gearing for 18" rim

Discussion in 'Parallel Universe' started by Singletrack_mind, May 19, 2012.

  1. Singletrack_mind

    Singletrack_mind Cave Man

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    I'm thinking about building up a set of dirt rims for my GS800 so I can quickly swap from street-appropriate tires on the OEM wheels to full knobbies on an 18" rear and a 21" but narrower front. My question here is about gearing. The 18" rim with a knobby on it will be a bit bigger in diameter than the OEM setup, so I would need to lower the gear ratio via a larger rear sprocket or smaller countershaft sprocket just to maintain status quo. Ideally though, I'd want even lower gearing for the dirt so I could use 2nd gear more of the time; the stock gearing is a bit too tall in the dirt even with a 17" wheel.

    I want to do this without exceeding the adjustment range on the chain tensioners so that swapping wheels is hopefully just a case of swapping wheels that have different sprockets. If I need to though, I could swap countershaft sprockets at the same time.

    The stock gearing is 16/42. I'm wondering about leaving it that way for the street, and going to something like 15/43 for the dirt.

    Has anyone else done this? Can you comment on what gearing choices you made?

    Thanks!
    #1
  2. ebrabaek

    ebrabaek Long timer

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    Here is what I did......
    http://www.advrider.com/forums/showthread.php?t=759024&page=5

    In summery.... the 45 was too much of a difference to me, and the bike although great in first.... were running too high rpm in 6'th....So I went back to a 42....still a tall first...but either sprocket would work within the stock tensioner, and chain on the bike....
    #2
  3. Singletrack_mind

    Singletrack_mind Cave Man

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    Thanks E; I'd found that other thread before, and followed the links Wooody provided too. What I haven't seen is anyone else swapping back and forth from the 17" on the street to the 18" in the dirt and I've been wondering if it would be possible to pick good gearing for both without running out of adjustment room.

    From your post though it sounds like I could put a 45T on the 18" wheel for dirt and revert to the 42T on the 17" for street and be happy. I don't care if I loose out on top speed when I have knobbies on.
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  4. ebrabaek

    ebrabaek Long timer

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    Yeppers.... You can.... at least with the stock chain on it..... I am an all mountain rider on a 7 inch 29'er, and share your like for single tracks....:D, and in my opinion.... the 18inch/45 combo makes for a great balance between average dirt road straights....and up and over's. If your unskilled in riding up and overs..... I would think two higher at 47 would be perfect....But I would have no fear at anything the bike could do with the 45...
    #4
  5. bodhizafa

    bodhizafa How hard can it be?

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    I have a stock chain on with 15 front and 45 rear sprockets. 18 inch wheel too.
    #5
  6. mousitsas

    mousitsas Long timer

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    The difference between a 18 and a 17 rear, is 1 tooth on the front sprocket exactly.
    #6
  7. Singletrack_mind

    Singletrack_mind Cave Man

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    Good to know. That is virtually the same as adding 3 teeth on the rear: stock gear ratio with 16/42 sprockets is 2.625; running 15/42 yields 2.8, and running 16/45 = 2.813.

    This would be great if I wanted to retain the stock effective ratio in the dirt, but I don't. I want it to be shorter. The OEM ratio is OK on the street, but too tall in the dirt. So, I might have to do something like this: stock 16/42 for the road; 15/45 dirt (3.0 ratio). That's a two tooth difference between the two set-ups, I'll have to go see if there is room in the chain adjusters for that.
    #7
  8. mousitsas

    mousitsas Long timer

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    Check this site out
    www.gearingcommander.com

    You can play with sprocket sizes and check the resulting chain lengths
    #8
  9. Singletrack_mind

    Singletrack_mind Cave Man

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    Thanks for the input folks. As I was totaling the bill for this project and finding that it was going to push $2500 by the time I was done (including a new shock to help the back end work like it should off road), a used KTM 690 came my way and it just seemed like a better way to go. Even with great tires and suspension, the GS would still weigh 150lbs more than the KTM & that's huge.

    I almost feel bad about backing away from pushing the 800 to it's limits, it is such a great bike and can be pushed amazingly far. I have had wonderful trips on it in the back country. In fact, it is BETTER than the 690 a lot of the time on a dual sport trip, but in those situations where it is too big, heavy and soft, the 690 just rocks.
    #9