frame flex and power transfer...

neeb
neeb Posts: 4,470
edited September 2008 in Workshop
A few weeks ago I brought up this thorny issue in another thread, and the consensus seemed to be that with a flexy frame power is lost, so stiff=faster.

I find this subject very interesting, as I know than my 725 steel frame feels a little flexier than my Ti and Alu ones (the Ti one is almost as comfortable though!). Anyway, today I stumbled across this, which if it is to be believed, suggests that power loss through flex isn't much of an issue:

http://www.bikethink.com/Frameflex.htm

Opinions anyone? I'm no expert and don't have any axes to grind either way, I'm just interested!

Comments

  • neeb wrote:
    A few weeks ago I brought up this thorny issue in another thread, and the consensus seemed to be that with a flexy frame power is lost, so stiff=faster!

    Stiff in which plane, though? Left-to-right flex would mean power loss, as this torsion is highly likely to result from pedalling. A little up-and-down flex in the rear stays is no bad thing, though, as it soaks up some of the lumps and bumps of the road surface and is unlikely to result in wasted pedalling effort.

    David
    "It is not enough merely to win; others must lose." - Gore Vidal
  • neeb
    neeb Posts: 4,470
    If the article is correct it is the lateral flex (horizontal torque) that is the biggest component though (this is what advertisers of stiff frames are always saying they are trying to reduce), and the force causing this is returned in the bottom part of the pedal stroke.

    I think what I find particularly interesting about this is that if you take it on face value, there might not be much difference in speed over a given distance between a stiff frame and a more flexible one (although the stiff frame will maybe be more responsive), but maybe there will be a different optimal pedaling technique for each? I sometimes think that it takes time to "adapt" to a new bike frame, maybe you intuitively sense the dynamics of the frame through the pedals and subtly adjust some aspect of pedaling technique to suit. That's how it sometimes feels to me at least.
  • How much do you weigh?

    High-end Reynolds steel frames have recommended rider weights when racing. I think for 531c it was 150lb, 130lb for 753r (record) and 200lb for the lowest 501.
    MTB
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  • balthazar
    balthazar Posts: 1,565
    There is more on the subject here:

    http://www.sheldonbrown.com/rinard/rina ... etest.html

    These tests are well known and somewhat old, but may add more grist to your discussions.
  • dennisn
    dennisn Posts: 10,601
    neeb wrote:
    A few weeks ago I brought up this thorny issue in another thread, and the consensus seemed to be that with a flexy frame power is lost, so stiff=faster.

    I find this subject very interesting, as I know than my 725 steel frame feels a little flexier than my Ti and Alu ones (the Ti one is almost as comfortable though!). Anyway, today I stumbled across this, which if it is to be believed, suggests that power loss through flex isn't much of an issue:

    http://www.bikethink.com/Frameflex.htm

    Opinions anyone? I'm no expert and don't have any axes to grind either way, I'm just interested!

    I'm with you. Very interesting. Mostly because someone is challenging the so-called
    conventional wisdom that frame makers keep throwing out there that "stiffer is better".
    This is a good thing, to say the least, because if something gets repeated enough(such as
    stiffer is better) it eventually becomes "fact" for many people even though it may be
    completely off the mark. Not that frame makers are lying to us. They may think it's true and
    can "prove" it just like this guy can "prove" it.

    Dennis Noward
  • aracer
    aracer Posts: 1,649
    The trouble with that analysis is that there an awful lot of (unstated) assumptions. The very fact they're unstated makes me dubious for a start about whether he actually fully understands the situation. The main ones are that the frame is perfectly elastic so returns all the energy that was put into flexing it (it isn't) end that no energy is lost through eccentric contraction of the rider's muscles - strangely he notes at the top how you'll get tired without doing any work, but doesn't then explore this any further.

    In reality I don't actually believe frame stiffness makes that much difference, but that's a long, long way from proving it.
    otherself wrote:
    High-end Reynolds steel frames have recommended rider weights when racing. I think for 531c it was 150lb, 130lb for 753r (record) and 200lb for the lowest 501.
    Are you really telling me that all those guys heavier than 9st 4lb shouldn't have been on 753 frames? Not really that big a market for those then :shock:
  • neeb
    neeb Posts: 4,470
    and that no energy is lost through eccentric contraction of the rider's muscles
    I think that's what I had in mind (without previously being familiar with the terminology) when I said that perhaps there is a different optimal pedaling technique for a more flexible frame. To avoid energy loss through eccentric contraction, maybe you learn to distribute muscular force differently during the course of the pedal stroke?
    How much do you weigh?
    60-65kg. It's a very subtle thing, but even at my weight pedaling feels a little different with different frames if I really push hard. It seems that you can almost feel the "spring" in a more flexible frame and learn to work with it rather than against it. I still prefer the feel of a stiffer frame if I am riding fast though.
  • I don't buy it: frame flex is the same action as a spring. your chain pulls the frame which deforms and stores energy, which is then released via the chain when it unflexes.

    Energy is conserved, so show me where the energy due to frame flex goes other than through the wheel, and I will believe it. Unless the bike frame gets hot, I can''t understand why frame flex is detremental.