650C rolling resistance higher

gierst said:

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.

"gierst" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Has anyone out there objective data on the difference in rolling

resistance

Quoted message said:

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

Quoted message said:

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

The following are some rolling resistance test of 700c and 26" mt bike tires that may help:
terrymorse.comrolres.htmlOpen ↗

"gierst" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Has anyone out there objective data on the difference in rolling

resistance

Quoted message said:

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

Quoted message said:

on smaller wheels? Is there a linear relation? Any info is welcome,

Sjef

bfd said:

The following are some rolling resistance test of 700c and 26" mt bike tires that may help:
terrymorse.comrolres.htmlOpen ↗

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.

Quoted message said:

"gierst" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Has anyone out there objective data on the difference in rolling

resistance

Quoted message said:

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

Quoted message said:

on smaller wheels? Is there a linear relation? Any info is welcome,

Sjef

Peter :

Quoted message said:

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

"Peter" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

bfd said:

The following are some rolling resistance test of 700c and 26" mt

bike tires

Quoted message said:

Quoted message said:

that may help: terrymorse.comrolres.htmlOpen ↗

While these rolling resistance measurements are very valuable, some

care

Quoted message said:

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

Quoted message said:

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.

Quoted message said:

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

Quoted message said:

a shorter but wider patch compared with the larger diameter wheel.

This

Quoted message said:

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

Quoted message said:

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

Phil Holman said:

"Peter" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

bfd said:

The following are some rolling resistance test of 700c and 26" mt

bike tires

Quoted message said:

Quoted message said:

that may help: terrymorse.comrolres.htmlOpen ↗

While these rolling resistance measurements are very valuable, some

care

Quoted message said:

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

Quoted message said:

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).

Quoted message said:

Quoted message said:

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

Quoted message said:

a shorter but wider patch compared with the larger diameter wheel.

This

Quoted message said:

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

Quoted message said:

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.

Quoted message said:

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.

"Peter" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

Phil Holman said:

"Peter" <[email hidden]> wrote in message "]news:[email hidden]...

Quoted message said:

bfd wrote:

>The following are some rolling resistance test of 700c and 26" mt

bike tires

Quoted message said:

>that may help: terrymorse.comrolres.htmlOpen ↗

While these rolling resistance measurements are very valuable, some

care

Quoted message said:

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

Quoted message said:

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

Quoted message said:

Quoted message said:

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

Quoted message said:

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.

Quoted message said:

Quoted message said:

Quoted message said:

Note that when comparing two tires of identical pressure and width

but

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

a shorter but wider patch compared with the larger diameter wheel.

This

Quoted message said:

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

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

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

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

rolling resistance for smaller wheels which empirically is not the

case.

Quoted message said:

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

Quoted message said:

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.

Quoted message said:

Quoted message said:

The effect is small and overshadowed by the increased force required

to

Quoted message said:

Quoted message said:

overcome the slightly less or identical resistance torque of a

smaller

Quoted message said:

Quoted message said:

diameter wheel. I.e. a 5 inch-lb resistance torque will result in a

5/R

Quoted message said:

Quoted message said:

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

Quoted message said:

of rolling resistance. The main energy waste comes about directly as

a

Quoted message said:

result of the deformation of the tire and tube materials in the

vicinity

Quoted message said:

of the contact patch. If the tire deformation is greater (as it is

with

Quoted message said:

a smaller diameter tire and shorter/wider contact patch), then there

is

Quoted message said:

more energy going into making the tire squirm and deform. This

process

Quoted message said:

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.

Quoted message said:

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

Quoted message said:

resistance with smaller diameter wheels, but the tire/tube contact

patch

Quoted message said:

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

Quoted message said:

resistance than typical pneumatic-tire wheels although the losses in

the

Quoted message said:

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

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

"Tim McNamara" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

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

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?

Quoted message said:

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.

Quoted message said:

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

Quoted message said:

From: gierst <[email hidden]>

Quoted message said:

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:

Quoted message said:

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:

www-ifia.fzk.deROLLW E.HTMOpen ↗

Quoted message said:

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:

physics.helsinki.firolling.htmlOpen ↗
legslarry.beerdrinkers.co.ukGS.htmOpen ↗
legslarry.beerdrinkers.co.ukJL.htmOpen ↗
ice.hpv.co.uk495000 Soapbox Tyres TRS.pdfOpen ↗

Tim McNamara said:

Quoted message said:

From: gierst <[email hidden]>

Quoted message said:

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:

Quoted message said:

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:

www-ifia.fzk.deROLLW E.HTMOpen ↗

Quoted message said:

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.

Tim McNamara:

Quoted message said:

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

Phil Holman said:

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.

Quoted message said:

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

Peter :..

Quoted message said:

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

In article said:

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?

Quoted message said:

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.

Quoted message said:

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.

Quoted message said:

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.

"Andrew Bradley" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

Phil Holman said:

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?

Quoted message said:

Quoted message said:

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

Peter said:

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.

Another two cents worth-- I think I first saw the Greenspeed/Ian Sims data (measured with a
small diameter roller) in "Human Power", an IHPVA publication <www.ihpva.org>? The article
appeared maybe 8-10 years ago? It would be worth reviewing that article for the test
proceedure--somehow I seem to recall that the tire pressure and/or load was varied in an attempt
to control the tire deflection...larger deflection on a small 4 inch diameter drum than on a
flat surface (or large drum).

Anyway, at that time I had access to some Crr data measured on large diameter drums (car-tire
industry-standard, about 5.6 feet diameter which gives 300 revs/mile), and the Greenspeed data
didn't seem to correlate at all. In some cases the Greenspeed data didn't even have the same rank
order as the large drum data. I didn't go much further with it, and of course there may have been
problems with my informal attempt at correlation.

-- Doug Milliken www.millikenresearch.com