I got a 86 250R quad. Mild Mods. Here is my scenario:

Before:
13/40 Gearing with 22's. Was a monster....had incredible holeshot.

Now:
13/40 Gearing with 18's. All I do is spin....lost my holeshot completely.

I just ordered a 15 tooth front.

Does anybody know if I this will make me get my holeshot back.

I wish I had a calculator or a formula where I could enter in my tire size and my sprocket sizes so I can get that final ration back so I can have my holeshot back and still handle great with my 18's.

My 22's were 25% (tread left) Pro-Wedges.
My new 18's are Trekkers.

Somebody...please help....I would love to get my exact ratio back.

 

I think I can help you out here. First I will assume that the tires are exacltly 22 and 18 inches tall. Of course this wiil not be the case so if you want to be really accurate, measure your tires and put the new numbers in the formulas.

22(3.14)=69.08 which is the circumference of the 22s
18(3.14)=56.52 which ''' ''' ''' '' '' '' 18s

40/13 = 3.07 current gear ratio

69.08/3.07=22.5 number of inces you travel for one countershaft rotation

to keep the ratio the same use the 18 circum and x for the new ratio for the formula:
56.52/x=22.5 doing the algebra gives:
x=2.512 (the new ratio you need to be the same with the 18s)
to get this ratio you will need 15-38 gearing since
38/15=2.53 which is pretty darn close to 2.512

so there you have it 15-38 gearing needed if tires are exacltly 18 and 22 tall

and you thought that algebra class was useless didn't you!

 

Thanks man! You're a lifesaver!

 

just remember that even when you get it geared the same as it was it will probably still not holeshot like it did. 22 inch tires are going to get better traction than 18s are, regardless of gearing. 18 inch tires for 8 inch rims will get more traction than 18s for 9 inch rims and 10 inch being the worse for traction. The trade off is better handling in the corners. But it should be much better on the holeshot when you re gear it.

 

Thanks again....I went out and measured my tires exactly. Old ones were 21.5, new ones are 17.5.

I adjusted the formula. My best bet is 15/38. I have a 14 a 15 and a 38 and a new chain on the way.

I could also use 13/33 or 14/35.

My final will be a 2.53 which is pretty darn close to 2.499 (original after my new calculations)...that's with the 15/38 of course. [img]i/expressions/face-icon-small-smile.gif[/img] I should gain back 99% of my holeshot! [img]i/expressions/face-icon-small-smile.gif[/img]

I love that formula! [img]i/expressions/face-icon-small-smile.gif[/img]

 

I just made a pretty cool calculator in MS Excel for just this kind of thing using your formula.....I would be glad to send it to you....thanks again man.

 

best of luck with the new steup. Just post back and let us know how the holeshot is. If you want to email me your excel program, I would love to see it.

 

If you look in the newest James Lucky catalog on about page 34, there is a chart that equates the ratio of front to rear sprockets for all sizes. You can also just divide the rear # by the front # and accomplish what the chart tells you. You will still have to calculate the tire size though.

Screamintrx's program would be easier.