3.7 v/cell)
Total Reduction Ratio = 9.97
Tire Circ. = 12.88
Est. Top Speed = 41.21mph
Ratio = 12.88 / 9.97 = 1.29

This is a very cool setup and runs awesome.

I am going to setup this system to run with 6S and push the motor rpms up to 40400.
To compensate and also to get the rollout/gearing ratio closet to 1:1 I will run a 13/46 setup with the following:
Tire diameter = 4.1"
Motor RPM = 40400 (@3.7 v/cell)
Total Reduction Ratio = 11.5
Tire Circ. = 12.88
Est. Top Speed = 42.85mph
Ratio = 12.88 / 11.5 = 1.12

Everything will be well within comfortable specs of operation. I have already run the 5S setup and will run the 6S setup tomorrow after work for a bit and report back on performance, smootheness, acceleration, temps, etc.

I have come to the conclusion that for the weight of the vehicle and size of the tires it take about 800-900 watts of power to push the truck the way I like it - so I don't expect that to change to much - it has been pretty consistent with 3S 4S and 5S setups. The question will be if the 6S and faster motor speed is better.

[rabosi]

Originally Posted by VintageMA

Lutach - I read a thread on another "reliable" forum (can't remember which) about a year ago by a really hardcore 1/10 on-road guy who was describing how the final drive ratio should be 1:1 with the rollout of the tires for highest efficiency of the setup.

............................

Unless I'm missing something, it seems like the weight of the vehicle could have a huge effect on the load to the motor (heat?) even if you have a 1:1 ratio. So there'd have to be a way to figure ideal power/weight to maintain desired heat level.

[VintageMA]

Originally Posted by rabosi

Unless I'm missing something, it seems like the weight of the vehicle could have a huge effect on the load to the motor (heat?) even if you have a 1:1 ratio. So there'd have to be a way to figure ideal power/weight to maintain desired heat level.

Like I said - I don't fully buy into the concept - but it does make some sense.

I guess weight comes into play the same way it does now. 1509/1512 for a buggy, 1515 for a truggy/mt, etc.

[VintageMA]

So I went out and tried the 6S setup with the 13/46 gearing running at about 40K rpm. I even turned up the punch control a bit on the MMM.

Results are that the temps were hitting up around 180* compared with around 150* for the 5S setup after 10 minutes of hard WOT running in a huge parking lot.

I will admit that the acceleration and startups were definitely a bit smoother, but this wasn't enough to convince me that for my driving style sticking to around 33-35krpm in my setups just seems to work best for me.

In a track situation with much more technical driving and only a bit of WOT I could see that faster rpm setups working very nicely.

There is no definitive answer here.

[BrianG]

I don't there is a definitive answer here either because there are so many variables.

If running at more or less constant high speeds, I can see the high rpms heating the motor up more. And really, smoothness is irrelevant at high speed running anyway.

If running where the speed varies a lot with little top-end speed, I can see setting up the system for higher rpm to give more control at the lower speeds.

So, there is no one-size-fits-all solution, but the 35k "golden rule" can work for all around use. I just prefer smoothness at low speeds along with high speed capability.