Frame stiffness

Discussion in 'Cycling Equipment' started by William, Oct 18, 2003.

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  1. William

    William Guest

    There seem to be several sacred cows on this newsgroup, and today I would like to question the frame
    stiffness one. If I understand it rightly, it says that frames can flex a bit and this is okay
    because the energy that goes into sideways flexing is "returned" on the upstroke. Flexing is only
    bad if it make the chainstays rub on the wheel or the chain fall off.

    But how is the energy returned? Is it returned in a way that makes my bike go forward faster? I
    imagined a thought experiment. Imagine I sit still on my beautifully lugged bicycle and then push
    backwards and forwards on the pedals. The hand-painted bottom bracket moves from side to side and
    the frame doesn't heat up or distort so no energy is being wasted. But my legs are getting tired and
    I'm not going forwards.

    It seems to me that the energy used to push the bottom bracket sideways is returned on the upstroke
    by pushing my leg back the other way, and not in any way by making the bike go forwards and that any
    design that minimises this sideways movement will make me go faster.

    W
     
    Tags:


  2. William <[email protected]> wrote in message
    news:<[email protected]>...
    > There seem to be several sacred cows on this newsgroup, and today I would like to question the
    > frame stiffness one. If I understand it rightly, it says that frames can flex a bit and this is
    > okay because the energy that goes into sideways flexing is "returned" on the upstroke.

    People may be thinking more along the lines of, if there is loss - it is small. I've not seen a
    good explanation of how the energy is returned or how it is wasted. "The frame doesn't heat up"
    doesn't cover it.

    IIRC people have calculated significant flex loss for sprinters on the assumption that the energy
    used to flex the frame is then dissipated.

    > Flexing is only bad if it make the chainstays rub on the wheel or the chain fall off.
    >
    > But how is the energy returned? Is it returned in a way that makes my bike go forward faster? I
    > imagined a thought experiment. Imagine I sit still on my beautifully lugged bicycle and then push
    > backwards and forwards on the pedals. The hand-painted bottom bracket moves from side to side and
    > the frame doesn't heat up or distort so no energy is being wasted. But my legs are getting tired
    > and I'm not going forwards.
    >
    > It seems to me that the energy used to push the bottom bracket sideways is returned on the
    > upstroke by pushing my leg back the other way, and not in any way by making the bike go forwards
    > and that any design that minimises this sideways movement will make me go faster.

    This is what a lot of people believe. British Cycling have moved into frame manufacture at some
    expense and the result is that David Millar pummels the opposition in the world tt. What more proof
    do you need?

    If there is loss, the energy must be going somewhere. Perhaps rider damping is a factor (the frame
    wouldn't heat up).

    For conservative pedalling, the "unwind" of the spring would have to go to the drivetrain by pushing
    a pedal towards an appropriately positioned foot or help in moving leg mass around or go into
    flexing the frame the other way on the next stroke or something...

    Andrew Bradley
     
  3. William wrote:

    > There seem to be several sacred cows on this newsgroup, and today I would like to question the
    > frame stiffness one. If I understand it rightly, it says that frames can flex a bit and this is
    > okay because the energy that goes into sideways flexing is "returned" on the upstroke. Flexing is
    > only bad if it make the chainstays rub on the wheel or the chain fall off.
    >
    > But how is the energy returned? Is it returned in a way that makes my bike go forward faster? I
    > imagined a thought experiment. Imagine I sit still on my beautifully lugged bicycle and then push
    > backwards and forwards on the pedals. The hand-painted bottom bracket moves from side to side and
    > the frame doesn't heat up or distort so no energy is being wasted. But my legs are getting tired
    > and I'm not going forwards.
    >
    > It seems to me that the energy used to push the bottom bracket sideways is returned on the
    > upstroke by pushing my leg back the other way, and not in any way by making the bike go forwards
    > and that any design that minimises this sideways movement will make me go faster.

    It helps push the crank back up the other side. There is a small energy loss depending on the
    material, because frame materials aren't perfect springs. Also, to some extent the tyres will scrub,
    and you'll never be able to get that energy back.

    However, if your gears don't shift themselves and your brakes don't rub, it's nothing to worry
    about. Look at how many TdF stages Sean Kelly won on his Vitus alu bike - that was about 1/3 as
    stiff as steel because they were using "normal" tube diameters. None of this Cannondale
    my-tubes-are-so-fat-I-don't-need-a-front-fender stuff. I don't think the Vitus frames would last
    long in hard use though.
     
  4. Tim McTeague

    Tim McTeague Guest

    Andrew Bradley wrote:
    >
    > This is what a lot of people believe. British Cycling have moved into frame manufacture at some
    > expense and the result is that David Millar pummels the opposition in the world tt. What more
    > proof do you need?

    Has anyone told you that you would make a great research scientist? Didn't think so.

    Tim McTeague
     
  5. Phil Holman

    Phil Holman Guest

    "Tim McTeague" <[email protected]> wrote in message news:[email protected]...
    > Andrew Bradley wrote:
    > >
    > > This is what a lot of people believe. British Cycling have moved
    into
    > > frame manufacture at some expense and the result is that David
    Millar
    > > pummels the opposition in the world tt. What more proof do you need?
    >
    > Has anyone told you that you would make a great research scientist?
    Didn't
    > think so.

    Andrew knows better. I took his statement to be tongue in cheek. We all know the real reason is
    because he has a superior power to drag ratio. Now if only he could work on his power to weight
    ratio........and his endurance......and his bike handling skills .........and his equipment
    ......his teamwork .......his people skills ..... and .....

    Phil Holman
     
  6. Stan Cox

    Stan Cox Guest

    Zog The Undeniable wrote:
    > William wrote:
    >
    ><snip> However, if your gears don't shift themselves and your brakes don't rub, it's nothing to
    >worry about. Look at how many TdF stages Sean Kelly won on his Vitus alu bike - that was about 1/3
    >as stiff as steel because they were using "normal" tube diameters. None of this Cannondale
    >my-tubes-are-so-fat-I-don't-need-a-front-fender stuff. I don't think the Vitus frames would last
    >long in hard use though.
    >
    Those Vitus frames were lovely, Light as well for the time. I used to work for a LBS that sold them
    but was warned off them because I am not light and because after a few years of club use they would
    go excessively 'whippy' and the owner would have to buy another frame. I would have thaough that
    Sean would have been lucky to get a years use out of a frame.

    Stan Cox

    --
    Brighton Excelsior CC Member Cat 3 Fat Bloke Brighton, UK
     
  7. Carl Fogel

    Carl Fogel Guest

    Zog The Undeniable <[email protected]> wrote in message news:<[email protected]>...

    [snip lateral frame flex question]

    > It helps push the crank back up the other side.

    [snip]

    Dear Zog,

    I'm not denying your well-worded explanation, just struggling to follow what seems obvious to
    everyone else. (I've been enjoying your posts all morning.)

    I don't really follow how a lateral flexing of the bicycle helps the upstroke or even results from
    the downstroke. I keep thinking that a sideways flexing must result from the wasted side-to-side
    motion of the rider's--well, let's say his hips. I want to believe, but I need help here.

    But even if I get past how an up-and-down pedal motion produces a sideways wiggle in a stiff metal
    frame (maybe the offset of the pedals from the center of the frame?), I'm still vague about when the
    downstroke flexing of the frame is returned during the upstroke.

    If I shove down on my left pedal and my frame flexes laterally, storing energy--

    (Come to think of it, which way does my rear axle flex with regard to my crank--left or right?)

    --my right pedal is coming up at the same time and supposed to be restoring energy, isn't it?

    How does my laterally flexing frame simultaneously store the energy from my left pedal going down
    while restoring it to my right pedal coming up?

    That is, what's the detailed sequence of store and restore for both pedals during a full cycle and
    where is my stupid frame at each point? I don't even know which way it's flexing.

    I hate being outwitted by these metal tubes.

    Your befuddled but potential worshipper,

    Carl Fogel
     
  8. Pete Biggs

    Pete Biggs Guest

    Andrew Bradley wrote:

    > If there is loss, the energy must be going somewhere. Perhaps rider damping is a factor (the frame
    > wouldn't heat up).

    The loss is the frame going sideways. Energy is being spent flexing the frame instead of driving
    the wheel.

    ~PB
     
  9. Phil Holman

    Phil Holman Guest

    "Carl Fogel" <[email protected]> wrote in message
    news:[email protected]...
    > Zog The Undeniable <[email protected]> wrote in message
    news:<[email protected]>...
    >
    > [snip lateral frame flex question]
    >
    > > It helps push the crank back up the other side.
    >
    > [snip]
    >
    > Dear Zog,
    >
    > I'm not denying your well-worded explanation, just struggling to follow what seems obvious to
    > everyone else. (I've been enjoying your posts all morning.)
    >
    > I don't really follow how a lateral flexing of the bicycle helps the upstroke or even results from
    > the downstroke. I keep thinking that a sideways flexing must result from the wasted side-to-side
    > motion of the rider's--well, let's say his hips. I want to believe, but I need help here.
    >
    > But even if I get past how an up-and-down pedal motion produces a sideways wiggle in a stiff metal
    > frame (maybe the offset of the pedals from the center of the frame?), I'm still vague about when
    > the downstroke flexing of the frame is returned during the upstroke.
    >
    > If I shove down on my left pedal and my frame flexes laterally, storing energy--
    >
    > (Come to think of it, which way does my rear axle flex with regard to my crank--left or right?)
    >
    > --my right pedal is coming up at the same time and supposed to be restoring energy, isn't it?
    >
    > How does my laterally flexing frame simultaneously store the energy from my left pedal going down
    > while restoring it to my right pedal coming up?
    >
    > That is, what's the detailed sequence of store and restore for both pedals during a full cycle and
    > where is my stupid frame at each point? I don't even know which way it's flexing.
    >
    > I hate being outwitted by these metal tubes.
    >
    > Your befuddled but potential worshipper,

    BB lateral flex is caused by the moment offset of the pedal from the neutral axis when applying a
    force normal to the direction of flexing. It is reversible. It is at a maximum when the down stroke
    pedal is horizontal and is a minimum when that pedal is at bottom dead center
    i.e. the return is in the bottom half of the power stroke. The frame material's restitution
    properties will govern the loss which will be in the region of 10%.

    If we use a power output of 300 watts at 90rpm with a BB flex of 5mm, a crude estimate of the losses
    will be *energy input into the frame with each power stroke times the number of power strokes per
    second times the percentage loss*.

    Using Hooke's Law (1/2kx^2), this will be .5k*x^2*3*.10 = .5*360*200*.005^2*3*.10 = .27 watt. IMO
    this loss is tiny compared to the psychological loss of riding a noodly frame that handles poorly.

    Phil Holman
     
  10. root

    root Guest

    In article <[email protected]>, Stan Cox
    <[email protected]> wrote:
    >Zog The Undeniable wrote:
    >> William wrote:
    [Vitus]
    >>
    >Those Vitus frames were lovely, Light as well for the time. I used to work for a LBS that sold them
    >but was warned off them because I am not light and because after a few years of club use they would
    >go excessively 'whippy' and the owner would have to buy another frame.

    That's a popular myth, albeit usually applied to steel.

    Some of them did suffer from the joints seperating. Mostly the carbon-fiber Vituses (Vitii?), not
    the aluminum ones.

    I rode mine for ~7 years, training and racing 5-8k miles/year and never noticed it getting any more
    'whippy' than it was when I started.

    Eric
     
  11. Tim Cain

    Tim Cain Guest

    "Stan Cox" <[email protected]> wrote in message
    news:[email protected]...
    >
    {snip stuff about small dia. Al frames]
    >
    > Those Vitus frames were lovely, Light as well for the time. I used to work for a LBS that sold
    > them but was warned off them because I am not light and because after a few years of club use they
    > would go excessively 'whippy'

    Shouldn't this be stored in the same round metal filing cabinet on the floor as the old canard
    regarding steel frames going "dead"?

    Unless you're *really* suggesting that the elastic modulus of the frame material changed?

    > and the owner would have to buy another frame. I

    Of course they would. Just like the owner of a steel frame that had gone "dead"...

    > would have thaough that Sean would have been lucky to get a years use out of a frame.
    >
    > Stan Cox
    >

    Tim.

    ---
    Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com).
    Version: 6.0.528 / Virus Database: 324 - Release Date: 16/10/03
     
  12. Scott Mann

    Scott Mann Guest

    "William" <[email protected]> wrote in message
    news:[email protected]...
    > There seem to be several sacred cows on this newsgroup, and today I would like to question the
    > frame stiffness one. If I understand it rightly, it says that frames can flex a bit and this is
    > okay because the energy that goes into sideways flexing is "returned" on the upstroke. Flexing is
    > only bad if it make the chainstays rub on the wheel or the chain fall off.

    I found this article by Keith Bontrager, entitled "Stiffness," to be somewhat informative.
    http://www.bontrager.com/keith/rants.asp?id=32
     
  13. Carl Fogel

    Carl Fogel Guest

    "Phil Holman" <[email protected]> wrote in message
    news:<[email protected]>...

    > "Carl Fogel" <[email protected]> wrote in message
    > news:[email protected]...

    > > Zog The Undeniable <[email protected]> wrote in message
    > news:<[email protected]>...
    > >
    > > [snip lateral frame flex question]
    > >
    > > > It helps push the crank back up the other side.
    > >
    > > [snip]
    > >

    [snip all of Fogel's confused question]

    >
    > BB lateral flex is caused by the moment offset of the pedal from the neutral axis when applying a
    > force normal to the direction of flexing. It is reversible. It is at a maximum when the down
    > stroke pedal is horizontal and is a minimum when that pedal is at bottom dead center
    > i.e. the return is in the bottom half of the power stroke.

    [snip]

    > Phil Holman

    Dear Phil,

    If I follow your description, my left pedal stroke flexes the frame sideways for the first
    quarter-turn, reaching maximum with the crank arms horizontal, and then somehow returns the stored
    energy for the second-quarter of the left-hand pedal's downstroke?

    This is exactly what I was hoping for--someone to connect the dots for me and work out what kind of
    energy is actually involved.

    I'm afraid that this is a dumb question, but is the result during the downward quarter of the
    left-pedal stroke:

    a) an increased downward force on the left pedal (seems unlikely)

    b) a reduced resistance on the pedal
    (what you may be implying)

    c) an upward force through the other pedal (maybe a bad interpretation of Zog's brief line: "It
    helps push the crank back up the other side.")

    d) all or some of the above (it's a solid crank from pedal to pedal)

    I take all your calculations on faith and am pleased that this frame-flex works out to very little
    wasted energy. I've asked this before, but either no one was interested, or else no one interested
    understood it as well as you and Zog do.

    Thanks,

    Carl Fogel
     
  14. Mark Hickey

    Mark Hickey Guest

    "Phil Holman" <[email protected]> wrote:

    >Andrew knows better. I took his statement to be tongue in cheek. We all know the real reason is
    >because he has a superior power to drag ratio. Now if only he could work on his power to weight
    >ratio........and his endurance......and his bike handling skills .........and his equipment
    >......his teamwork .......his people skills ..... and .....

    ... add a front derailleur to his time trial bike.

    Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame
     
  15. Phil Holman

    Phil Holman Guest

    "Carl Fogel" <[email protected]> wrote in message
    news:[email protected]...
    > "Phil Holman" <[email protected]> wrote in message
    news:<[email protected]>...
    >
    > > "Carl Fogel" <[email protected]> wrote in message
    > > news:[email protected]...
    >
    > > > Zog The Undeniable <[email protected]> wrote in message
    > > news:<[email protected]>...
    > > >
    > > > [snip lateral frame flex question]
    > > >
    > > > > It helps push the crank back up the other side.
    > > >
    > > > [snip]
    > > >
    >
    > [snip all of Fogel's confused question]
    >
    > >
    > > BB lateral flex is caused by the moment offset of the pedal from the neutral axis when applying
    > > a force normal to the direction of
    flexing.
    > > It is reversible. It is at a maximum when the down stroke pedal is horizontal and is a minimum
    > > when that pedal is at bottom dead center
    > > i.e. the return is in the bottom half of the power stroke.
    >
    > [snip]
    >
    > > Phil Holman
    >
    > Dear Phil,
    >
    > If I follow your description, my left pedal stroke flexes the frame sideways for the first
    > quarter-turn, reaching maximum with the crank arms horizontal, and then somehow returns the stored
    > energy for the second-quarter of the left-hand pedal's downstroke?
    >
    > This is exactly what I was hoping for--someone to connect the dots for me and work out what kind
    > of energy is actually involved.
    >
    > I'm afraid that this is a dumb question, but is the result during the downward quarter of the
    > left-pedal stroke:
    >
    > a) an increased downward force on the left pedal (seems unlikely)
    >
    >b) a reduced resistance on the pedal
    > (what you may be implying)

    No, but what happens is, there is more pedal travel on the top quarter part of the stroke relative
    to rear wheel travel (~1%) according to my numbers. Energy is force times distance so you're storing
    some energy in flexing the frame and not in turning the back wheel up to the half way point of the
    downstroke.

    There is one percent less travel on the lower quarter of the pedal stroke as the deflection
    unwinds and you've exerted a little less energy for the same amount of work on the back wheel as
    the top quarter.

    >
    > c) an upward force through the other pedal (maybe a bad interpretation of Zog's brief line: "It
    > helps push the crank back up the other side.")

    It all ends up pushing the cranks around to provide power to the back wheel. Did you ever have a
    clockwork trainset as a kid. If you didn't hold the wheels stationary they would turn as you wound
    it up. When you stopped winding, the wheels would still turn until the spring ran down. This is a
    very similar situation but with much smaller deflections and energy storage. For a bicycle there is
    also deflection in every part of the drive train and not just the BB. Crank arms and pedal axles
    twist and bend, chains stretch, back wheels wind up torsionally. You can apply the brakes and stand
    on a forward pedal to see how much you can make the pedal move without the back wheel turning. It's
    about 1/4 inch on my regular bike and 1/2 inch on the front crank of my tandem. Even if all this
    energy was lost it would still be small (perhaps 2%). Most of it is returned and the actual loss is
    more like .2%.

    >
    > d) all or some of the above (it's a solid crank from pedal to pedal)

    I think I've covered it.

    >
    > I take all your calculations on faith and am pleased that this frame-flex works out to very little
    > wasted energy.

    My calculations are just good enough to show that the losses are small. In an all out sprint
    deflections are larger and losses greater but the stiffness consideration is probably prevention of
    wheels rubbing the brakes, chains rubbing the derailleur cage and general handling through tight
    bends at speed.

    Phil Holman
     
  16. Jobst Brandt

    Jobst Brandt Guest

    Phil Holman writes:

    about frame flex:

    > It all ends up pushing the cranks around to provide power to the back wheel. Did you ever have a
    > clockwork trainset as a kid. If you didn't hold the wheels stationary they would turn as you wound
    > it up. When you stopped winding, the wheels would still turn until the spring ran down. This is a
    > very similar situation but with much smaller deflections and energy storage.

    I don't think that is a good parallel to cyclic frame deflection. Nothing happens with the frame
    after the pedal stroke where the bicycle is back to its neutral position.

    It's often better to consider the entire stroke of the pedals for a typical revolution. The bicycle
    moves at a reasonably constant speed through several crank revolutions so any variability in any
    part of the stroke is not rejected as a rebound or unused work. The pedals turn at a rate that is as
    constant as the wheel rotation. Side sway of the BB is also not a rebound and absorbs no energy
    since it is working against a metal spring (the frame) where no energy is dissipated.

    > For a bicycle there is also deflection in every part of the drive train and not just the BB. Crank
    > arms and pedal axles twist and bend, chains stretch, back wheels wind up torsionally. You can
    > apply the brakes and stand on a forward pedal to see how much you can make the pedal move without
    > the back wheel turning. It's about
    > 1/4 inch on my regular bike and 1/2 inch on the front crank of my tandem. Even if all this energy
    > was lost it would still be small (perhaps 2%). Most of it is returned and the actual loss is
    > more like .2%.

    I'm not sure where such numbers arise, but I suspect they depend on the rider and many other factors
    but I doubt that 0.2% is a measurable value.

    >> I take all your calculations on faith and am pleased that this frame-flex works out to very
    >> little wasted energy.

    > My calculations are just good enough to show that the losses are small. In an all out sprint
    > deflections are larger and losses greater but the stiffness consideration is probably prevention
    > of wheels rubbing the brakes, chains rubbing the derailleur cage and general handling through
    > tight bends at speed.

    I don't see how you can calculate this, considering that the losses are with the rider, if any, not
    in the machinery of the bicycle.

    Jobst Brandt [email protected]
     
  17. Phil Holman

    Phil Holman Guest

    [email protected] wrote in message news:<[email protected]>...
    > Phil Holman writes:
    >
    > about frame flex:
    >
    > > It all ends up pushing the cranks around to provide power to the back wheel. Did you ever have a
    > > clockwork trainset as a kid. If you didn't hold the wheels stationary they would turn as you
    > > wound it up. When you stopped winding, the wheels would still turn until the spring ran down.
    > > This is a very similar situation but with much smaller deflections and energy storage.
    >
    > I don't think that is a good parallel to cyclic frame deflection. Nothing happens with the frame
    > after the pedal stroke where the bicycle is back to its neutral position.

    It's a stretch but I use it to illustrate that torsional and bending deflections incurred when
    powering a bicycle store energy and just because the drivetrain isn't infinitely stiff doesn't mean
    losses are incurred. The clockwork motor being the antithesis of infinitely stiff.

    >
    > It's often better to consider the entire stroke of the pedals for a typical revolution. The
    > bicycle moves at a reasonably constant speed through several crank revolutions so any variability
    > in any part of the stroke is not rejected as a rebound or unused work. The pedals turn at a rate
    > that is as constant as the wheel rotation. Side sway of the BB is also not a rebound and absorbs
    > no energy since it is working against a metal spring (the frame) where no energy is dissipated.

    Except for a small hysteresis loss.

    >
    > > For a bicycle there is also deflection in every part of the drive train and not just the BB.
    > > Crank arms and pedal axles twist and bend, chains stretch, back wheels wind up torsionally. You
    > > can apply the brakes and stand on a forward pedal to see how much you can make the pedal move
    > > without the back wheel turning. It's about
    > > 1/4 inch on my regular bike and 1/2 inch on the front crank of my tandem. Even if all this
    > > energy was lost it would still be small (perhaps 2%). Most of it is returned and the actual
    > > loss is more like .2%.
    >
    > I'm not sure where such numbers arise, but I suspect they depend on the rider and many other
    > factors but I doubt that 0.2% is a measurable value.

    Exactly, it's so small as to be negligible. The question being answered here is how much more
    usuable power will be available with a stiffer frame. My answer is a negligible amount.

    >
    > >> I take all your calculations on faith and am pleased that this frame-flex works out to very
    > >> little wasted energy.
    >
    > > My calculations are just good enough to show that the losses are small. In an all out sprint
    > > deflections are larger and losses greater but the stiffness consideration is probably prevention
    > > of wheels rubbing the brakes, chains rubbing the derailleur cage and general handling through
    > > tight bends at speed.
    >
    > I don't see how you can calculate this, considering that the losses are with the rider, if any,
    > not in the machinery of the bicycle.

    By what mechanism are you saying the losses are with the rider? I believe my calculations, crude as
    they are, agree that the losses are not significant in the machinery. My estimate is probably fairly
    conservative but good enough to determine a rough order of magnitude. I don't think we need to run a
    finite element model to conclude that frame flex within the limit of inducing chain or wheel rub or
    poor handling will not result in any significant losses of power to the rear wheel.

    Phil Holman
     
  18. Tom Ace

    Tom Ace Guest

    Phil Holman wrote:

    > > I don't see how you can calculate this, considering that the losses are with the rider, if any,
    > > not in the machinery of the bicycle.
    >
    > By what mechanism are you saying the losses are with the rider?

    Phil was responding to Jobst, but to offer an answer--

    The losses can be in your legs. The frame may be close to 100% elastic (no significant hysteretic
    loss) but the energy doesn't necessarily go into driving the bicycle.

    Imagine you're tightening a bolt with a ratchet and socket, and you have a long extension that's not
    stiff torsionally and winds up with each stroke. You won't turn the bolt as many degrees with each
    stroke as you would with a stiff extension. Where does the energy go that was stored in winding up
    the extension?

    Tom Ace
     
  19. Jobst Brandt

    Jobst Brandt Guest

    Tom Ace writes:

    > The losses can be in your legs. The frame may be close to 100% elastic (no significant hysteretic
    > loss) but the energy doesn't necessarily go into driving the bicycle.

    > Imagine you're tightening a bolt with a ratchet and socket, and you have a long extension that's
    > not stiff torsionally and winds up with each stroke. You won't turn the bolt as many degrees with
    > each stroke as you would with a stiff extension. Where does the energy go that was stored in
    > winding up the extension?

    Oops! There is a rebound with that example and one that doesn't advance the bolt. In a bicycle,
    there is no rebound, the foot never retreating from its most advanced position of the pedal
    revolution. This is the point I made earlier.

    Jobst Brandt [email protected]
     
  20. Tom Ace

    Tom Ace Guest

    [email protected] wrote:

    > > Imagine you're tightening a bolt with a ratchet and socket, and you have a long extension that's
    > > not stiff torsionally and winds up with each stroke. You won't turn the bolt as many degrees
    > > with each stroke as you would with a stiff extension. Where does the energy go that was stored
    > > in winding up the extension?
    >
    > Oops! There is a rebound with that example and one that doesn't advance the bolt. In a bicycle,
    > there is no rebound, the foot never retreating from its most advanced position of the pedal
    > revolution. This is the point I made earlier.

    Bear with me please, I missed any earlier post where this was mentioned.

    I'm still not convinced. There isn't the same kind of rebound as in the ratchet example, but there
    is a stroke, and downward force on the pedal has different effects at different parts of the stroke.
    You can't apply much drive torque near the bottom of the stroke. If the frame springs back at that
    point, is the energy returned really going primarily into driving the bicycle?

    Tom Ace
     
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