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help changing gear ratio

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help changing gear ratio

Postby Iceman9c » November 17th, 2003, 5:01 pm

when changing the driveshaft gear down to a lower tooth, do you have to also change the jack shaft gear. The jack shaft is 21 tooth, the drive shaft is 39 tooth and the chain pitch is 66w. What is the next size down from a 21/39, for 94' polaris 600.
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help changing gear ratio

Postby Burky » November 17th, 2003, 11:32 pm

It depends on how much slack ends up in your chain. The tensioner will take it up, but any time you bend a chain in an additional loop, you lose the efficency of the chain and increase the risk of failure. You also increase the loads on the tensioner. Best deal is to change the chain as well to match the suggestions in the Polaris gearing chart. It tells you what gear combos go with what chain length. Someone on here with Polaris background should have the info, or know where it's located.
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help changing gear ratio

Postby Shockman » November 17th, 2003, 11:43 pm

You can drop to 21/38 with 66 pitch , or try 22/39 should still work.
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help changing gear ratio

Postby Iceman9c » November 18th, 2003, 5:53 am

right now I have a 12/39 66w, and I want more bottom head. Thats what I should do?
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help changing gear ratio

Postby Shockman » November 18th, 2003, 3:41 pm

If I understand correctly you have 21/39 now.
you want to gear for low end?
If this is the case you need to go opposite
21/40 = higher ratio lower top speed
21/38 22/39 = lower ratio higher top speed
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help changing gear ratio

Postby Iceman9c » November 19th, 2003, 3:26 pm

ok thanks, I get it now!
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help changing gear ratio

Postby funkadelic101 » November 20th, 2003, 5:34 am

Iceman9c.... Hopefully you come back to read this. OK, first, you need a calculator. Then to figure out you gear ratios, you divide the top gear into the bottom gear. ie, 21/39= 1.85, this means that for every 1.85 revolutions that your jackshaft makes, your driveshaft makes one. If you want a better low end speed, you need to make that number higher, (ie 20/40=2.00) There are cases in sleds, where gearing for a higher number can actually increase both low end and top end. Next, you should find out what your chaincase center to center is, this will enable you to figure out what gear ratios work with what chain in your specific chaincase. I am going to assume that you have 9 tooth drivers at a 2.52" pitch in your sled also. (stock)

OK, with the gearing you have right now....

RPM / GR x SC / 12 x 60 / 5280 = MPH ..... GR=gear ratio, SC= sprocket circumference

8300 / 1.85 x (9 x 2.52) / 12 x 60 / 5280 = 96 MPH.... This is assuming you reach 8,300 as a top RPM.

To figure out chain size.... Take your center to center distance number (CD).....

(CD x 2) /.375 + (TS + BS) / 2 = chain pitch TS & BS mean top sprocket and bottom sprocket..... THe chain pitch number you get won't be rounded off, so you need to round the number you get up to the next even number, as that chains pitch can only be an even number, and this will tell you what size chain you need. SO if you look at the equation, it only serves you to realize that the top and bottom sprocket get added up, so you only need to know the total amount of teeth to get a certain pitch chain for your sled. You have 21/39, an even 60 gear teeth. 20/40 is also 60 gear teeth, so the chain size will remain the same. And the ratio is 2.00.

Check for MPH.... 8,300/2.00x(9x2.52)/12x60/5280= 89 MPH.

SO my guess is you never really got to the 96 MPH top speed, but you got around low 90's, which this 2.00 ratio will get you to 90 faster, with it having better low end, and it may even let you pull a couple more RPM's and you may achieve the same top end, only get there a LOT faster. There's a bunch of variables here ( chaincase size, and top RPM) LONG post, I know, but hopefully it can help more people out there.
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help changing gear ratio

Postby brettshouse » November 20th, 2003, 3:21 pm

I asked a question today on another forum about this very topic. so glad i checked over here.

great info...

any suggestions for a ZRT 600 (1997) my clutch engags at 5000rpm.

Thanks guys.

Brett.
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help changing gear ratio

Postby funkadelic101 » November 20th, 2003, 5:49 pm

Three things really need to be known before gearing advice is really given, I will assume that you have 9 tooth drivers with a 2.52" pitch. (stock) First thing, what is your stock gearing at?? Next, find out at your dealer, what is the center to center distance for your sleds chaincase.

After we get those two things down, you should be able to say what it is you want your sled to do, or be better for. Get me those three tidbits of info. (I could look up the first two, but it is your sled)

Being that a triple does have a higher top RPM gives you more room, but another good thing to know is what RPM does your sled and its pipes like to sit at. That way I can figure gearing to work with top RPM, and also, what is the highest top speed you have achieved on the sled. More good info I could use for better advice.
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help changing gear ratio

Postby Iceman9c » November 20th, 2003, 8:19 pm

Thanks Guys for the great help!
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