Wheel Base and Downhill Speed

Discussion in 'Recumbent bicycles' started by Stratrider, May 7, 2003.

Thread Status:
Not open for further replies.
  1. Stratrider

    Stratrider Guest

    Many years ago (OK. 20 or more) the thought with downhill skis was: "longer skis are faster than
    shorter ones". Could the same be said about wheel base?

    Jim Reilly Reading, PA
     
    Tags:


  2. Jim Reilly wrote:

    > Many years ago (OK. 20 or more) the thought with downhill skis was: "longer skis are faster than
    > shorter ones". Could the same be said about wheel base?

    It *could*. Whether it contains any truth is another matter entirely. If no corners are involved,
    speed is mostly due to:

    1. aerodynamics, and
    2. the bravery / stupidity / lack of imagination of the rider

    If it's a twisting descent, factor two becomes the more important...

    Dave Larrington - http://legslarry.crosswinds.net/
    ===========================================================
    Editor - British Human Power Club Newsletter
    http://www.bhpc.org.uk/
    ===========================================================
     
  3. Doug Huffman

    Doug Huffman Guest

    Yes, and with similar accuracy.

    There is an aerodynamic relationship with increasing length and drag. Hydrodynamically it is
    expressed as hull speed = 1.34 x WL^1/2.

    "stratrider" <[email protected]> wrote in message
    news:[email protected]...
    > Many years ago (OK. 20 or more) the thought with downhill skis was: "longer skis are faster than
    > shorter ones". Could the same be said about wheel base?
    >
    > Jim Reilly Reading, PA
     
  4. Seth Jayson

    Seth Jayson Guest

    Years ago when I thought about physics and did some math, I was under the impression that, for a
    given mass on SKIS, youR increased your speed by increasing aerodynamics and reducing friction twixt
    snow and ski. (longer ski meaning more surface area, meaning less pressure per area, yielding less
    friction, as I remember)

    For a given slope, mass, and aerodynamic setup, then, would not your speed increase be only due to
    ski length. I'm not sure if this was ever true, it was just a set of physics problems to demonstrate
    competence with the week's lessons.

    Even if this IS true then it isn't applicable to wheelbase, as that's a completely different set of
    phenomena.

    As Rich P. has pointed out here many times, above 15mph or so, virtually all speed increase is
    gained by lessening wind resistance... (assuming all have already thrown the brain aside and are
    free-falling)
     
  5. Cletus Lee

    Cletus Lee Guest

    In article <[email protected]>, [email protected] says...
    > As Rich P. has pointed out here many times, above 15mph or so, virtually all speed increase is
    > gained by lessening wind resistance... (assuming all have already thrown the brain aside and are
    > free-falling)
    >
    >
    Kinda gives a new meaning to the term 'Terminal Velocity'.

    --

    Cletus D. Lee Bacchetta Giro Lightning Voyager http://www.clee.org
    - Bellaire, TX USA -
     
  6. "Doug Huffman" <[email protected]> wrote in message news:[email protected]...
    > Yes, and with similar accuracy.
    >
    > There is an aerodynamic relationship with increasing length and drag. Hydrodynamically it is
    > expressed as hull speed = 1.34 x WL^1/2.

    Sorry, this is related to wavemaking drag and has no relationship to aerodynamics.

    Len
     
  7. Don

    Don Guest

    Another vote for aerodynamics being more important than wheel base. There is hill that I go down a
    lot. It is a curvy bike trail through trees (close) and then it bottoms out down low along a
    beautiful river.

    I have found that (on my Haluzak) if (1)I hit the top hard and then coast or (2)hammer all the way
    down my maximum speed only varies by 2 mph between the two methods. I hit 25-27 mph and all the
    pedaling in the world does not increase that. So my now preferred method of doing that downhill is
    to hammer the top and then just glide (coast) effortlessly through the curves on my land luge. Don
     
  8. A&B

    A&B Guest

    I think Steve Delaire recently posted that one thing they found in coast downs and fastness tests
    was that the longer bikes were faster but they didn't know why. bill "yet another erroneous
    factoid" g

    Don wrote:
    >
    > Another vote for aerodynamics being more important than wheel base. There is hill that I go down a
    > lot. It is a curvy bike trail through trees (close) and then it bottoms out down low along a
    > beautiful river.
    >
    > I have found that (on my Haluzak) if (1)I hit the top hard and then coast or (2)hammer all the way
    > down my maximum speed only varies by 2 mph between the two methods. I hit 25-27 mph and all the
    > pedaling in the world does not increase that. So my now preferred method of doing that downhill is
    > to hammer the top and then just glide (coast) effortlessly through the curves on my land luge. Don
     
  9. Tom Sherman

    Tom Sherman Guest

    a&b wrote:
    >
    > I think Steve Delaire recently posted that one thing they found in coast downs and fastness tests
    > was that the longer bikes were faster but they didn't know why.

    All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
    distance between the center of mass of the bicycle/rider combination and the center of mass of the
    earth will be less, and of course gravitational attraction varies inversely with the square of the
    distance between the masses in question.

    Tom Sherman - Various HPV's Quad Cities USA (Illinois side)
     
  10. A&B

    A&B Guest

    Proof, with references please ;-) bill "and then a miracle happened" g

    Tom Sherman wrote:

    >
    > All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
    > distance between the center of mass of the bicycle/rider combination and the center of mass of the
    > earth will be less, and of course gravitational attraction varies inversely with the square of the
    > distance between the masses in question.
    >
    > Tom Sherman - Various HPV's Quad Cities USA (Illinois side)
     
  11. Tom Sherman

    Tom Sherman Guest

    a&b wrote:
    >
    > Proof, with references please ;-) bill "and then a miracle happened" g

    The relevant reference is "Philosophiae Naturalis Principia Mathematica"; Newton, Sir Isaac; 1687.

    Tom Sherman - Various HPV's Quad Cities USA (Illinois side)
     
  12. I think that speed down hills has more to do with overall handling characteristics than with any
    one factor.

    FWIW, my Wishbone Classic is a lot more secure-feeling at really high speeds (40+) than my
    Pharobike.

    The Wishbone has a much longer wheelbase and a much higher center of gravity than the itty-bitty
    Pharobike.

    Maybe it's just altered perception from being so low, but once I get the Pharobike up to the high
    30s I don't really *want* to go much faster. I'm having enough fun. I think I hit 41 or 42 once.

    One time I had the Wishbone up to 52 mph.

    Gearing might also be a factor... I have the Pharobike rigged with a top end gear of something like
    110-115 inches (depends on the tire I'm using). So at 35 mph I'm pedaling pretty fast, which upsets
    the bike. The Wishbone has a top end of 125 inches or so, which means that I don't start really
    flailing until a much higher speed.
     
  13. Tom Sherman <[email protected]> wrote in news:[email protected]:

    >
    > a&b wrote:
    >>
    >> I think Steve Delaire recently posted that one thing they found in coast downs and fastness tests
    >> was that the longer bikes were faster but they didn't know why.
    >
    > All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
    > distance between the center of mass of the bicycle/rider combination and the center of mass of the
    > earth will be less, and of course gravitational attraction varies inversely with the square of the
    > distance between the masses in question.
    >
    > Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

    Technically true, but in practice it's meaningless. A human being would not be able to tell the
    difference without _very_ precise measuring systems.

    Perhaps the longer wheelbase has less induced rotation due to irregularities in the road surface
    (cracks, holes, etc). A short wheelbase will rotate more about its cross axis due to these
    irregularities. This increased rotation means that more load is placed on the contact surfaces due
    to torque, so the tires will deform more. Deformed tires mean more kinetic energy lost until the
    tires go back to their proper shape.

    Or, to look at it differently. You have a long board with a fixed hinge at one end, a weight in the
    middle, and you holding it toward the other end. The farther out you go, the less force you have to
    exert to raise your end of the board one inch because you get more torque and the distance you raise
    the weight is less. On a bicycle, the hinge is the rear tire contact point, the weight is you, and
    the person lifting is the front tire contact point when it hits a ridge in the road. The farther
    forward the front tire is, the less force is necessary to raise the front of the bike over the
    obstacle. All of that force is applied through the tire, deforming it. The process of deforming and
    returning to its correct shape absorbs kinetic energy from the bicycle, slowing it down.

    That, at least, is my best guess.

    -Bill
     
  14. Scott

    Scott Guest

    Tom Sherman <tsherma[email protected]> wrote in message news:<[email protected]>...
    > a&b wrote:
    > >
    > > I think Steve Delaire recently posted that one thing they found in coast downs and fastness
    > > tests was that the longer bikes were faster but they didn't know why.
    >
    > All else being equal, a lower bicycle will be faster downhill than a taller bicycle since the
    > distance between the center of mass of the bicycle/rider combination and the center of mass of the
    > earth will be less, and of course gravitational attraction varies inversely with the square of the
    > distance between the masses in question.
    >
    > Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

    Tom: Seems like maybe you know the math. Could you take a hypothetical situation, like a Haluzak vs.
    your lowracer, and calculate the speed difference on a given hill--irrespective of wind resistance?
    That is, based purely on the gavitational attraction business, how much faster IS a lowracer than
    some taller bikes, all other things being equal? It would be interesting to know, but this has got
    to be a pretty small number.

    Come to think of it, if I recall this correctly, if I, on my "high racer" and you on your lowracer
    zoomed down a hill at the same speed, wouldn't I age LESS by the time we finished? Or have I got the
    "twins' paradox" reversed? Anyway, seems like we could make some sort of physics argument for one
    type of bike over the other based on relative ageing! :)

    Scott Haluzak high! Greenspeed low...

    Scott
     
  15. Dave Lehnen

    Dave Lehnen Guest

    Tom Sherman wrote:
    > a&b wrote:
    >
    >>Proof, with references please ;-) bill "and then a miracle happened" g
    >
    >
    > The relevant reference is "Philosophiae Naturalis Principia Mathematica"; Newton, Sir Isaac; 1687.
    >
    > Tom Sherman - Various HPV's Quad Cities USA (Illinois side)

    I'm pretty sure nobody is serious about this, but lowering the CG by 2 feet increases gravitational
    pull by about .000019%, but you're also running through denser low-altitude air. If the center of
    pressure drops only one foot, air density is about .0036% greater, more than offsetting the huge
    gravitational gain.

    Dave Lehnen
     
  16. Seth Jayson

    Seth Jayson Guest

    I can't wait to see the answer to this one. The number will be so small that I'll bet a single fly
    in the chest on the way down could compensate for the "speed increase"

    Tom was jest funnin' with us. You're talking about a difference of inches between two numbers
    measured in thousands of miles.
     
  17. Bill Hamilton wrote:
    > Perhaps the longer wheelbase has less induced rotation due to irregularities in the road surface
    > (cracks, holes, etc). A short wheelbase will rotate more about its cross axis due to these
    > irregularities. This increased rotation means that more load is placed on the contact surfaces due
    > to torque, so the tires will deform more. Deformed tires mean more kinetic energy lost until the
    > tires go back to their proper shape.
    >
    > Or, to look at it differently. You have a long board with a fixed hinge at one end, a weight in
    > the middle, and you holding it toward the other end. The farther out you go, the less force you
    > have to exert to raise your end of the board one inch because you get more torque and the
    > distance you raise the weight is less. On a bicycle, the hinge is the rear tire contact point,
    > the weight is you, and the person lifting is the front tire contact point when it hits a ridge in
    > the road. The farther forward the front tire is, the less force is necessary to raise the front
    > of the bike over the obstacle. All of that force is applied through the tire, deforming it. The
    > process of deforming and returning to its correct shape absorbs kinetic energy from the bicycle,
    > slowing it down.
    >
    > That, at least, is my best guess.
    >
    > -Bill

    I think there are some remarks to be made here: first of all: how do you ensure that the two bikes
    compared _only_ differ in length, i.e. how sure are you the frontal area is exactly the same, the
    shape factor is exactly the same, the same tires used on the same pressure (exactly!) thus exactly
    the same rolling resistance. And most importantly in this case, the total weight is the same. With
    all the rest truely the same (takes some effort ;-) a bit extra weight increases the gravitational
    pulling power and consequently the equilibrium speed (speed where all resistances summed make up for
    the gravitation). And guess which will be (slightly) heavier: the shorter or the longer bike....
    Indeed. Unfortunately, you'll get punished for this little gain on the other side of the mountain

    Kees, Hurricane on Stelvio's as fast uphill as my DF, but o so much faster down, van Malssen
     
  18. Tom Sherman

    Tom Sherman Guest

    Andrew Douglas wrote:
    >
    > I think that speed down hills has more to do with overall handling characteristics than with any
    > one factor.
    >
    > FWIW, my Wishbone Classic is a lot more secure-feeling at really high speeds (40+) than my
    > Pharobike.
    >
    > The Wishbone has a much longer wheelbase and a much higher center of gravity than the itty-bitty
    > Pharobike.
    >
    > Maybe it's just altered perception from being so low, but once I get the Pharobike up to the high
    > 30s I don't really *want* to go much faster. I'm having enough fun. I think I hit 41 or 42
    > once....

    My Sunset is very similar in overall configuration to the Pharobike LowFat. I find that the handling
    becomes more stable with speed, even when I am pedaling at a cadence of 140+ rpm [1]. I am perfectly
    comfortable descending at 50+ mph, so I suspect the issue probably one of steering geometry
    differences between the two bikes.

    [1] I spin out at about 160 rpm.

    Tom Sherman - Various HPV's Quad Cities USA (Illinois side)
     
Loading...
Thread Status:
Not open for further replies.
Loading...