Right now I have 20" tires {factory} on my Banshee but I want to use 25" tires for the snow. How can I gear it so it acts the same with the 25's as it does with the 20"s. The drive sprocket has an appr diameter of 2.5" and the rear sprocket is appr 7.75".
I had posted this topic before but there was some confusion with atvbbs and making "personal posts" and some other stuff about how my new email address still isn't reflected in my profile yet.
So if you responded to the prior post, please do so again to this post.
thanks

 

SINCE TALLER TIRES GIVE MORE TOP AND TAKES AWAY FROM THE BOTTOM END, YOU WOULD NEED TO GEAR MORE FOR BOTTOM END. I DON'T KNOW HOW THE RATIO CHANGES WITH THE DIFFERENT SIZE TIRES-GOOD QUESTION.

RICK

 

You need to put a 12 tooth front sprocket on to get somwhere neer the same gear ratio you have now.

 

First, diameter is not the best way to measure sprockets. The most common is to count how many teeths you have (then you can go to your dealer and say "I want a 12 teeth sprocket!!". Stock banshee should be 14 teeth in front and 41 rear.
I had done it for my 22"...I should have used a theorical 12.7 teeth front sprocket...wait a second and I'll tell you (you'll have to change both front and rear).
Here we go.
Your stock ratio is 41/14 (=2.9), that means your axle makes 2.9 revolutions for one at the exit of the gearbox. 1 revolution of the axle makes you go 125" with 20" tires. With 25" tires, you will go 157" inches. So your ratio should be of 3.6 instead of 2.9. 3.6, that's a 11.4 front sprocket and 41 rear (you have to choose between 11 and 12), or 50.2/14 (!), or 43.2/12, with any of these, you shouldn't be too far from the stock gearing.
I don't know if it's done properly, but it does make sense

 

Well if you want to know thw correct answer to your question it is that for every inch you go up in tire size over stock you need to drop one tooth on the front sprocket since you didn't give me the year of your banshee I'll assume that it is an 89 or newer since the sprocket size on the older ones is one tooth smaller you would need to put a 9 tooth front sprocket on your banshee in stead of the 14 tooth stock one. this may be hard to find. you should probably change your rear sprocket instead. you would need to go with a 46 tooth rear sprocket since the formula work the opposite on the rear. Hope this helps if you have any other questions let me know I own a race shop in nebraska

 

cdracing...
1) where can I get a 46 tooth sprocket? the highest I've found is 44.
2) how do you do the actual math to get the exact answer? or is the "tooth per inch" rule of thumb accurate enough?

 

OK I did some searching and found this link...http://pub20.ezboard.com/bhondatrx250r
so let me see if this makes sense.
1- i need to find the difference between the tires i have and the tires i want.
i have 20" tires, i want 25" tires so the difference is 5".
2- i divide the difference (5") by the current tire size. (5 divided by 20 = .25)
3- i reserve that number aside and label it. (.25=a)
4- i determine the ratio of current sprockets by dividing the rear number of teeth (41) by the front number of teeth (14)...(41 divided by 14= 2.93)
5- i reserve that number and label it. (2.93=b)
6- since i want to go to a larger rear tire i use the formula...(1+a)x b= new sprocket ratio to achieve previous ratio with smaller tires.
7- do the math 25-20=5, 5/20=.25, .25=a
41/14=2.93, 2.93=b
1+.25=1.25
1.25x2.93= 3.6625
3.6625= the new sprocket ratio i need to turn 25" tires like it turns the existing 20" tires.
8- i look on a gearing chart http://www.sidewindersprockets.com/images/ratioa.gif and determine which sprocket combination comes closest to 3.6625.
according to this chart my choices are... 11:41, 12:44, 13:48, 14:51.

Now all I need is someone with extensive math knowledge to tell me if my cifers -n- gazintuhs werkt riht.

ps thanks to those who asked the question before me and provided the skoolin i misst.

 

That's the same thing I said a bit above...said in a different way, but you get the same results...and from the same numbers.
I would go 12/43 ( or 12/44 if you can't find 43).
The 1"/1 tooth formula is crap. A kitchen recipe. It works, yes (maybe), but is unprecise, and has no real basis. No one knows where it comes from...it's empirical. To me, it's better doing 3 seconds of basic maths, and knowing exactly what is good, and from there decide what you can do (like decide between 43 and 44 when you're supposed to use a 43.2 teeth sprocket).
Again, I'd go for the 12/43. Acceleration will be a bit less violent, which is good for snow.

 

I need this info again and thought it might help someone.

 

Your sprocket formula is correct, but you are going the wrong way about measuring your tires. Measure the total circumference around each tire. Do not be surprised when one is larger than the other. In your case you will use the largest tire size for your calculations. All tire companies use different ATTS (actual tire tread size) measurements so a 25" Mud Machine may measure larger than that of say a 25" Relator.