650C rolling resistance higher

Discussion in 'Cycling Equipment' started by Gierst, Jun 26, 2003.

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  1. Gierst

    Gierst Guest

    Has anyone out there objective data on the difference in rolling resistance between a 700c and 650c
    wheel? I do not mean lower kinetic energy, because of lower mass, but more friction between road and
    tire, because of smaller diameter. Is it really substantial, or is it a matter of feeling less
    stable on smaller wheels? Is there a linear relation? Any info is welcome,

    Sjef
     
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  2. Peter

    Peter Guest

    gierst wrote:
    > Has anyone out there objective data on the difference in rolling resistance between a 700c and
    > 650c wheel? I do not mean lower kinetic energy, because of lower mass, but more friction between
    > road and tire, because of smaller diameter. Is it really substantial, or is it a matter of feeling
    > less stable on smaller wheels? Is there a linear relation? Any info is welcome,

    Bicycling Science, Whitt & Wilson, MIT Press has some data and indicates that resistance should
    increase in inverse proportion to the wheel radius (half the radius would double the rolling
    resistance). However with high-pressure tires rolling resistance is a pretty small part of the
    total. I don't feel at a significant disadvantage on my Bike Friday with 451 wheels and coast
    downhill at about the same speed as on my other bikes. Feels just as stable at 50 mph as well.
     
  3. Jon Bond

    Jon Bond Guest

    "gierst" <[email protected]> wrote in message news:BB213FFB.166AD%[email protected]...
    > Has anyone out there objective data on the difference in rolling
    resistance
    > between a 700c and 650c wheel? I do not mean lower kinetic energy, because of lower mass, but more
    > friction between road and tire, because of smaller diameter. Is it really substantial, or is it a
    > matter of feeling less
    stable
    > on smaller wheels? Is there a linear relation? Any info is welcome,
    >
    > Sjef

    No numbers here, but I would assume that any loss you'd get from that would just about be nullified
    by the smaller aerodynamic profile (?) of the wheel.

    Jon Bond
     
  4. Bfd

    Bfd Guest

    The following are some rolling resistance test of 700c and 26" mt bike tires that may help:
    http://www.terrymorse.com/bike/rolres.html

    "gierst" <[email protected]> wrote in message news:BB213FFB.166AD%[email protected]...
    > Has anyone out there objective data on the difference in rolling
    resistance
    > between a 700c and 650c wheel? I do not mean lower kinetic energy, because of lower mass, but more
    > friction between road and tire, because of smaller diameter. Is it really substantial, or is it a
    > matter of feeling less
    stable
    > on smaller wheels? Is there a linear relation? Any info is welcome,
    >
    > Sjef
     
  5. Peter

    Peter Guest

    bfd wrote:
    > The following are some rolling resistance test of 700c and 26" mt bike tires that may help:
    > http://www.terrymorse.com/bike/rolres.html

    While these rolling resistance measurements are very valuable, some care must be used when comparing
    tires of different diameters. Many rolling resistance measurement stands use a cylindrical drum
    rolling against the tire under test with a fixed load. Unless the drum is very large in diameter,
    this measurement procedure can mask the effects due to changes in wheel size.

    Note that when comparing two tires of identical pressure and width but of different diameters there
    will be a change in the shape of the contact patch on the road. The wheel with a smaller diameter
    will have a shorter but wider patch compared with the larger diameter wheel. This also means the
    sidewall of the tire on the smaller wheel will be deformed more at the point of contact. But if the
    test is performed with a small diameter drum pressed against the tires on the two wheels then the
    two contact patches will have almost the same shape (essentially identical if the drum size is much
    smaller than either wheel size). Therefore a major factor affecting the rolling resistance of
    different wheel sizes on the road (with a flat surface) cannot be measured with a test setup that
    involves a small-diameter curved drum pressed against the tire tread.

    Jobst's tests were all performed on tires of nearly equal diameters and was used to show differences
    between tires. And Terry's mountain bike tire comparison also looks at tires that are all the same
    size. Therefore the use of a drum roller is reasonable in each of these studies separately, but this
    type of measurement should not be used to compare rolling resistance of different size wheels. In
    addition, the two sets of data can't be used for the purpose of evaluating the effects of tire size
    since they were done with very different types of tires and loadings and may not have been performed
    on the same test stand.

    >
    > "gierst" <[email protected]> wrote in message news:BB213FFB.166AD%[email protected]...
    >
    >>Has anyone out there objective data on the difference in rolling
    >
    > resistance
    >
    >>between a 700c and 650c wheel? I do not mean lower kinetic energy, because of lower mass, but more
    >>friction between road and tire, because of smaller diameter. Is it really substantial, or is it a
    >>matter of feeling less
    >
    > stable
    >
    >>on smaller wheels? Is there a linear relation? Any info is welcome,
    >>
    >>Sjef
    >>
    >
     
  6. Peter :
    > I don't feel at a significant disadvantage on my Bike Friday with 451 wheels and coast downhill at
    > about the same speed as on my other bikes. Feels just as stable at 50 mph as well.

    At 50MPH you might be into aerodynamic gains with your small wheels.

    Andrew Bradley
     
  7. Phil Holman

    Phil Holman Guest

    "Peter" <[email protected]> wrote in message news:[email protected]...
    > bfd wrote:
    > > The following are some rolling resistance test of 700c and 26" mt
    bike tires
    > > that may help: http://www.terrymorse.com/bike/rolres.html
    >
    > While these rolling resistance measurements are very valuable, some
    care
    > must be used when comparing tires of different diameters. Many rolling resistance measurement
    > stands use a cylindrical drum rolling against the tire under test with a fixed load. Unless the
    drum
    > is very large in diameter, this measurement procedure can mask the effects due to changes in
    > wheel size.

    On the contrary, it isolates the effect of wheel size. The difference in contact patch is only a
    small effect.

    >
    > Note that when comparing two tires of identical pressure and width but of different diameters
    > there will be a change in the shape of the contact patch on the road. The wheel with a smaller
    > diameter will
    have
    > a shorter but wider patch compared with the larger diameter wheel.
    This
    > also means the sidewall of the tire on the smaller wheel will be deformed more at the point of
    > contact. But if the test is performed with a small diameter drum pressed against the tires on
    > the two wheels then the two contact patches will have almost the same shape (essentially
    > identical if the drum size is much smaller than either wheel size). Therefore a major factor
    > affecting the rolling
    resistance
    > of different wheel sizes on the road (with a flat surface) cannot be measured with a test setup
    > that involves a small-diameter curved drum pressed against the tire tread.

    This is not the major determining factor with differences in wheel diameter. What you have shown
    above would probably demonstrate a lower rolling resistance for smaller wheels which empirically is
    not the case. The effect is small and overshadowed by the increased force required to overcome the
    slightly less or identical resistance torque of a smaller diameter wheel. I.e. a 5 inch-lb
    resistance torque will result in a 5/R force to over come it. Obviously this reduces with a larger R
    (a larger wheel).

    Phil Holman
     
  8. Peter

    Peter Guest

    Phil Holman wrote:
    > "Peter" <[email protected]> wrote in message news:[email protected]...
    >
    >>bfd wrote:
    >>
    >>>The following are some rolling resistance test of 700c and 26" mt
    >>
    > bike tires
    >
    >>>that may help: http://www.terrymorse.com/bike/rolres.html
    >>
    >>While these rolling resistance measurements are very valuable, some
    >
    > care
    >
    >>must be used when comparing tires of different diameters. Many rolling resistance measurement
    >>stands use a cylindrical drum rolling against the tire under test with a fixed load. Unless the
    >
    > drum
    >
    >>is very large in diameter, this measurement procedure can mask the effects due to changes in
    >>wheel size.
    >
    >
    > On the contrary, it isolates the effect of wheel size. The difference in contact patch is only a
    > small effect.

    It makes the test only valid if you spend your day riding over small diameter drums instead of
    roads. The contact patch is where the vast majority of rolling resistance occurs (some also occurs
    in the bearings but this is tiny for properly maintained wheels).

    >>Note that when comparing two tires of identical pressure and width but of different diameters
    >>there will be a change in the shape of the contact patch on the road. The wheel with a smaller
    >>diameter will
    >
    > have
    >
    >>a shorter but wider patch compared with the larger diameter wheel.
    >
    > This
    >
    >>also means the sidewall of the tire on the smaller wheel will be deformed more at the point of
    >>contact. But if the test is performed with a small diameter drum pressed against the tires on
    >>the two wheels then the two contact patches will have almost the same shape (essentially
    >>identical if the drum size is much smaller than either wheel size). Therefore a major factor
    >>affecting the rolling
    >
    > resistance
    >
    >>of different wheel sizes on the road (with a flat surface) cannot be measured with a test setup
    >>that involves a small-diameter curved drum pressed against the tire tread.
    >
    >
    > This is not the major determining factor with differences in wheel diameter. What you have shown
    > above would probably demonstrate a lower rolling resistance for smaller wheels which empirically
    > is not the case.

    Only if you argue that greater deformation will somehow lead to less energy loss - clearly nonsense.
    The smaller diameter tire will have greater deformation in the contact patch area leading to greater
    rolling resistance.

    > The effect is small and overshadowed by the increased force required to overcome the slightly
    > less or identical resistance torque of a smaller diameter wheel. I.e. a 5 inch-lb resistance
    > torque will result in a 5/R force to over come it. Obviously this reduces with a larger R (a
    > larger wheel).

    On the contrary, think about where energy is wasted - that is the cause of rolling resistance. The
    main energy waste comes about directly as a result of the deformation of the tire and tube materials
    in the vicinity of the contact patch. If the tire deformation is greater (as it is with a smaller
    diameter tire and shorter/wider contact patch), then there is more energy going into making the tire
    squirm and deform. This process transforms some of the kinetic energy into heat in the tire and
    slows you down. The resistance you're talking about above is in the bearings which is very small in
    comparison to the tire losses for a properly lubricated ball bearing hub. Both effects lead to
    increased rolling resistance with smaller diameter wheels, but the tire/tube contact patch area is
    the main source of internal energy loss for typical bicycle wheels. Rail bikes with steel wheels on
    rails can have far less rolling resistance than typical pneumatic-tire wheels although the losses in
    the hub bearings will be the same in both cases. Unfortunately when the measurements are done using
    a small-diameter drum the effects of changing the contact patch shape are largely lost.
     
  9. Phil Holman

    Phil Holman Guest

    "Peter" <[email protected]> wrote in message news:[email protected]...
    > Phil Holman wrote:
    > > "Peter" <[email protected]> wrote in message news:[email protected]...
    > >
    > >>bfd wrote:
    > >>
    > >>>The following are some rolling resistance test of 700c and 26" mt
    > >>
    > > bike tires
    > >
    > >>>that may help: http://www.terrymorse.com/bike/rolres.html
    > >>
    > >>While these rolling resistance measurements are very valuable, some
    > >
    > > care
    > >
    > >>must be used when comparing tires of different diameters. Many rolling resistance measurement
    > >>stands use a cylindrical drum rolling against the tire under test with a fixed load. Unless the
    > >
    > > drum
    > >
    > >>is very large in diameter, this measurement procedure can mask the effects due to changes in
    > >>wheel size.
    > >
    > >
    > > On the contrary, it isolates the effect of wheel size. The
    difference in
    > > contact patch is only a small effect.
    >
    > It makes the test only valid if you spend your day riding over small diameter drums instead of
    > roads. The contact patch is where the vast majority of rolling resistance occurs (some also
    > occurs in the
    bearings
    > but this is tiny for properly maintained wheels).

    When talking about rolling resistance of bicycle wheels, the discussion should only be about the
    losses at the contact patch.

    >
    > >>Note that when comparing two tires of identical pressure and width
    but
    > >>of different diameters there will be a change in the shape of the contact patch on the road. The
    > >>wheel with a smaller diameter will
    > >
    > > have
    > >
    > >>a shorter but wider patch compared with the larger diameter wheel.
    > >
    > > This
    > >
    > >>also means the sidewall of the tire on the smaller wheel will be deformed more at the point of
    > >>contact. But if the test is performed with a small diameter drum pressed against the tires on
    > >>the two
    wheels
    > >>then the two contact patches will have almost the same shape (essentially identical if the drum
    > >>size is much smaller than either wheel size). Therefore a major factor affecting the rolling
    > >
    > > resistance
    > >
    > >>of different wheel sizes on the road (with a flat surface) cannot be measured with a test setup
    > >>that involves a small-diameter curved
    drum
    > >>pressed against the tire tread.
    > >
    > >
    > > This is not the major determining factor with differences in wheel diameter. What you have shown
    > > above would probably demonstrate a
    lower
    > > rolling resistance for smaller wheels which empirically is not the
    case.
    >
    > Only if you argue that greater deformation will somehow lead to less energy loss - clearly
    > nonsense. The smaller diameter tire will have greater deformation in the contact patch area
    > leading to greater
    rolling
    > resistance.

    The amount of deformation is only part of the equation. Shorter but wider contact patches generate
    less rolling resistance than longer but narrower patches given identical amounts of tire deflection.
    This is observed in cases where the same tires but different widths are compared. The wider tire has
    identical or even less rolling resistance at the same pressure.

    >
    > > The effect is small and overshadowed by the increased force required
    to
    > > overcome the slightly less or identical resistance torque of a
    smaller
    > > diameter wheel. I.e. a 5 inch-lb resistance torque will result in a
    5/R
    > > force to over come it. Obviously this reduces with a larger R (a larger wheel).
    >
    > On the contrary, think about where energy is wasted - that is the
    cause
    > of rolling resistance. The main energy waste comes about directly as
    a
    > result of the deformation of the tire and tube materials in the
    vicinity
    > of the contact patch. If the tire deformation is greater (as it is
    with
    > a smaller diameter tire and shorter/wider contact patch), then there
    is
    > more energy going into making the tire squirm and deform. This
    process
    > transforms some of the kinetic energy into heat in the tire and slows you down.

    Yes, but this is converted into a resisting torque which the rider has to overcome.

    >The resistance you're talking about above is in the bearings which is very small in comparison to
    >the tire losses for a properly lubricated ball bearing hub.

    No I'm not referring to this at all. Converting tire deflection into a resisting torque is the key
    to understanding how contact shape and wheel radius effect the energy required to overcome the
    rolling resistance. You originally brought up ..... "Bicycling Science, Whitt & Wilson, MIT Press
    has some data and indicates that resistance should increase in inverse proportion to the wheel
    radius (half the radius would double the rolling resistance)" For identical amounts of tire
    deflection (lets increase the smaller wheel's tire pressure to achieve this) the larger diameter
    wheel will have less rolling resistance.

    Both effects lead to increased rolling
    > resistance with smaller diameter wheels, but the tire/tube contact
    patch
    > area is the main source of internal energy loss for typical bicycle wheels. Rail bikes with steel
    > wheels on rails can have far less
    rolling
    > resistance than typical pneumatic-tire wheels although the losses in
    the
    > hub bearings will be the same in both cases. Unfortunately when the measurements are done using a
    > small-diameter drum the effects of changing the contact patch shape are largely lost.

    Rolling resistance is a result of the compression of the tire/wheel. When a wheel rolls on a surface
    it compresses at an area directly below the center of the wheel. This sets up a compression force
    and a rebound force. At the area where the wheel compresses, the forces are higher than in the
    rebound stage due to the internal friction of the materials. This internal friction is known as
    hysteresis. This sets up a resisting torque which we call rolling resistance. The actual resisting
    force calculated from the resisting torque is an inverse function of the wheel radius.

    Phil Holman
     
  10. Peter

    Peter Guest

    Phil Holman wrote:
    > "Peter" <[email protected]> wrote in message news:[email protected]...
    >
    >>Phil Holman wrote:
    >>
    >>>"Peter" <[email protected]> wrote in message news:[email protected]...
    >>>
    >>>
    >>>>bfd wrote:
    >>>>
    >>>>
    >>>>>The following are some rolling resistance test of 700c and 26" mt
    >>>>
    >>>bike tires
    >>>
    >>>
    >>>>>that may help: http://www.terrymorse.com/bike/rolres.html
    >>>>
    >>>>While these rolling resistance measurements are very valuable, some
    >>>
    >>>care
    >>>
    >>>
    >>>>must be used when comparing tires of different diameters. Many rolling resistance measurement
    >>>>stands use a cylindrical drum rolling against the tire under test with a fixed load. Unless the
    >>>
    >>>drum
    >>>
    >>>
    >>>>is very large in diameter, this measurement procedure can mask the effects due to changes in
    >>>>wheel size.
    >>>
    >>>
    >>>On the contrary, it isolates the effect of wheel size. The
    >>
    > difference in
    >
    >>>contact patch is only a small effect.
    >>
    >>It makes the test only valid if you spend your day riding over small diameter drums instead of
    >>roads. The contact patch is where the vast majority of rolling resistance occurs (some also
    >>occurs in the
    >
    > bearings
    >
    >>but this is tiny for properly maintained wheels).
    >
    >
    > When talking about rolling resistance of bicycle wheels, the discussion should only be about the
    > losses at the contact patch.
    >
    >
    >>>>Note that when comparing two tires of identical pressure and width
    >>>
    > but
    >
    >>>>of different diameters there will be a change in the shape of the contact patch on the road. The
    >>>>wheel with a smaller diameter will
    >>>
    >>>have
    >>>
    >>>
    >>>>a shorter but wider patch compared with the larger diameter wheel.
    >>>
    >>>This
    >>>
    >>>
    >>>>also means the sidewall of the tire on the smaller wheel will be deformed more at the point of
    >>>>contact. But if the test is performed with a small diameter drum pressed against the tires on
    >>>>the two
    >>>
    > wheels
    >
    >>>>then the two contact patches will have almost the same shape (essentially identical if the drum
    >>>>size is much smaller than either wheel size). Therefore a major factor affecting the rolling
    >>>
    >>>resistance
    >>>
    >>>
    >>>>of different wheel sizes on the road (with a flat surface) cannot be measured with a test setup
    >>>>that involves a small-diameter curved
    >>>
    > drum
    >
    >>>>pressed against the tire tread.
    >>>
    >>>
    >>>This is not the major determining factor with differences in wheel diameter. What you have shown
    >>>above would probably demonstrate a
    >>
    > lower
    >
    >>>rolling resistance for smaller wheels which empirically is not the
    >>
    > case.
    >
    >>Only if you argue that greater deformation will somehow lead to less energy loss - clearly
    >>nonsense. The smaller diameter tire will have greater deformation in the contact patch area
    >>leading to greater
    >
    > rolling
    >
    >>resistance.
    >
    >
    > The amount of deformation is only part of the equation. Shorter but wider contact patches generate
    > less rolling resistance than longer but narrower patches given identical amounts of tire
    > deflection. This is observed in cases where the same tires but different widths are compared. The
    > wider tire has identical or even less rolling resistance at the same pressure.

    But we're not talking about an inherently wider tire in this case - we're talking about two tires
    that are identical in pressure and width but one is smaller in diameter. The area of the contact
    patch is therefore the same in both cases (since the pressures are equal), but the smaller diameter
    wheel has to be pushed down more in the area of the contact patch to create that area. Therefore it
    suffers from greater deformation and has a wider but shorter contact patch - and also has more
    rolling resistance as a direct result of this. More deformation implies more flexing implies more
    heat generation implies greater energy losses.

    That's completely different from the case you're imagining of two tires with the same diameter but
    where one is wider than the other. In that case if the pressures are the same the wider tire will
    have less deformation. But with two otherwise identical tires but of different diameters we can't
    satisfy your assumption above of 'identical tire deflection'. False assumptions lead to bad
    conclusions.
    >
    >>>The effect is small and overshadowed by the increased force required
    >>
    > to
    >
    >>>overcome the slightly less or identical resistance torque of a
    >>
    > smaller
    >
    >>>diameter wheel. I.e. a 5 inch-lb resistance torque will result in a
    >>
    > 5/R
    >
    >>>force to over come it. Obviously this reduces with a larger R (a larger wheel).
    >>
    >>On the contrary, think about where energy is wasted - that is the
    >
    > cause
    >
    >>of rolling resistance. The main energy waste comes about directly as
    >
    > a
    >
    >>result of the deformation of the tire and tube materials in the
    >
    > vicinity
    >
    >>of the contact patch. If the tire deformation is greater (as it is
    >
    > with
    >
    >>a smaller diameter tire and shorter/wider contact patch), then there
    >
    > is
    >
    >>more energy going into making the tire squirm and deform. This
    >
    > process
    >
    >>transforms some of the kinetic energy into heat in the tire and slows you down.
    >
    >
    > Yes, but this is converted into a resisting torque which the rider has to overcome.

    No need for any conversion. It's already in the form of a force from the ground pushing back against
    the leading edge of the contact patch of the tire. This force is slightly greater than the rebound
    force of the trailing edge of the contact patch as that part of the tire leaves the ground (the
    forces are unequal due to energy losses in the tire/tube material as they flex and heat up in the
    area of the contact). The net difference of these two forces is the rolling resistance drag force
    pushing back on the bike. Why insist on converting anything?

    >
    >
    >>The resistance you're talking about above is in the bearings which is very small in comparison to
    >>the tire losses for a properly lubricated ball bearing hub.
    >
    >
    > No I'm not referring to this at all. Converting tire deflection into a resisting torque is the key
    > to understanding how contact shape and wheel radius effect the energy required to overcome the
    > rolling resistance. You originally brought up ..... "Bicycling Science, Whitt & Wilson, MIT Press
    > has some data and indicates that resistance should increase in inverse proportion to the wheel
    > radius (half the radius would double the rolling resistance)"

    Which I stated initially and fully agree with. And my posts since then have just established the
    physical reason why that statement is true and pointed out that doing the testing with small
    diameter drums will generally not be valid for comparing tires of different diameters.

    > For identical amounts of tire deflection (lets increase the smaller wheel's tire pressure to
    > achieve this) the larger diameter wheel will have less rolling resistance.

    Now you're introducing another variable which was not done in the Bicycling Science book when they
    concluded that there is an inverse relationship. Their conclusion was reached under the assumption
    that the tires were otherwise identical - same width, same construction, same pressure - and that
    the only variable was the wheel diameter. With such an increase in pressure I expect that we would
    not see anywhere near a doubling of the rolling resistance for a halving of the diameter.

    I saw a published paper a while back that claimed to prove that 406 size Tioga tires have less
    rolling resistance than 622 size tires of similar construction. The error in the paper was that they
    used a test rig with small diameter drums to measure the rolling resistance and as a result they
    erroniously reached conclusions that were opposite to the part of Bicycling Science you just quoted.
    >
    > Both effects lead to increased rolling
    >
    >>resistance with smaller diameter wheels, but the tire/tube contact
    >
    > patch
    >
    >>area is the main source of internal energy loss for typical bicycle wheels. Rail bikes with steel
    >>wheels on rails can have far less
    >
    > rolling
    >
    >>resistance than typical pneumatic-tire wheels although the losses in
    >
    > the
    >
    >>hub bearings will be the same in both cases. Unfortunately when the measurements are done using a
    >>small-diameter drum the effects of changing the contact patch shape are largely lost.
    >
    >
    > Rolling resistance is a result of the compression of the tire/wheel. When a wheel rolls on a
    > surface it compresses at an area directly below the center of the wheel.

    Actually the compression occurs just forward of the center of the wheel
    (i.e. in the front half of the contact patch) and this is quite important since it means that the
    force from the ground on this part of the tire has a component backward in the horizontal
    direction in addition to an upward component.

    > This sets up a compression force and a rebound force. At the area where the wheel compresses, the
    > forces are higher than in the rebound stage due to the internal friction of the materials. This
    > internal friction is known as hysteresis. This sets up a resisting torque which we call rolling
    > resistance.

    The portion of the resistance due to tire contact patch effects exerts a direct retarding force. I
    agree with everything you state just above paragraph except when you call it a "resisting torque".
    It is a direct force backwards on the bicycle of some number of pounds of force. Note that this is
    also in the correct units to use for rolling resistance: it will be some number of pounds (or
    Newtons) of drag. But torque is measured in lb-ft, not lbs., so no, we cannot call a torque rolling
    resistance - the two are not the same type of quantity. You correclty point out that your "rebound
    force" is less than your "compression force". The difference of the horizontal components of these
    two forces directly give you the rolling resistance drag force. Why complicate things by imagining
    that they have to be converted into torques and then converted back into rolling resistance? You
    already have the answer in the form you want.

    On the other hand, the small resistance due to bearing friction is manifested as a torque since it
    acts to retard the turning of the hub and this in turn slows the rim rotation through the tension in
    the spokes. The resisting force of the bearings at the hub is therefore reduced by the ratio of the
    radius of the bearing race divided by the radius of the wheel resulting in a very small retarding
    force on the bike since the torques (force x eff. lever distance) have to balance. That's why I
    thought you were talking about bearing friction when you brought up the issue of torque - it's a
    useful concept for the bearing friction but is not necessary when talking about resistive forces due
    to the deformation losses in the tire/tube at the contact patch. But I see now that this was not
    your intention and we are in agreement that bearing friction can be neglected.

    > The actual resisting force calculated from the resisting torque is an inverse function of the
    > wheel radius.

    Which you should note is exactly the conclusion I gave in the very first response provided by anyone
    in this thread (although I didn't use the word torque and still object to it here as stated above).

    Unfortunately I've seen improperly done tests used to try to deny that relationship between
    resistance and wheel radius and the flaw in the tests was that they used a small drum pressed
    against the tires under test. Such tests are fine for comparing tires of a fixed size against each
    other but give misleading results favoring the smaller tires when they are used to compare tires of
    substantially different diameters. That's why I objected when someone presented just such tests and
    suggested that they should be used in studying the effects of wheel diameter on rolling resistance.
    The specific charts shown also had other discrepancies that would make reaching clear conclusions
    impossible - the loads were different and the tires were of very different types. Far too many
    uncontrolled variables to separate out the effect of wheel size by itself.
     
  11. Tim McNamara

    Tim McNamara Guest

    Please, folks, learn to rewrap quoted material. And use shorter text lines, too. It would make this
    *so* much easier to read!
     
  12. Phil Holman

    Phil Holman Guest

    "Tim McNamara" <[email protected]> wrote in message
    news:[email protected]...
    > Please, folks, learn to rewrap quoted material. And use shorter text lines, too. It would make
    > this *so* much easier to read!

    Sorry about that. Mine automatically wraps at 72 characters.

    Phil Holman
     
  13. Phil Holman

    Phil Holman Guest

    It was a lot of work to rewrap plus it didn't work anyway so I have snipped most of the text and
    will try to focus on the essence of the discussion. How many characters are you using Peter?

    > No need for any conversion. It's already in the form of a force from the ground pushing back
    > against the leading edge of the contact patch of the tire. This force is slightly greater than the
    > rebound force of the trailing edge of the contact patch as that part of the tire leaves the ground
    > (the forces are unequal due to energy losses in the tire/tube material as they flex and heat up in
    > the area of the contact). The net difference of these two forces is the rolling resistance drag
    > force pushing back on the bike. Why insist on converting anything?

    Because if you don't know how to do this then it's just a leap of faith. If you think that the
    difference between the compression force and the rebound force is equal to the rolling resistance
    drag force then the previous statement is confirmed. It requires a little more mathematical
    conversion than just subtraction.

    >
    > Actually the compression occurs just forward of the center of the wheel (i.e. in the front half of
    > the contact patch) and this is quite important since it means that the force from the ground on
    > this part of the tire has a component backward in the horizontal direction in addition to an
    > upward component.

    A single force has net zero normal components. What is happening here is the vertical force on the
    front half of the contact patch creates a bigger torque than the vertical force on the rear half.
    The difference is the net torque and from this, by dividing by wheel radius, we obtain the drag
    force, aka rolling resistance. For the rolling resistance to be an inverse function of the tire
    radius, the resisting torques will all be identical. You are correct when you state that the 650
    tire will have a larger hysteresis loss but this is offset by the shorter contact patch length. The
    centers of pressure of the forward and rear contact patch halves being indentically disproportionate
    to make the resultant torques identical.

    Phil Holman
     
  14. Tim McNamara

    Tim McNamara Guest

    > From: gierst <[email protected]>

    > Has anyone out there objective data on the difference in rolling resistance between a 700c and
    > 650c wheel? I do not mean lower kinetic energy, because of lower mass, but more friction between
    > road and tire, because of smaller diameter.

    No one clarified a significant part of the question from the OP. Rolling resistance is not friction
    between the road and the tire. It is the energy lost in flexing the rubber tread, fabric casing and
    rubber inner tube. These are called "hysteresis" (sp?) losses.

    At the point where the rubber meets the road, the tire flattens out a bit. This is referred to as
    the "contact patch." That patch, which results from the deformation of the tire, involves flexing at
    the front or leading edge and the rear or trailing edge. From what I've read, most of the hysteresis
    loss (perhaps all? I've never asked about this) occurs at the leading edge of the contact patch.

    From the discussions about rolling resistance over the years in this newsgroup it seems that, all
    other things being equal, the smaller diameter wheel will have more rolling resistance. "All other
    things" includes inflation pressure, tire construction, rubber composition, tread thickness, etc.
    Rarely in the bicycle world are "all other things" equal, so it is very difficult to compare from
    one size wheel to the next.

    Greenspeed argue against this, however, reporting that:

    > There is also a excellent selection of 20" tyres available, and we have found that contrary to
    > popular belief, the 20" tyres have a LOWER rolling resistance than 26" tyres of the same
    > construction. We have tested over 50 different tyres from all over the world, and selected the
    > best. The Tioga Comp Pools tyres we use, were found to give the lowest rolling resistance coupled
    > with the best grip, yet they have lasted over 10,000kms on tour.

    The Greenspeed RR tests are referenced below and have been graphed here:

    http://www-ifia.fzk.de/IFIA_Webseiten/Personal/fuchs/ROLLW-E.HTM

    > Is it really substantial, or is it a matter of feeling less stable on smaller wheels? Is there a
    > linear relation?

    In most scientific measurements, the method of measurement has an effect on the outcome of
    measurement, which is what this discussion has focused on.

    There have been hundreds of discussions about this over the past decade, and I would recommend that
    the OP check out www.google.com and click on the "Groups" link. From there you can search this
    newsgroup with the keywords "rolling resistance." You ought to get several thousand posts on this
    topic. ;-)

    Here are some other links:

    http://www.physics.helsinki.fi/~tlinden/rolling.html
    http://www.legslarry.beerdrinkers.co.uk/tech/GS.htm
    http://www.legslarry.beerdrinkers.co.uk/tech/JL.htm
    http://www.ice.hpv.co.uk/495000_Soapbox_Tyres_TRS.pdf
     
  15. Peter

    Peter Guest

    Tim McNamara wrote:
    >>From: gierst <[email protected]>
    >
    >
    >>Has anyone out there objective data on the difference in rolling resistance between a 700c and
    >>650c wheel? I do not mean lower kinetic energy, because of lower mass, but more friction between
    >>road and tire, because of smaller diameter.
    >
    >
    > No one clarified a significant part of the question from the OP. Rolling resistance is not
    > friction between the road and the tire. It is the energy lost in flexing the rubber tread, fabric
    > casing and rubber inner tube. These are called "hysteresis" (sp?) losses.
    >
    > At the point where the rubber meets the road, the tire flattens out a bit. This is referred to as
    > the "contact patch." That patch, which results from the deformation of the tire, involves flexing
    > at the front or leading edge and the rear or trailing edge. From what I've read, most of the
    > hysteresis loss (perhaps all? I've never asked about this) occurs at the leading edge of the
    > contact patch.
    >
    > From the discussions about rolling resistance over the years in this newsgroup it seems that, all
    > other things being equal, the smaller diameter wheel will have more rolling resistance. "All other
    > things" includes inflation pressure, tire construction, rubber composition, tread thickness, etc.
    > Rarely in the bicycle world are "all other things" equal, so it is very difficult to compare from
    > one size wheel to the next.
    >
    > Greenspeed argue against this, however, reporting that:
    >
    >
    >>There is also a excellent selection of 20" tyres available, and we have found that contrary to
    >>popular belief, the 20" tyres have a LOWER rolling resistance than 26" tyres of the same
    >>construction. We have tested over 50 different tyres from all over the world, and selected the
    >>best. The Tioga Comp Pools tyres we use, were found to give the lowest rolling resistance coupled
    >>with the best grip, yet they have lasted over 10,000kms on tour.
    >
    >
    > The Greenspeed RR tests are referenced below and have been graphed here:
    >
    > http://www-ifia.fzk.de/IFIA_Webseiten/Personal/fuchs/ROLLW-E.HTM
    >
    >
    >>Is it really substantial, or is it a matter of feeling less stable on smaller wheels? Is there a
    >>linear relation?
    >
    >
    > In most scientific measurements, the method of measurement has an effect on the outcome of
    > measurement, which is what this discussion has focused on.

    The GreenGear reference doesn't appear to specify the details of the measurement method, but it
    looks like data that I've seen published which used a small diameter drum rolling on the tire under
    test to evaluate rolling resistance. As I pointed out previously in this thread that method is not
    suitable for evaluating differences due to wheel diameter. In the real world with a tire rolling on
    a rather flat road surface the smaller tire will have a shorter but wider contact patch with
    greater deflection of the tread at the center as compared to an otherwise identical tire (same
    construction, width, pressure, etc.) which has a larger diameter. But when using a drum with a much
    smaller diameter than either tire the contact patches will have essentially the same shape.
    Therefore this test ignores a very significant difference in the real-world behavior of small vs.
    large diameter tires and can't be used to compare overall rolling resistance differences resulting
    from wheel size changes.
     
  16. Tim McNamara:

    > At the point where the rubber meets the road, the tire flattens out a bit. This is referred to as
    > the "contact patch." That patch, which results from the deformation of the tire, involves flexing
    > at the front or leading edge and the rear or trailing edge. From what I've read, most of the
    > hysteresis loss (perhaps all? I've never asked about this) occurs at the leading edge of the
    > contact patch.

    There'll be energy loss at the front and energy payback at the back. But does that mean the
    hysteresis losses occur at the front?

    The losses are best considered as the result of an energy-dissipating process not something
    happening at a particular point .

    Andrew Bradley
     
  17. Phil Holman <[email protected]> wrote:

    >For the rolling resistance to be an inverse function of the tire radius, the resisting torques will
    >all be identical.

    Certainly true, but this is working backward from the result which is in question experimentally.

    >You are correct when you state that the 650 tire will have a larger hysteresis loss but this is
    >offset by the shorter contact patch length. The centers of pressure of the forward and rear contact
    >patch halves being indentically disproportionate to make the resultant torques identical.

    Somethings not clear. All the power losses are due to hysteresis in the tyre since there is no other
    mechanism for energy dissipation in the model, No? Or do you mean something else?

    Andrew Bradley
     
  18. Peter :..

    >
    > The GreenGear reference doesn't appear to specify the details of the measurement method, but it
    > looks like data that I've seen published which used a small diameter drum rolling on the tire
    > under test to evaluate rolling resistance. As I pointed out previously in this thread that method
    > is not suitable for evaluating differences due to wheel diameter. In the real world with a tire
    > rolling on a rather flat road surface the smaller tire will have a shorter but wider contact patch
    > with greater deflection of the tread at the center as compared to an otherwise identical tire
    > (same construction, width, pressure, etc.) which has a larger diameter. But when using a drum with
    > a much smaller diameter than either tire the contact patches will have essentially the same shape.
    > Therefore this test ignores a very significant difference in the real-world behavior of small vs.
    > large diameter tires and can't be used to compare overall rolling resistance differences resulting
    > from wheel size changes.

    You don't like these tests, but you agree with the inverse proportion to diameter rule, but you
    don't like Phils explanation of it. And you think the inverse proportion rule breaks down completely
    for small test-drums. Is that the situation?

    Do you know an inverse proportion model can be derived simply from tyre deformations?

    Andrew Bradley
     
  19. Tim McNamara

    Tim McNamara Guest

    In article <[email protected]>, Peter <[email protected]> wrote:

    > The GreenGear reference doesn't appear to specify the details of the measurement method, but it
    > looks like data that I've seen published which used a small diameter drum rolling on the tire
    > under test to evaluate rolling resistance.

    As I've searched around the newsgroup archives, the only RR test rig drum diameter that has been
    specified is Jobst's, which was 2 meters- twice the diameter of a 700C wheel. What are you
    considering a "small" drum?

    > As I pointed out previously in this thread that method is not suitable for evaluating differences
    > due to wheel diameter. In the real world with a tire rolling on a rather flat road surface the
    > smaller tire will have a shorter but wider contact patch with greater deflection of the tread at
    > the center as compared to an otherwise identical tire (same construction, width, pressure, etc.)
    > which has a larger diameter.

    So, the shape of the contact patch is related to the proportion of the tire cross section to the
    major diameter? In whch case a 650 x 25 will have a shorter, wider contact patch than a 700 x 25 at
    the same pressure and assuming the tires are of identical construction. Are my assumptions in
    agreement with yours?

    A 700 x 28 has a shorter, wider contact patch than a 700 x 23; in Jobst's RR tests for Avocet, that
    was shown to result in lower rolling resistance. By extension, then, the 650 x 25 ought to have
    lower rolling resistance than the 700 x 25.

    > But when using a drum with a much smaller diameter than either tire the contact patches will have
    > essentially the same shape.

    I'm not yet convinced this is an accurate assumption. I'm having trouble visualizing it. And if it's
    accurate, I'm not sure it's meaningful. Too bad Jobst hasn't weighed in on this, since he's actually
    conducted rolling resistance tests unlike most of the rest of us.

    > Therefore this test ignores a very significant difference in the real-world behavior of small vs.
    > large diameter tires and can't be used to compare overall rolling resistance differences resulting
    > from wheel size changes.

    While you may be right, I remain unconvinced.
     
  20. Phil Holman

    Phil Holman Guest

    "Andrew Bradley" <[email protected]> wrote in message
    news:[email protected]...
    >
    > Phil Holman <[email protected]> wrote:
    >
    > >For the rolling resistance to be an inverse function of the tire radius, the resisting torques
    > >will all be identical.
    >
    > Certainly true, but this is working backward from the result which is in question experimentally.

    I agree that this whole thing is shrouded in complications making the inverse function
    questionable. I've been looking at contact patches of 700 and 650 tires and calculated chord
    lengths, widths and compression heights by taking a slice through a toroid to obtain the same
    approximate contact area (crude).

    700 650 compression .038" .040" length 2.00" 1.98" width .380" .384"

    We are looking at a 1% decrease in offset of the centers of pressure for a 650. To balance this (and
    force fit the questionable inverse radius proportionality) a 1/2% load increase on the forward
    contact area and a
    1/2% load decrease on the rear contact area would be required to maintain the same wheel load. Does
    this sound reasonable for a .002" additional deflection at the high spot?

    >
    > >You are correct when you state that the 650 tire will have a larger hysteresis loss but this is
    > >offset by the shorter contact patch length. The centers of pressure of the forward and rear
    > >contact patch halves being indentically disproportionate to make the resultant torques identical.
    >
    > Somethings not clear. All the power losses are due to hysteresis in the tyre since there is no
    > other mechanism for energy dissipation in the model, No? Or do you mean something else?

    The energy is only lost through hysteresis in the tire. I may not have been entirely clear on that
    point previously.

    Phil Holman
     
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