chainring/cog size and efficiency

Discussion in 'Cycling Equipment' started by Andrew Lee, Jul 6, 2003.

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  1. Andrew Lee

    Andrew Lee Guest

    I was always under the impression that for a given ratio that a larger chainring/cog combination
    would be more efficient assuming all else equal (chainline straight...). Thus a 52/13 would be more
    efficient than a 44/11, though both give ratios of 4:1. The graph showing the efficiency of an XT
    44/32/22 11-32 drivetrain in 15 different gears seems to imply the same.
    http://www.selbst-machen.de/speedhub_vs_xt.gif In this graph, they don't show any duplicate
    ratios, but it is clear that as the chain was shifted onto larger cogs, the efficiency went up
    (looking at the graph on top with the blue, green and yellow dots from left to right). When
    they switch down to the 32 chainring from the 44 or 32 to 22, the efficiency takes a big drop
    and it looks like if they had chosen to duplicate or com close to duplicating the same ratio
    in the 32 as the 44 ring, the efficiency would have been lower in the 32. This all fits in
    with my previous thinking. A study by Spicer of Hopkins University (overview of it at
    http://www.jhu.edu/news_info/news/home99/aug99/bike.html ) seems to confirm this - larger rear
    cogs are more efficient. However, they found that higher chain tensions mean higher
    efficiency... something that they didn't expect and couldn't explain (seems backwards from
    what you would expect with friction losses?). They did not compare different chainring/cog
    size combination of the same ratio that I can tell from the web site link. A later study base
    on Spicer's data found this:

    A googled post from alt.rec.bicycles.recumbent:

    >However, in an article in the fall 2000 "Human Power" called "Efficiency of bicycle chain drives:
    >results at constant velocity and supplied power"
    John
    >and Claire Walton did an analysis comparing chain and sprocket efficiency
    at a
    >constant supplied power and vehicle speed.
    >
    > Using data from the previous Spicer HP article, they found that at constant power and vehicle
    > speed, the efficiencies were 92% for the 11
    tooth,
    >90.5% for the 15 tooth, and 88.5% for the 21 tooth.
    >
    > Another cycling myth (of many) bites the dust!
    >
    > Tailwinds (and nine tooth sprockets) Rich Pinto Bacchetta Bicycles

    This confuses me. The implication is that the higher chain tension from using a smaller chainring
    allows it to be more efficient at a constant power and speed (same gear ratio?) Without actually
    seeing the Walton's paper, it seem like maybe they are pulling something out of Spicer's data that
    is somewhat suspect. It seems to directly conflict with the first graph that I referenced too... but
    I don't know how that graph was made (does it say in the sidebar? I don't read German.) and under
    what conditions... constant power?

    What do you all think of this? The reason that I was thinking about this was because I was comparing
    the benefits of a smaller set of rings for an all around bike... 48/36 12-27 vs. 52/39 13-29 for
    example. I know the actually differences would be miniscule, but it's fun to think about. Lighter
    for the smaller setup, but less efficient (if I am right about larger rings being more efficient). I
    kind of think that if there is any measureable efficiency gain with the larger rings, even 1/2%, it
    would overcome that slight gain in weight. Of course, it would also eliminate the need to find a 110
    bcd double, which are less available nowadays. Why don't cyclocross bikes have gearing like 52/44
    14-29 (availability of 14 starting cogs nowadays?).
     
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  2. Andrew Lee <whatsupandrewathotmaildotcom> wrote:

    ><snip>A study by Spicer of Hopkins University (overview of it at
    >http://www.jhu.edu/news_info/news/home99/aug99/bike.html ) seems to
    confirm
    > this - larger rear cogs are more efficient. However, they found that
    higher
    > chain tensions mean higher efficiency... something that they didn't expect and couldn't explain
    > (seems backwards from what you would expect with friction losses?).

    Confusing web page!

    Speaking of chain tension they show a picture of somebody lifting the chain off the ring with
    fingers. Do they mean "resting" chain tension?

    Assuming that's not what is meant, with the same chainring you can only have a higher chain tension
    _and_ a larger sprocket by increasing the resistance. Combined with lower RPM (to maintain the
    constant power) this could be more efficient, for one thing it would seem there might be less energy
    dissipation in the "unloaded" part of the slower wrapping chain.

    > > Using data from the previous Spicer HP article, they found that at constant power and
    > > vehicle speed, the efficiencies were 92% for the 11
    > tooth,
    > >90.5% for the 15 tooth, and 88.5% for the 21 tooth.
    > >
    > > Another cycling myth (of many) bites the dust!

    > This confuses me.

    Yes it is confusing and they could be missing the point entirely. The "myth" lives on for the
    moment. What they may have found is that big gears make for drivetrain efficiency. But it's surely
    not the lack of teeth on the sprocket that causes the effect. (A bigger chainring might have been
    even more efficient)

    The main consideration in cadence is the rider, not the drivetrain, cadence needs to be a
    constant in the investigation of drivetrain efficiency. If the researchers haven't understood
    that, they've goofed.

    Maybe someone who has read the papers can enlighten us.

    Andrew Bradley
     
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