Help with chain-ring size vs. chain length...-
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  1. #1
    mtbr member
    Join Date
    Sep 2011

    Help with chain-ring size vs. chain length...

    I'm building my first SS MTB and I'm working with vertical dropouts and trying to avoid a chain tensioner at least for now until I decide if I like SS or not and put more money into it.

    I'm ~1/2 link off from a good chain tension. I know I could buy a 1/2 link, but my local co-op has Plenty of chain-rings that I can have for free, so I'd rather just swap the ring. I currently have a 38 up front and 16 in rear. What would you recommend I swap to to make that 1/2 link difference?

  2. #2
    mtbr member
    Reputation: J.B. Weld's Avatar
    Join Date
    Aug 2012
    I think that unless the trails you ride have no hills, or unless you have super human strength that gearing is going to all but guarantee that you won't enjoy SS mountain biking much. I'm not a fan of the "magic ratio" and think a tensioner is a much better option because you can use any gearing you want, but if you want to go without one I would recommend going smaller on the front and/or bigger in the back.

    Sorry I can't help out with the right combo but there is a chart that may help somewhere on this forum. Search magic ratio.

  3. #3
    mtbr member
    Reputation: jmmUT's Avatar
    Join Date
    Sep 2008
    Here is the uber technical breakdown. To | Magic Gear

    But the very rough rule of thumb is every tooth equals about 1/8" moving back or forth.

  4. #4
    mtbr member
    Reputation: mack_turtle's Avatar
    Join Date
    Jan 2009
    38/16 is great if you are riding a 100% flat course, or you don't mind walking and carrying your bike a whole lot. 32/16 might be more like it.

    to make 1/2 link difference, you could put a 17t cog on the back, but that would still be a ridiculous ratio for SS off-road.

    you can also try this:
    but remember that the calculations you get from something like that are only as good as the perfection of parts on your bike. if your chain is new, it's going to "stretch" a bit, and your teeth are going to wear down. as soon as that starts, which is basically right away, those numbers mean nothing and you will need some sort of tensioner device.

  5. #5
    mtbr member
    Join Date
    Sep 2011
    Thanks for the advice. The 38 in front is simply because that's what chain-ring came on the cranks I had laying around. I figured smaller rather than larger was the way to go in the front, but I guess I should go even smaller than I expected. I'll try out a 32 and work from there to get the chain tension right. I did get 17 and 18 tooth sprockets with the wheel, but it seems like running a smaller chain-ring would be better than putting a larger cog on the rear. I don't have the tool yet to get the rear cog off anyway, and I have the allen wrench to swap the chain-ring.

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