Gear ratio: 40:18=38:20?- Mtbr.com
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  1. #1
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    Gear ratio: 40:18=38:20?

    I have a single speed convert Specialized Cross Trail expert with vertical drop outs.
    Right now the gearing is 40:18... little too aggressive for the mini climbs I'm currently doing.

    I am not running a tensioner. The chain is nice and tight.

    After doing some research, I found an article on Sheldon Browns site that stats one tooth is equal to one tooth front or rear (paraphrasing)... that make sense.

    so if I went to a 38/20 configuration I should ideally be able to keep my current chain tension without a tensioner correct?
    Attached Thumbnails Attached Thumbnails Gear ratio: 40:18=38:20?-546142_10150943372540977_716470976_12122296_1839891724_n.jpg  

    Last edited by yosshaa; 05-17-2012 at 03:38 PM.

  2. #2
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    Here is the original article I'm siting/verifying my understanding.


    Singlespeed Bicycle Conversions


    "Changing either sprocket size by one tooth is the equivalent of moving the axle 1/8" (4 mm). Thus, if I wanted a 5.75 gain ratio (75" / 6 meter gear), I might first try a 42/15, this gives a gain ratio of 5.77 (75.6" / 6.05 m gear). If the chain was too loose, I could take up 1/8" (4 mm) of axle movement by replacing the 42 with a 43. This 43/15 combination would raise my gain ratio to 5.91 (77.4" / 6.19 m). Alternately, I could get the same axle position with a 42/16 --5.41 (70.9" / 5.67 m)."

  3. #3
    jl
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    38/20 should work. The tension won't be exactly the same, it'll probably be slightly looser. You probably won't be able to do more than 2-teeth without a significant change in tension.

    MTBR.com Single Speed Forum - Single Speed FAQ
    Last edited by jl; 05-17-2012 at 03:00 PM. Reason: more info
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  4. #4
    Just Ride
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    To me 40/18 would be the same as 38/20 as far as how the pedaling feels.
    I really don't know what the difference would be.
    SS ==> Nut up or Shut up!

  5. #5
    conjoinicorned
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    I would easily notice the difference, it's not huge but it's real

    I agree with jl, the tension will be very close and should work but be aware that manufacturer tolerances could work against you, usually only an issue if your magic gear is already very tight. If you are dedicated to the SS cause you can file a very small amount of the dropout and/or hub axle which will give you a slight amount of adjustment either way. This doesn't affect strength or geared usage but can look scary for resale.
    what would rainbow unicorn do?

  6. #6
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    Thanks everybody for the replies!

    Looking at the gear ratio category and coming from a road bike background I think I would definitely notice the difference.

    This little rat bike is staying single speed. I like the simplicity of it.

  7. #7
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    If that does not work for you, try looking here to find a magic gear.

  8. #8
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    Read this, do the math, and be enlightened.

    Gain Ratios--A New Way to Designate Bicycle Gears

    Mind your own religion.

  9. #9
    Just Ride
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    I mean in theory the feel should be the same, though in practice I'm sure you can feel it, even though I wouldn't think so. Which is harder to push up a hill for example? 40/18 or 38/20. they'd both kill me. My "magic" gear is 32/18...I think, I just threw my 16 on so I could get the perfect chain length for the tensioner to work pushing up on my converted cannondale.
    SS ==> Nut up or Shut up!

  10. #10
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    Quote Originally Posted by Cormac View Post
    I mean in theory the feel should be the same, though in practice I'm sure you can feel it, even though I wouldn't think so. Which is harder to push up a hill for example? 40/18 or 38/20. they'd both kill me. My "magic" gear is 32/18...I think, I just threw my 16 on so I could get the perfect chain length for the tensioner to work pushing up on my converted cannondale.
    If you still need a chain tensioner then its not a magic gear.
    Mind your own religion.

  11. #11
    Just Ride
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    Quote Originally Posted by PoisonDartFrog View Post
    If you still need a chain tensioner then its not a magic gear.
    Vertical dropouts and no EBB. Guess I misunderstood what magical gear meant.
    SS ==> Nut up or Shut up!

  12. #12
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    Quote Originally Posted by Cormac View Post
    Vertical dropouts and no EBB. Guess I misunderstood what magical gear meant.
    A magic gear is a gear combination that provides sufficient chain tension without the need for a tensioning device.

  13. #13
    (Ali)
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    If you can feel a difference from 32:18 to 38:20, then you will feel a greater difference from the latter to 40:18:

    32/18 = 1.78
    38/20 = 1.90 (%6.7 harder than previous)
    40/18 = 2.22 (%16.8 harder that previous)

    Ali

  14. #14
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    well i changed from 40-18 to 37-21... the chain seemed to be a little looser but within an acceptable range. went on a 2 hour trek and had the chain fall off twice... both times to the inside. Both times when charging steep climbs. no biggie.

    I was at 980 calories burned (via garmin) and decided to sprint down the street to my house to hit 1000.
    Chain slipped to the outside I flipped over the bars.
    taco'd my front brake disc, bent the skewers and bent my fork/broke the tip that holds the skewer.
    jacked up my shoulder knee and right hand... other than that all is well.
    Back to the drawing board I think I'm going to take some links out of the chain and try a more standard 34-18 set up.

  15. #15
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    moral of the story 1 tooth up 1 tooth down can effect the chain tension.

    live and learn I guess

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