That magic ratio... can I calculate what it is?- Mtbr.com
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  1. #1
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    That magic ratio... can I calculate what it is?

    I found the magic ratio of my hardtail with vertical dropouts. It's 32T chainging, 14T cog.

    Ultimately, I want to put a 16T in the rear and swap out the chainring for something bigger. That way I just have gears with more teeth to bite into. But I still want the same ratio of 32-14, just with xx-16. But anyway, that's later down the road...

    But let's keep the conversation simple, since I wasn't exactly an ace in math... let's say I keep the 32T chainring up front, but I want to swap out my 14T cog in the rear for something else. If I throw in a 15T cog, that's 1 additional tooth. With my chain being tight @ 32-14, if I move to 32-15, would adding 1 full link be the magic ticket? Or would I have to move the 14T cog to a 16T cog? Would that be the magic ratio at that point?

    I'm just trying to figure out, mathematically, how I can adjust the links on my chain in ordinance with increasing the size of my rear cog while still achieving the magic ratio.

  2. #2
    one chain loop
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    34:16. according to fixmeup!, if its too tight, file the axle.
    everything sucks but my vacuum cleaner.

  3. #3
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    34:16... then what? Do I add 1 link? 2?

  4. #4
    one chain loop
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    a link. eat your breakfast every morning and keep those pearly whites clean.
    everything sucks but my vacuum cleaner.

  5. #5
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    Quote Originally Posted by fishcreek
    a link. eat your breakfast every morning and keep those pearly whites clean.
    Thanks?

  6. #6
    one chain loop
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    i guess?
    everything sucks but my vacuum cleaner.

  7. #7
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    Quote Originally Posted by fishcreek
    i guess?
    <3?

  8. #8
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    Just get a half link chain

  9. #9
    one chain loop
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    Quote Originally Posted by njbiker66
    Just get a half link chain
    wow.
    everything sucks but my vacuum cleaner.

  10. #10
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    Quote Originally Posted by njbiker66
    Just get a half link chain
    I considered that, but I read mixed reviews about their strength... so I was putting a back seat on that idea.

    I think I'll be going with a Rennen, since they seem to get really good reviews. That way if I decide I want to do some hardcore flat road riding I can at least swap out for a bigger cog or chainring on the fly and just adjust the Rennen accordingly, rather than worrying about the chain length and all.

  11. #11
    local trails rider
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    Chain length needs to be pretty close, to get it within the adjustment range of a simple tensioner. You may get away with adding or subtracting a tooth or two.

    Adding in one end of the drivetrain AND subtracting the same amount at the other end should work. Like: 34/16 and 36/14, keeping the total number of teeth the same.

  12. #12
    bikexor
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    32t/14t = 2.2857

    Xt/16t = 2.2857 (Xt=front chainring)

    Xt=36.571427t.... Good Luck!

  13. #13
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    Quote Originally Posted by Roasted
    I found the magic ratio of my hardtail with vertical dropouts. It's 32T chainging, 14T cog.

    Ultimately, I want to put a 16T in the rear and swap out the chainring for something bigger. That way I just have gears with more teeth to bite into. But I still want the same ratio of 32-14, just with xx-16. But anyway, that's later down the road...

    But let's keep the conversation simple, since I wasn't exactly an ace in math... let's say I keep the 32T chainring up front, but I want to swap out my 14T cog in the rear for something else. If I throw in a 15T cog, that's 1 additional tooth. With my chain being tight @ 32-14, if I move to 32-15, would adding 1 full link be the magic ticket? Or would I have to move the 14T cog to a 16T cog? Would that be the magic ratio at that point?

    I'm just trying to figure out, mathematically, how I can adjust the links on my chain in ordinance with increasing the size of my rear cog while still achieving the magic ratio.
    Adjustment: 4 teeth = 1 link

    If you want to swap the cog in the back for something larger you'll have to go to an 18T and add one link to your chain. The chain will be looser but maybe you can take up slack.

    BTW the term is not magic ratio (ratio has nothing to do with fit). It's magic gear(ing).
    Last edited by pacman; 07-02-2009 at 11:45 AM. Reason: deleted wrong comparison

  14. #14
    local trails rider
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    Quote Originally Posted by pacman
    Adjustment: 4 teeth = 1 link.
    Almost, but not not exactly, I think.
    I used to run one converted bike 32/16.
    To gear it for the streets, I tried 36/16 but it was too tight for the cranks to turn freely.
    At 36/15 the tensioner has some slack to take up, with a new chain.

  15. #15
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    People always seem to get this Magic gear thing messed up. To keep the exact same chain tension you have to add or subtract the same number of teeth from the front AND back. This will keep the chain at the same angle from ring to cog and result in the same chain tension. using your example, you currently have 32:14. So other ratios that would work are:

    34:16
    36:18
    38:20
    30:12

    If you are just add four teeth to one side, the chain tension will be different. If you tried 32:18, your chain would looser because the chain is traveling more horizontal from ring to cog. And concerning the chain length, adding a total of 4 teeth would require 1 additional chain link.

  16. #16
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    Quote Originally Posted by perttime
    Almost, but not not exactly, I think.
    I used to run one converted bike 32/16.
    To gear it for the streets, I tried 36/16 but it was too tight for the cranks to turn freely.
    At 36/15 the tensioner has some slack to take up, with a new chain.
    It's a close approximation. And certainly something to use when you have sliders or EBB or chains with different wear.

    at 32/16 the ideal chainstay is 16.952"
    at 36/16 the ideal chainstay is 16.925"

  17. #17
    local trails rider
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    The 32/16 was already slightly on the tight side with a new chain, so it did not take much to get it too tight.

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