Motor kv and some other math

[fpvmiller]

As of now I have a handle ( I believe ) on how to select the pinion for the tire size I am running. I am still trying to figure out how the kv is part of this mix. I can only find equations that show the theoretical rpm from one motor to another. So some math which I will try to make simple.

Using the granite 3s blx setup for baseline:

Internal gear ratio = 37t / 13t = 2.85
Spur-pinion ratio = 57t / 15t = 3.8
Final drive ratio = 3.8 * 2.85 = 10.83
Tire circumference = 130mm * 3.14 = 408.2
Tire rollout = 408.2 / 10.83 = 37.69

So on a granite 3s blx if I wanted to use a larger tire (3.8) I would drop down to a 13t pinion to keep the rollout similar.
57t / 13t = 4.38 spur-pinion ratio
4.38 * 2.85 = 12.48 final drive ratio
Tire circumference = 149mm * 3.14 = 467.86 ( 3.8 badlands )
467.86 / 12.48 = 37.49

So a 13t pinion on the stock motor with 3.8 tires would be close to the stock rollout and should result in no overheating.

Now, when we drop kv or go to a bigger motor or do both (bigger motor and lower kv) how does that play in?

I haven't found the part of how to calculate the motor change. The formula above that I found just takes into account tire size.

The formula for motor rpm I found is:
( Kv * voltage ) / ( spur / pinion ) = RPM

Stock granite 3s - ( 3200Kv * 12.6 ) / 3.8 = 10,610
Blx185 - ( 2050Kv * 12.6 ) / 3.8 = 6,797

So going to the blx185 on the granite gearing shows a decreased rpm. To compensate, I would think you could use a pinion higher than 15t. Let's try 23t pinion.

57 / 23 = 2.48
( 2050Kv * 12.6 ) / 2.48 = 10,415 rpm.

Fairly close to the 3200Kv 3S stock setup as far as rpm goes.

But that also doubles the rollout on 3.8 tires. So where does this mesh? I had been going on information I pooled from many places to try to choose a motor but keep getting roundabout answers like I just provided here.

:sigh: it's actually a bit easier on drones to match motor size, kv, cell count and propellers

 

[nino]

It's quite easy if you go from what you have. If your top speed is 50 with a 12t pinion, it will be 50*(15/12) with a 15t = 62.5.

Similarly, if you have 100mm OD wheels, going to 120mm OD will increase your top speed 20% (to 60).

KV is the same. Go from a 4000KV to a 2000KV motor and you'll need double the pinion teeth for the same speed!

Most of this stuff scales linearly. (Theoretically, of course - air resistance and other factors like pushing the limits of your lipos factor in too.)

 

[fpvmiller]

It's quite easy if you go from what you have. If your top speed is 50 with a 12t pinion, it will be 50*(15/12) with a 15t = 62.5.

Similarly, if you have 100mm OD wheels, going to 120mm OD will increase your top speed 20% (to 60).

KV is the same. Go from a 4000KV to a 2000KV motor and you'll need double the pinion teeth for the same speed!

Most of this stuff scales linearly. (Theoretically, of course - air resistance and other factors like pushing the limits of your lipos factor in too.)

This is the conclusion I was almost drawing to but wanted to make sure. Especially with as many speedrunners here having more experience with heat and gearing.

So the lower kv motor definitely benefits from a bigger pinion without the risk of overheating as it would be with going to a bigger pinion on a stock motor.

 

[tow_glinka]

One thing you shouldn't forget: The 3.8 tires are heavier. This will also make the motor heat up more, as it has to acclerate more rotational mass (and make the car eventually more sluggish).

 

[fpvmiller]

One thing you shouldn't forget: The 3.8 tires are heavier. This will also make the motor heat up more, as it has to acclerate more rotational mass (and make the car eventually more sluggish).

This is where I was trying to compensate with the lower kv (more torque). Bigger can lower kv should have no problem turning 3.8 tires but would be thin on the top speed.

My logic is to use a lower kv for the grunt and then leverage the pinion for the top speed.

Or is that just not possible?

 

[tow_glinka]

This is where I was trying to compensate with the lower kv (more torque). Bigger can lower kv should have no problem turning 3.8 tires but would be thin on the top speed.

My logic is to use a lower kv for the grunt and then leverage the pinion for the top speed.

Or is that just not possible?

That is exactly how you do it, everything fine. You could optimally also go with a bit of a longer can to make sure that you have enough extra torque, but then an ESC upgrade might also be needed.

I mean, if you go 3.8 tires you could also go full retard and put a Leopard Hobby 4065 or some random cheap 4068 in there

 

[fpvmiller]

That is exactly how you do it, everything fine. You could optimally also go with a bit of a longer can to make sure that you have enough extra torque, but then an ESC upgrade might also be needed.

I mean, if you go 3.8 tires you could also go full retard and put a Leopard Hobby 4065 or some random cheap 4068 in there

That is the plan ? which is why I am testing on my disposable (to me) blx185 out of my kr6s because if smokes it smokes but trying to gauge how it performs now to make a motor purchase

On that note I did a test run.

The setup:
Modded granite is essentially an outcast 4s
blx185 system
22t pinion 57t spur
3.8 badlands
Adjustable motor mount to set mesh properly
No binding that I can feel by hand whether motor/center driveshaft is connected/disconnected
3S 5000mah 50C dynamite reaction2

Varied driving terrain (grass, dirt, random asphalt, pine needles, pine cones) and some high speed passes on some fairly tamed asphalt.

After a full pack the temps read:
Esc 46°C external
Battery 44°C external
Motor 72°C external center of can

Research and notes tells me I'm overgeared yet I have seen similar results with a 13t and 15t pinion. So where is the motor heating up at? There is no cooling fan on the motor because again it came out of the kr6s and there was no fan with it. I have read many people throw out some crazy numbers like "I don't let it get hotter than 120F" which I think is difficult even with a cooling fan.

Any other tips/ideas/info people? I'm thrown too because my hobbywing escs have their thermal cutoffs set at 105°C and that is the lowest cutoff??

*** Edit: I think I know where the heat is coming from but I can't test it until I get to the lhs tomorrow. ***

 

[nino]

72°C is hot but I wouldn't call it overheating... I got 90+ in a Kraton on 6s stock pinion, some full throttle bashing on grass.

 

[tow_glinka]

Well to be honest, if you get 72°C that is not a problem. And sounds like a beast...

And for the record, full retard wasn't meant badly...I mean, you have seen my Malvion build @fpvmiller , so I'm basically going full retard myself. Although mine gets pretty hot atm, I will have to look into that.....on the other hand it eats my Arrowmax 7200mah 3S packs in under 15 minutes, so what do I expect. xD

 

[fpvmiller]

@nino @tow_glinka

Thanks for the reassurance. I hammered the pack until my smc alarm went off. Checked temps as soon as I got the body off and that was what I got. I feel like the motor might be a tad hot compared to the esc and battery temps so I need a couple of minor/small items from the lhs to see if my heat source theory is correct.

But I will say this, the 4074 2050kv on 22t pinion and granite gearing with 3.8 tires is crazy. I won't be going back to the 2.8 anytime soon. It seems a bit faster but I have no way of checking speed right now and I still have to roll in the throttle to keep from doing flips. Haven't tried to do standing backflips but I think it might, IDK. I prefer to stay wheels on the ground unless I'm jumping lol