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Old 03-12-2010, 03:04 PM
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Sprocket sizes.......

Hello everybody, I just recently put the big 3 on my 2008 kfx 450r. Shortly after this i broke my back sprocket somehow, or something like that anyways it had to be replaced. So i took it to the dealer to get fixed and they gave me a new back sprocket. Soon as i went for a rip up the road i noticed a difference. I had way more pull on bottom end and in 5th gear it comes up in a wheelie with very little effort. It's fun but i do a lot of trail riding and for some reason it feels a lot slower. Like it revs really high in 5th and just doesn't seem as fast. Could they have put different size sprockets on? Is there a way to tell? And is there anyway i can fix this problem without loosing the awesome bottom end i gained? Any help would be appreciated I'd like to have this problem fixed for this summer coming. Thanks
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Old 03-12-2010, 04:24 PM
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Count your teeth. Stock is 14/38 i'm pretty sure. I run a 15/38 or 15/36 if its more open.
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Old 03-12-2010, 05:51 PM
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I counted them and i counted 39 back 13 front is this the rason why its so slow?
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Old 03-12-2010, 07:32 PM
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Quote: Originally Posted by KFX450r_15 View Post
I counted them and i counted 39 back 13 front is this the rason why its so slow?
Hahaha hoooly fuck, you must be able to walk faster than that thing in first, pinned!

Yes, that is absolutely the reason. Swap them both out (14, or even a 15 front, and 38 rear) and you'll be golden.

Lowering the number in the front = less speed, more acceleration. Raising the number in the back = the same thing (Simple gear ratios). Doing both = a very slow KFX with waaaaay too much bottom end, and not nearly enough top. I think they're geared too low from the factory, which, as previously mentioned, is 14/18. 13/39 is retarded (Didn't know anyone even made a 13 tooth front for these things, unless it shares a sprocket with another machine, that it came from).
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Old 03-12-2010, 08:56 PM
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I would do some searching on here for broken sprockets and case savers. The OEM sprockets break or shear bolts pretty easy and often times under power the chain slaps the cases and cracks them. Many people on here are running case savers to protect from the chain damaging the aluminum cases and many are running aftermarket sprockets or drill out the holes on the sprockets and replace them with larger diameter bolts. Just something else to think about while you're getting your gear ratios sorted back out. -Keith
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Old 03-12-2010, 10:45 PM
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i run 14/39 in the dunes . and 15/38 in the dirt .
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Old 03-13-2010, 08:31 AM
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Thanks guys for your replies, So 15 front 38 back is the way to go? am i expected to loose a lot of bottom end?
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Old 03-13-2010, 12:55 PM
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Thumbs up Tech.Tip:Gear Ratios FAQ

Quote: Originally Posted by KFX450r_15 View Post
Thanks guys for your replies, So 15 front 38 back is the way to go? am i expected to loose a lot of bottom end?



Gear ratios got you in a bind?

Another popular question that I’ve seen floating around the message boards is in regards to gearing and gear ratios.

“What is gearing down?”
“Do I need a larger or smaller front sprocket for more top speed?”
“What effects will my ATV have if I switch to smaller tires?”

I’m sure you get my point by now. Well I hope this quick overview will enlighten everyone just a tad.

Larger - front sprocket = more top speed, less acceleration
Smaller - front sprocket = more acceleration, less top speed
Larger - rear sprocket = more acceleration, less top speed
Smaller - rear sprocket = less acceleration, more top speed
Taller - rear tire = more top speed, less acceleration
Shorter - rear tire = more acceleration, less top speed
1" @ the rear tire roughly translates to 2 teeth on the rear sprocket.
2-3 teeth on the rear sprocket roughly translates to 1 tooth on the front sprocket
"Taller" ratios mean more top speed, less acceleration, smaller numerically.
"Shorter" ratios mean less top speed, more acceleration, larger numerically.

To obtain a gear ratio, divide the number of teeth on your rear sprocket by the number of teeth on your front. For example, 13-tooth front and 40-tooth rear.

40 divided by 13 = 3.07

If you had a 11 front and 37 rear that = 3.36

If you had 3.36 and you went to 3.07 that would be considered moving to a "taller" gear, because you're going to achieve a higher top speed with it.

If you had 3.07 and went to 3.36 that would be considered moving to a "shorter" gear, because you'll be acceleration more and achieving less top speed than before.


Keep in mind, depending on engine power rating, you will NOT be able to rev out last gear in every ratio! If you have a 14 tooth front sprocket and 30-tooth rear sprocket on a stock 250cc quad, you can hang that idea out to dry, because it’ll never be able to pull it with 100% efficiency.



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Old 03-13-2010, 05:28 PM
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With the big 3 installed, you'll have more than enough torque with a 15 tooth front and stock rear. Wouldn't even worry about it.
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Old 03-13-2010, 06:29 PM
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Thank you everybody for your replies. I am going to go with a 15 front and 38 on back. I'm going to order off ebay tonight. Just one more quick question. How hard is it to put the sprockets on by yourself? I never did it before and i don't want to have to take it to a dealer because the closest one to me is like a 2 hour drive.
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