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Thread: Rule of four?

  1. #1
    giddy up!
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    Rule of four?

    I need a little help here in regards to the "rule of four". Typically the rule of four says that if you add two teeth to the cog and subtract two teeth from the ring you shouldn't need to move your wheel at all(assuming we're talking about a frame with track ends).

    Here's the question....will the rule of four apply if I just subtract 4 teeth on the chainring and shorten the cahin?

    Reason I ask is this....I know that a 36x20 works on a yeti arc without a tensioner....will a 32x20 also work, with a shorter chain of course?

    Thanks for the help fellas,

    B
    www.thepathbikeshop.com

  2. #2
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    May work but the chain will be tight.

  3. #3
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    Quote Originally Posted by DiRt DeViL
    May work but the chain will be tight.
    Why's that?

    B
    www.thepathbikeshop.com

  4. #4
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    Quote Originally Posted by donkey
    Why's that?

    B
    The trouble is that when you add teeth, you increase the diameter of the chainring. This makes the chain run slightly steeper angles (relative to a reference point like the chainstay) from the cog to the chainring. The steeper angles make the distance that the chain needs to travel a little tiny bit longer and requires some trigonometry to account for. Therefore, if you shorten the chain, it'll probably be close, but just a little too short.

  5. #5
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    I did that setup on my nieghboor SS and 32:20 had the chain tight, the same happened on a converted frame that I had for a while using the same combination but on any 2:1 combo was perfect.

    It will come down to the chainstay lenght of the bike and I have no idea on the measurements of the Yeti.

  6. #6
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    Rules schmules

    Quote Originally Posted by donkey
    you add two teeth to the cog and subtract two teeth from the ring you shouldn't need to move your wheel at all
    That is, of course, true, but the rule is that you can also add or subtract a total of four from either ring or cog or both. For example, you can go from a 42:20 to a 46:20 (+4:0) or a 48:18 (+6:-2) with a change in chain length that's a multiple of 1/2 link. It's not perfect though. Run your ideas through fixmeup, and you'll see they are very close, depending on what size you started with. The radii of the ring and cog cause small errors to the rule. For the most options, you need a half-link though.

  7. #7
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    Correct me if I am wrong, but I would think it would work, but might be a little LOOSE. His plan is to subtract 4 teeth from the chainring, which will cause the chain to be more horizontal, making teh angle the chain must travel from ring to cog smaller. That should result in a little extra chain length. This is of course taking into account removing a chain link. Am I not thinking straight?

  8. #8
    Recovering Weight Weenie
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    Don't be afraid...break out the dremmel!
    Have Terminaut help you....

  9. #9
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    Correct!

    Quote Originally Posted by bikeny
    Correct me if I am wrong, but I would think it would work, but might be a little LOOSE. His plan is to subtract 4 teeth from the chainring, which will cause the chain to be more horizontal, making teh angle the chain must travel from ring to cog smaller. That should result in a little extra chain length. This is of course taking into account removing a chain link. Am I not thinking straight?
    If the 36:20 is a perfect fit (48" chain), a 47" chain (32:20) will be .042" too long, the slack will let the chain dip .59".
    Last edited by pacman; 12-23-2004 at 12:41 PM.

  10. #10
    giddy up!
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    Quote Originally Posted by Padre
    Don't be afraid...break out the dremmel!
    Have Terminaut help you....
    I happen to be dremmeling as we speak.....cleaning up a monkey fork:-)

    No way am I taking a dremmel to the little Yeti..this things a classic!

    B
    www.thepathbikeshop.com

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