ratios

[randomb0b123]

i dont quite understand this, is a ratio of 5 faster or 3.75?

"Go Kart Gear Ratio

The gear ratio on your mini bike or go kart is the ratio between the number of teeth on the clutch, and the number of teeth on the drive axle sprocket.

One of the most common gear ratios for go karts is 11-12 teeth on the clutch sprocket, and 60 teeth on the axle sprocket. A very common mini bike ratio is 11-12 teeth on the clutch, and 72 teeth on the axle sprocket.

To find the gear ratio, simply divide the axle teeth by the clutch teeth.

For example:

60 on the axle divided by 12 on the clutch = 5.

However,

80 on the axle divided by 16 on the clutch also = 5.

Both of these sprocket combinations would result in the same performance on your kart!

As the gear ratio increases, you can get a better top speed. However, you sacrifice power. "

 

[Russ2251]

3.75 will be faster but will cost you in torque losses and therefore requires more hp to get you up to full speed.

 

[Jerryburger]

If you know what motor you're going to run, and what you plan on doing with the kart/minibike, that'll give you some insight on what to shoot for.
I ride on mostly flat terrain, and as a kind of "This is how fast various motors have carried me" thing, these are pretty good figures to shoot for.....

2hp B/S: 25 mph. (Have had it up to 30, but it took a veeery looong block of straightaway to do it.)

3hp B/S: 35 mph (My son and I took a lloooong run one time and got it up to 39mph with both of us on it, down a slight hill.)

5hp B/S: (...a very worn-out 5hp B/S) managed about 42mph on the same vehicle as above. Again, it was a pretty sick motor, but the off-the-line acceleration was much better.

6.5hp Blue Terror: Same gearing, kart same as the above examples, an easy 48 mph.

8hp B/S on the same kart, geared way to tall, (3:1 ratio with 16" wheels) managed to hit 51 mph.)


Except for the last example, all the motors were on the old Lowrider trike, with a 4.5:1 ratio (45/10) with 16" diameter wheels. While top speed has advanced pretty well, the huge difference in acceleration from the B/S 3hp to the B/S 5hp and above makes it worth running with either it or the Blue Terror 6.5 hp.
If you let us know what motor/kart combo you plan on building, I'm sure a lot of people will jump in with their advice on this.
Also, to prevent the "I read some where that a guy achieved 50 mph with a B/S 2hp...." crap; I humbly insert the disclaimer "Your results may vary."

 

[randomb0b123]

ive got an 8hp B&S ohv brand new, thinking about 45/12 gears or 60/10 gears because im not sure if i want acceleration or top speed, back wheels are 2' tractor tires, and im 170lbs

 

[ZnsaneRyder]

Those are BIG tires! How about a jackshaft to go lower. That will give you more torque, since your wheels are big enough to get you going fast already, you just need the torque to turn them! Even with a lower ratio of 72T/12T, the speed calculator still shows 43mph @ 3600 RPM which is still fast for those big heavy tires if you want torque.

There's a ratio calculator @ http://jimsitton.net/ratiocalc/ and download the exe. It allows up to 3 ratio reductions (jackshafts) and may help better figure it out.

Using that, if you did a 12T clutch, 17T sprocket on jackshaft, then a 12T sprocket from jackshaft to a 72T on your axle, you would get 30.2Mph @3600RPM (governed) or 42Mph @5000RPM (wide open)