Does Stiffness Matter?

Discussion in 'General Cycling Discussions' started by bpsmith, 5 Feb 2018.

  1. EatSleepRideRepeat

    EatSleepRideRepeat AKA Martin from Wales

    Location:
    West Wales
    A lot of youtube chatter, and attempted technical explanation. No amount of talk will convince me that you can load energy into an object (frame) and get the same amount out. Maths and science says other wise. You will lose power with a flexy frame, part of the loss will be dispersed in heat.
     
    Drago, FishFright and 400bhp like this.
  2. 400bhp

    400bhp Guru

    Pretty much this. A couple of other points:
    I wonder if the mounting of the bike on a static trainer means the effect is over egged? The trainer effectively channels the force the only way out it knows, which is kinetic energy through the back wheel. In the real world, some of that energy could actually be absorbed through you alternate leg for example?

    As per later on in the thread, there will be some loss through kinetic energy in the sideways movement. Think of Newton's cradle.

    In my head I think where there may be an advantage in a flex frame is on rougher surfaces, where some dampening effect might be of benefit.
     
  3. Globalti

    Globalti Legendary Member

    A bike frame is not designed to store then release energy like a prosthetic running blade or a competition snow ski, both of which are designed to bounce you back into the next movement in the sequence.
     
  4. andrew_s

    andrew_s Veteran

    Location:
    Gloucester
    You haven't done 26% more climbing on bike 1.
    The averages are per ride, which is fairly meaningless in this comparison.

    Both bike 1 and bike 2 are 52 feet per mile, so climbing will have had minimal impact on the difference in speed.
     
    bpsmith likes this.
  5. Alan O

    Alan O Über Member

    Location:
    Liverpool
  6. OP
    OP
    bpsmith

    bpsmith Veteran

    Absolutely. I was looking at the average elevation per ride, and neglected the fact that the the rides were shorter on Bike 2.

    Just spotted that in work break and came back to post it up. You beat me to it.

    So the stats work out very nicely indeed. For 2017, I appear to have ridden 2 different style bikes of similar weight, but different characteristics including stiffness and aerodynamics.

    The stats show that, at my average speed, there appears to be very little difference in speed whatsoever.

    Very interesting indeed.
     
  7. OP
    OP
    bpsmith

    bpsmith Veteran

    Actually just mentioned this to a cycling mate and he asked how the figures stack up if I ordered them in top average speed order and removed the group rides.

    Of the top 30 fastest rides, only 5 were on Bike 2 and only 1 was a group ride. The top 10 fastest rides were all on Bike 1.

    Perhaps there is something in the Stiffness/Aero frame argument?
     
  8. Smokin Joe

    Smokin Joe Legendary Member

    It's not how stiff the frame is that matters, but how stiff the legs are.
     
  9. Car manufacturers fit flexible driveshafts because they transmit power more efficiently. Not
     
    bpsmith likes this.
  10. nickyboy

    nickyboy Norven Mankey

    Perhaps one should consider the physics of a flexing as opposed to stiff bike.

    If the flexing is lateral and there are no heat losses in the flexing process, there is no meaningful energy loss compared to a 100% stiff bike with no lateral flexing

    Where you do get a big difference is flexing resulting in vertical movement of the rider. That's why full suss MBs are so inefficient. The rider tends to bounce up and down and that definitely is a major waste of energy in terms of conversion into forward movement

    But lateral flexing...I don't think it does anything meaningful
     
  11. There are heat losses in the flexing process, although the numbers will be infintissimal.
     
  12. I like Skol

    I like Skol Hold my beer and watch this..........

    Location:
    Ashton-under-Lyne
    What do you think a dual mass flywheel is? And what about the doughnut in the discovery rear propshaft.......
     
    Drago likes this.
  13. DMF is to protect engine electrics from excessive vibration. The old rubber doughnuts etc were cheaper than a proper UJ.

    These items exists not at all in the interest of transmitting power efficiently, and in both cases actually detract from efficiency in favour of other considerations.
     
  14. Alan O

    Alan O Über Member

    Location:
    Liverpool
    I don't think anyone is suggesting that a bicycle's drivetrain should be flexible.
     
  15. I don't recall suggesting anyone did.
     
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