Getting KV/gear ratio right

[bugboy1641]

Hi everyone,

Just been looking over Vedders website where he talks about Ratios, Motor KV and Voltage.

http://vedder.se/2014/10/chosing-the-right-bldc-motor-and-battery-setup-for-an-electric-skateboard.

As I'm still pretty new to the whole E-skateboard forum its taken me a while to, I guess, gather all the necessary information.

My design brief is/was:
90mm wheels
8S voltage but maybe switch to 9s. I fly quads that use 3S so was just gonna stick with the 3S but I'm actually planning on an upgrade to my quad to 4s and figured I could just go 4S and swap out between the two and save on batteries.
Top speed of around 40kmh but cruising at 25-30kmh. Auckland has this 10+km long seafront, which is perfect for cruising along plus the odd blast past the joggers.

I had decided on a single HK 245KV 6364 and a 3:1 gearing, 44:14 to be exact(3.14:1) Nothing bought yet.

Using his calculations, I actually should be looking for an 286KV(or equivalent) motor and a 3.66:1 (44:12) gearing to get that magical 8600rpm?

Could someone let me know if my gearing/motor is acceptable or is it better to get it spot on as per Vedders?

Many thanks in advance,

Dan

 

[torqueboards]

You can use Vedder's calculations to calculate your own. As for his default suggestion (it's regardless of battery voltage and motor kv) he aims for 8600 RPM.

Shooting for 35km/h - (35 / (0.084 * pi * (8600 / 60) * 3.6) = 0.257 which is 11/0.257) = 1:3.9 gearing
Ex. 30mph or 48km (48 / (0.084 * pi * (8600 / 60) * 3.6) = 0.352 which is 11/0.352) or 44/125 and 125/44 = 2.84 = 1:2.84 gearing

You can use this calculator. http://web2.0calc.com/

*Note that this gear ratio is independent of battery voltage and motor kv.
*From the ESC perspective, we should run on as high voltage and low current as possible.
*I have discovered that a good trade-off is at around 60V (quite safe for 10s or 12s lipos), where the efficiency is good and the voltage is not too problematic to handle.

Since we want to run the motor at 8.6k RPM, we need a KV of 8600 / 45.6 = 188kv.

12S with 1:3.9 gearing and 35km/h or 21mph top speed we need a 188KV motor. Although, no motors at 188kv in 50mm.

10S with KV of 8600 / (10 * 3.8) = 226kv. Which there are 225 KV motors.

83mm wheels, 10S Lipo, 1:3.9 gearing, 35km/h top speed and 50mm 225kv motor.

I don't think it's set in stone though and you are free to use whichever gearing you want. You might have some performance losses but IMO I think it's hardly noticeable.

Ex. If you are on 6S. 8600 / 22.8 = 377KV motor - IMO These motors aren't that great and I wouldn't recommend using them at all.

The best setup is similar to what Ben mentions however for people who use lower voltages it's just not as efficient with higher voltages which doesn't mean it won't work.

21mph = 1:3.90 gearing
10S and 225KV Motor

30mph = 1:2.84 gearing
12S 188KV Motor or 200KV motor.

IMO Gearing ratio of 1:2 to 1:4. 1:2 is lower resistance which increases top speed. 1:3 increases torque.

 

[bugboy1641]

Thanks Torqueboards,

For optimum efficiency, the system (Motor/voltage/ratio) all need to work within its defined range in order to get the most performance or efficiency.
I think I've got a pretty good idea of what I want this board to do but I still want to it to be as efficient as possible.

I've got your 12S ESC and mounts winging their way to me so hopefully I can start building something physical soon!! For me 12S is overkill but I think eventually I'll want more POWER!

Cheers