## Recommended Gearing for 350kv

Lightweight / Folding / Portable EVs - seats optional
theice45   10 µW

Posts: 6
Joined: Apr 24 2016 8:25pm

### Recommended Gearing for 350kv

Hey guys,
I'm new around here, so please let me know if I'm violating any sort of rules.

I have a Turnigy Aerodrive SK3 - 4250-350kv motor around, and was thinking about building a board (have access to a 3d printer, so I could make a motor mount and such easily)

I was wondering if you guys could help me figure out a nice gearing for it (on 76mm wheels)
I'm aiming for a minimum of 15mph in terms of speed, but with the 350kv, I'm worried about torque.

I was looking at a 1:2.864 ratio due to nice sizing on the pulleys for the wheels.

Any thoughts would be greatly appreciated.

thepronghorn   100 W

Posts: 231
Joined: Mar 27 2011 8:07pm
Location: Bay Area, California

### Re: Recommended Gearing for 350kv

Your motor is a bit smaller than normal. It may be a bit of a tall order to ask for 15mph from that motor. Generally (for a single motor), you want at least a 50mm diameter motor.

Figuring out gearing is just a matter of doing the math.

Multiply Kv times voltage to get motor rpm.
Divide by reduction to get wheel rpm.
Multiply by circumference of wheel (in inches) to get inches/min.
Convert to miles/hour. (multiply by 60/12/5280) (60 min/hour, 12 in/ft, 5280 ft/mile)
This is your unloaded speed. A rough estimate of top speed would be to multiply by 0.8.

For you, assuming a 5S 18.5V battery, a 2.864 reduction on 76mm wheels would net you 20mph unloaded so about 16mph loaded if your motor doesn't burn up getting there. 16mph probably requires about 800W (just a guess, my longboard doesn't go that fast), which is gonna be pushing your little motor to its thermal limit.

WeeChumlee   1 kW

Posts: 346
Joined: Jul 08 2015 9:47am

### Re: Recommended Gearing for 350kv

Hi
Don't want to burst your bubble but only try that if you really have no use for the motor at all.
Might work on a flat, with a kick push start, but even then I think it will probably run very hot. Glancing sideways at a hill will pop it almost immediately I would wager.

theice45   10 µW

Posts: 6
Joined: Apr 24 2016 8:25pm

### Re: Recommended Gearing for 350kv

thepronghorn wrote:Your motor is a bit smaller than normal. It may be a bit of a tall order to ask for 15mph from that motor. Generally (for a single motor), you want at least a 50mm diameter motor.

Figuring out gearing is just a matter of doing the math.

Multiply Kv times voltage to get motor rpm.
Divide by reduction to get wheel rpm.
Multiply by circumference of wheel (in inches) to get inches/min.
Convert to miles/hour. (multiply by 60/12/5280) (60 min/hour, 12 in/ft, 5280 ft/mile)
This is your unloaded speed. A rough estimate of top speed would be to multiply by 0.8.

For you, assuming a 5S 18.5V battery, a 2.864 reduction on 76mm wheels would net you 20mph unloaded so about 16mph loaded if your motor doesn't burn up getting there. 16mph probably requires about 800W (just a guess, my longboard doesn't go that fast), which is gonna be pushing your little motor to its thermal limit.
WeeChumlee wrote:Hi
Don't want to burst your bubble but only try that if you really have no use for the motor at all.
Might work on a flat, with a kick push start, but even then I think it will probably run very hot. Glancing sideways at a hill will pop it almost immediately I would wager.
Don't worry, I appreciate the feedback.
So for a more sane setup, based around what I've been reading, I'm now thinking of a 6S setup on either an Alien 6355 Outrunner 230KV, or 5065 270KV at the same gearing as mentioned above (1:2.864)

I've calculated that those should get me to around 13-15mph respectively, am I correct that this is a more sane setup?

WeeChumlee   1 kW

Posts: 346
Joined: Jul 08 2015 9:47am

### Re: Recommended Gearing for 350kv

If you are going single motor then definitely go for a 63mm.
The 6355, 230KV would, at 6S, easily get you to where you want to be.
What gears exactly you wanting to use? (Tooth numbers)

theice45   10 µW

Posts: 6
Joined: Apr 24 2016 8:25pm

### Re: Recommended Gearing for 350kv

WeeChumlee wrote:If you are going single motor then definitely go for a 63mm.
The 6355, 230KV would, at 6S, easily get you to where you want to be.
What gears exactly you wanting to use? (Tooth numbers)
14:40,

Would it be detrimental to buy the 270kv? I don't mind going faster, but I'm still trying to learn how much torque corresponds to kV.

WeeChumlee   1 kW

Posts: 346
Joined: Jul 08 2015 9:47am

### Re: Recommended Gearing for 350kv

The 50mm motor is great for a dual but for a single motor build they just do not provide enough torque.
Definitely go 63mm for a single. Not just KV but sheer size is the main contributor here.

theice45   10 µW

Posts: 6
Joined: Apr 24 2016 8:25pm

### Re: Recommended Gearing for 350kv

WeeChumlee wrote:The 50mm motor is great for a dual but for a single motor build they just do not provide enough torque.
Definitely go 63mm for a single. Not just KV but sheer size is the main contributor here.
Alright, great!
Any chance you have some recommendations for motors of that size?
The alien power system one I referenced seems to have great reviews, yet the shipping cost is killing me..
Worse case, I'll get it from them, just wondering.

WeeChumlee   1 kW

Posts: 346
Joined: Jul 08 2015 9:47am

### Re: Recommended Gearing for 350kv

I am in the UK so I got all my motors from Alien but in the US you have http://diyelectricskateboard.com and Hobby king.
Hobby king SK3 motors have a good rep but are hard to get hold of.

theice45   10 µW

Posts: 6
Joined: Apr 24 2016 8:25pm

### Re: Recommended Gearing for 350kv

WeeChumlee wrote:I am in the UK so I got all my motors from Alien but in the US you have http://diyelectricskateboard.com and Hobby king.
Hobby king SK3 motors have a good rep but are hard to get hold of.
http://diyelectricskateboard.com/produc ... 0kv-2650w/

so this motor should work nicely?
I'm concerned about it being sensored (in sensor-less setup, they mention it needing a kick on startup, is this true?)

LEVer   10 kW

Posts: 719
Joined: Nov 07 2012 3:29am
Location: SoCal
Contact:

### Re: Recommended Gearing for 350kv

theice45 wrote:
WeeChumlee wrote:I am in the UK so I got all my motors from Alien but in the US you have http://diyelectricskateboard.com and Hobby king.
Hobby king SK3 motors have a good rep but are hard to get hold of.
http://diyelectricskateboard.com/produc ... 0kv-2650w/

so this motor should work nicely?
I'm concerned about it being sensored (in sensor-less setup, they mention it needing a kick on startup, is this true?)
Unless you want to ride your E-board like a hoverboard, a little kick before starting is no biggie. The motor will be fine even when used "unsensored."