What is the difference between 4s and 66% TX rate on 6s?

[chilly81]

I got one car that I’m thinking might be better on 4s but most of my other stuff is 3s/6s focused. So I was just wondering what is the difference if I just use a 6S battery set my throttle dual rate to 66%? Won’t the ESC be putting out 2/3 of the voltage which would effectively be 4s voltage? And at that point, it’s just a matter of watt hours… so I would run a smaller amp hour 6S then I would a 4s to get equivalent runtimes.

Any other factors I am overlooking that make it not all that simple?

 

[Hector_Fisher]

This recent thread came to mind that had some decent insight on the topic. Sounds like you're thinking along the same lines.
https://www.arrmaforum.com/threads/reduce-throttle-percentage-or-lowercell-count.61195/

They will probably feel pretty similar overall. If it simplifies your setup and doesn't require new dedicated batteries, then I'd do the 6s on lower throttle percentage. That should be $0 to try out right? And if you're happy with that setup after trying, then why bother buying 4s lipos just to try out?

 

[Dan B.]

As long as you’re running a 6s ESC it will be fine. Even turning down the throttle with 6s on a 4s ESC can fry it.

 

[MMoelmann]

I do this sometimes and the only issue I run into which is annoying is your brake power is also affected. So you may need to increase your brake force in the ESC so it is strong enough when you lower the throttle %, but then you need to be more delicate when braking using 100% power.

 

[LibertyMKiii]

Pretty darn close between the two. In theory, the "maths" say it should be about the same.
In reality, the punch on the 6s setup will be much higher.

 

[loryriga]

Brushless motors speed is adjusted by changing the commutation frequency of the coils while the supplied voltage is always the same, so if you connect a 6s lipo, you are actually applying 25V to your motor, no matter what your throttle limit is set to. Applying a higher voltage will result in a higher torque since the generated magnetic field is stronger

 

[chilly81]

Some good points

@LibertyMKiii you had an interesting note about ESC ripple in mid band that’s good to know about. It will just be an MT 410 basher, I’m not a speed runner so I’m not actually sure what ripple is at this point lol but hopefully it won’t matter much? I’ll definitely keep an eye on purpose though.

As far as braking, I would think it would scale the same way as forward throttle wouldn’t it? It’s the same channel on the radio after all. No?

Obviously, I can run a smaller pinion, but I’d like the ability to switch between four and 6S packs, depending on what I have convenient at the time. I can now, but I feel like it wheelies at 3% throttle on 6s lol

Brushless motors speed is adjusted by changing the commutation frequency of the coils while the supplied voltage is always the same, so if you connect a 6s lipo, you are actually applying 25V to your motor, no matter what your throttle limit is set to

Oh, very interesting! So it’s not simply adjusting voltage it’s more analogous to something like pulse width modulation where it’s not just a dumb 100% constant supplied voltage?

So maybe it would still work out OK but not for the reason I was thinking. That would explain what. @LibertyMKiii mentioned about it still having much more torque on 6S.

 

[LibertyMKiii]

Some good points

@LibertyMKiii you had an interesting note about ESC ripple in mid band that’s good to know about. It will just be an MT 410 basher, I’m not a speed runner so I’m not actually sure what ripple is at this point lol but hopefully it won’t matter much? I’ll definitely keep an eye on purpose though.

As far as braking, I would think it would scale the same way as forward throttle wouldn’t it? It’s the same channel on the radio after all. No?

Obviously, I can run a smaller pinion, but I’d like the ability to switch between four and 6S packs, depending on what I have convenient at the time. I can now, but I feel like it wheelies at 3% throttle on 6s lol

Correct there is voltage ripple that occurs in the mid-range throttle however it does not seem to impact the RTR setups as badly has the high amperage Castle ESCs.

I had a Kraton 6s and tried the lower throttle switch setting. I forget now if it was 25% or 50% it would make you lose the brakes. Only the 75% setting retained brakes but they were weak.

If you are doing this for a child then I understand, but if it is for yourself I would just constrain yourself and learn on 100%.

 

[LibertyMKiii]

So maybe it would still work out OK but not for the reason I was thinking. That would explain what. @LibertyMKiii mentioned about it still having much more torque on 6S.

If you look at the math parts of this it is not telling the full story... On 6s you will have the 3600 watts available instead of 2400.
The real difference is that the throttle limiting will not let you go into the full RPMs capable on 6s. The power is noticeably different on 6s.

My Kraton 6s would not wheelie on 4s but absolutely would on 6s. It was a rocket with the speed gear installed.

 

[ChemEng]

If you are doing this for a child then I understand, but if it is for yourself I would just constrain yourself and learn on 100%.

I use the MMX8s aux wire on channel 3 (linked to a knob on my NB4) to adjust the Max Throttle Adjustment. I turn it WAY DOWN and let my 4 year old drive my kraton or mojave.

Is this bad for the ESC? He loves it and i can adjust it up as he gets better...

 

[LibertyMKiii]

I use the MMX8s aux wire on channel 3 (linked to a knob on my NB4) to adjust the Max Throttle Adjustment. I turn it WAY DOWN and let my 4 year old drive my kraton or mojave.

Is this bad for the ESC? He loves it and i can adjust it up as he gets better...

Pull the logs and check the ripple voltage. Chances are its fine...

 

[loryriga]

Oh, very interesting! So it’s not simply adjusting voltage it’s more analogous to something like pulse width modulation where it’s not just a dumb 100% constant supplied voltage

Exactly

 

[LibertyMKiii]

I made a visualization of this.
You can see the 6s red line has higher power, but they both stop around the same RPM when the 6s setup is limited to 66% throttle.

Then comparing when the 6s is allowed to run 100% it goes beyond the RPMs of the 4s for considerably higher speeds and power.

(note these lines are just drawn in MS Paint and resemble the power curve of a piston engine more so than electric) but the point is hopefully understood with the visualization: