Gearing question

Discussion in 'Recumbent bicycles' started by Tom Blum, Sep 29, 2003.

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  1. meb

    meb New Member

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    I haven't found the article that you are refering to with Google search. If you can post a link, that might help.

    Here are data from a study concluding just the opposite:


    “ Measurements of Efficiency of Chain and Shaft Drives Section 1.2.2 Contributed by Chris Juden, CTC Technical Officer, [email protected]. The best work I know of on
    this subject was
    commissioned by Fichtel & Sachs AG. Comparisons of single-speed, multi-speed hub and derailleur gearing were published in Radmarkt Nr.12/1983 and I am aware of other
    work within that company which compared the efficiency of chains in
    various states of neglect. Here's a summary of results, percentage efficiency values estimated from the graphs printed in Radmarkt.
    New, clean, lubricated chain drives


    _______1-spd____3-spd Hub Gear____6-spd Derailleur
    Power___________Low 1:1 _High_____24T 19T 13T
    50W____96.0_____90.6 93.4 87.3_____94.2 94.1 92.1
    100W___97.3_____92.8 95.7 90.9_____96.2 96.4 94.9
    200W___98.1_____94.0 96.9 92.9_____97.4 97.6 96.9
    400W___99.0_____95.0 97.9 93.9_____98.1 98.4 97.8


    Used chain (8000km), no rust, lubricated 100W 94-96%, 200W 97-98%
    Neglected used chain (7000km), rusty, dry 100W 88%, 200W 93%
    We can draw some interesting conclusions from these data. They confirm that hub gears are a little less efficient than derailleur, even in direct drive, and show that they
    work better in low than high ratio. With a derailleur: running the chain around the little pulleys takes only about 1W. And a misaligned chain is much less wasteful than small
    sizes of the sprocket, especially at low power levels. Indeed: at 50W the out of line 24T does a bit better than the in line 19T! 13T is not even very small by
    today's standards, but is clearly rather inefficient. At very high power levels however, alignment may become as important as size.”
     


  2. meb

    meb New Member

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    The biggest source of friction losses on the chain are a result of the links pivoting about their pins relative to the adjacent link(s). The more the link pivots, the more the frictional loss. The chain will be pivoting about the link when traveling around the chainring, sprockets, derailleur jockey wheels, and any pulleys tensioners or idlers on the chain.

    On an 11T sprocket, a link pivots a great deal to navigate the sprocket turn.
    On a 48T chainring, the link pivoting very little to circle the chainring.
    On a 38T chainring, the links pivots slightly more, but it is still a relatively small amount, therefor losses are small so the efficiency of a 38T chainring is only slightly smaller than the 48T ring.
    However, for an equal gear ratio, a 38T chainring would be associated with a smaller sprocket.

    A 48T/14T chainring/sprocket combo would have a similar gear ratio to 38T/11T combo.

    The pivoting angles are much larger for these associated sprockets than the chainrings, hence more friction lost on the sprocket than the chainring. The chainlink is pivoting more about the 11T sprocket than the 14T sprocket, therefor more losses on the 11T sprocket.

    The second big loss in derailleur based chain systems, is from a lateral twisting of chains when the chainring and sprocket are not aligned. When you are on say a middle sprocket and middle triple chainring, you generally have the sprockets and chainring aligned, so minimal losses here. When you have a small sprocket selected and say a middle chainring, you have the chain twisting laterally between the chainring and sprocket and are introducing an extra 1-2 percent loss from the lateral twist in addition to the 2-3 percent loss you have from the smaller sprocket size.

    The derailleur jockey wheels are generally seeing greater pivots than on the sprockets, but since they are located on the low tension side of the chainloop, the friction is relatively small-frictional loss is proportional to the tension on the chain when it pivots.

    Often, bents have a lot of pulleys re-routing chains so there are more losses hear, particularly if on the high tension portion of the chain loop.

    Rider can obtain best efficiencies when riding by selecting gears such that the chainring selected and sprocket are as best aligned as possible. Keeping a ratio where your legs are more efficient usually is primary to selecting a higher chain drive efficiency.

    The mechanic could increase drivetrain efficiency by proportionately increasing sprocket and chainring size. The downside, more weight, cost (check out chainring prices for 54-104T chainrings) and slightly higher aerodynamic drag (the net should generally be low since the chainring is usually pushing air in front of some other component that would have otherwise needed to break wind-particularly rider (bents) or sprockets(uprights)).
    Remember, heavier cogsets increse rolling resistance and add weight you need to take uphill.
     
  3. John Foltz

    John Foltz Guest

    >> 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.
    >>
    Rich was referring to this? http://www.jhu.edu/news_info/news/home99/aug99/bike.html

    It's interesting in that, if this was the original research, someone came along a year later and
    reached the opposite conclusion using the same data?

    Also interesting, the study showed that lube was not a factor in efficiency. Chain tension was, but
    you can't pre-tension the chain in a derailleur system; so any tension must come from the
    application of power by the cyclist. (IOW, higher efficiencies only happen under high-load
    conditions.)
    --

    John Foltz --- O _ Baron --- _O _ V-Rex 24 --- _\\/\-%)
    _________(_)`=()___________________(_)= (_)_____
     
  4. On Wed, 01 Oct 2003 21:57:29 -0400, John Foltz <[email protected]> wrote:
    >>> 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.
    > >>
    >Rich was referring to this? http://www.jhu.edu/news_info/news/home99/aug99/bike.html
    >
    >It's interesting in that, if this was the original research, someone came along a year later and
    >reached the opposite conclusion using the same data?

    That's the original experiment, yes. Here's Rich Pinto's explanation about the revised conclusion,
    from the same thread I mentioned:

    => The Spicer test was set up with a single 52 tooth chainring in the front, and had equal power
    => and RPM applied to it, regardless of the rear cog chosen. This led to the Spicer studies
    => observation that the largest rear wheel sprocket was most efficient.
    =>
    => That conclusion is correct, but the real question (TM, thanks to Dave L) is what is the effect
    => of rear cog and front chainring changes at constant vehicle power and speed? BTW, this Real
    => Question never occurred to me until the Walton followup!
    =>
    => Here is a quote from the Walton article "When appied power, crank RPM, and chainring size are
    => held constant, the velocity of the vehicle and the force applied to the rear wheel must vary.
    => Since the same work and chainring RPM are producing different velocities, a different force
    => must be reacting against the wheel" ...
    =>
    => "Similarly, in figure 3 (Spicer article) the chain tension is kept constant. At constant
    => tension, larger sprockets are more efficient, but they would be delivering more torque" (to
    => the rear wheel)
    =>
    => Using only the measurement data generated by Spicer for his analysis, Walton concluded with
    => this..."The surprising and counterintuative result is that the smaller sprockets have greater
    => estimated chain efficiency at constant vehicle velocity and applied power than the larger
    => sprockets" and I think this may be the simple answer to your question Ken ..."Therefore, the
    => increased efficiency from the higher chain tension is more important than the loss of
    => fficiency from having the smaller sprocket."
    =>
    => It was Spicers conclusion that much of the transmission power loss was not converted to
    => heat, but was converted into vibration instead. The higher tension chains showed less loss
    => from vibration, and those gains in efficiency were more important than loss from using
    => smaller sprockets.

    Ken Kobayashi [email protected] http://solarwww.mtk.nao.ac.jp/kobayashi/personal/
     
  5. On Wed, 01 Oct 2003 16:51:14 GMT, "Tom Blum" <[email protected]> wrote:

    >That's just the kind of scientific support I was seeking for my percieved sensations.

    Actually it doesn't support your sensations, I'm sorry. The study found that _smaller_ gears (small
    chainring and cog) are more efficient than large gears (large chainring and larger cog), if the
    vehicle speed, power input and gear ratio are the same. If I read your message correctly, it's the
    opposite of your experience.

    I wonder if there is another factor not considered in the above study, namely frame flex. Presumably
    the frame flexes from side to side as you pedal, robbing some power. The amount of flex should be
    proportional to chain tension, which is higher with smaller gears (assuming same cadence and power
    input). This effect would not have been measured by Spicer's experiment - he used a very sturdy
    workbench instead of a real bike frame, and an electric motor which produced a smooth rotation.

    Ken Kobayashi [email protected] http://solarwww.mtk.nao.ac.jp/kobayashi/personal/
     
  6. John Foltz wrote:

    > It's interesting in that, if this was the original research, someone came along a year later and
    > reached the opposite conclusion using the same data?

    One Stuart Burgess, at Bristol University, also concluded that bigger was better following a study
    in 1997/8. One wonders whose experiments most closely duplicated the Real World(tm)...

    Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
    ===========================================================
    Editor - British Human Power Club Newsletter
    http://www.bhpc.org.uk/
    ===========================================================
     
  7. Tom Blum

    Tom Blum Guest

    Ken Kovayashi said: "Actually it doesn't support your sensations, I'm sorry. The study found that
    _smaller_ gears (small chainring and cog) are more efficient than large gears (large chainring and
    larger cog), if the vehicle speed, power input and gear ratio are the same"

    It might still be true. I was pedaling with what I call constant force. Therefore, the chain tension
    would be constant, so the first data would apply.

    At any rate, I'm beginning to believe it's mostly physological.

    I will continue to work on efficient spinning in my quest for improvement.

    --
    Miles of Smiles,

    Tom Blum Winter Haven, Florida Homebuilts: SWB Tour Easy Clone Speed Machine Clone High Racer Clone
    www.gate.net/~teblum
     
  8. meb

    meb New Member

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    Does anybody know of a link for the Walton article. I still get zero hits on the stated search

    Maybe if we could see the article, we could see if some other variable was in play – such as chainring/sprocket not aligned or frame flex, or maybe he is not keeping gear ratios and or cadence constant. The quote posted stating:

    “Here is a quote from the Walton article "When applied power, crank RPM, and chainring size are held constant, the velocity of the vehicle and the force applied to the rear wheel must vary. Since the same work and chainring RPM are producing different velocities, a different force must be reacting against the wheel"

    suggests the passage in Walton refers to a test in which gear ratios were not held constant-else velocity would not be different.

    The Spicer article doesn’t support the proposition that smaller is more efficient for a fixed gear ratio-merely that bigger is no more efficient than smaller.

    For a given gear ratio, chain tension reduced to 38/48 by going from the big chainring to smaller chainring would offset by an increase in chainlink bends by 48/38 more at the sprocket and chainring-an offset.
     
  9. meb

    meb New Member

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    Tom,

    Found some more data while looking to see if this elusive Walton et al. article was online.
    This Chester Kyle & Frank Berto study test agrees with the earlier work published by Juden and the observation Spicer that smaller sprockets are less efficient.
    It can be seen at: http://www.ihpva.org/pubs/HP52.pdf

    Has some great collection of data points comparing derailleur and gearhub efficiency

    Shimano
    Ultegra Mtbk. Cassette

    Chain
    Ring


    80W… 34……30……26……23……20……18……16……14……12
    22……93.1………….92.8………….92.6………….91.7………….90.7
    32……89.4………….90.0………….89.5………….90.9………….86.9
    44……92.1………….91.0………….94.4………….93.8………….91.1

    150W..34…… 30……26……23……20……18……16……14……12
    22……94.6………….94.6………….94.5………….93.8………….91.8
    32……92.9………….92.5………….93.0………….93.0………….91.0
    44……93.8………….93.6………….95.0………….94.8………….93.3

    200W..34…….30……26……23……20……18……16……14……12
    22……95.0………….94.5………….94.2………….93.9………….91.9
    32……93.6………….93.1………….93.6………….93.8………….91.4
    44……94.2………….93.9………….95.9………….97.1………….93.7


    Kyle et al. supports the quantitative observations of Juden and the qualitative disclosure of Spicer that increasing chain tension (more power) increases efficiency.

    However, as one can readily see, the higher tension does not usually offset one-to-one the lower efficiency of a smaller sprocket.

    Both the Juden and Kyle et al. data show increased efficiency with respect to chain tension is sublinear.

    A smaller chainring results in greater chain tension for a given power at constant crank rpm, resulting in a higher efficiency on the chain tension component of the efficiency equation.

    Since the efficiency increase is sublinear, large cog vs. the small-cog efficiency advantages will differ with differing power. More power will favor the large cog/large sprocket combination, less power will favor the small cog/small chainring approach.

    Using the data with combination of closest gear ratios most similar chainline twist, you would have near identical velocities for the fixed cadence of the test, and minimize the difference in the chainline chain twist:

    ……………………….…44/34..vs..22/16………44/26.vs..22/12
    Gear”26”X1.5……..33.3……….35.4…………43.6.………47.2
    chainline
    twist(cm)…..……….22.4…..….13.7………….13.7…....22.4
    efficiency
    80W…………………....92.1……..91.7………….93.0….…..90.7
    150W………………..…93.9…..…93.8………….93.6…..…..91.8
    200W………………..…94.2……..93.9………….93.9…..…..91.9

    This Kyle et al. data shows the chainline twist is more significant than sprocket size, particularly since the frictional losses at the sprocket are partially offset by higher efficiencies due high tension as noted by both Kyle et al. and Spicer.

    It still shows bigger sprockets and chainrings are better unless you go to extremes with the more dominant issue of chainline twist. If you have some idlers proximate the drive side of the chainrings or sprockets, a big enough decrease increase in efficiency could result from the chainline twist.

    All these tests were done under constant cadence. Under constant speed conditions you would reduce cadence inversely proportionate to the gear ratio increase.
    If maintaining constant speed, you could reduce cadence when switching to a smaller sprocket and since power is torque X rpm, for a given power, the lower cadence speed would result in higher torque for the same power with the result of higher chain tension and increased efficiency.

    Under specific conditions-due the sublinear effect of higher tension on efficiency, with constant low-power constant-speed conditions, you could get higher chaindrive efficiencies with smaller sprockets than larger sprockets, and with enough extra chainline twist you could find higher efficiencies at elevated power levels on a lower sprocket.

    The Browning data likewise is consistent:
    On the data points with with minimal gear ratio differences and low chainline chain-twist we see:

    Browning 12 speed


    Chain Sprockets
    ring

    Ratios……..32…….23………17………12
    (26X1.95)
    30…………24.1…..33.6.….45.4.……64.4
    38…………30.6…..42.5.….57.6.……81.5
    48…………38.6…..53.7.….72.7……103.0

    80W……….32…….23……..17…...…..12
    30………….91.1…..91.3…..91.9…….90.9
    38………….92.5…..92.5…..90.7…….89.8
    48………….91.6…..91.2…..91.1…….88.8

    150W………32……..23……..17……....12
    30………….93.8……92.5…..92.9……92.2
    38………….93.9……93.3…..93.5…...93.4
    48………….94.5……93.8…..93.7…...91.8

    200W……….32……. 23……..17…..…..12
    30…………..92.7…..92.8…..92.7……93.4
    38…………..95.2…..94.3…..94.4……93.2
    48…………..94.2…..94.0…..94.1……93.5

    Closest ratios:
    ……………………..…..48/23…..…38/17…..48/17….38/12
    Gear”
    (26”X……..………..….53.7………57.6….…72.7…..…81.5
    1.95)
    chainline
    twist cm…………….….10……...….0………….5…………5

    efficiency
    80W………………….…..91.2…….90.7………91.1…….88.8
    150W………………….…93.8…….93.5………93.7……..91.8
    200W………………….…94.0…….94.4………94.1……..93.5
     
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