Thumb test

Tim McNamara said:

The ordinal ranking of the top ten tires was (widths are actual): Deda
Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28 tubular);
Michelin Pro2 Race (700 x 25); Continental Ultra Gator (700 x 23),
Mistuboshi Trimline (650B x 37), Panaracer Pasela (700 x 35); Clement
Criterium (700 x 21); Avocet Cross (700 x 35); Avocet Duro (700 x 28).

I wonder when this was done. The comparisons with most of these tires
just no longer matter. Certainly neither the Clement del Mondo nor the
Criterium are available any longer, and if they were, they would cost a
bleeding fortune. They were beautiful, handmade silk tires, but their
kind has gone extinct.

Or, it may be that someone has resurrected the names, though not the tires.

--

David L. Johnson

__o | When you are up to your ass in alligators, it's hard to remember
_`\(,_ | that your initial objective was to drain the swamp. -- LBJ
(_)/ (_) |

Blair P. Houghton said:

Quoted message said:

You seem determined to miss every point--including the nail. 🙂

I'm going to sit here and tell you flat-out that you just
projected.

Here, you're fond of balloons, do this:

Take a balloon, preferably the kind that are shaped
like a baloney rather than a basketball. Inflate it.

Now press the nose of the balloon against your hand.

Notice what happens to the tension of the rubber against
your palm. It decreases. The rubber relaxes there.
It contracts towards its uninflated size.

That isn't increasing the tension in the rubber,
it's relieving it.

isn't that because because it's in contact with your hand? as i
understand it, if your hand was entirely frictionless, tension would
remain as the skin of the balloon has to be in equilibrium.

Quoted message said:

It's still under inflated tension, because its uninflated
size is like a centimeter in diameter and you've inflated
it to a diameter of several inches.

But it's less.

You can make that end wrinkle and slew, which you can't
do with the fully-tensioned sides.

I'd say you shouldn't go in so much for the physics
discussions until you've had a few more birthday parties.
You need more of a grounding in real data to understand
a lot of this stuff.

--Blair

David L. Johnson said:

Ben C said:

Well, the larger tyre requires less pressure for a given casing tension.
I read that here recently and am still getting my head around it. It's
also mentioned here by Jobst Brandt:

http://www.sheldonbrown.com/brandt/rim-support.html

"[...] unit casing tension is equivalent to inflation pressure times the
radius of curvature divided by pi [...]".

I was a bit surprised by this at first, but then if you think pressure
is force per unit area, if you increase the area of the inside of the
casing, you need more force for a given pressure. Not sure if this
reasoning is bogus or not though.

It is right, of course. And, the area increase is roughly proportional to
the tire width.

Quoted message said:

So if the thumb test measures casing tension, then the larger tyre does
require less pressure to get to the same thumb-hardness.

But a thumb-press is not measuring casing tension. It is moving the
casing perpendicular to the direction of the tension, so feels nothing
from that directly, only the internal pressure.

10 points. go to the top of the class.

Tim McNamara said:

The ordinal ranking of the top ten tires was (widths are actual): Deda
Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28 tubular);
Michelin Pro2 Race (700 x 25); Continental Ultra Gator (700 x 23),
Mistuboshi Trimline (650B x 37), Panaracer Pasela (700 x 35); Clement
Criterium (700 x 21); Avocet Cross (700 x 35); Avocet Duro (700 x 28).

I just went looking for the Dedas (of course I want them! for I
must go faster than any other 43-year-old solo huffer on Pecos
Road!) and someone has included a partial table of the rolling-
resistance coefficients here:

========
http://www.woodlandscycling.org/index.php?option=com_joomlaboard&func=view&catid=5&id=5335

The rule of thumb is that .0001 of Crr, equates to about 1 watt at 25-30 mph. So, for example, the Pro 2 Race takes 4 more watts than the Deda Tre Giro

Deda Tre Giro d'Italia......... 0.0038
Michelin Pro 2 Race...........0.0042
Vittoria Diamante Pro Rain....... 0.0044
Continental GP Force (rear specific).. 0.0057
Contintal Podium (Tubular).......0.0060
Specialized S-Works Mondo........0.0061
Continental GP 3000...........0.0067
Tufo Hi-Composite Carbon (Tubular)...0.0077
==========

Quoted message said:

There is not a Web version of this article. You'll have to get a copy
from Vintage Bicycle Press, with which I have no connection other than
being a subscriber. I'm intrigued as the results support some things
that I have previously dismissed as myth and lore. Being a rather
skeptical sort, I have to question whether these results are accurate,
being that they seem to fly in the face of previous tests, or whether
the results point out some things that haven't been taken into account.

I think the 2:1 ratio of Crr's in that table and the
obvious mixing of sizes in the list indicates that
unless you buy exactly those items that were tested
you will not be able to tell by looking at the tire
specs whether it will have a high Crr or a low one.

I.e., get the ones that look cool, and go improve the
engine.

--Blair

David L. Johnson said:

Ben C said:

Well, the larger tyre requires less pressure for a given casing tension.
I read that here recently and am still getting my head around it. It's
also mentioned here by Jobst Brandt:

http://www.sheldonbrown.com/brandt/rim-support.html

"[...] unit casing tension is equivalent to inflation pressure times the
radius of curvature divided by pi [...]".

I was a bit surprised by this at first, but then if you think pressure
is force per unit area, if you increase the area of the inside of the
casing, you need more force for a given pressure. Not sure if this
reasoning is bogus or not though.

It is right, of course. And, the area increase is roughly proportional to
the tire width.

Quoted message said:

So if the thumb test measures casing tension, then the larger tyre does
require less pressure to get to the same thumb-hardness.

But a thumb-press is not measuring casing tension. It is moving the
casing perpendicular to the direction of the tension, so feels nothing
from that directly, only the internal pressure.

Dear Dave,

A thumb press measures the casing tension.

At 100 psi, the inflated tire is motionless.

But the air pressure on each side of the tire is unequal.

What stops the tire from moving is the not-truly-perpendicular tension
of the casing. The casing can't be truly flat. It must always be at at
slight angle if there's any pressure differential, just as a rope
cannot be stretched truly horizontal against the force of gravity.

When you apply a 30-pound pressure against the taut casing, your thumb
moves the casing inward.

(Or outward, if you're inside the tire--it doesn't matter.)

Then your thumb stops moving.

The tire pressure remains at 100 psi.

What stops your thumb from moving is the local increase in tire casing
tension on the doughnut-shaped trampoline kept taut by internal air
pressure.

To repeat, the tire pressure does not change.

It's the tension of the casing that changes until it's enough to
balance the forces again and stop your thumb from moving. Otherwise,
your thumb would move endlessly because of the net force.

Hang a 100-pound weight from a pulley and let it sit on the ground.
That's the mistaken raw air-pressure idea.

Put a 50-pound weight on the other end of the rope. That's your thumb
pressure in this mistaken theory.

The 100-pound weight just sits there. The 50-pound weight just hangs
there. There's no motion, unlike your thumb against the tire casing.
The 50-pound weight (or a 50-pound push from the ground) can't raise a
100-pound weight.

Here's how the tire actually works.

Put the 100-pound weight on a trampoline. The weight accelerates
toward the ground. The trampoline surface sags, going into tension, no
longer at that perpendicular angle.

When the forces balance, the weight stops moving, just as a tire stops
expanding when the forces of its inflation and its casing tension
match. No net force, no motion.

Hang the 50-pound weight from the pulley again and attach the rope to
the motionless 100-pound weight.

Now the 100-pound weight rises, just as the tire casing moves under
your thumb. Then it stops, just as the tire casing stops moving under
your thumb. The weight still weighs 100 pounds, just as the tire
pressure remained 100 psi.

The weight simply moves to where all the forces balance again. Since
the weight (and air pressure) never changed, the motion just tells you
where the tension balances the thumb/pulley force. The 100-pound
weight simply moves upward to where a 50-pound weight would push the
trampoline.

Indirectly, the thumb press does measure air pressure, since air
pressure will determine casing tension. And that's good enough for
bicycles. But the air pressure never changes. Only the tension and the
small angles at which it resists your thumb change when you press
against a tire, a trampoline, a drum head, or a stretched rubber band.

Cheers,

Carl Fogel

In article <[email hidden]>,

Ben C said:

Does the "thumb test" (squeezing a tyre to see if it's hard enough)
measure tyre pressure or casing tension?

I get the tire pressure by measuring the loaded roll-out
of the tire, then reading the pressure from a chart:

Roll out, 90 psi: 2099 mm
...
Roll out, 120 psi: 2107 mm

--
Michael Press

In article
<[email hidden]>,

Tim McNamara said:

The ordinal ranking of the top ten tires was (widths are actual): Deda
Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28 tubular);
Michelin Pro2 Race (700 x 25); Continental Ultra Gator (700 x 23),
Mistuboshi Trimline (650B x 37), Panaracer Pasela (700 x 35); Clement
Criterium (700 x 21); Avocet Cross (700 x 35); Avocet Duro (700 x 28).

Conspicuously absent is the Avocet Fasgrip.

--
Michael Press

jim beam said:

Blair P. Houghton said:

Quoted message said:

You seem determined to miss every point--including the nail. 🙂

I'm going to sit here and tell you flat-out that you just
projected.

Here, you're fond of balloons, do this:

Take a balloon, preferably the kind that are shaped
like a baloney rather than a basketball. Inflate it.

Now press the nose of the balloon against your hand.

Notice what happens to the tension of the rubber against
your palm. It decreases. The rubber relaxes there.
It contracts towards its uninflated size.

That isn't increasing the tension in the rubber,
it's relieving it.

isn't that because because it's in contact with your hand? as i
understand it, if your hand was entirely frictionless, tension would
remain as the skin of the balloon has to be in equilibrium.

Quoted message said:

It's still under inflated tension, because its uninflated
size is like a centimeter in diameter and you've inflated
it to a diameter of several inches.

But it's less.

You can make that end wrinkle and slew, which you can't
do with the fully-tensioned sides.

I'd say you shouldn't go in so much for the physics
discussions until you've had a few more birthday parties.
You need more of a grounding in real data to understand
a lot of this stuff.

--Blair

Dear Jim,

You're probably right about the friction. It's the equivalent of
twisting part of the bulging rubber surface and pulling it into a knot.

In any case, it doesn't really matter. Total tension must incease

The only way to reduce the tension in a local spot on an expanded
balloon surface is to contract it.

Obviously, its contraction must raise the tension in the elastic
surface all around it, so the the edges are pulling all around the hand
and trying to force it outward.

Consider a trampoline--if the section under your foot contracts, then
the rest of the trampoline must tighten. The total tension must be
greater, not the same or less.

Consider a taut string--if you pull a section together in the middle so
that it hangs slack, then tension must increase in the rest of the
string. The total tension must be greater, not the same or less.

Back to the balloon.

Since the same total force (all the outward air pressure) is now
contained by a smaller total skin (the whole original balloon surface
less amount of local contraction), the tension must rise all around the
edge of your hand--a slightly smaller bag is constraining the same
volume of gas. The rest of the ballon must bulge out as much as your
hands pushed it inward.

The only way that total tension can drop is if you press inward on the
entire surface (raise the exterior air pressure).

Cheers,

Carl Fogel

In article <[email hidden]>,

Michael Press said:

In article
<[email hidden]>,

Tim McNamara said:

The ordinal ranking of the top ten tires was (widths are actual): Deda
Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28 tubular);
Michelin Pro2 Race (700 x 25); Continental Ultra Gator (700 x 23),
Mistuboshi Trimline (650B x 37), Panaracer Pasela (700 x 35); Clement
Criterium (700 x 21); Avocet Cross (700 x 35); Avocet Duro (700 x 28).

Conspicuously absent is the Avocet Fasgrip.

Avocet Duro is the FasGrip design. It was formerly maked 700 x 32 and
is now marked 700 x 28 to reflect reality. I have the old ones which
said "FasGrip" on the label. Now they say "Carbon 12." The testers
only used the one size of Avocet slick road tires.

In article <[email hidden]>,

David L. Johnson said:

Tim McNamara said:

The ordinal ranking of the top ten tires was (widths are actual):
Deda Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28
tubular); Michelin Pro2 Race (700 x 25); Continental Ultra Gator
(700 x 23), Mistuboshi Trimline (650B x 37), Panaracer Pasela (700
x 35); Clement Criterium (700 x 21); Avocet Cross (700 x 35);
Avocet Duro (700 x 28).

I wonder when this was done. The comparisons with most of these
tires just no longer matter. Certainly neither the Clement del Mondo
nor the Criterium are available any longer, and if they were, they
would cost a bleeding fortune. They were beautiful, handmade silk
tires, but their kind has gone extinct.

Or, it may be that someone has resurrected the names, though not the
tires.

The tests were done over three days in 2006. Some of the tires used as
reference were old stock, such as the del Mundos. I don't know about
the Clement Criterium- didn't somebody buy the Clement name and try to
reissue tires made in Thailand under that label a few years back? The
Mitsuboshi Trimline is also recently out of production.

As I said, I am skeptical about accepting the results at face value. I
really don't know how good the methodology was and how well confounds
were controlled for.

In article <[email hidden]>,

Blair P. Houghton said:

Tim McNamara said:

The ordinal ranking of the top ten tires was (widths are actual):
Deda Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28
tubular); Michelin Pro2 Race (700 x 25); Continental Ultra Gator
(700 x 23), Mistuboshi Trimline (650B x 37), Panaracer Pasela (700 x
35); Clement Criterium (700 x 21); Avocet Cross (700 x 35); Avocet
Duro (700 x 28).

I just went looking for the Dedas (of course I want them! for I must
go faster than any other 43-year-old solo huffer on Pecos Road!) and
someone has included a partial table of the rolling- resistance
coefficients here:

========
http://www.woodlandscycling.org/index.php?option=com_joomlaboard&func=
view&cat id=5&id=5335

The rule of thumb is that .0001 of Crr, equates to about 1 watt at
25-30 mph. So, for example, the Pro 2 Race takes 4 more watts than
the Deda Tre Giro

Deda Tre Giro d'Italia......... 0.0038
Michelin Pro 2 Race...........0.0042
Vittoria Diamante Pro Rain....... 0.0044
Continental GP Force (rear specific).. 0.0057
Contintal Podium (Tubular).......0.0060
Specialized S-Works Mondo........0.0061
Continental GP 3000...........0.0067
Tufo Hi-Composite Carbon (Tubular)...0.0077
==========

Those numbers match the findings of TOUR Magazine's measurements in
10/05, according to the table published in Bicycle Quarterly as a
comparison to their study.

Quoted message said:

Quoted message said:

There is not a Web version of this article. You'll have to get a
copy from Vintage Bicycle Press, with which I have no connection
other than being a subscriber. I'm intrigued as the results support
some things that I have previously dismissed as myth and lore.
Being a rather skeptical sort, I have to question whether these
results are accurate, being that they seem to fly in the face of
previous tests, or whether the results point out some things that
haven't been taken into account.

I think the 2:1 ratio of Crr's in that table and the obvious mixing
of sizes in the list indicates that unless you buy exactly those
items that were tested you will not be able to tell by looking at the
tire specs whether it will have a high Crr or a low one.

I.e., get the ones that look cool, and go improve the engine.

The conclusion of the authors was that they would choose tires based not
only on rolling resistance but also other issues, such as comfort,
durability, etc. The authors are randonneurs and one author did find
that he rode significantly faster times on a 600K and 1000K brevet
(setting personal bests and course records) while his times on his
regular tires in other brevets were about typical. He felt that was an
indication that there was something to the results of their testing.

Ben C said:

Does the "thumb test" (squeezing a tyre to see if it's hard enough)
measure tyre pressure or casing tension?

Yes.

In article
<[email hidden]>,

Tim McNamara said:

In article <[email hidden]>,

Michael Press said:

In article
<[email hidden]>,

Tim McNamara said:

The ordinal ranking of the top ten tires was (widths are actual): Deda
Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28 tubular);
Michelin Pro2 Race (700 x 25); Continental Ultra Gator (700 x 23),
Mistuboshi Trimline (650B x 37), Panaracer Pasela (700 x 35); Clement
Criterium (700 x 21); Avocet Cross (700 x 35); Avocet Duro (700 x 28).

Conspicuously absent is the Avocet Fasgrip.

Avocet Duro is the FasGrip design. It was formerly maked 700 x 32 and
is now marked 700 x 28 to reflect reality. I have the old ones which
said "FasGrip" on the label. Now they say "Carbon 12." The testers
only used the one size of Avocet slick road tires.

OK, but the 28 mm Duro is 66 thread per inch side wall
while the 25 mm Road is 127 tpi. Big difference.

--
Michael Press

Quoted message said:

Ben C said:

Does the "thumb test" (squeezing a tyre to see if it's hard enough)
measure tyre pressure or casing tension?

[...]

Thank you to everyone for the explanations.

carlfogel> Thumb pressure measures casing tension.

carlfogel> [...]

carlfogel> Think of a trampoline.

carlfogel> Air pressure does not hold the trampoline up, any more than
carlfogel> it holds the trampoline down. What holds the tramoline taut
carlfogel> is the springs pulling it tight at the edges. Push down, and
carlfogel> the trampoline dents easily. Push a little further, and more
carlfogel> force is needed. The resistance is nicely progressive . . .

carlfogel> Just like a tire.

carlfogel> A tire is a doughnut-shaped trampoline. What stretches the
carlfogel> tire tight in all directions is the expanding spring of the
carlfogel> air pressure.

carlfogel> [...]

I am leaning towards this interpretation. Here is the explanation of
casing tension:

jobst.brandt> Casing stress is arrived upon by cutting across the
jobst.brandt> circular minor diameter of the tire (the tire is a
jobst.brandt> circular cross section having no structural belt as radial
jobst.brandt> tires do to change that) and take the two halves as solid
jobst.brandt> sections being pressed apart by inflation pressure. That
jobst.brandt> gives the lineal separation force which is the casing
jobst.brandt> tension.

Now, suppose I literally do cut across the minor diameter of my tyre, in
two places. I now have a piece of basically fabric hosepipe with no air
in it. It's a bit curved, but we'll pretend it's straight.

I now insert a piece of dowel into either end (to keep the hose
cylindrical). I attach clamps securely around the ends and attach the
clamps to strong springs such that they pull the hose out until it's in
the same tension as it was when it was on the bike and pumped up to
100psi. There's just atmospheric pressure in the hose (let's say I make
a small puncture somewhere, or the dowel is air-permeable, or it's a
clincher tyre anyway and therefore not a hose but more like a section of
guttering).

Now I squeeze it, against the force of the springs. We assume for the
sake of simplicity that the casing itself doesn't extend when stressed
(otherwise the length of the section I cut out has to be taken into
account).

Does it feel basically the same as it did when it was on the bike? If
so, it does seem that I'm measuring casing tension. The trampoline
springs are doing exactly the same job that the air was.

On the other hand I also believe this:

jobst.brandt> When a tire is pressed against a flat surface, the tire
jobst.brandt> flattens until the flat contact area times inflation
jobst.brandt> pressure equal the load.

If you think of the end of my thumb as a disk, with an area of half a
square inch, then to press that disk completely flat against either the
thin or the fat tyre will require the same force-- a force equivalent to
50lb.

But, at that point, the penetration of the disk along its normal into
the narrower tyre will be deeper, because of its narrower radius of
curvature.

If instead of asking, how hard do to I need to press to get the disk
flat against the tyre, I ask, how hard do I need to squeeze the tyre for
my thumb and forefinger to move 1/8 inch closer together, then for a fat
tyre and a thin tyre at the same pressure, I will have to squeeze the
fat tyre harder. This squares (at least qualitatively so far) with the
reasoning about trampoline springs.

carlfogel said:

Quoted message said:

But a thumb-press is not measuring casing tension. It is moving the
casing perpendicular to the direction of the tension, so feels nothing
from that directly, only the internal pressure.

Dear Dave,

A thumb press measures the casing tension.

No. By that logic, the road also "measures" casing tension -- since there
is no significant difference between the thumb pushing with a given force
on the tire, and the road pushing up. But that would mean that the size
of the contact patch would decrease with tire width (at a fixed pressure),
since the casing tension increases with tire width.

--

David L. Johnson

__o | If all economists were laid end to end, they would not reach a
_`\(,_ | conclusion. -- George Bernard Shaw
(_)/ (_) |

David L. Johnson said:

Quoted message said:

The ordinal ranking of the top ten tires was (widths are actual):
Deda Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28
tubular); Michelin Pro2 Race (700 x 25); Continental Ultra Gator
(700 x 23), Mistuboshi Trimline (650B x 37), Panaracer Pasela (700
x 35); Clement Criterium (700 x 21); Avocet Cross (700 x 35);
Avocet Duro (700 x 28).

Quoted message said:

I wonder when this was done. The comparisons with most of these
tires just no longer matter. Certainly neither the Clement del
Mondo nor the Criterium are available any longer, and if they were,
they would cost a bleeding fortune. They were beautiful, handmade
silk tires, but their kind has gone extinct.

Quoted message said:

Or, it may be that someone has resurrected the names, though not the
tires.

It's not the tires themselves that are interesting but the relation of
inflation pressure to RR for tires with essentially smooth tread.

http://www.sheldonbrown.com/brandt/rolling-resistance-tubular.html

The RR curves for smooth tires are nearly identical but with a
multiplier. One could take one of the curves and multiply it to
generate the others. Exceptions are those that have significant tread
profile that causes tread squirm losses. Conspicuous are the two
Specialized tires with raised center ridge.

Tubular tires, that have less RR than all the clinchers, are out of
place by the losses caused by elastomeric rim glue while their slopes
shows that they have inherently low RR due to their thin casings and
tread, and the thin latex inner tubes. The effect of inflation
pressure (slope) for them diminishes with less lossy material.

All the smooth tires have a characteristic that at infinite pressure
(to the left) approaches zero RR. As is apparent, the tubulars have a
constant offset from that characteristic. When I first saw these
curves, I thought they would be apparent to observers and resolve age
old questions. Instead they gave rise to endless speculation and
interpretation.

Track glue was used for a purpose that remained unknown after WWII
when those who understood the problem had retired and left us with two
kinds of glue, much like john Starley left us with tied and soldered
spokes. No one seemed to understand the purpose.

Track tires users in events like the 1000m and 4000m pursuit know that
these events are won on 1/100 of seconds and that RR in that realm is
important. Track tires were made with bare cloth base tapes to be
coated with track glue and mounted on rims with track glue. Track
glue is partially dried shellac that hardens as non elastic cement.

Rubberized tubular base tapes do not lend themselves to use with track
glue.

Jobst Brandt

Ben C said:

Quoted message said:

Ben C said:

Does the "thumb test" (squeezing a tyre to see if it's hard enough)
measure tyre pressure or casing tension?

[...]

Thank you to everyone for the explanations.

carlfogel> Thumb pressure measures casing tension.

carlfogel> [...]

carlfogel> Think of a trampoline.

carlfogel> Air pressure does not hold the trampoline up, any more than
carlfogel> it holds the trampoline down. What holds the tramoline taut
carlfogel> is the springs pulling it tight at the edges. Push down, and
carlfogel> the trampoline dents easily. Push a little further, and more
carlfogel> force is needed. The resistance is nicely progressive . . .

carlfogel> Just like a tire.

carlfogel> A tire is a doughnut-shaped trampoline. What stretches the
carlfogel> tire tight in all directions is the expanding spring of the
carlfogel> air pressure.

carlfogel> [...]

I am leaning towards this interpretation. Here is the explanation of
casing tension:

jobst.brandt> Casing stress is arrived upon by cutting across the
jobst.brandt> circular minor diameter of the tire (the tire is a
jobst.brandt> circular cross section having no structural belt as radial
jobst.brandt> tires do to change that) and take the two halves as solid
jobst.brandt> sections being pressed apart by inflation pressure. That
jobst.brandt> gives the lineal separation force which is the casing
jobst.brandt> tension.

Now, suppose I literally do cut across the minor diameter of my tyre, in
two places. I now have a piece of basically fabric hosepipe with no air
in it. It's a bit curved, but we'll pretend it's straight.

I now insert a piece of dowel into either end (to keep the hose
cylindrical). I attach clamps securely around the ends and attach the
clamps to strong springs such that they pull the hose out until it's in
the same tension as it was when it was on the bike and pumped up to
100psi. There's just atmospheric pressure in the hose (let's say I make
a small puncture somewhere, or the dowel is air-permeable, or it's a
clincher tyre anyway and therefore not a hose but more like a section of
guttering).

Now I squeeze it, against the force of the springs. We assume for the
sake of simplicity that the casing itself doesn't extend when stressed
(otherwise the length of the section I cut out has to be taken into
account).

Does it feel basically the same as it did when it was on the bike? If
so, it does seem that I'm measuring casing tension. The trampoline
springs are doing exactly the same job that the air was.

On the other hand I also believe this:

jobst.brandt> When a tire is pressed against a flat surface, the tire
jobst.brandt> flattens until the flat contact area times inflation
jobst.brandt> pressure equal the load.

If you think of the end of my thumb as a disk, with an area of half a
square inch, then to press that disk completely flat against either the
thin or the fat tyre will require the same force-- a force equivalent to
50lb.

But, at that point, the penetration of the disk along its normal into
the narrower tyre will be deeper, because of its narrower radius of
curvature.

If instead of asking, how hard do to I need to press to get the disk
flat against the tyre, I ask, how hard do I need to squeeze the tyre for
my thumb and forefinger to move 1/8 inch closer together, then for a fat
tyre and a thin tyre at the same pressure, I will have to squeeze the
fat tyre harder. This squares (at least qualitatively so far) with the
reasoning about trampoline springs.

Dear Ben,

Basically, an inflated elastic skin automatically occupies the lowest
possible tension shape. If you force a deformation anywhere locally,
the tension rises.

It might be easier to work through the geometry than devise spring
devices inside short sections. Air and rubber are sufficient.

The simplest tension figure that we can describe is a straight line,
say a rubber string stretched tight between two fixed points.

If we push the rubber string in any direction, the force increases the
tension.

Push the tight rubber string sideways at any point and the distance
obviously increases because a straight line is the shortest distance
between two points.

Instead of pushing the rubber string sideways, grab a point on the
rubber string and pull it toward either fixed point. Tension drops on
one side, but must rise to more than the original tension on the other
side--your pull has introduced a third fixed point and stretched a
section of the original rubber string further and tauter.

Let's get rid of those pesky fixed points.

An endless rubber string is just a rubber band. Imagine a rubber band
laid flat to form an air-tight seal between two plates.

Pump air into the sealed space and the rubber band will automatically
expand to form a circle with even pressure all around.

Push inward on either side of the rubber band circle, and the circle
will start to flatten.

The air pushed out of that section doesn't vanish. It's pushed into
the unflattened areas--which must bulge outward and cause the tension
to rise in the rubber band.

If there's just the right friction on the two points where the rubber
band is flattened, you could get a local tension lowering in the
chord, but that would just raise the tension even higher in the
unflattened sections of the rubber band.

A thumb press requires flattening two spots, one where our thumb is,
and one (or more) to oppose the thumb press. Locally, we can try to
lower tension by flattening a curve into a chord, but that relies on
friction to keep the flattened area from adjusting its tension as the
rest of the rubber band expands.

We could simplify to a single chord instead of two by grabbing two
spots on the rubber band and pulling them together (or apart). Either
way, the tension must rise in the rubber band.

If we pull the two points apart, the tension is obvious between our
fingers.

If we push the two points together, the rest of the rubber band has to
bulge as the more air is squeezed into what's left of the circle.

So much for one-dimensional rubber bands. Let's do a two-dimensional
surface, such as a trampoline.

Let's skip the ordinary flat trampoline and move straight to an
endless trampline--the two-dimensional surface of a sphere.

If we inflate a round balloon, every part of its surface automatcally
goes to the lowest possible even tension.

Mark a circle on the surface and label it "trampoline"--the tension
from inflation serves as the tension springs for an ordinary flat
trampoline.

We can push two (or more) spots inward on our balloon. The force will
cause of the balloon to bulge outward and its skin elsewhere to
increase in tension. If friction where we push our hands together
reduces the tension locally (a chord is shorter than a curve), then
there's even less balloon skin left elsewhere to constrain the same
amount of air and tension and pressure must rise even further.

Transform a sphere into a toroid and we have a tire that's still an
endless trampoline. The only way to reduce total tension is to press
inward everywhere by raising atmospheric pressure. Otherwise, the tire
must bulge out somewhere else whenever we push it inward (or outward)
locally with a thumb.

The most intuitive way to see what happens may be to imagine an inner
tube or tire force-filled with water. The skin of the toroid is
obviously tight. Push inward on it locally anywhere, and you know that
the water is bulging it outward elsewhere and raising the tension.

Less intuitively, pulling the water-filled tube outward anywhere must
also raise the tension. You have to pull at two points and apply force
to distort the tube. Pulling on the tube must increase its tension. As
soon as you let go and release the tension, the tube snaps back into
its original form, which is a slightly lower pressure.

Just like an air-filled tire--push it inward or outward locally, and
you raise the tension and pressure. As soon as you stop pushing, the
deforming tension is lost and the tire snaps back into its original
shape.

Cheers,

Carl Fogel

In article <[email hidden]>,

Quoted message said:

Tim McNamara said:

Quoted message said:

>> Does the "thumb test" (squeezing a tyre to see if it's hard
>> enough) measure tyre pressure or casing tension?

Quoted message said:

Quoted message said:

> Directly the latter and indirectly the former, I think.

Quoted message said:

Quoted message said:

This is what I was starting to think too. What this means of
course is that if you have a fat tyre and a thin tyre that feel
the same, the fat tyre will actually be at a lower pressure (as it
typically should be), and have higher rolling resistance.

Quoted message said:

After I typed this, I thought that the thumb test would also be
influenced by the stiffness of the tire wall: the stiffness of the
fabric casing, the thickness and durometer of the rubber, etc.

... not more than the un-inflated tire.

That makes sense, although what I was thinking about was flexing
uninflated tires which have a noticeable resistance.

Quoted message said:

Obviously some tires have a stiff tread that doesn't deflect even
when flat, an example is a motorcycle tire. I assume those who use
their thumbs are also smart enough not to do that to a knobby tread
or a thick road tread. The bare side wall on older tires could give
an accurate feel, but even that has been taken from us by the "I get
too many flat tires" folks. We may revert to the non-pneumatic tire
era soon.

Judging by the number of cyclists I see every day with no pump or repair
tools, that might not be a bad idea for some.

In article <[email hidden]>,

Quoted message said:

David L. Johnson said:

Quoted message said:

The ordinal ranking of the top ten tires was (widths are actual):
Deda Tre Giro d'Italia (700 x 24); Clement del Mondo (700 x 28
tubular); Michelin Pro2 Race (700 x 25); Continental Ultra Gator
(700 x 23), Mistuboshi Trimline (650B x 37), Panaracer Pasela (700
x 35); Clement Criterium (700 x 21); Avocet Cross (700 x 35);
Avocet Duro (700 x 28).

Quoted message said:

I wonder when this was done. The comparisons with most of these
tires just no longer matter. Certainly neither the Clement del
Mondo nor the Criterium are available any longer, and if they were,
they would cost a bleeding fortune. They were beautiful, handmade
silk tires, but their kind has gone extinct.

Quoted message said:

Or, it may be that someone has resurrected the names, though not
the tires.

It's not the tires themselves that are interesting but the relation
of inflation pressure to RR for tires with essentially smooth tread.

http://www.sheldonbrown.com/brandt/rolling-resistance-tubular.html

The RR curves for smooth tires are nearly identical but with a
multiplier. One could take one of the curves and multiply it to
generate the others. Exceptions are those that have significant
tread profile that causes tread squirm losses. Conspicuous are the
two Specialized tires with raised center ridge.

Interestingly, the BQ study found that tire pressure had less impact on
rolling speed than other factors (assuming the results are accurate).

Quoted message said:

Tubular tires, that have less RR than all the clinchers, are out of
place by the losses caused by elastomeric rim glue while their slopes
shows that they have inherently low RR due to their thin casings and
tread, and the thin latex inner tubes. The effect of inflation
pressure (slope) for them diminishes with less lossy material.

The BQ test tried some of the tires with butyl and latex tubes. The
tires rolled slower with latex tubes. They did note that these latex
tubes were not as thin as those in tubular tires.

As I've stated though, the results of the test are different enough that
I am somewhat skeptical, and wonder about the contribution of confounds.
Someone with a better understanding of this stuff than I would have to
read the test and its methodology.

In article <[email hidden]>,

Michael Press said:

In article <[email hidden]>,

Tim McNamara said:

In article
<[email hidden]>,

Michael Press said:

In article <[email hidden]>,
Tim McNamara <[email hidden]> wrote:

> The ordinal ranking of the top ten tires was (widths are
> actual): Deda Tre Giro d'Italia (700 x 24); Clement del Mondo
> (700 x 28 tubular); Michelin Pro2 Race (700 x 25); Continental
> Ultra Gator (700 x 23), Mistuboshi Trimline (650B x 37),
> Panaracer Pasela (700 x 35); Clement Criterium (700 x 21);
> Avocet Cross (700 x 35); Avocet Duro (700 x 28).

Conspicuously absent is the Avocet Fasgrip.

Avocet Duro is the FasGrip design. It was formerly maked 700 x 32
and is now marked 700 x 28 to reflect reality. I have the old ones
which said "FasGrip" on the label. Now they say "Carbon 12." The
testers only used the one size of Avocet slick road tires.

OK, but the 28 mm Duro is 66 thread per inch side wall while the 25
mm Road is 127 tpi. Big difference.

It is, and I thought about that too. Interestingly I found that some of
my own riding supported some of the things they wrote. For example, I
tried the Rivendell Rolly Polys and found them slower than the
Continental Ultra 2000 (both 700 x 28) that I usually use. The Avocet
700 x 25 is simply unavailable in shops locally. Only one shop bothered
to stock Avocet road tires and they are long gone. They couldn't keep
the 700 x 25 (marked 7000 x 28 in the old days) in stock- I was never
able to buy a set.

Someday I'll mail order a couple of them. None of the local bike shops
want to deal with Avocet, they are too much hassle I guess.