Gear ratio question- Mtbr.com
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  1. #1
    offroader
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    Gear ratio question

    What's the difference between running 32x18, 34x20, or 22x13 ratios?

    I'm currently running a 34x20 which is probably equivalent to a 32x19. If 22x13 is the same ratio wouldn't it be better to run the smaller cog/chainring to save weight? What other factors are there in choosing a gear ratio?
    Last edited by CupOfJava; 04-19-2010 at 06:21 PM.

  2. #2
    Mtbr Forum Sponsor - Homebrewed Components
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    faster wear is probably the biggest thing. If your cog is half the size, it'll probably wear out twice as fast. Same with the chainring.
    but they are all a little different from each other as far as ratio goes.
    32-18: 1:1.777777
    34-20: 1:1.7
    22-13: 1:1.6923
    but i know what you mean....

  3. #3
    Over the bars...
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    Also I believe a smoother spin with more points of engagement. But if we are Single do we really want to be engaged. Now there is the real question... Cheeze!!

  4. #4
    offroader
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    Quote Originally Posted by egggman
    Also I believe a smoother spin with more points of engagement. But if we are Single do we really want to be engaged. Now there is the real question... Cheeze!!
    Smoother spin with the bigger gears? What do you mean by points of engagement. Thanks

  5. #5
    Over the bars...
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    I may be off on what points of engagement mean but in this case I believe it is how many teeth contact the chain at any given point. I also did a little Sheldon Brown reading and he has some good info on reasons for big or small cogs and chainrings.

    http://www.sheldonbrown.com/singlespeed.html

  6. #6
    offroader
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    Ah thanks for the link. I get a little better understanding now.

    Bigger
    Good:
    * Slightly less friction
    * Longer chain/sprocket life
    * less chain tension

    Bad:
    * Slightly heavier
    * Chainstay clearance may be a problem on some frames

    Smaller
    Good
    * Slightly lighter
    * More log-jumping clearance
    * More chainstay clearance

    Bad
    * Rapid chain/sprocket wear
    * Greater chain tension (increased liklihood of the axle slipping in the frame)

  7. #7
    Over the bars...
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    I started to copy/paste this but then got lazy and posted the link instead. Good on ya!!

  8. #8
    offroader
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    Has anyone noticed any other difference besides the ones mentioned here? I sort of get the feeling that a bigger (34T) front ring causes my legs to rotate slower compared to a smaller ring (32T). Is this in my head?

  9. #9
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    Quote Originally Posted by CupOfJava
    Has anyone noticed any other difference besides the ones mentioned here? I sort of get the feeling that a bigger (34T) front ring causes my legs to rotate slower compared to a smaller ring (32T). Is this in my head?

    If the sprocket in the rear stays the same size, your legs will rotate slower with a bigger ring up front.
    Ride more!

  10. #10
    offroader
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    Quote Originally Posted by asphaltdude
    If the sprocket in the rear stays the same size, your legs will rotate slower with a bigger ring up front.
    I guess it's in my head then.

  11. #11
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    Bigger cogs have more chain wrap and are less likely to skip under pedalling. Sheldon Brown and others suggest going no lower than 16 teeth at the rear. Shimano seem to agree.

  12. #12
    I am Walt
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    Size of rings, cogs, wear, etc. aside, the difference in the ratios is the gear inches - the measure of how much "work" you do. The formula is as follows:
    Front ring / Rear cog x wheel diameter (26 or 29) = gear inches (measure of how far the wheel goes with each pedal revolution).

    Examples:
    29'er: 32 / 18 x 29 = 51.6 gear inches
    26'er: 32 / 18 x 26 = 46.2 gear inches
    29'er: 34 / 18 x 29 = 54.8 gear inches

    This shows why the same ratio on a 29'er vs. a 26'er is "harder", and why, to CupOJava's question, the bigger front ring causes his legs to rotate slower...it's a taller gear.
    Ride more; post less...

  13. #13
    offroader
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    Quote Originally Posted by nuffink
    Bigger cogs have more chain wrap and are less likely to skip under pedalling. Sheldon Brown and others suggest going no lower than 16 teeth at the rear. Shimano seem to agree.
    Ah thanks, so that's why the magic gear ratios seem to be 32-18 and 32-20.

  14. #14
    Monkey Junkie
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    Here's the gear inch calculator I use: http://software.bareknucklebrigade.c...it.applet.html

    comes in handy sometimes when trying to get the best gear inch with the variety of cogs/rings I have for different bikes.

  15. #15
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    I can't say its a huge difference, but I've been using a 36t front ring for a while and it feels a bit nicer and seems to be holding up well.

  16. #16
    Ovaries on the Outside
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    Gear ratios- something for SSers to fuss about besides which alt bar they want and whether to have susp or rigid.

    Maybe I come from a different riding perspective, but if my chainring will hit something on the way over, it is time to walk.

    I'm thinking of upping to 42t in the front.

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