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  1. #1
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    sram x.o short cage chain wrap capacity

    I'm setting up a 1x9 and would like the short cage to keep a higher tension on the chain. Is there any reason a short cage would not work with a 34 tooth front ring? I noticed in the QBP catalog that it mentions a 32t max for chain wrap capacity, whatever that means. Anyone out there going renegade with a 34 or more? It seems like the short cage would be used on a lot of DH bikes which would surely have a larger ring. someone show me the light.

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  3. #3
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    while that was a very amusing link, the literature I found proved no real help for a 1x9 setup.
    The formula takes the difference between the rings in front and back. With a single ring of 34 in the front, that number is larger than the difference in a 44/32/22 setup, which would be 22, and then add the cassette difference. anyway, the point of the post is to bring up the question of how does the chain wrap formula change when only using one front ring, because taking the difference of 34 and zero doesn't seem to make sense, because it would state that a derailleur and chain could work with a 44t triple but not a 34t in a single ring setup. I know there are details left out, but do you follow me?
    does chain wrap capacity not apply in a single ring setup?
    let's leave the math out and hear from someone who has used the short cage with more than a 32t and no problems. i always appreciate practice over theory.

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    It's time for a cup of cup of strong coffee.

    As you said, the formula takes in the difference in chainrings from the largest to the smallest. In a single speed there's only one, in your case 34t, so the largest and smallest are the same, and the difference is zero.

    You'd count a difference of 34t only if you wanted the RD to be able to take up all the slack when the chain fell off onto the BB.

    At this point, slap your forehead and say DOH!!

    BTW- most SS systems can use short RD's since they only need to take up slack from the cassette.
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    haha, doh!!
    i guess the difference IS zero as opposed to subtracting zero. thanks

    I did uncover another formula which takes into account proper gear use.
    I suppose it would go like (34t + 11tcog) - (34t + 34tcog) = 23
    they used this because the big ring would never use the biggest cog and vice versa.

    anyway, thanks FBinNY, you gave me a definite duh moment.

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    The formula remains the same, and it works to give you a smooth running drivetrain.

    You may stray outside of the calculated results of the formula, but the outcome is not guaranteed to run smoothly (especially with SRAM derailleurs).

    Specs for SRAM X0 Short Cage
    Maximum capacity = 30T
    Mininum sprocket = 11T
    Maximum sprocket = 34T

    Maximum chainring = 34T
    Minimum chainring = 34T

    Chain wrap capacity required from your drivetrain (assuming that you want to use a 11-34T cassette):

    X = (34T - 34T) + (34T - 11T)
    X = 0 + 23T
    X = 23T

    As you can see, the chain wrap capacity required from your drivetrain is 23T and this number is well within the limits of the X0 Short Cage RD.

    This tells you that the required capacity is determined by the range of the cassette alone when there is only a single chainring up front.

  7. #7
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    Quote Originally Posted by standard3x
    This tells you that the required capacity is determined by the range of the cassette alone when there is only a single chainring up front.
    this statement sums up the mathematical theory I was looking for to go with my simple logic. thanks

  8. #8
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    Quote Originally Posted by nikwashere
    haha, doh!!
    i guess the difference IS zero as opposed to subtracting zero. thanks

    I did uncover another formula which takes into account proper gear use.
    I suppose it would go like (34t + 11tcog) - (34t + 34tcog) = 23
    they used this because the big ring would never use the biggest cog and vice versa.

    anyway, thanks FBinNY, you gave me a definite duh moment.
    Might help, too, if your math notation was correct
    (34 - 11) + (34 - 34) = 23
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    the math was correct, just backwards. sorry, i dropped the negative sign.
    anyway, this was the theory i was referencing from another thread.

    "A 12-30 with 36x52 requires "34T" of capacity. But in reality, you don't use 36/12 or 52/30 (or at least you shouldn't). So you really need wrap for 52/27 to 36/13 or (52+27)-(36+13) = 30T."

    granted the concept needed is a little different, the math worked all the same, and again, got me to an answer.

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    No, the math cannot be correct if backwards, math doesn't work that way. The chain wrap capcity is the difference between the chainrings plus the difference in the cogs. You can adjust for what you will or will not use, but that's not how it's calculated...so for your example of a single ring it's as I stated before. Did you sleep through math class?
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  11. #11
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    wow, i never knew a negative integer could make someone so upset. I think I got my answers already, I hope this doesn't keep you up at night. thanks though

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    Lighten up guys, it's the internet.

    Anyway this reminds me of an old Abbott & Costello line.

    After Costello can't do a simple math problem
    Abbott asks "didn't you go to school, stupid"
    Costello, "Yeah, and I came out the same way!"

    You probably had to be there
    cheers
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    The key to solving any problem is to understand and address the underlying cause.

  13. #13
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    Quote Originally Posted by nikwashere
    wow, i never knew a negative integer could make someone so upset. I think I got my answers already, I hope this doesn't keep you up at night. thanks though
    My apologies, but was looking at your original math, which wasn't showing -23 as an answer...I don't have a problem with reworked formulas and negative integers particularly....that's my story and I'm sticking to it!
    "...the people get the government they deserve..."
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  14. #14
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    3x9: long cage
    2x9: medium cage
    1x9: short cage

    Your derailleur won't necessarily explode if you use a short cage with 2x9 either. The cage size isn't dependant upon the front chainring size, but the amount of difference between more than one chainring.

  15. #15
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    if i am running a 1X7 and my chain falls of of the front at gears 1 and 2 in the back. a short cage derailleur would be the solution?

  16. #16
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    No, this isn't necessary the root of your problem.

    Look towards chainline, a bent ring, bent hanger or a damaged chain first. Also check for flexing when you apply your weight onto the driveside crank - again, this will be a chainline issue.


    Quote Originally Posted by rlrocks04
    if i am running a 1X7 and my chain falls of of the front at gears 1 and 2 in the back. a short cage derailleur would be the solution?

  17. #17
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    Quote Originally Posted by rlrocks04
    if i am running a 1X7 and my chain falls of of the front at gears 1 and 2 in the back. a short cage derailleur would be the solution?
    No, the RD has nothing to do with what happens up front. (minor exception, for nit-pickers, a bend cage might cause chain to fall of when back peadlling)

    Your chain falling off is an indication of poor chain line. Measure the chain line carefully, and if your chainring is far inboard of the middle cog in back, you might have to get a longer spindle, or better yet (because it's free) move the chainring to the outer position if you converted a double or triple crank.

    Other things you could do are get a chainring intended for single speed use, which won't have any shifting gates which are often are a cause of derailling, replace the chain with one whose inner plates have more bellmouth, to improve pickup when coming at an angle, or use a chain retainer device, either a "bashwich" or one of those things that look sort of like a front derailleur and ensure that the chain cannot come off when pedalling forward.

    Lot's of things could help, but the one thing that can't is changing the RD.
    fb
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    The key to solving any problem is to understand and address the underlying cause.

  18. #18
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    So a Shimano road derailleur with a capacity of 23t is all I really need for my 1x9 set up. Except, the reach is wrong, right? I mean, it won't play nice with anything taller than a 25t cog. So where's the MTB derailleur that gets to a 34t cog with the narrowest capacity?
    "Bikes aren't fast--people are fast. Bikes are overpriced. It's an important distinction."---BikeSnob NYC

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    Quote Originally Posted by GrantB
    So a Shimano road derailleur with a capacity of 23t is all I really need for my 1x9 set up. Except, the reach is wrong, right? I mean, it won't play nice with anything taller than a 25t cog. So where's the MTB derailleur that gets to a 34t cog with the narrowest capacity?
    This is where mechanical skill comes a factor. I don't know whether a 25t cog capacity road RD will clear a 34t cog, but there are many short cage MTB RDs that will. It's an issue of a sufficiently long pantograph, and the "B" adjustment. Most short and long cage RDs of the same series use the same bodies. Adjust the "B" screw & measure the chain so the upper pulley is pulled back & down, clearing the larger cogs as you shift to them.

    Before giving up, you might mount that road RD, bring in the "B" screw, and rotate the idler cage counter clockwise as if the chain were tight and see if you can get it to clear.
    AS the NYS lottery folks say "you never know".
    fb
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    The key to solving any problem is to understand and address the underlying cause.

  20. #20
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    thanks a lot should the chain be relatively tight on a 1X7 rig?

  21. #21
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    Quote Originally Posted by FBinNY
    Before giving up, you might mount that road RD, bring in the "B" screw, and rotate the idler cage counter clockwise as if the chain were tight and see if you can get it to clear.
    AS the NYS lottery folks say "you never know".

    Sure, why not? Thanks.
    "Bikes aren't fast--people are fast. Bikes are overpriced. It's an important distinction."---BikeSnob NYC

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