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  1. #1
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    Gear Size and Mechanical Advantage

    Hi all, this is my first thread here so please be patient with me. I've looked around for information on this topic, but haven't really been able to find what I'm looking for.

    So I recently converted my 1990 Trek 950 to SS and have been having fun with gear ratios and such. I started out with 46-16 gearing, which was a tad high for riding around town and definitely too high for trails. I switched to 36-13 which is just slightly lower and pretty much perfect for speedy commuting and cruising.

    However, I'm curious if there is more torque/mechanical advantage when a larger cog and chainring are used but with the same gear ratio as a smaller cog/chainring. For example, would there be a noticeable difference running 46-16 vs 40-14? The gear ratio is the same but it seems like there may be more torque using larger gears. I am probably wrong though. Please let me know if you have any input on the matter. Thanks!

  2. #2
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    Mechanically there is no difference is it 40/20 or 30/15 - there should be no difference in torque (because the gear ratio is the same). The only thing is the larger the cog the more 'round' goes the chain which is good (maybe that encreases efficiency a little and reduces wear).

    However, It's obvious that the bigger the cog is the better is the chainwrap and the chain is less likely to skip - the load is transmitted with the larger amount of teeth so it also will not wear as fast as the smaller cog.

    Usually cogs smaller than 16t are not recommended to use on a SS. I think that the bigger is the better for road or commuting. Though, for MTB the ground clearence of a big chainring may be an issue. So 32t ring is usually used on standard cranks unless you need a gear bigger than 2:1.

  3. #3
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    Thanks for the info, Igoreha. Yes, a 13t cog is probably too small, especially because I took it off an 8-speed cassette so the teeth aren't very long like they are on single specific cogs. I'll keep an eye out for a 44t chainring so I can run a 16t cog like you said and keep about the same ratio as I've been happy with.

    Is 2:1 fairly standard gearing for trail riding? It seems like one could go a bit higher and power up all but the most brutal climbs and not be as limited on the flats and downhills. I suppose I'll just have to figure it out on my own though.

  4. #4
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    Depends on your terrain for your gearing. For a 26" wheel, if the climbs aren't sustained, something about a 2:1 should be good. If the climbs are longer and steeper, then accordingly lower until you can get up most of them - you'll always have something that will end up making you push some or all of it... This gearing is a bit low for most street usage - unless you are willing to cruise or spin your brains out!
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  5. #5
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    ATBScott - thanks, that's more or less what I expected. I can't wait to start hitting up the trails on the SS!

  6. #6
    CB2
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    A little more to it it than whose got the biggest.

    Bicycle Cranks

    Your wheel size and size of your gears effect the "leverage" too.

  7. #7
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    While there may be no mechanical advantage to running a large ring/cog combo over a small ring/cog combo to achieve the same gear, I believe there are other advantages, and I think some out there may agree with me. I think the difference may be akin to the difference between a 26" wheel and a 29"wheel. Maybe something about the moment of inertia of the larger ring being greater, and thus the larger ring wants to stay spinning more. I used to run a 32x20 (29.2.25 tire, 175 mm cranks), which yeilds a ratio of 1.6 to 1. Now I run 36x22 (same tires and cranks), which yeilds a 1.63 to 1 ratio. It is a tiny bit harder, but I feel that once you get it rolling, it is easier to keep rolling. Kind of like the difference between rolling along on your road bike in the 53t ring as opposed to the 39t ring... Personally, I'm totally sold on the bigger ring/cog thing.
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  8. #8
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    I would be interested in hearing others thoughts on this since I'm new to the whole SS thing I run a 32-20 set up for all my Colorado riding.


    I tapped that

  9. #9
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    Depending on your setup you can also run a dingle with a mountain gearing like 32x18 on one end and more road gearing like 36x14 on the other. Just run 2 chainrings up front and 2 cogs in the back. The chain will fit in either location without changing any settings. Although if you are running a tensioner as I suspect you probably are that could be a problem.

    FWIW I run 2 teeth over a 2:1 for most stuff in AZ. If it is spinning and flat I will run a 2:1 or even 1 under (32x15) if it is an epic with tons of climbing I will run up to 4 teeth over (32:20 or even 22). (same with my 29er but the ratios are 32x18, 32x20 or 32x22 which isn't as easy as a 26er 32x22 but that is a ridiculous gear on a 26er anyways).
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  10. #10
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    Quote Originally Posted by rockcrusher View Post
    Depending on your setup you can also run a dingle with a mountain gearing like 32x18 on one end and more road gearing like 36x14 on the other. Just run 2 chainrings up front and 2 cogs in the back. The chain will fit in either location without changing any settings. Although if you are running a tensioner as I suspect you probably are that could be a problem.
    That is a fantastic idea, I hadn't ever heard of a dingle. So if I'm understanding this correctly, as long as the number of teeth add up to the same number the chain length will be equal? For example, I could run my current 36/13 on the high end and 33/16 on the low end? I do have a tensioner but it's not really doing much right now because the 36/13 is almost the magic gear. How much chain slack is acceptable?

  11. #11
    The need for singlespeed
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    Quote Originally Posted by merzperson View Post
    So if I'm understanding this correctly, as long as the number of teeth add up to the same number the chain length will be equal?
    It'll be close, but not quite equal.

  12. #12
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    Quote Originally Posted by PAmtbiker View Post
    While there may be no mechanical advantage to running a large ring/cog combo over a small ring/cog combo to achieve the same gear, I believe there are other advantages, and I think some out there may agree with me. I think the difference may be akin to the difference between a 26" wheel and a 29"wheel. Maybe something about the moment of inertia of the larger ring being greater, and thus the larger ring wants to stay spinning more. I used to run a 32x20 (29.2.25 tire, 175 mm cranks), which yeilds a ratio of 1.6 to 1. Now I run 36x22 (same tires and cranks), which yeilds a 1.63 to 1 ratio. It is a tiny bit harder, but I feel that once you get it rolling, it is easier to keep rolling. Kind of like the difference between rolling along on your road bike in the 53t ring as opposed to the 39t ring... Personally, I'm totally sold on the bigger ring/cog thing.
    I'm into the bigger chainrings myself. I run 36x20 on my 26er, which works out well here in my region of CA as long as I keep in decent shape. After a full season on the bike I'm seeing very little wear on the chainrings and KMC SS chain.
    Attached Thumbnails Attached Thumbnails Gear Size and Mechanical Advantage-dsc01520.jpg  


  13. #13
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    Bigger gears will also translate into less tension on the chain which could affect ease of pedalling and does affect chain life.
    Well, it was a good try.

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