I have already worked this all out on paper but I am curious what you guys may have to add.

13/38 sprocket raio on 25" tires was pushing me at about 60 mph.

Now I have 13/34 ratio on 22" tires. Where will this put me on top end? How fast would it go if I put the 25" back on it?

Thanks for any input.

 

Chris,

By my (extremely rough) guesstimate, you gained 11% more speed when you went to the smaller sprocket. However, when you went to the 22" tires you lost 12% of your speed. Here's how it breaks down:

13/34 gearing and 25" tires = 66 mph.

13/34 gearing and 22" tires = 59 mph.

I arrived at this wholly unscientific conclusion by dividing the two gear ratios (2.92 and 2.62) and getting the difference between the two (11%) and by dividing the circumference (pi squared x radius) of the two tires (123" for 25s, 108" for 22s) to get 12%. In other words, you only lost about 1 mph.

 

Mike,

You get an "A". That is what I came up with also. That is about what it feels like but my engine is really screaming on top end. It didn't do that before. It never has seen that high of a rpm before. Makes me a little nervous.

 

The only problem that I see is:

Circumference = 2 * Pi * r

Area = Pi * r squared

Old: 25" circumference = 78.57"
New: 22" circumference = 69.14"

For each revolution of the engine you cover:
Old: 13/38 * 78.57 = 26.88"
New: 13/34 * 69.14 = 26.44"

To figure the RPMs of the engine at 60 mph for your old setup:
60 mph / 60 mins = 1 miles per minute
inches in mile = 63360
60 / 60 * 63360 / 26.88 = 2357 rpm

If we assume the same RPMs with the new setup we get:
2357 * 26.44 / 63360 * 60 = 59.01 mph

You might also gain a few more RPMs by reducing the rotational mass of the tire. The lighter it is the faster you can spin it given the same amount of power.

Hope this helps,

 

<<
Pi * r squared
>>



Kev, I don't know about in TN but in TX pie are not square, Pie are round. [img]i/expressions/face-icon-small-happy.gif[/img]

Charlie

 

Yeah, we know: pie are round, corn bread are square, and White Lilly biscuits are the best!

 

kevin_staton,

That's not the type of equation I used but comes up with the basically same answers.

After riding last night I beleive that I need to lower my ratio a little more. These 22&quot; tires are just so much easier to turn.

Thanks for the help everyone.

 

The ratios will work out to be very close to each other. However, the smaller (and lighter) tires will make it FEEL like you have more power, as they will have less rotational mass, and will be easier to spin. Also, there is the 'tire stretch' theory, which states, in theory, at least, that a tire will 'expand' a certain % with a given RPM. There are a LOT of variables, such as tire stiffness, initial squat, inflation pressure, etc.
Compare it to a drag racer. Notice how much taller those tires get when up to speed, as compared to sitting still? Well the same thing happens in an ATV at speed. The shorter, stiffer tires will not 'stretch' as much, which combined with less rotational mass, and less rolling resistance, will in a way make it act as if it is geared lower than when compared to the taller tires and the other sprockets (which in theory should have resulted in the same gearing).

I have an Xplorer, and it never ceases to amaze me how much speed can be gained by simply running up the inflation pressure in the tires to reduce rolling resistance. Of course, it makes for a rougher, bouncy ride, but for drag racing on smooth roads, it makes a NOTICEABLE difference.

 

Looks like we have some pretty smart people in these forums.

JaMeS

 

One thing people are forgetting from the &quot;equation&quot; is the occurance of tire growth. Watch a top fuel dragster from the front and you'll see the best example. Depending on HP generated and RPM, this can account for a great deal of gearing change.

KT