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
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
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