Weird rolling resistance

Discussion in 'Cycling Archive' started by Nick Maclaren, Sep 11, 2015.

  1. I found that my bicycle could roll down a 1:50 slope on smooth
    concrete brick but not on smoothish tarmac. This was at c. 1.5m/s.
    The bizarre thing is that means the resistive force was c. 2.1
    newtons, and I was losing 32 watts. But, if I scale that up to
    my optimal condition speed of 7 m/s, I get 150 watts, which is
    ridiculous, because I am pretty sure that I am losing 40-50 watts
    of rolling resistance and 75-85 watts of air resistance at that
    speed (16 MPH, fully upright, working clothes). And I know that
    my effective aerobic capacity is only 125 watts, based on lots
    of different experiments.

    Or perhaps rolling resistance is NOT linear in speed, after all!


    Regards,
    Nick Maclaren.
     
    Nick Maclaren, Sep 11, 2015
    #1
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  2. Nick Maclaren

    soup Guest

    Rolling resistance is NOT speed related at all it being given by the
    coefficient of sliding friction X the normal force exerted by an
    object.

    At speed you need to account for skin friction, form drag et al as well
    as rolling resistance.

    Total drag is related to the square of the speed so...
     
    soup, Sep 11, 2015
    #2
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  3. Er, that's dragging resistance. Rolling resistance of a pneumatic
    tyre is due to losses in the tread and sidewalls, to cope with the
    'flat patch' moving round the tyre, and the losses in the bearings
    and drive system. Both are proportional to speed. See Bicycling
    Science.

    But the aspect that seems to be ignored in most research is the
    vibrational loss from the road surface, which dominates the others
    in my experience. And that experiment makes me wonder whether it
    is more-or-less a constant with speed. If so, that would explain
    a lot - not least how racing cyclists can manage with narrow tyres
    without losing a catastrophic amount of energy over poor surfaces.


    Regards,
    Nick Maclaren.
     
    Nick Maclaren, Sep 12, 2015
    #3
  4. Nick Maclaren

    soup Guest


    Perhaps a difference in nomenclature but

    From:-
    https://en.wikipedia.org/wiki/Rolling_resistance

    " In analogy with sliding friction, rolling resistance is often
    expressed as a coefficient times the normal force. "
    Speed is not even mentioned.

    In my above I should have used 'a coefficient' rather than "the
    coefficient of sliding friction" sorry for any confusion.
     
    soup, Sep 12, 2015
    #4
  5. No, the FORCE is not speed-dependent, but it means that the power
    lost (which is what is almost always used in a cycling context)
    is proportional to speed. Yes, I agree that the terms rolling
    resistance and air resistance are abused in cycling contexts to
    refer to the power, but that is the case.

    Anyway, my point was that I now have evidence that the losses due
    to road surface are NOT proportional to speed. I haven't seen
    that aspect mentioned in any of the research, though I hypothesised
    it might be the case many decades ago. What surprised me was that
    my (one) measurement indicates that the losses may be effectively
    constant with speed.


    Regards,
    Nick Maclaren.
     
    Nick Maclaren, Sep 12, 2015
    #5
  6. Nick Maclaren

    Ian Smith Guest

    On Sat, 12 Sep 2015 12:36:36 +0100 (BST)
    I'd suggest that the component of rolling resistance due to road
    surface imperfections will reach a plateau (or even decline!) as speed
    increases once the 'undulations' are fully absorbed or accomodated by
    tyre deflection alone rather than frame/rider moving up and down. It's
    basically the inverse of 'feeling every bump' when going too slowly.
    <disclaimer>This hunch is about as unscientific as it
    gets.</disclaimer> :)
     
    Ian Smith, Sep 12, 2015
    #6
  7. Not really (to the last). My calculations on overly simplistic
    models indicate that is precisely what happens. In practice, it
    is what happens on corrugations (e.g. cattle grids and roads that
    have been grated for resurfacing). I can think of how to investigate
    it properly, but it's not an easy task.


    Regards,
    Nick Maclaren.
     
    Nick Maclaren, Sep 12, 2015
    #7
  8. Nick Maclaren

    Mike Causer Guest

    On Sat, 12 Sep 2015 23:00:41 +0100 (BST)
    This may be of interest:
    http://lustaufzukunft.de/pivit/comfort/vibration.html

    Mike
     
    Mike Causer, Sep 13, 2015
    #8
  9. Nick Maclaren, Sep 13, 2015
    #9
  10. Nick Maclaren

    Rob Morley Guest

    Rob Morley, Sep 13, 2015
    #10
  11. Most bicycle manufacturers are very much behind the times. For
    most riders, such tyres will be both faster and much more comfortable.
    It's only the people who routinely ride at 20+ MPH that get the
    benefit from narrow ones.

    The dogma that narrow tyres roll better dates from the days of latex
    and linen! 25 psi would have been the absolute maximum for such a
    tyre in those days.


    Regards,
    Nick Maclaren.
     
    Nick Maclaren, Sep 13, 2015
    #11
  12. Skinny ones, then...

    There's a (non-racing) trike here with 26x4.0 tyres.
     
    Dave Larrington, Sep 14, 2015
    #12
  13. I am sure that there are races where those would be the right ones!
    And quite possibly where a trike would be faster than a bike :)


    Regards,
    Nick Maclaren.
     
    Nick Maclaren, Sep 14, 2015
    #13
  14. http://www.whiteicecycle.com/ springs to mind, FSVO of "race".
    (4.5" - not slicks though.)
     
    Alan Braggins, Sep 14, 2015
    #14
  15. Nick Maclaren

    Rob Morley Guest

    Fat bikes are fat, but they're not very common - ordinary "mountain
    bikes" are very common and mostly capable of running 60mm tyres.
    I would be interested to see how 100mm tyres performed against 60mm on
    "normal" road and path surfaces.
     
    Rob Morley, Sep 14, 2015
    #15
  16. My normal town/commute bike is a old hardtail has light weight cross
    country tyres, ie 500g and a smidge under 60mm. and suspension forks.

    smooths out roads/shared paths etc, and on rougher sections of towpaths
    can dent CX riders pride as they can't maintain the same pace, over the
    rougher sections.

    I think it would be quite hard getting a like for like test fat tyres
    and fat bike and normal MTB tyres and bike.

    there is also the magic ruler that makers use for tyres and MTB tyres
    can differ quite a lot, My bike has Hans Dampf tyre on the front it's
    closer to 70 than 60, most folks report it blows up big.

    Roger Merriman
     
    Roger Merriman, Sep 15, 2015
    #16
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