Thumb test

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?

Directly the latter and indirectly the former, I think.

Tim McNamara said:

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?

Directly the latter and indirectly the former, I think.

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.

Ben C said:

Tim McNamara said:

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?

Directly the latter and indirectly the former, I think.

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.

Hm. I'm not sure, and I'm just a fly on the wall in this forum, but
this makes little sense to me. Doesn't the larger tire/tyre require
more pressure to get to the same hardness as a small tire? The large
tire has more rubber, so it's got more "stretchiness" or elasticity
across it's cross section. Further, wouldn't a large tire with the same
"thumb feel" as a thin tire (ie, really hard), have less rolling
resistance than a large tire with less pressure? I must be
misunderstanding your statement, sorry....

Quoted message said:

Ben C said:

Tim McNamara said:

In article <[email hidden]>,
Ben C <[email hidden]> wrote:

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

Directly the latter and indirectly the former, I think.

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.

Hm. I'm not sure, and I'm just a fly on the wall in this forum, but
this makes little sense to me. Doesn't the larger tire/tyre require
more pressure to get to the same hardness as a small tire?

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.

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

Quoted message said:

The large tire has more rubber, so it's got more "stretchiness" or
elasticity across it's cross section. Further, wouldn't a large tire
with the same "thumb feel" as a thin tire (ie, really hard), have less
rolling resistance than a large tire with less pressure?

A hard tyre basically has less rolling resistance than if it's soft. But
rolling resistance depends on pressure rather than on casing tension, or
at least, most of the graphs you see are of pressure against RR.

I think this also starts to explain the counter-intuitive result that
fatter tyres have lower RR than thinner tyres at the same pressure.
There are some explanations given in terms of fatter tyres needing to
deform less which I don't fully understand. But if at equal pressures
the fat tyre actually feels harder, it's easier to see intuitively that
it would roll better.

I might make a graph of RR against casing tension (as opposed to
pressure) for different tyre diameters...

E C McDougall said:

Quoted message 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:

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:

Hm. I'm not sure, and I'm just a fly on the wall in this forum, but
this makes little sense to me. Doesn't the larger tire/tyre require
more pressure to get to the same hardness as a small tire? The large
tire has more rubber, so it's got more "stretchiness" or elasticity
across it's cross section. Further, wouldn't a large tire with the
same "thumb feel" as a thin tire (ie, really hard), have less
rolling resistance than a large tire with less pressure? I must be
misunderstanding your statement, sorry...

The thumb test accurately exposes the thumb to tire pressure. How
that gets to the rim is another problem, but to flatten a portion of
the tire by pressing on it is directly related to pressure (assuming
the tire is a >23mm cross section). I'm assuming the thumb is not
that of a giant and has a width of 10-15mm contact.

The size of the tire does not affect that sensation, just as it
doesn't affect the contact patch area. What is different is the
compliance of the tire when loaded. A fat tire can absorb more
deflection than a narrow one.

Jobst Brandt

Ben C said:

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

Dear Ben,

Mostly, it measures the operator's belief in his sensitivity to
pressing one squashy pad against a less squashy pad as the contact
surface broadens.

(It's good enough for riding around.)

But you were asking about casing tension versus air pressure.

Thumb pressure measures casing tension.

Yes, there's 100 psi pushing the casing outward against your thumb.

But when you push the tire a quarter of an inch inward locally,
there's still 100 psi pushing against your thumb.

The 100 psi didn't go away. It didn't increase significantly due to
the tiny change in tire volume. But something stopped you from pushing
any further.

Think of a trampoline.

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

Just like a tire.

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

To dent a tire, you must overcome tension in every direction around
the dent. The deeper you try to push the casing in, the more tension
you must overcome.

_________________________ _ = tire casing
||||||||||||||||||||||||| | = 100 psi upward force

-> ______ ______ <- exaggerated shortening for dent
\/ pulling casing in against tension

Since 100 psi of air pressure is trying to keep the rest of the tire
casing tight everywhere else, you won't get very far.

Here's a simple demonstration that everyone knows--push on a tire
valve. There's no side tension holding the metal valve stem up against
your finger because the metal valve is a special case in the otherwise
uniform doughnut.

Notice that the valve doesn't move (dent) as you slowly apply more and
more pressure. It just sits there until your thumb pressure matches
the air pressure resisting it.

At that point, the valve moves dramatically because almost all the
resisting pressure is no longer being applied--the end of the valve is
just waving around inside the 100 psi air chamber.

But if you were simply pushing a tiny metal piston down into a deep
metal tube set into the tire, you'd push harder and harder without any
movement--and then the piston would begin to move steadily inward and
keep moving steadily because the air pressure resisting your thumb had
been overcome.

Here's another simple demonstration with a bicycle.

Toss a metal tool with a rounded handle less than an inch thick on the
garage floor--a slim socket wrench handle will work well.

Now roll your roughly 1-inch wide 700c front tire at 100 psi onto the
thin round handle.

Lean on the handlebars.

Presumably you weigh well over 100 pounds, but it's unlikely that you
can make the tire touch the garage floor on both sides of the round
metal handle.

A local pressure of 100 psi is not enough to overcome the casing
tension of a tire inflated all the way around to 100 psi.

Cheers,

Carl Fogel

Carl Fogel said:

Here's another simple demonstration with a bicycle.

Quoted message said:

Toss a metal tool with a rounded handle less than an inch thick on
the garage floor--a slim socket wrench handle will work well.

Quoted message said:

Now roll your roughly 1-inch wide 700c front tire at 100 psi onto
the thin round handle.

Quoted message said:

Lean on the handlebars.

Quoted message said:

Presumably you weigh well over 100 pounds, but it's unlikely that
you can make the tire touch the garage floor on both sides of the
round metal handle.

Quoted message said:

A local pressure of 100 psi is not enough to overcome the casing
tension of a tire inflated all the way around to 100 psi.

Invalid experiment!

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

Remember, how tight must a wire be pulled so that it doesn't sag at
midspan? Casing tension does not come into play at tire-to-ground
contact where it has no curvature, only inflation pressure. In
contrast your complex experiment, involves pressure, casing curvature
in two directions with cord angle and more.

Jobst Brandt

Quoted message said:

Carl Fogel said:

Here's another simple demonstration with a bicycle.

Quoted message said:

Toss a metal tool with a rounded handle less than an inch thick on
the garage floor--a slim socket wrench handle will work well.

Quoted message said:

Now roll your roughly 1-inch wide 700c front tire at 100 psi onto
the thin round handle.

Quoted message said:

Lean on the handlebars.

Quoted message said:

Presumably you weigh well over 100 pounds, but it's unlikely that
you can make the tire touch the garage floor on both sides of the
round metal handle.

Quoted message said:

A local pressure of 100 psi is not enough to overcome the casing
tension of a tire inflated all the way around to 100 psi.

Invalid experiment!

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

Remember, how tight must a wire be pulled so that it doesn't sag at
midspan? Casing tension does not come into play at tire-to-ground
contact where it has no curvature, only inflation pressure. In
contrast your complex experiment, involves pressure, casing curvature
in two directions with cord angle and more.

Jobst Brandt

Dear Jobst,

The point is that the casing tension must be overcome in all
directions to make a dent in the doughnut-like surface.

The dent does not raise the air pressure significantly.

But the dent does require dragging material inward in all directions,
material that's being held in tension over the rest of the doughnut by
100 psi.

Consider the reverse case, an imaginary thumb pushing outward from
inside the tire. What would stop it from pulling the tire wildly out
of shape?

Stick a nail inside a tubeless tire, a nail large enough to make a
bulge in the uninfalted tire.

When you inflate the tire, the air pressure will pushe the tire
outward in all directions.

The increasing tension, however, will pull the tire onto the nail and
puncture the tire.

Cheers,

Carl Fogel

Carl Fogel said:

Quoted message said:

Quoted message said:

Here's another simple demonstration with a bicycle.

Quoted message said:

Quoted message said:

Quoted message said:

Toss a metal tool with a rounded handle less than an inch thick on
the garage floor--a slim socket wrench handle will work well.

Quoted message said:

Quoted message said:

Quoted message said:

Now roll your roughly 1-inch wide 700c front tire at 100 psi onto
the thin round handle.

Quoted message said:

Quoted message said:

Quoted message said:

Lean on the handlebars.

Quoted message said:

Quoted message said:

Quoted message said:

Presumably you weigh well over 100 pounds, but it's unlikely that
you can make the tire touch the garage floor on both sides of the
round metal handle.

Quoted message said:

Quoted message said:

Quoted message said:

A local pressure of 100 psi is not enough to overcome the casing
tension of a tire inflated all the way around to 100 psi.

Quoted message said:

Quoted message said:

Invalid experiment!

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

Remember, how tight must a wire be pulled so that it doesn't sag at
midspan? Casing tension does not come into play at tire-to-ground
contact where it has no curvature, only inflation pressure. In
contrast your complex experiment, involves pressure, casing
curvature in two directions with cord angle and more.

Quoted message said:

The point is that the casing tension must be overcome in all
directions to make a dent in the donut-like surface.

It may do that but for bicycle tires that have little curvature with
respect to the minor diameter even these distortions are small. This
has no effect on pressing against a flat surface as I pointed out.

Quoted message said:

The dent does not raise the air pressure significantly.

....and who said it did?

Quoted message said:

But the dent does require dragging material inward in all
directions, material that's being held in tension over the rest of
the donut by 100 psi.

That may be a dynamic effect, the one that causes rolling resistance,
but it has no effect on contact pressure between tire and road.

Quoted message said:

Consider the reverse case, an imaginary thumb pushing outward from
inside the tire. What would stop it from pulling the tire wildly
out of shape?

The reverse case does not apply. There is no curvature at the ground
contact and thumb pushing is not a dynamic effect.

Quoted message said:

Stick a nail inside a tubeless tire, a nail large enough to make a
bulge in the uninfalted tire.

You're grasping at straws. All this does not apply to the contact
pressure with the road or a thumb flattening the tire locally.

Quoted message said:

When you inflate the tire, the air pressure will push the tire
outward in all directions.

Quoted message said:

The increasing tension, however, will pull the tire onto the nail
and puncture the tire.

And how does this apply to contact pressure with an essentially flat
surface?

Jobst Brandt

Quoted message said:

Carl Fogel said:

Quoted message said:

> Here's another simple demonstration with a bicycle.

Quoted message said:

Quoted message said:

> Toss a metal tool with a rounded handle less than an inch thick on
> the garage floor--a slim socket wrench handle will work well.

Quoted message said:

Quoted message said:

> Now roll your roughly 1-inch wide 700c front tire at 100 psi onto
> the thin round handle.

Quoted message said:

Quoted message said:

> Lean on the handlebars.

Quoted message said:

Quoted message said:

> Presumably you weigh well over 100 pounds, but it's unlikely that
> you can make the tire touch the garage floor on both sides of the
> round metal handle.

Quoted message said:

Quoted message said:

> A local pressure of 100 psi is not enough to overcome the casing
> tension of a tire inflated all the way around to 100 psi.

Quoted message said:

Quoted message said:

Invalid experiment!

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

Remember, how tight must a wire be pulled so that it doesn't sag at
midspan? Casing tension does not come into play at tire-to-ground
contact where it has no curvature, only inflation pressure. In
contrast your complex experiment, involves pressure, casing
curvature in two directions with cord angle and more.

Quoted message said:

The point is that the casing tension must be overcome in all
directions to make a dent in the donut-like surface.

It may do that but for bicycle tires that have little curvature with
respect to the minor diameter even these distortions are small. This
has no effect on pressing against a flat surface as I pointed out.

Quoted message said:

The dent does not raise the air pressure significantly.

...and who said it did?

Quoted message said:

But the dent does require dragging material inward in all
directions, material that's being held in tension over the rest of
the donut by 100 psi.

That may be a dynamic effect, the one that causes rolling resistance,
but it has no effect on contact pressure between tire and road.

Quoted message said:

Consider the reverse case, an imaginary thumb pushing outward from
inside the tire. What would stop it from pulling the tire wildly
out of shape?

The reverse case does not apply. There is no curvature at the ground
contact and thumb pushing is not a dynamic effect.

Quoted message said:

Stick a nail inside a tubeless tire, a nail large enough to make a
bulge in the uninfalted tire.

You're grasping at straws. All this does not apply to the contact
pressure with the road or a thumb flattening the tire locally.

Quoted message said:

When you inflate the tire, the air pressure will push the tire
outward in all directions.

Quoted message said:

The increasing tension, however, will pull the tire onto the nail
and puncture the tire.

And how does this apply to contact pressure with an essentially flat
surface?

Jobst Brandt

Dear Jobst,

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

Cheers,

Carl Fogel

In article <[email hidden]>,

Ben C said:

Tim McNamara said:

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?

Directly the latter and indirectly the former, I think.

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.

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.

Tim McNamara said:

Quoted message 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:

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. 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.

Jobst Brandt

In article <[email hidden]>,

Quoted message said:

Ben C said:

Tim McNamara said:

In article <[email hidden]>,
Ben C <[email hidden]> wrote:

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

Directly the latter and indirectly the former, I think.

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.

Hm. I'm not sure, and I'm just a fly on the wall in this forum, but
this makes little sense to me. Doesn't the larger tire/tyre require
more pressure to get to the same hardness as a small tire?

All other things being equal, the wider tire will have higher casing
tension (called "hoop stress," IIRC) at the same PSI. But it will take
more air to achieve that PSI because of course the volume of the tire is
larger.

Quoted message said:

The large tire has more rubber, so it's got more "stretchiness" or
elasticity across it's cross section. Further, wouldn't a large tire
with the same "thumb feel" as a thin tire (ie, really hard), have
less rolling resistance than a large tire with less pressure? I must
be misunderstanding your statement, sorry....

Because of the fabric casing, a tire doesn't stretch much. It's not
like a balloon. The casing bears the load of the tension, not the
rubber.

Interestingly enough, there is a lot of data showing that- all other
things being equal- a wider tire has lower rolling resistance than a
narrower tire. But of course, all things are not equal. Wider tires,
for example, need a heavier casing because the tension is higher. There
is a fair amount of debate whether rolling resistance tests are
consistent with the behavior of tires in the real world. The problem is
that real world testing introduces all kinds of weird confounds that
make accurate measurement difficult.

The most recent attempt I know of is in the latest Bicycle Quarterly
(formerly "Vintage Bicycle Quarterly"😉, which compared 16 different
tires in a roll-down test on a soapbox derby course. The results
suggested that at moderate speeds (10.6 to 16.9 mph) some tires were 20%
faster than others (the Deda Tre Giro d'Italia 700 x 24 being the
fastest and the Rivendell Nifty Swifty 650B x 32 being the slowest);
tires with cotton casings rather than nylon appeared to perform better
when other factors were similar; tires with soft rubber compounds fared
better than those with harder compounds (the reverse of the findings
when steel rollers are used, apparently); subjective impression of
"fastness" do not correlate well with actual performance; and air
pressure made less difference that tire construction factors (size,
tread, rubber thickness, etc). The results also showed that tires roll
faster in hot weather than cold.

It was an interesting study but I am inclined to take it with a grain of
salt as yet. Some of the results appear confounding to me, which may be
due to there having been significant errors of measurement or that there
are real differences of which I don't understand the causes. Some
things were obvious- large tread features were generally problematic;
thinner rubber was faster than thicker rubber for example, wider tires
were faster (three widths of the Michelin Pro2 Race were tested- about a
2% difference). These things are known. What seemed to make an
interesting difference was the tires' ability to absorb surface
irregularities. The shocker was the Avocet Cross, with an inverted
tread like a car tire and very thick rubber, came in 8th!

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).

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.

In article <[email hidden]>,

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.

If you have an inflation pressure of 100 psi, a tire with more inside
surface area will have a casing under greater tension because there are
more square inches. If I grok correctly.

Tim McNamara said:

Quoted message 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

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

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.

Quoted message said:

If you have an inflation pressure of 100 psi, a tire with more
inside surface area will have a casing under greater tension because
there are more square inches. If I grok correctly.

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

The above mentioned formula reduces to just that. For cord stress,
adjusting for 45 degree bias ply SQR(2) gets involved but this is
about casing tension which is the same regardless of fabric structure.

Jobst Brandt

Quoted message said:

Carl Fogel said:

Quoted message said:

> Here's another simple demonstration with a bicycle.

Quoted message said:

Quoted message said:

> Toss a metal tool with a rounded handle less than an inch thick on
> the garage floor--a slim socket wrench handle will work well.

Quoted message said:

Quoted message said:

> Now roll your roughly 1-inch wide 700c front tire at 100 psi onto
> the thin round handle.

Quoted message said:

Quoted message said:

> Lean on the handlebars.

Quoted message said:

Quoted message said:

> Presumably you weigh well over 100 pounds, but it's unlikely that
> you can make the tire touch the garage floor on both sides of the
> round metal handle.

Quoted message said:

Quoted message said:

> A local pressure of 100 psi is not enough to overcome the casing
> tension of a tire inflated all the way around to 100 psi.

Quoted message said:

Quoted message said:

Invalid experiment!

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

Remember, how tight must a wire be pulled so that it doesn't sag at
midspan? Casing tension does not come into play at tire-to-ground
contact where it has no curvature, only inflation pressure. In
contrast your complex experiment, involves pressure, casing
curvature in two directions with cord angle and more.

Quoted message said:

The point is that the casing tension must be overcome in all
directions to make a dent in the donut-like surface.

It may do that but for bicycle tires that have little curvature with
respect to the minor diameter even these distortions are small. This
has no effect on pressing against a flat surface as I pointed out.

Quoted message said:

The dent does not raise the air pressure significantly.

...and who said it did?

Quoted message said:

But the dent does require dragging material inward in all
directions, material that's being held in tension over the rest of
the donut by 100 psi.

That may be a dynamic effect, the one that causes rolling resistance,
but it has no effect on contact pressure between tire and road.

Quoted message said:

Consider the reverse case, an imaginary thumb pushing outward from
inside the tire. What would stop it from pulling the tire wildly
out of shape?

The reverse case does not apply. There is no curvature at the ground
contact and thumb pushing is not a dynamic effect.

Quoted message said:

Stick a nail inside a tubeless tire, a nail large enough to make a
bulge in the uninfalted tire.

You're grasping at straws. All this does not apply to the contact
pressure with the road or a thumb flattening the tire locally.

Quoted message said:

When you inflate the tire, the air pressure will push the tire
outward in all directions.

Quoted message said:

The increasing tension, however, will pull the tire onto the nail
and puncture the tire.

And how does this apply to contact pressure with an essentially flat
surface?

Jobst Brandt

Dear Jobst,

Here's an explanation of how tire tension resists thumbs (and larger
thing).

The tire is inflated to 100 psi.

The force of the air pressure pushes the tire outward, expanding the
tire.

Two forces oppose what would otherwise be an endless expansion (given
enough soap-bubble matterial).

First one atmosphere of air pressure pushes back.

When your pumping raises the pressure inside the tire to match the
outside atmosphere, the second force takes over--casing tension.

The doughnut trying to expand in all directions puts the tire casing
in tremendous tension.

In the case of our tire, the almost sideways tension is enough to hold
back the force of 100 pounds per square inch that's not canceled out
by 1 atmosphere on each side of the tire.

Once the forces match, the tire stops expanding and becomes motionless
and achieves a Buddah-like contentment--no net force, no net movement.

But you can disturb its repose by pushing on it with your thumb. The
instant that you push on it with 10 pounds of force (or 20 or 50), the
tire flees from your touch because the forces are no longer balanced.

In a moment, you'd sprain your thumb as it banged into the rim,
except--

Well, it's not the air pressure stopping your thumb. The air pressure
doesn't rise to fight your thumb. It remains 100 psi everywhere inside
the tire.

Yet obviously the forces have balanced, since your thumb stops moving
inward, even though you maintain your hopeful pressure against the
tire.

What's changed is the casing tension, which has risen until it plus
the 100 psi match the atmosphere and the pressure of your thumb.

Yes, the casing tension does depend on the air pressure, which is what
turns the doughnut shape into a taut 3-d trampoline.

But the motion that begins when you push on the tire stops only
because of the increasing local tension.

A dramatic example of this in reverse is when the bead of an inflated
tire slips off the rim for a few inches. Suddenly, there's a local
area of exposed inner tube that's too stretchy to supply the necessary
tension.

The inner tube swells wildly, as if a monster thumb had pushed it
outward from inside the tire.

(It doesn't matter whether you push in or out.)

If the inner tube were strong enough and elastic enough, you might end
up with a huge aneurysm. More likely, the inner tube splits and
explodes with a bang.

You can also work through the physics with a trampoline, a rope, a
pulley, and two weights, one large and one small.

Hang a 100 pound weight from the pulley and let it sit on the ground.
Its downward force represents the outward air pressure in the mistaken
view of what happens.

Dangle a 50 pound weight from the rope on the other end of the pulley.

Drat.

The 100 pound weight doesn't move. Neither does the 50 pound weight.

Doesn't look like pressing on a tire with your thumb.

Now let's see how a tire really works.

Heave the 100 pound weight onto the trampoline. It accelerates
downward until the tension on the trampoline skin matches the
force of gravity.

Then the weight stops and just sits there, with the trampoline bulging
but motionless--no net force, no movement.

Just like a tire expanding outward with 100 psi of air pressure and
enormous tension.

Now attach the rope to the weight, run it over the pulley, and hang
the 50 pound weight from it.

Instead of sitting still, the 100 pound weight rises. It still weighs
100 pounds, so that hasn't changed, any more than the air pressure
changes inside a tire when you push on it with your thumb.

All that's changed is the tension of the trampoline, which no longer
has to be tight enough to hold up 100 pounds--it rises to the point
where a 50 pound weight would cause it to sag.

Then the 100 pound weight stops moving because the forces have
balanced again.

All that changes is the tension.

Cheers,

Carl Fogel

Quoted message said:

The point is that the casing tension must be overcome in all
directions to make a dent in the doughnut-like surface.

Jobst just told you that it's not a dent.

In fact, flattening the tire at the contact patch relieves
the casing tension.

It doesn't overcome it.

Quoted message said:

The dent does not raise the air pressure significantly.

Doesn't need to. Just needs to increase the
contact surface.

Same thing happens when you press with your thumb.

Try it sometime.

Quoted message said:

But the dent does require dragging material inward in all directions,
material that's being held in tension over the rest of the doughnut by
100 psi.

Consider the reverse case, an imaginary thumb pushing outward from
inside the tire. What would stop it from pulling the tire wildly out
of shape?

See, now you should be getting a clue. That side is causing
the casing tension; increasing the pressure from that side
increases the casing tension; pressing from the outside
relieves the casing tension. The casing is under tension at
all stretch other than zero stretch. Tubes stretch a lot,
tires stretch a little (spokes stretch a teeny bit).

Quoted message said:

Stick a nail inside a tubeless tire, a nail large enough to make a
bulge in the uninfalted tire.

When you inflate the tire, the air pressure will pushe the tire
outward in all directions.

The increasing tension, however, will pull the tire onto the nail and
puncture the tire.

Where do you get this stuff?

Have you ever done anything even remotely like this?

How do you know whether the nail would be more, less,
or equally likely to pierce the surface when the tube
is pulling backwards against the point when uninflated
or thinned-out by the expansion when inflated or has a
tension tending to pull its intermolecular bonds apart
when inflated? I think depending on the type of rubber,
the relative sizes of the tire and nail, and the sharpness
of the point, it could be any of those answers.

--Blair

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.

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

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.

--

David L. Johnson

__o | Arguing with an engineer is like mud wrestling with a pig... You
_`\(,_ | soon find out the pig likes it!
(_)/ (_) |