(Small) Tire Science? Contact Patch Wrinkles?

On Jul 3, 9:27 am, "Jeff Potter (of OutYourBackdoor.com)"
<[email hidden]> wrote:
[ ]

Quoted message said:

I recall that the German research that was quoted mentioned a couple
things that weren't discussed (buried in here:http://groups.google.com/group/rec.bicycles.tech/browse_thread/thread...).
Offhand, it seems that there's a bulge in front of and behind a
contact patch and furthermore there might even be a concavity---I saw
this in a graphic. These effects seem to be due to bias-ply casing
wrinkling as it flattens into the contact patch.

PS: It seems like some of the loss caused by the bulge is because it's
a "hill" that has to be rolled over---the German article translation
used the awkward term "tilting over an edge"---but I'm combining hints
from the 2 (unusual) resistances they list: "unreeling" and "pull." I
think that "unreeling" might've been corrected as "coasting."

Jeff Potter (of OutYourBackdoor.com) said:

On Jul 3, 9:27 am, "Jeff Potter (of OutYourBackdoor.com)"<[email hidden]> wrote:

[ ]

Quoted message said:

I recall that the German research that was quoted mentioned a couple
things that weren't discussed (buried in here:http://groups.google.com/group/rec.bicycles.tech/browse_thread/thread...).
Offhand, it seems that there's a bulge in front of and behind a
contact patch and furthermore there might even be a concavity---I saw
this in a graphic. These effects seem to be due to bias-ply casing
wrinkling as it flattens into the contact patch.

PS: It seems like some of the loss caused by the bulge is because it's
a "hill" that has to be rolled over---the German article translation
used the awkward term "tilting over an edge"---but I'm combining hints
from the 2 (unusual) resistances they list: "unreeling" and "pull." I
think that "unreeling" might've been corrected as "coasting."

I guess the question is does any of this really make a difference? I
think getting a bike that fits and making sure you are FIT make more
of a difference than worrying about whether your tires or wheels have
more resistance. If you're in shape, you'll go fast. If not, then the
most aero bike isn't going to get you up that hill any faster!

Jeff Potter (of OutYourBackdoor.com) said:

I saw the long chat here last fall about rolling resistance.

I have a few more questions...

How do small wheels relate to bike science, tire behavior and energy
losses? There are increased angle of attack and increased tire squat
(I think). I would think that some losses could be reduced with extra
supple casing and wider tires at comparable or higher pressures. But I
suppose those changes bring on their own losses?

(I guess suppleness reduces energy loss but increases the contact
patch... But it looks like there's still debate as to how this affects
real world losses on "rough" roads---with Jan saying this is important
and Jobst saying it isn't...)

Probably most small wheel science would relate to 20" but Moulton
16-17" have been studied a lot, even if not used often. What happens
as size goes down to the 12" wheels that some micro bikes use? Is good
performance possible?...

My experience is that I out-coast tri/TT bikes, upright tandems and
even Zzipper faired Easy Racers on an ISO 305-mm (Primo Comet) / ISO
406-mm (Japanese Tioga Comp Pool) wheelset, so the rolling resistance
penalty must not be too bad.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

In article <[email hidden]>,

Jeff Potter (of OutYourBackdoor.com) said:

I saw the long chat here last fall about rolling resistance.

I have a few more questions...

How do small wheels relate to bike science, tire behavior and energy
losses? There are increased angle of attack and increased tire squat
(I think). I would think that some losses could be reduced with extra
supple casing and wider tires at comparable or higher pressures. But
I suppose those changes bring on their own losses?

From what I have read, Alex Moulton tested his small wheel bike against
a regular bike (his own Hetchins road bike) and found that the small
wheel bike was about 2% more efficient as a total package. I think you
can find the particulars in one of Tony Hadland's books and there might
be something on the Web about it too. Bear in mind that there were
significant differences that make it difficult: hub gear versus
derailleur, small wheel versus large, suspension versus none.

Quoted message said:

(I guess suppleness reduces energy loss but increases the contact
patch... But it looks like there's still debate as to how this
affects real world losses on "rough" roads---with Jan saying this is
important and Jobst saying it isn't...)

I don't see any reason why a more supple tire would have an increased
contact patch, all other things being equal. I also don't know that
there has been much discussion of suppleness vis-a-vis rough roads. I
think the disagreement between Jan and Jobst is whether measuring RR on
pavement versus a steel drum would change the ordinal ranking of the
tires in terms of rolling resistance. Otherwise they were pretty
broadly in agreement, I thought. There were a number of questions about
noise and errors of measurement in Jan's data using a roll-down test to
try to determine RR, but those concerns were not unique to Jobst.

Quoted message said:

Probably most small wheel science would relate to 20" but Moulton
16-17" have been studied a lot, even if not used often. What happens
as size goes down to the 12" wheels that some micro bikes use? Is
good performance possible?

Small wheels also seem to affect handling---mostly due to change of
contact patch size and less pneumatic trail? A big wheel has a much
longer patch, right? There might be bigger factors, of course... But
frame/fork dimensions should be altered to give stability if small
wheels are used, right?

Unfortunately it's comparing apples to oranges. Small wheel bikes often
have odd steering geometries so it is difficult to estimate the effects
of the wheel size. Moulton tested steering geometries empirically,
having forks make with really long dropouts that allowed for quickly
changing the effective fork offset and thus geometric trail. The old F
frame Moultons tended to feel pretty normal, at least the few I have
ridden did. I've only briefly ridden one AM series Moulton.

Quoted message said:

The small wheel question seems to relate to another tire behavior
question that seemed to get overlooked last fall.

Quoted message said:

I recall that the German research that was quoted mentioned a couple
things that weren't discussed (buried in here:
http://groups.google.com/group/rec.bicycles.tech/browse_thread/thread/e0a8bca8
6795ebb5/6bc6e9842c888dd5?lnk=st&q=german+research+rolling+resistance&rnum=1&h
l=en#6bc6e9842c888dd5).
Offhand, it seems that there's a bulge in front of and behind a
contact patch and furthermore there might even be a concavity---I saw
this in a graphic. These effects seem to be due to bias-ply casing
wrinkling as it flattens into the contact patch. There was brief,
indirect discussion of bias vs. radial casing, with the idea that bike
tires are made somewhat on the bias---and so perhaps this results in
the tire wrinkling mentioned? The conclusion of the German research
said that radial bike tires like the handmade Rinki's would roll
better in small sizes (large, too?) because they avoid this wrinkling?
(In part?)

This is beyond my knowledge base, hopefully someone who knows more about
it will post. I recall reading that radial bike tires were tried by
Michelin and that the testers did not like how the tires felt to ride,
although I don't know what their objection was.

Quoted message said:

At any rate, it seems that small wheels would end up with more intense
bulging, etc., unless offset by other factors.

I don't see why. Inflation pressure is one of the most central factors
in how much the tire bulges out at the contact patch. PSI is PSI no
matter the major diameter of the wheel.

Quoted message said:

Tire science...a swamp!

Thanks, Jeff Potter

Tim McNamara said:

Inflation pressure is one of the most central factors
in how much the tire bulges out at the contact patch. PSI is PSI no
matter the major diameter of the wheel.

The length of the tire's contact patch at a given PSI is a function of
the tire's diameter-- and it seems to vary by more than the
proportional change in diameter. The bigger the wheel, the lower the
feasible tire pressure for any given width, and the lower the casing
deflection (and thus RR losses) for any given pressure and width.

I've been using 700x60 Schwalbe Big Apple slicks, and they are a
revelation. I can easily run them at pressures below 30psi without
bottoming or noteworthy rolling resistance. This is contrary to my
experience with 26", let alone 20", tires in comparable widths.

Chalo

In article <[email hidden]>,

Chalo said:

Tim McNamara said:

Inflation pressure is one of the most central factors in how much
the tire bulges out at the contact patch. PSI is PSI no matter the
major diameter of the wheel.

The length of the tire's contact patch at a given PSI is a function
of the tire's diameter-- and it seems to vary by more than the
proportional change in diameter. The bigger the wheel, the lower the
feasible tire pressure for any given width, and the lower the casing
deflection (and thus RR losses) for any given pressure and width.

Interesting point. I was thinking in terms of the area of the contact
patch and didn't think about it's shape. Time for Carl Fogel to break
out his stamp pad and graph paper over at Fogel Labs.

Quoted message said:

I've been using 700x60 Schwalbe Big Apple slicks, and they are a
revelation. I can easily run them at pressures below 30psi without
bottoming or noteworthy rolling resistance. This is contrary to my
experience with 26", let alone 20", tires in comparable widths.

A friend used the 7000 x 60 Big Apples for quite a long time and loved
them. At 50 psi he seemed to have no trouble with riding centuries. It
was amusing to watch the reactions of people on 700 x 23s when he turned
up for rides with those huge tires. They expected him to be pushing
along at 13 mph but he had no trouble rolling along at 18-20 and didn't
get dropped on downhills.

I don't recall riding non-knobby tires of 26" or smaller with similar
width, so it would be comparing apples to oranges. I did have a Birdy
folder for several years with 18" (355-28) tires and suspension. With
the Schwalbe Stelvio tires, it rolled as well as any of my road bikes
with 700C wheels- as far as i could tell, at least. With the stock
Birdy tires, it was noticeably slower. The current owner puts it in a
suitcase and takes it on business trips all over the world; he's put it
to much better use than I ever did.

Tim McNamara said:

In article <[email hidden]>,

Chalo said:

Tim McNamara said:

Inflation pressure is one of the most central factors in how much
the tire bulges out at the contact patch. PSI is PSI no matter the
major diameter of the wheel.

The length of the tire's contact patch at a given PSI is a function
of the tire's diameter-- and it seems to vary by more than the
proportional change in diameter. The bigger the wheel, the lower the
feasible tire pressure for any given width, and the lower the casing
deflection (and thus RR losses) for any given pressure and width.

Interesting point. I was thinking in terms of the area of the contact
patch and didn't think about it's shape. Time for Carl Fogel to break
out his stamp pad and graph paper over at Fogel Labs.

[snip]

Dear Tim,

The contact patch area for a bicycle tire tends toward a favored size,
despite inflation. Straightforward inflation theory works well for
pistons in metal-walled cylinders, but not for inflated canvas-sided
toroids pressed against flat, unyielding surfaces.

Here's a graph, showing measured versus predicted sizes:

http://i17.tinypic.com/2j3jpqc.jpg

At over ~70 psi, the contact patch of a 700 x 25 becomes larger and
larger than expected, instead of shrinking to suit the mistaken theory
that area times inflation must equal load. At 120 psi with a 100-lb
load, measured patches are 30%~35% larger than straightforward air
pressure predicts.

This effect at over ~70 psi is probably due to the rubber mostly
curving away at the edges of the small contact patch, creating a ring
of low-pressure. Most of the patch is pressing down at the inflation
pressure, but the pressure fades away to zero toward the edges,
creating a larger contact patch.

At under ~70 psi, things reverse. The same tire's contact patch
refuses to expand as much as expected. At 30~40 psi, measured contact
patches are only 62%~80% as large as predicted.

This effect at under ~70 psi is probably due to the rubber pressing
down with extra pressure at the edges of the large contact patch,
instead of the pressure fading away.

The source of the force is quite obvious--at low pressures, the
sidewalls are being bent visibly outward like curved leaf-springs
against the air pressure that tries to keep them in more circular
arcs.

As Chalo says, the contact patch tends to lengthen more than it
spreads. Think of the tire as a series of hoops, like rubber cheerios
on a ring. With more load (or lower inflation), the tire works by
squashing the original central hoop a little more so that new hoops on
either side touch the ground, start to bend, and support the new load.

Cheers,

Carl Fogel

In article <[email hidden]>,

Tim McNamara said:

A friend used the 7000 x 60 Big Apples for quite a long time

^^^^^^^
It was a funny looking bike- he's really tall!

In article <[email hidden]>,

Quoted message said:

Tim McNamara said:

In article <[email hidden]>,

Chalo said:

Tim McNamara wrote:
>
> Inflation pressure is one of the most central factors in how
> much the tire bulges out at the contact patch. PSI is PSI no
> matter the major diameter of the wheel.

The length of the tire's contact patch at a given PSI is a
function of the tire's diameter-- and it seems to vary by more
than the proportional change in diameter. The bigger the wheel,
the lower the feasible tire pressure for any given width, and the
lower the casing deflection (and thus RR losses) for any given
pressure and width.

Interesting point. I was thinking in terms of the area of the
contact patch and didn't think about it's shape. Time for Carl
Fogel to break out his stamp pad and graph paper over at Fogel Labs.

[snip]

Dear Tim,

The contact patch area for a bicycle tire tends toward a favored
size, despite inflation. Straightforward inflation theory works well
for pistons in metal-walled cylinders, but not for inflated
canvas-sided toroids pressed against flat, unyielding surfaces.

Here's a graph, showing measured versus predicted sizes:

http://i17.tinypic.com/2j3jpqc.jpg

I remember that discussion and the attempts to explain the observed
information. It's striking how well your measurement's and Tom's fit.
I can't recall- were both of you using the same make/model tire?

<snip>

Quoted message said:

As Chalo says, the contact patch tends to lengthen more than it
spreads. Think of the tire as a series of hoops, like rubber cheerios
on a ring. With more load (or lower inflation), the tire works by
squashing the original central hoop a little more so that new hoops
on either side touch the ground, start to bend, and support the new
load.

Yes, that's a simpler and better visual than my explanation would have
been.

Tim McNamara said:

In article <[email hidden]>,

Quoted message said:

Tim McNamara said:

In article <[email hidden]>,
Chalo <[email hidden]> wrote:

> Tim McNamara wrote:
> >
> > Inflation pressure is one of the most central factors in how
> > much the tire bulges out at the contact patch. PSI is PSI no
> > matter the major diameter of the wheel.
>
> The length of the tire's contact patch at a given PSI is a
> function of the tire's diameter-- and it seems to vary by more
> than the proportional change in diameter. The bigger the wheel,
> the lower the feasible tire pressure for any given width, and the
> lower the casing deflection (and thus RR losses) for any given
> pressure and width.

Interesting point. I was thinking in terms of the area of the
contact patch and didn't think about it's shape. Time for Carl
Fogel to break out his stamp pad and graph paper over at Fogel Labs.

[snip]

Dear Tim,

The contact patch area for a bicycle tire tends toward a favored
size, despite inflation. Straightforward inflation theory works well
for pistons in metal-walled cylinders, but not for inflated
canvas-sided toroids pressed against flat, unyielding surfaces.

Here's a graph, showing measured versus predicted sizes:

http://i17.tinypic.com/2j3jpqc.jpg

I remember that discussion and the attempts to explain the observed
information. It's striking how well your measurement's and Tom's fit.
I can't recall- were both of you using the same make/model tire?

<snip>

Quoted message said:

As Chalo says, the contact patch tends to lengthen more than it
spreads. Think of the tire as a series of hoops, like rubber cheerios
on a ring. With more load (or lower inflation), the tire works by
squashing the original central hoop a little more so that new hoops
on either side touch the ground, start to bend, and support the new
load.

Yes, that's a simpler and better visual than my explanation would have
been.

Dear Tim,

I measured a single tire, nominally 700x26 from 30 to 120 psi in 10
psi increments.

Tom measured 5 tires of various widths from 60 to 120 psi in 10 psi
increments.

When I averaged the data points for his 5 tires, his results were
ridiculously close to my single-tire data points--the blue and yellow
lines on the graph pretty much match.

I think that the graph of my areas at 30-40-50 psi is jagged,
suggesting that more measurements would produce a smoother curve, but
the general trend seemed to be clear--the tire just didn't spread out
nearly as much as expected.

The sidewall tension probably gives a progressive spring-style
resistance. By the time the tire flattens into impact-puncture
dimensions, the originally round cross-section of the tire has been
enormously distorted against the resistance of the air pressure on the
sidewalls.

Another way to appreciate the sidewall force is to hold a tire off the
ground and then imagine how hard you'd have to pull each sidewall
outward to produce the same flattening at the contact patch. The tire
surface is stretched taut, like a 3-D curved trampoline--bulging it in
either direction takes a lot of force.

At high pressures, the sidewalls bulge very little, and the very edges
of the contact patch press against the ground with less than inflation
pressure because the sidewall is pulling them away from the ground.

But once the load is high enough (or the pressure low enough) to
flatten a good deal of rubber against the ground, the tables turn, and
the sidewall switches to pushing the edges down even harder than
inflation pressure.

Tread thickness and sidewall stiffness probably influence things a
bit, but the main factor is likely to be the cross-section diameter of
the unloaded tire.

Given the same inflation and load . . .

A very wide tire should act like the test tires at high pressure, with
a shorter, rounder contact patch with low-pressure edges that's larger
than pressure x area predicts. (Fewer hoops are bending.)

A very thin tire should act like the test tires at low pressure, with
a long contact patch with high-pressure edges that's smaller than
pressure x area predicts. (More hoops are bending.)

From a practical point of view, the shorter, rounder contact patch
involves less rolling resistance because there's less sidewall
bending.

But the longer contact patch may give better traction because the
pressure is greater than inflation at the edges and the longer strip
_may_ bridge small slippery spots better when cornering.

Here's an exaggerated comparison:

same load, same inflation

narrow tire, wide tire,
slightly larger area slightly smaller area
long, thin contact patch short, wide contact patch

xxxxxxxxxxx XXXXXXX
<-bridges-> <slips>
..1234567.. 1234567 <--same slippery patch

Cheers,

Carl Fogel

Carl, any idea how this stuff relates to small wheels? 20"? 12"? Wide
and narrow tires? Low pressure, high pressure. Thanks, JP

PS: Carl, have you heard of the bulge/bump in front of the contact
patch? Behind it? How about a concavity after the bulge but before the
patch? --Due to bias ply? Did you see the mention of the losses in
that German research from "tilting over the edge" of the bulge? ---JP

On Thu, 05 Jul 2007 05:10:47 -0700, "Jeff Potter (of

OutYourBackdoor.com) said:

Carl, any idea how this stuff relates to small wheels? 20"? 12"? Wide
and narrow tires? Low pressure, high pressure. Thanks, JP

Dear Jeff,

Contact patches probably work the same way for any round cross-section
tire.

Given a wide enough range of inflation and load . . .

At high pressure the dominant effect will be the ring of low-pressure
contact, where the rubber is curving away from the ground. This
rounder contact patch will have an absolute area smaller than simple
inflation theory predicts. It will have lower rolling resistance
because the sidewalls are hardly bending at all, so they waste less
energy in internal friction. (Similarly, thin sidewalls are more
efficient, since less material is bending.)

At low pressure, the dominant effect will be the high-pressure contact
at the edges, caused by the sidewalls bending enough to function as
C-shaped springs. This long contact patch will have an absolute area
larger than simple inflation theory predicts.

The transition point depends on load, inflation, and cross section. As
load increases (or inflation decreases), the angle of the sidewall
meeting the ground becomes steep enough that the spring-effect becomes
dominant.

In real life, the practical details goof up all sorts of things.

As Sheldon points out, if you have a wide tire and a thin tire at the
same pressure, at least one of them is at the wrong pressure. We use
thin tires on 700c rims because those rims can't handle high inflation
pressures with very wide tires.

A smaller rim can handle higher inflation and reduces wind drag, but
it increases real-life rolling resistance in that it hits real-life
road irregularities at a steeper angle, so more energy goes into
bouncing the bike and rider up and down.

The history of the Moulton small-wheel bicycle offers a good example
of such practical trade-offs. In 1983, Moultons came with 17 inch
wheels:

http://www.moultoneers.net/moultam.html

In 1998, the new series switched to 20 inch wheels:

http://www.moultoneers.net/newnew.html

The size of the rims and tires must deal with the practical problems
of wind drag, rolling resistance, suspension, gearing, and
availability. A very small tire and rim can reduce wind drag and
theoretical rolling resistance, but it can require more suspension,
special gearing, and terrible supply problems.

Cheers,

Carl Fogel

Quoted message said:

On Thu, 05 Jul 2007 05:10:47 -0700, "Jeff Potter (of

OutYourBackdoor.com) said:

Carl, any idea how this stuff relates to small wheels? 20"? 12"? Wide
and narrow tires? Low pressure, high pressure. Thanks, JP

Dear Jeff,

Contact patches probably work the same way for any round cross-section
tire.

Given a wide enough range of inflation and load . . .

At high pressure the dominant effect will be the ring of low-pressure
contact, where the rubber is curving away from the ground. This
rounder contact patch will have an absolute area smaller than simple
inflation theory predicts. It will have lower rolling resistance
because the sidewalls are hardly bending at all, so they waste less
energy in internal friction. (Similarly, thin sidewalls are more
efficient, since less material is bending.)

At low pressure, the dominant effect will be the high-pressure contact
at the edges, caused by the sidewalls bending enough to function as
C-shaped springs. This long contact patch will have an absolute area
larger than simple inflation theory predicts.

The transition point depends on load, inflation, and cross section. As
load increases (or inflation decreases), the angle of the sidewall
meeting the ground becomes steep enough that the spring-effect becomes
dominant.

In real life, the practical details goof up all sorts of things.

As Sheldon points out, if you have a wide tire and a thin tire at the
same pressure, at least one of them is at the wrong pressure. We use
thin tires on 700c rims because those rims can't handle high inflation
pressures with very wide tires.

A smaller rim can handle higher inflation and reduces wind drag, but
it increases real-life rolling resistance in that it hits real-life
road irregularities at a steeper angle, so more energy goes into
bouncing the bike and rider up and down.

The history of the Moulton small-wheel bicycle offers a good example
of such practical trade-offs. In 1983, Moultons came with 17 inch
wheels:

http://www.moultoneers.net/moultam.html

In 1998, the new series switched to 20 inch wheels:

http://www.moultoneers.net/newnew.html

The size of the rims and tires must deal with the practical problems
of wind drag, rolling resistance, suspension, gearing, and
availability. A very small tire and rim can reduce wind drag and
theoretical rolling resistance, but it can require more suspension,
special gearing, and terrible supply problems.

Cheers,

Carl Fogel

very informative summary.

One last part of the question: Does anyone know anything about the
bulges, concavity, wrinkles that relate to the leading and trailing
edges of the contact patch? And maybe relate to the bias-ply
construction vs. radial? As I mentioned the German research mentions
this stuff but no one else has ever said a word about it. Thanks, JP

Jeff Potter (of OutYourBackdoor.com) said:

One last part of the question: Does anyone know anything about the
bulges, concavity, wrinkles that relate to the leading and trailing
edges of the contact patch? And maybe relate to the bias-ply
construction vs. radial? As I mentioned the German research mentions
this stuff but no one else has ever said a word about it.

Bicycle Quarterly did a big article on this last year.

--

John ([email hidden])

John Thompson said:

Jeff Potter (of OutYourBackdoor.com) said:

One last part of the question: Does anyone know anything about the
bulges, concavity, wrinkles that relate to the leading and trailing
edges of the contact patch? And maybe relate to the bias-ply
construction vs. radial? As I mentioned the German research mentions
this stuff but no one else has ever said a word about it.

Bicycle Quarterly did a big article on this last year.

It did?

I thought I had all those... Title? Issue? Thanks, JP