Gearing Question

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rdsrf
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Gearing Question

Post by rdsrf »

I recently changed my gearing from 13/47 (stock?) to 12/47. I'm still on the fence as to which I prefer. Both have their advantages and disadvantages in the terrain I ride. I plan on replacing the chain and sprockets in the near future and I'm doing a bit of experimentation to decide on what to go with. I've been told that going up or down a tooth on the countershaft sprocket is equal to about a 4 tooth difference on the rear sprocket. Assuming enough chain to create a the same wheelbase for either set-up, is there any advantage/disadvantage of a 12/47 gear set compared to say, a 13/51? Maybe the engineering types can comment on this one. Thanks in advance for any input!
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canyncarvr
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Post by canyncarvr »

Do the math to reach your own conclusions. Simply divide the driven (rear) by the driving (countershaft) to get the ratio. That ratio number that will be relative to the samely derived number on some other sprocket set. Why that matters: As the number of teeth change on either sprocket, a tooth makes more or less of a difference.

Exaggerate for the purpose of example. If you have a 2 tooth CSS (countershaft sprocket), adding one tooth will effect a 50% change in teeth. A larger difference 'natch than adding a tooth to a 12T CSS setup.

Hence: 'I've been told that going up or down a tooth on the countershaft sprocket is equal to about a 4 tooth difference on the rear sprocket.' is not always true.

Might as well figure the ratio and know.

Stock final drive on the 'H' KDX is 13/47. It is generally agreed that a rear sprocket larger than 49T will require a chain guide replacement. The sprocket gets larger with every tooth..pretty soon the two don't fit together.


Changing the rear sprocket is more expensive than changing the front. It takes longer to change, too. Those are downsides to changing the rear sprocket should you choose to do that.

Depending on the wear of your chain, you won't fit the OEM 108 linked chain on a 13/50, either. You will need 110.

You can change the front sprocket between 12/13 in a couple minutes. The OEM 108L chain will run on either setup, too (12-13/47).

I don't know the physics of chain mechanics..but I have been told by types that should know that there is a relationship between the measurement of angle of a sprocket (determined by diameter) to the pitch of chain (distance between pins). A 520 chain will run fine on a 12T CSS considering that relationship, but that's the small-end limit. 11T would be too 'sharp' an angle.

Certainly any sprocket of fewer teeth will wear more than one of more teeth. Saying a 12T wears too fast may fit someone's particular choice of setup, but it's not wearing too fast because it isn't suited for the application. It's just smaller, is all.

Maybe the engineering types can indeed comment further..but this moyuck thinks 12/47 beats 13/51 in enough ways to be the sensible choice.

..for me, anyway.

Now...type of sprockets (brands, material made from, design) that's another story, 'eh?

Good luck!

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rdsrf
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Post by rdsrf »

CC, thanks for taking the time to write that response. Lots of good info there. 12/47 indeed seems a better choice than 13/51. For the moment I'm going to ride the 12/47. When I replace the chain/sprockets as a set I may wind up going with a 13/49 as a compromise for somewhat lower gearing than stock, but slightly better wear characteristics than with the 12T and not having to change the chain guide.

Is there rule of thumb for increasing/decreasing chain length when changing the size of the sprockets?
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Post by MX500 »

Yes, cut a link off the chain if the extenders have to be past their limit in order for the chain to have proper tension, buy a longer chain if you can't move the rear axle far enough forward.

How long have you been riding? This should be common sense unless you are very new to the sport.
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Post by canyncarvr »

rd: You're welcome. More than likely a lot of blather already known or not needed to BE known. Written more for the casual reader and general overview. To any extent it was helpful, that's a good thing. In the case of none of it being of any use to a reader...go read something else.

Rule of thumb?

Probably is such a thing around somewhere...I'm not aware of one.

Chain length choices will vary depending on method/extent of snail/bolt/swingarm slot adjustment. Two lengths will generally cover whatever you would reasonably want to use on your KDX. 108 for OEM gearing (13/47) and 12/47. 110 for 13/50 and larger (rear).

While I'm sure some have taken links off a 108 length chain when the limits of adjustment were reached, that's a matter of ignoring the obvious: The chain is shot and the final drive should've been replaced already.

Common sense? Duck. Water. Back. Consider. :neutral:

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Post by Jeb »

>|<>QBB<
MX500 wrote:How long have you been riding? This should be common sense unless you are very new to the sport.
???
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Post by canyncarvr »

Jeb wrote:???
Help me help you understand.

What is it exactly that you do NOT understand? How long have you been reading? I see nothing that is not quite plain unless you are new to the world of reading...the literate.

:hmm:

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Post by rdsrf »

>|<>QBB<
MX500 wrote:Yes, cut a link off the chain if the extenders have to be past their limit in order for the chain to have proper tension, buy a longer chain if you can't move the rear axle far enough forward.

How long have you been riding? This should be common sense unless you are very new to the sport.
With all due respect Mr. MX500, I ask questions to save myself time, aggravation, and money. If there is a mathematical formula or rule of thumb that I can use to buy a chain that is exactly the right length, I aim to use it. If I can't find that information, then I have to use the trial and error/common sense method. I try to work smarter, not harder.
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Post by Indawoods »

Do not be afraid to ask questions. There is no need to spend money on trial and error if we have the answers here.

Most do not respond in this manner.

MX500 ...And it's 2 links removed not one to mate the chain again... hence the 108 and 110 reference. :wink:
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Post by Jeb »

>|<>QBB<
canyncarvr wrote:
Jeb wrote:???
Help me help you understand.

What is it exactly that you do NOT understand? How long have you been reading? I see nothing that is not quite plain unless you are new to the world of reading...the literate.

:hmm:
I just didn't understand why the implication that rd didn't have common sense. I thought it was a reasonable question and was actually looking forward to reading the response
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