Calculating motor pinion size and top speed

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Motorider

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Arrma RC's
Does anyone know the final drive ratio of the kraton not to include the tires? Trying to figure out what pinion I will need with my tekin t8 1700kv motor. Im thinking 17t to maintain stock speed.

Thanks,
shaun
 
Last edited:
4.3 is the final according to the Arrma Comparison chart. Here's the link. Just select your vehicle.

https://www.arrma-rc.com/vehicle-comparison/AR102601/AR102604

That is actually the diff drive ratio. The front and rear diff has a 10t pinion and 43t ring gear. That is your 4.3:1 ratio. Final drive is a combination of motor pinion to spur and rear diff ratio. stock is a 12t pinion and a 50t spur for a transmission ratio of 4.17:1. I am running the outcast tires, which are a bit taller than stock at 6.77" tall.

Calculations to get vehicle speed per 1000 rpm of motor.
4.17:1 for the trans
4.3:1 for the diff
17.92:1 = 4.3 x 4.17 = final drive ratio
21.27" = 6.77" x 3.14 = tire circumference
55.8 = 1000 / 17.92 = tire rpm per 1000 motor rpm
1186.9" = 55.8 x 21.27" = vehicle speed in inches/minute
99.9' = 1186.9" / 12 = vehicle speed in ft/minute
5994' = 99.9' x 60 = vehicle speed in ft/hr
1.124 = 5994' / 5280' = miles/hr per 1000 rpm of motor.

Motor Speed calculations
Lipo cell = 3.7v
4s = 14.8v
motor is stock 2050kv
max speed is 14.8v x 2050kv = 30340 rpm

6s = 22.2v
22.2v x 2050kv = 45,510rpm

Vehicle theoretical top speed
4s/12t pinion top speed
30,340rpm / 1000 rpm = 30.34 = our calculation is in mph per 1000 rpm
1.124mph x 30.340 = 34.1mph = theoretical top speed. This does not inclulde motor slip, wind drag, vehicle drivetrain losses, or tire slip.

6s/12t pinion top speed
45,510 / 1000rpm = 45.51
1.124mph x 45.51 = 51.14mph

Whew all that to get vehicle speed. Now you can just substitute for different pinion teeth to get wheel speed. I will be running a 1700kv teken 4038 motor and 17t pinion. With that combination I will have a near stock speed of 40.16mph on 4s and 60.28mph on 6s. This gearing should work great. I may even be able to step up to an 18 or 19 tooth pinion, heat permitting. Again these are theoretical top speeds and do not account for any losses associated with physics.

I hope this can help guys gear their Arma vehicles without just playing the guessing game. Good luck!
 
I use RC gear lite of the Google play store it's been pretty close so far and I use it everyday in my store for customers to see what changing things "COULD POSSIBLY" do.
 
What about any info on going up or down on both pinion and spur gears? I if you go larger or smaller and keep the same ratio, how does it affect the load and rpm of motor
 
It doesn’t change if the ratio doesn’t change. Theoretically the larger gear set should be more durable however.
 
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