How to determine gearing?

[Panhead5496]

Long story short, I'm going back electric in my quad build http://www.diygokarts.com/vb/showthread.php?t=40679 2-stroke motors are too finicky for the family members to deal with...electric will be much simpler.

I ordered a 500 watt 36v motor set up, and need to figure out gearing. It has a 11tooth sprocket on the motor, so what should I be going for on the rear axle? It has 9" tires.

Chain size is 25H, so need a compatible sprocket.

Really not looking for much speed here, more for pulling up hills and such. Top speed is not the important factor here as much as lower end torque. With that said, I don't want a 8 mph snail, but don't want to be going 20 mph either!

Appreciate any suggestions guys!

 

[itsid]

Aha.. so you don't have the sprocket nor motor that came with the ATV then?
too bad

I hope you still ordered a geared motor (like you had)
so your axle sprocket won't end up being two feet in diameter

Since it's a fairly low powered motor, and we talk Kidsize here;
I think it's fair to say that you can just gear for max speed

Assumtions: targetspeed 15 mph, motorspeed 3200 rpms
so we talk 63'ish sprocket teeth on the axle

9" * Pi = 28.27" (rounded wheel circumference)
15 mph * 1056 = 15840 ipm (inches per minute)


15840 ipm / 28.27" = 560 rpm (WHEEL rpm)
3200 rpm / 560 rpm = 5.71 (rounded gear ratio)

5.71 * 11 (sprocket teeth) = 63 (axle teeth)

What I tend to do to get a verification for such numbers
is to do the same thing from the other perspective.. ( in real world metrics I'm afraid)
to see if it's within the limiting factor of motor torque.

Total weight of vehicle with passender (assuming 80kg or ~176lbs)
max torque of the motor (you can read that off the spec sheet I'm assuming 2 Nm here)
A value to remember: 15mph = 6.7 m/s

Wheel diameter to wheel radius
23 cm / 200 = 0.115 m (radius needed in meters so a factor of 100 to scale)
80kg * 0.115m = 9.2 kgm

Hoping for about 2m/s² acceleration (~ 1/5 of earth's gravitation is joyfull )
making it
9.2 kgm * 2 m/s² = 18.4 kgm²/s² (or what it's also called : 18.4 Nm of torque)

Now we can take that and divide by the motor torque

18.4 Nm / 2 Nm = 9.2 (min gear ratio to reach 2m/s² acceleration)

oops we're out

2 Nm * 5.71 = 11.72 Nm is all we have at the wheel..

11.72 Nm / 9.2 kgm = 1.24 m/s² is all the acceleration we could achieve with that gear ratio

that's still within my tolerance for a kid's toy reaching top speed in just over 5 seconds
6.7 m/s / 1.24 m/s² = 5.4 s (rounded of course)

it'd even allow a cent clutch to live on another day IMHO.
So you can stick with 5.71:1 ratio but it's not exactly snappy

And playing with all these numbers (keep in mind some are assumptions!)
and pushing them back and forth through those calculations you can find your ideal ratio

Sprockets? IDK some scootershops sell #25s

'sid