Gear inch = gear inch
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  1. #1
    iks
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    Gear inch = gear inch

    I printed out a table of gear inches to compare the ranges of a double and a triple using the same rear cog set. My question is: does the same gear inch number equate to the same speed at the same revolutions (cadence) and the same climbing effort ot efficiency even if the pairing are different: for example a 30x20 = 40.50 = 39x26.

    What I am really atfer is to find out whether I can manage a double on certain hills or whether I will need a triple. I tend to stand on the pedals rather than spin up hills. I don't like to peddle furiously while going slow up a hill. The ten-year old in me remembers standing up on those three speeds.

  2. #2
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    Yes.

    If the ratio is the same the speed is the same at the same cadence.
    We have nothing to lube but our chains.

  3. #3
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    With short hills, almost anything works. With long hills, you can't stand the whole way. If you're going up long hills and you think you might need the triple, you probably do.

    And yes, a 30x20 is the same as a 39x26.

  4. #4
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    skip gear inches...

    Just use the formula to find the equivalent gear ratio. For example, to find the equivalent of a 30/21 with a 39T chainring: 21/30 x 39= 27.3. This tells you that a 39/27 is just a bit higher than a 30/21.

    Using the little ring does not means that you'll be pedaling furiously while seated. All it does is add three lower ratios. If you have a shimano 12-27 for example, the only lower ratios that are useable are the 30/21, 30/24 and 30/27. You can certainly use the smaller cogs with the little ring, up to the 14T. They just duplicate ratios that are also available with the 39T middle ring. Do get a 39T middle ring, not a 42.

    The other thing to keep in mind is that the middle ring and largest cog is really not a useable combination with a triple. The middle ring on a triple is in about the same loccation as the big ring on a double. This makes the chain line too extreme, except for brief use.

    If a 39/27 low gear is not enough, then consider a compact. A 52/36 will add one lower ratio.
    Last edited by C-40; 08-15-2006 at 01:50 PM.

  5. #5
    iks
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    Makes sense. I have also been puzzling over what a compact is.

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  7. #7
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    A normal crank usually comes with a 52 or 53 tooth outer ring and a 39 tooth inner ring.

    A compact crank usually comes with a 50 tooth outer ring and a 34 tooth inner ring.

    The rings are not interchangable because the hole spacing is different.
    Before you criticize someone walk a mile in their shoes. That way when you criticize them you'll be a mile away & you'll have their shoes.

  8. #8
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    compact..

    The most common compact is a 50/34. It creates a 13% lower gear with the same largest cog (34/39 = .87). You also lose most of your top gear. 12/50 x 53 = 12.72. This tells you that a 50/12 is only a slightly larger gear than a 53/13. With a 27T low gear, the lowest ratio is like a 39/31, It's probably a better option than a triple for hill riding.

    The downside is that the wide spacing of a 50/34 or 52/36 creates the need to shift 1 more cog every time a shift is made between the chainrings. A 50/36 avoids that extra cog shifting, but you lose one top gear in exchange for one on the low end.

    Campy covers this situation a bit better by offering a 13-29 cassette.

  9. #9
    iks
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    slow ones

    For the slow ones: when you use the formula 21/30 x 39 = 27.3 does the 21/30 refer to the cassette and the 39 refer to the lower ring of a double?

  10. #10
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    Just divide the number of teeth on the chain ring (front) by the number of teeth on the cog (rear). The lower the number, the easier the pedaling and the slower the speed at the same cadence.

    Some people like to multiply the result of this division by the diameter of the wheel in inches (to get gear inches), but if you're always dealing with 27-inch wheels, this last multiplication doesn't help your understanding any.

  11. #11
    iks
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    A related question: of two gear ratios that are the same does it make a difference in terms of enegry required whether the front chainring is larger and the cog smaller or vice versa?

  12. #12
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    If the ratio is the same, choose the one that gives the straightest chain line (i.e., the one where the cog and the ring are most nearly in the same plane).

    A small, perhaps miniscule, consideration is to choose the larger ring and cog. This bends the chain more gently and gives you a (probably insiginificant) increase in efficiency.

    Note that in your question, "vice versa" does not apply. If two gear ratios are the same, then both the ring and the cog will be bigger or smaller together. You won't have one larger and one smaller and vice versa.

  13. #13
    iks
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    Thanks for your answers John. If the same drivetrain could produce duplicate ratios or gear-inches (e.g. a pairing of 40 in the front and 10 in the rear produces a ratio of 4, and 32 and 8 also produces a ratio of 4) would the first pairing produces any more power at the rear hub (Archimedes lever?). Of is this nonsense because the crank length and ratios are the same. See what I am getting at? Maybe it also never happens.

  14. #14
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    It's nonsense. The power is the same. if you need more power, stand up or pedal harder.

  15. #15
    iks
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    Thanks. The circle on gearing has closed!

  16. #16
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    Quote Originally Posted by C-40
    The downside is that the wide spacing of a 50/34 or 52/36 creates the need to shift 1 more cog every time a shift is made between the chainrings.

    "The need" ???? In order to achieve what aim?

  17. #17
    NeoRetroGrouch
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    Quote Originally Posted by Bob Ross
    "The need" ???? In order to achieve what aim?
    To end up with the same difference gear ratios as you would get from shifdting a 53/39. - TF
    "Don't those guys know they're old?!!"
    Me, off the back, at my first 50+ road race.

  18. #18
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    uniform gearing progression...

    Quote Originally Posted by Bob Ross
    "The need" ???? In order to achieve what aim?
    If you've ever plotted a uniform gearng progression, you would know that the shift between a 53 and 39 usually requires a 2-cog shift. For example, if you're in the big ring and the road gets steeper, it's time to shift to the little ring when the 53/23 is not low enough. The next lowest ratio is a 39/19, a two cog shift. In this same situation with a 50/34, you need to shift one more cog to the 34/17 to get the next lowest gear (a 3-cog shift. No matter what the situation is, the larger jump between the chainrings requires one more cog to be shifted after changing from one ring to the other.

  19. #19
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    Quote Originally Posted by C-40
    If you've ever plotted a uniform gearng progression, you would know that the shift between a 53 and 39 usually requires a 2-cog shift. For example, if you're in the big ring and the road gets steeper, it's time to shift to the little ring when the 53/23 is not low enough. The next lowest ratio is a 39/19, a two cog shift. In this same situation with a 50/34, you need to shift one more cog to the 34/17 to get the next lowest gear (a 3-cog shift. No matter what the situation is, the larger jump between the chainrings requires one more cog to be shifted after changing from one ring to the other.

    I'm not sure I understand what this is achieving. (Or, perhaps I just don't see a significant advantage to a "uniform gearing progression" ...versus, um, I dunno, just riding the bike?)

    Seems to me if you're in the big ring and the road gets steeper, it's time to shift to the little ring when the 53/23 is not low enough...now you're in the 39/23, badda-boom, badda-bing, done.

    Oh, it's a really steep hill & you want even easier gearing? Downshift one cog; now it's easier.

    "Uniform"? That's what cops and cheerleaders wear.

  20. #20
    NeoRetroGrouch
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    Quote Originally Posted by Bob Ross
    I'm not sure I understand what this is achieving. (Or, perhaps I just don't see a significant advantage to a "uniform gearing progression" ...versus, um, I dunno, just riding the bike?)

    Seems to me if you're in the big ring and the road gets steeper, it's time to shift to the little ring when the 53/23 is not low enough...now you're in the 39/23, badda-boom, badda-bing, done.

    Oh, it's a really steep hill & you want even easier gearing? Downshift one cog; now it's easier.

    "Uniform"? That's what cops and cheerleaders wear.
    On the right terrain, I shift rings often.

    One case: When I'm coming to a hill that is going to need the small ring, I usually downshift just before I get there or just as it starts to go up. This prevents shifting under power which can give problems. Since I'm not on the hill yet, I want the same ratio (or just a little easier) that I am in on the big ring. With a 39/53, I drop it to the small ring and give it two clicks at the rear and I'm fine. With the wider spread of the compact, I would need 3 clicks. No big deal once in a while, but when going all out trying to hang with a group on a series of rollers,...

    TF
    "Don't those guys know they're old?!!"
    Me, off the back, at my first 50+ road race.

  21. #21
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    Quote Originally Posted by TurboTurtle
    With a 39/53, I drop it to the small ring and give it two clicks at the rear and I'm fine. With the wider spread of the compact, I would need 3 clicks. No big deal once in a while, but when going all out trying to hang with a group on a series of rollers,...
    Ah, okay, I get it. I tend to do that sort of thing instinctively...never occurred to me that those 2 vs 3 clicks might make the difference between hanging with the kids & getting dropped. Thanks.

  22. #22
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    Quote Originally Posted by Bob Ross
    Ah, okay, I get it. I tend to do that sort of thing instinctively...never occurred to me that those 2 vs 3 clicks might make the difference between hanging with the kids & getting dropped. Thanks.
    Most of those "kids" are over 50. Old does not mean slow. - TF
    "Don't those guys know they're old?!!"
    Me, off the back, at my first 50+ road race.

  23. #23
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    Quote Originally Posted by TurboTurtle
    Old does not mean slow.
    I hope not, I'm 45!

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