Limitless Limitless 8s gear ratio???

[Tzahn]

Hi everyone
My limitless setup is castle xlx- Tp 4070(cm) 8mm shaft motor- gear ratio 25/42 running new 8s 8000mah batteries.
I've seen people running their limitless on 6s with Tp motor & castle xlx hitting 140 mph & more.
With my setup on 8s it hits 122.5 mph, that's on 25/42 gearing with the motor not getting hot.
I wonder what gear ratio should i run???
Thanks guys

 

[gotnitro]

@slick2500 could move this over to the limitless forum you will get more answers there.

 

[Tzahn]

@slick2500 could move this over to the limitless forum you will get more answers there.

Can't find a limitless forum....

 

[GMballistic]

Can't find a limitless forum....

Link: https://www.arrmaforum.com/forums/arrma-6s/?prefix_id=78

Hi everyone
My limitless setup is castle xlx- Tp 4070(cm) 8mm shaft motor- gear ratio 25/42 running new 8s 8000mah batteries.

I wonder what gear ratio should i run???

Why not just experiment which is what most of us speed runners do by going up a tooth at a time.

Alternatively try a speed calculator for a rough estimate of top speed achievable with your set up.
Link: https://www.radiocontrolinfo.com/information/rc-calculators/rc-car-calculator/

What KV is your motor btw?

 

[slick2500]

Thread moved to 6s section.

 

[Tzahn]

Link: https://www.arrmaforum.com/forums/arrma-6s/?prefix_id=78


Why not just experiment which is what most of us speed runners do by going up a tooth at a time.

Alternatively try a speed calculator for a rough estimate of top speed achievable with your set up.
Link: https://www.radiocontrolinfo.com/information/rc-calculators/rc-car-calculator/

What KV is your motor btw?

Don't really know, all I know is that it's a custom made Tp 4070 8mm shaft motor - should be around 1900-2000 kv but not really sure.
Spool is 40

 

[LibertyMKiii]

Don't really know, all I know is that it's a custom made Tp 4070 8mm shaft motor - should be around 1900-2000 kv but not really sure.
Spool is 40

Knowing the motor specs would help me guide you.
My fear is that if you have a standard TP 4070 they are rated for 50k rpms.

With voltage drop you would be around 26volts and 2000kv (26*2000 = 52,000 rpms)

Assuming that is correct maybe something in the ballpark of 29T would put you over 140mph. The safest thing to do as mentioned is change 1 tooth on the pinion. Do a run and check motor temps after. Pull the data logs and look at your ESC temps, voltage drop, amps, and motor rpms. Then adjust again.

Hopefully you also have a cap pack on that XLX? They see very bad ripple voltage during speed runs and that oscillation in voltage can over stress them resulting in ?

 

[Tzahn]

Thanks, the motor is definitely not standard tp but custom made, I do have cap pack on my xlx.
By 29t you mean spur gear ?

 

[LibertyMKiii]

Thanks, the motor is definitely not standard tp but custom made, I do have cap pack on my xlx.
By 29t you mean spur gear ?

IF you truly have a 40 or 42T spur gear then I am recommending a 29T pinion. (Limitless comes with 34T spur normally)

Without knowing your motor specs it is a shot in the dark...

 

[BashingBrian]

With voltage drop you would be around 26volts and 2000kv (26*2000 = 52,000 rpms)

@LibertyMKiii ... as above, is this how you calculate the rpms..??
Also why do you say with voltage drop..?? wouldn't that motor rev even more at full voltage therefore going way past its limit..??
I'm just here trying to learn..?

 

[LibertyMKiii]

@LibertyMKiii ... as above, is this how you calculate the rpms..??
Also why do you say with voltage drop..?? wouldn't that motor rev even more at full voltage therefore going way past its limit..??
I'm just here trying to learn..?

Correct, voltage * KV however KV is not always what is on the label (more on this below)
8s is 4.2v * 8 cells which = 33.6V
In reality the voltage drops during a speed run (less so when geared for genera bashing.)

This is an example of voltage drop on my MMX8s setup with two 4s 6,000 mAh CNHL 70C packs in series to make 8s.
The red line across the top is the voltage. You can see as the amperage and throttle increased the voltage began to drop. It went from 33.1V down to 27.7 volts. In this example I only reached 94% throttle due to running out of road, but I suspect the voltage would have gone down around 26-27v.

So I cannot calculate my speed assuming my 2000kv motor is going to be turning rpms relative for 33.6 volts. The reality is that peak speed is going to be the RPMs based on the voltage drop.
Knowing the real KV of the motor is also valuable. In my case the BLX 2050 KV motor is actually a 1916 KV motor.
Plugging those values into a calculator my run was 132mph. (only 94% throttle!!!)

 

[BashingBrian]

So in reality you want to do one stock setup run, pinion/spur etc and then come back and analyse the data then work from there..??
Of course choose your components correctly too would be a great start too..?

Is a motor analyser worth the money to buy..?? I've seen you can use the drill method to work out kv ?

Another question .. what's the safe max rpm for say a 50000rpm motor, where does it make its max power..??

 

[LibertyMKiii]

So in reality you want to do one stock setup run, pinion/spur etc and then come back and analyse the data then work from there..??
Of course choose your components correctly too would be a great start too..?

Is a motor analyser worth the money to buy..?? I've seen you can use the drill method to work out kv ?

Another question .. what's the safe max rpm for say a 50000rpm motor, where does it make its max power..??

I think if you are serious about speed running the motor analyzer is a great tool to help you understand your setup and changes to make to it. Like my 2050kv motor it was actual KV difference by 132kv lower.

As far as motors and max power rpm range I know a decent amount about 40 and 42mm motors, but 56 and 58mm motors is an unknown territory for me. With 40mm motors I would say peak power is between 32k-45k rpms.

With that said behavior and ideal RPM range can be different on 6s versus 8s on the same motor. There are a few motor dynos available online that measure/analyze the motors acceleration spinning up a weighted flywheel. They provide back data on ideal timing and power curve/rpms. I just don't believe they were ever engineered for these sort of power levels. It was designed mostly for track racing RC cars with smaller motors and running significantly less gearing/power levels.