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Discussion Starter · #1 ·
hi guys im changing my rear tyres from stock 20" to 18" wheels and front 21" to 20"

im happy with the gearing on the bike so i need to change the sproket to keep it the same when i drop wheel diameter. my theory is that smaller wheels engine will be to short geared so i plan to go one tooth smaller on the front., what do you all think.
 

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Other way around. 1 tooth up on the front will start to bring your gearing back to where you are now.
 

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If he's only going from 20 to 18 would he want to change the back instead? I know front is a lot cheaper but if he wants to stay close to the same I'd think changing back would be more precise. I was thinking 1 tooth in the front = 2 1/2 in the rear or close to that.
-Voodoo
 

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Would really have to do some calculations to get an exact gearing.
You're spot on with the +1 on the front = - 2 1/2 on the rear.
Less about 3 teeth on the rear or +1 tooth on the front should take up a 2" diameter loss on the wheel size.
 

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Discussion Starter · #5 ·
Would really have to do some calculations to get an exact gearing.
You're spot on with the +1 on the front = - 2 1/2 on the rear.
Less about 3 teeth on the rear or +1 tooth on the front should take up a 2" diameter loss on the wheel size.

i asked the guys at the shop to help me maintain stock gearing and this is what they sent me

stock front sprocket 14t / change to 13t
stock rear sprocket 38t/ change to 39t

do you guys think this will solve my problems and keep gearing same with the 2" diameter loss

i cant figure out why they sent me both sprockets i agree with you guys just go up one in the front but can you explain thier reasoning and if it will work

cheers guys

'
 

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You changed the radius of your rear tires by a factor of 0.9 so you need to do the same to your gearing to keep it the same (linear velocity = angular velocity of rear tires x radius of rear tires). You need to end up with 0.9x38/14 = 2.443 (stock is 38/14 = 2.714). The closest way to get this is either 37/15 which gives you a ratio of 2.467, or 34/14 which gives you 2.429. What your dealer says is close if you were changing FROM an 18" rear TO a 20" rear.
 

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Wise guy is right. They have sent you sprockets as if you were going from an 18" rear up to a 20".
Arse about for what you want to do.
I would go the 37/15 combination. Larger front always makes for better chain & chain slider life.
 

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Another thing is that some times your 18" tire is actually really 17". Or your 18" is really 19". I dont remember for sure but I think older LTRs actually came with 16" rears but say 18" on the side.

I know my kenda and turf tammers said 18" on the sides but where really 17"
 

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Discussion Starter · #9 ·
You changed the radius of your rear tires by a factor of 0.9 so you need to do the same to your gearing to keep it the same (linear velocity = angular velocity of rear tires x radius of rear tires). You need to end up with 0.9x38/14 = 2.443 (stock is 38/14 = 2.714). The closest way to get this is either 37/15 which gives you a ratio of 2.467, or 34/14 which gives you 2.429. What your dealer says is close if you were changing FROM an 18" rear TO a 20" rear.


this sounds like some awseom info, i have already rang up and ordered the 15tooth and the 37 tooth, i was just wondering if thier is a need to downsize on the rear, im aware its in attempt to maintain gearing but i was wondering if it will be to tall gearded for motorcross know, my plan is to mount the 15 tooth with stock rear 38 and if thiers not enough top end then try the 37, but thanks for the info has helped alot.
 

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Discussion Starter · #10 ·
im still not happy guys, its like this i race mx

got the 18s love them for stability
stock gearing not right revs to much 14/38
tried one tooth up in front its better 15/38
tried one tooth smaller in rear dont waste your money it doesn;t work 15/37

ive got pipe, jetting, igniton map unplugged, opened airbox and i cant catch a complete stock ktm off the line i need another idea to try guys any ideas much wanted
 

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Try 14/37 you already have that with you so try it. Or 14/36.

If14/38 is to hi
15/38 is better
few options would be go 14/37-14/36 or 15/39-15/40
Each time you change the front you are making 2 1/2 teeth change to the rear. Most people change the front sprocket because its cheaper. To fine tune you really only need to do the back.
I'm betting 14/36 is going be what ya like best but try the 14/37 and it might not cost ya more money if ya like that.
 

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I'm running mxr-6's or turftamers on my 525 and usually run 15/38 on more open mx tracks. But if the track has a lot of 180's or is a little tighter 15/38 is a little tall and then I run 14/36. I hope this helps ya.
 
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