Why do Sportsmen 500's have 12" rims in front and 10's in back?

 

Well, I’m not a tree farmer (or THE Tree Farmer) but wheel sizes have nothing to do with gear ratios. What matters is the circumference of the tire, or you could measure it using the diameter, but circumference would be the more correct way to go about it by my thinking. Wheels size has no more to do with “gear ratios” (what we’re really discussing here is revolutions in front relative to revolutions in back) than does hub size or bolt pattern. Although wheel WIDTH will effect the tire’s diameter and circumference. The same tire on a wide wheel will be “shorter” than on a narrow wheel.

In other words, the only thing that matters is the distance the outside of your “circle” travels in one complete revolution, front compared to rear. And from your question, I’m not even sure why that matters. Are you trying to solve a problem or just interested in theorizing?

 

Good response, FloodRunner! You make a valid point.

However, would you like to become even MORE precise? I didn't think so, but I'm going to tell you how, anyway!

(Attention; individuals offended by technical discussion, avert your eyes.)

Tire diameter is a good first approximation when considering gear ratios.

Yet, the actual determining dimension is: the laden rolling radius.

Meaning, the laden rolling radius is less than half the unladen tire diameter. Laden, that is, with the tire bearing its portion of the quad and rider's total weight, the bottom radius is "squished." Thus, in one revolution, the quad travels less that pi times the diameter of the unladen tire.

So much for "laden," in "laden rolling radius."

Now, for "rolling." The bottom radius dimension shows some elasticity when rolling, in contrast to when it's standing still. Rubber just naturally stretches and compresses; air expands and contracts. Thus, the rolling radius differs from the static radius, based upon the engine torque and rolling resistance loads applied to the tire.

Now, for comparison, and the level of precision of ATV Connection Forum discussions, "tire diameter" is a pretty good starting point. But it ain't the whole story in tire design and testing.

Tree Farmer

 

Thanks for clearing that up Jim! I enjoyed the disclaimer too (hehehe).

Now to answer BushHog’s “new” question, small diameter wheels in large diameter tires give a softer ride. Think about it like this. In the rear of your example the outside diameter of the circle that is your wheel and tire has more rubber (sidewall) in it than the front. This takes advantage of sidewall flexibility to offer a ride that conforms more to the terrain. This sidewall flex in the front would have an undesirable effect on our ability to steer so there’s more steel (wheel) in your circle up there to reduce flex.

Now of course there are the variables of tire pressure, sidewall construction and terrain but to answer your general question in a general way, that’s the story.

Did I miss anything TF?

 

Yes, I found the original post a little lengthy so I whittled it down a good bit. Thanks, there, Tree Farmer. I think I'll try a 26" tire in the back and 25" tires in front. I may even get some of those "big center,less rubber" 12" rims Floodrunner spoke of. Have a nice day, or night if you are in China.