Why does low pressure cost mileage?

On 2/7/2012 11:02 AM, wrote:

...
The reason is that the pressure is not even on the contact patch, due
to the sidewall acting somewhat like a scissors jack when its curved
shape distorts.

At low pressures, a halo of high pressure forms around the edges of
the contact patch.

At high pressures, a halo of low pressure forms around the edges.


Is there any imaging of this phenomenon?
I would not expect the thin/flexible walls of a typical narrow road
bicycle tire to contribute much of any support simply through their own
shape.

Only in a narrow range of medium pressures does the contact patch
spread out at an even pressure to match our expectations.


I would guess,,, not so.

With any non-belted bicycle tire, the tire inflation doesn't matter much
to the rolling resistance since they suffer from tread squirm at all
pressures. While a higher pressure results in a smaller contact patch,
the tread squirm decreases in size (due to the smaller contact patch)
but also occurs at a higher pressure.

On 02/08/2012 03:44 PM, DougC wrote:
On 2/7/2012 11:02 AM, wrote:

...
The reason is that the pressure is not even on the contact patch, due
to the sidewall acting somewhat like a scissors jack when its curved
shape distorts.

At low pressures, a halo of high pressure forms around the edges of
the contact patch.

At high pressures, a halo of low pressure forms around the edges.


Is there any imaging of this phenomenon?
I would not expect the thin/flexible walls of a typical narrow road
bicycle tire to contribute much of any support simply through their own
shape.

Only in a narrow range of medium pressures does the contact patch
spread out at an even pressure to match our expectations.


I would guess,,, not so.

With any non-belted bicycle tire, the tire inflation doesn't matter much
to the rolling resistance since they suffer from tread squirm at all
pressures. While a higher pressure results in a smaller contact patch,
the tread squirm decreases in size (due to the smaller contact patch)
but also occurs at a higher pressure.


What tread?

"DougC" wrote in message
...
On 2/7/2012 11:02 AM, wrote:

...
The reason is that the pressure is not even on the contact patch, due
to the sidewall acting somewhat like a scissors jack when its curved
shape distorts.

At low pressures, a halo of high pressure forms around the edges of
the contact patch.

At high pressures, a halo of low pressure forms around the edges.


Is there any imaging of this phenomenon?
I would not expect the thin/flexible walls of a typical narrow road
bicycle tire to contribute much of any support simply through their own
shape.

Only in a narrow range of medium pressures does the contact patch
spread out at an even pressure to match our expectations.


I would guess,,, not so.

With any non-belted bicycle tire, the tire inflation doesn't matter much
to the rolling resistance since they suffer from tread squirm at all
pressures. While a higher pressure results in a smaller contact patch, the
tread squirm decreases in size (due to the smaller contact patch) but also
occurs at a higher pressure.


DougC,
Are you saying that tread squirm is a much greater contributor to rolling
resistance of non-belted bicycle tires than the tire inflation pressure? For
all values of inflation pressure? For all tread depths? For all road
surfaces? If not, please clarify. If so, then do you contend that a silk
track sew-up on smooth concrete will not have much difference in rolling
resistance if it is inflated to 1 bar or if it is inflated to 8 bar?
Thanks,
Kerry

On Wed, 08 Feb 2012 14:44:51 -0600, DougC
wrote:

On 2/7/2012 11:02 AM, wrote:

...
The reason is that the pressure is not even on the contact patch, due
to the sidewall acting somewhat like a scissors jack when its curved
shape distorts.

At low pressures, a halo of high pressure forms around the edges of
the contact patch.

At high pressures, a halo of low pressure forms around the edges.


Is there any imaging of this phenomenon?
I would not expect the thin/flexible walls of a typical narrow road
bicycle tire to contribute much of any support simply through their own
shape.

Only in a narrow range of medium pressures does the contact patch
spread out at an even pressure to match our expectations.


I would guess,,, not so.

With any non-belted bicycle tire, the tire inflation doesn't matter much
to the rolling resistance since they suffer from tread squirm at all
pressures. While a higher pressure results in a smaller contact patch,
the tread squirm decreases in size (due to the smaller contact patch)
but also occurs at a higher pressure.

Dear Doug,

Yes, measurements of the contact patches of bicycle tires confirmed
that they act like car tires, where the non-linear relationship
between pressure and area is well-known. Tire patches were 20~30%
smaller/lower than predicted at high/low pressures. In other words,
the tires stubbornly try to remain at the sweet spot in the middle,
where the sidewall effects are minimized. Tom Schmitz put my
measurements of a single tire to shame with a wide variety of much
more carefully-done tests--and the curve of his average measurements
overlaid my single-tire tests:
http://groups.google.com/group/rec.b...d6c10253d9f70c

The scissors-jack effect doesn't depend on the thickness of the walls
of the tire--a good deal of force is needed to distort the curve of
the sidewalls, which leads to the halo of high pressure at low
pressures.

As for the halo of low-pressure at high pressures, that's easier to
understand. Assume a 150 psi inflation. Pressure at the center of the
tire patch is 150 psi. Near the edges, the tire starts to curve away
from the ground, until the 150 psi tire's pressure on the ground drops
to zero.

In the end, fairly simple physics explains the easily reproducible
results. Just pump a bicycle tire to 100 psi, ink it, lower it with a
100 lb weight onto paper (or glass as Tom did), measure the two
dimensions of the ellipse, and then repeat at 10 psi increments for a
range from 40 to 150 psi. As I mentioned, the effect is well-known in
automobile tire engineering.

Cheers,

Carl Fogel

On Wed, 08 Feb 2012 12:40:30 -0500, "(PeteCresswell)"
wrote:

Per Scott Dorsey:
My uncle Ralph claimed that you could never overfill a tire, that you should
just get a pump, and pump air in until you can't get any more in there and
that was the right amount of air.

Surprisingly enough, he was not killed in a car accident.

That one went right over my head.

Can somebody explain it for the temporarily humor-impaired?

Got nothing to do with humor unless you have the uncle.
It's about uncles with bad ideas, which they thought was common sense.
If somebody would argue with Scott's uncle about that air pressure, he
might say something like,
"You don't know much about equalization, do you? Common sense."

I had one like that.
I was his favorite nephew, because I fished with him, and usually
brought the beer.
And I liked his jokes. Man, he could tell jokes.
First time we went together in my car, he was totally shocked when he
saw me fasten my seat belt.
Seat belts were pretty new then.
Started yelling at me,
"What the hell you doing that for??!!"
I told him it was safer.
He looked at me like I as stupidest man alive, and said,
"Safe?!! What if you go in the river and can't get loose?!"

Anyway, you have to be there.

--Vic

On 09/02/12 10:07, wrote:
On Wed, 08 Feb 2012 14:44:51 -0600,
wrote:

On 2/7/2012 11:02 AM, wrote:

...
The reason is that the pressure is not even on the contact patch, due
to the sidewall acting somewhat like a scissors jack when its curved
shape distorts.

At low pressures, a halo of high pressure forms around the edges of
the contact patch.

At high pressures, a halo of low pressure forms around the edges.


Is there any imaging of this phenomenon?
I would not expect the thin/flexible walls of a typical narrow road
bicycle tire to contribute much of any support simply through their own
shape.

Only in a narrow range of medium pressures does the contact patch
spread out at an even pressure to match our expectations.


I would guess,,, not so.

With any non-belted bicycle tire, the tire inflation doesn't matter much
to the rolling resistance since they suffer from tread squirm at all
pressures. While a higher pressure results in a smaller contact patch,
the tread squirm decreases in size (due to the smaller contact patch)
but also occurs at a higher pressure.

Dear Doug,

Yes, measurements of the contact patches of bicycle tires confirmed
that they act like car tires, where the non-linear relationship
between pressure and area is well-known. Tire patches were 20~30%
smaller/lower than predicted at high/low pressures. In other words,
the tires stubbornly try to remain at the sweet spot in the middle,
where the sidewall effects are minimized. Tom Schmitz put my
measurements of a single tire to shame with a wide variety of much
more carefully-done tests--and the curve of his average measurements
overlaid my single-tire tests:
http://groups.google.com/group/rec.b...d6c10253d9f70c

The scissors-jack effect doesn't depend on the thickness of the walls
of the tire--a good deal of force is needed to distort the curve of
the sidewalls, which leads to the halo of high pressure at low
pressures.

As for the halo of low-pressure at high pressures, that's easier to
understand. Assume a 150 psi inflation. Pressure at the center of the
tire patch is 150 psi. Near the edges, the tire starts to curve away
from the ground, until the 150 psi tire's pressure on the ground drops
to zero.

In the end, fairly simple physics explains the easily reproducible
results. Just pump a bicycle tire to 100 psi, ink it, lower it with a
100 lb weight onto paper (or glass as Tom did), measure the two
dimensions of the ellipse, and then repeat at 10 psi increments for a
range from 40 to 150 psi. As I mentioned, the effect is well-known in
automobile tire engineering.

Dear Carl,

I imagine that with the aid of some integral calculus, the contact
pressure variation and patch size can be explained. It would be
interesting to see the results. One day maybe I'll try tackling such a
problem, unless someone beats me to it.

--
JS.

On Feb 8, 4:45*pm, Vic Smith wrote:

First time we went together in my car, he was totally shocked when he
saw me fasten my seat belt.
Seat belts were pretty new then.
Started yelling at me,
"What the hell you doing that for??!!"
I told him it was safer.
He looked at me like I as stupidest man alive, and said,
"Safe?!! What if you go in the river and can't get loose?!"

Anyway, you have to be there.

Being here is often enough.
http://groups.google.com/group/rec.b...f6f355644c4c08

DR

Vic Smith wrote:
On Wed, 08 Feb 2012 12:40:30 -0500, "(PeteCresswell)"
wrote:

Per Scott Dorsey:
My uncle Ralph claimed that you could never overfill a tire, that you should
just get a pump, and pump air in until you can't get any more in there and
that was the right amount of air.

Surprisingly enough, he was not killed in a car accident.

That one went right over my head.

Can somebody explain it for the temporarily humor-impaired?

Got nothing to do with humor unless you have the uncle.
It's about uncles with bad ideas, which they thought was common sense.

Precisely. And, it's possible that all he ever used was a foot pump so
there was some limit to the pressure he could get in the tire. But he
harbored a pretty serious delusion and it is a miracle he wasn't killed
by it.

If somebody would argue with Scott's uncle about that air pressure, he
might say something like,
"You don't know much about equalization, do you? Common sense."

I knew better than to even try.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Scott Dorsey wrote:
Vic Smith wrote:
On Wed, 08 Feb 2012 12:40:30 -0500, "(PeteCresswell)"
wrote:

Per Scott Dorsey:
My uncle Ralph claimed that you could never overfill a tire, that you should
just get a pump, and pump air in until you can't get any more in there and
that was the right amount of air.

Surprisingly enough, he was not killed in a car accident.

That one went right over my head.

Can somebody explain it for the temporarily humor-impaired?

Got nothing to do with humor unless you have the uncle.
It's about uncles with bad ideas, which they thought was common sense.

Precisely. And, it's possible that all he ever used was a foot pump so
there was some limit to the pressure he could get in the tire. But he
harbored a pretty serious delusion and it is a miracle he wasn't killed
by it.

If somebody would argue with Scott's uncle about that air pressure, he
might say something like,
"You don't know much about equalization, do you? Common sense."

I knew better than to even try.
--scott


Hey Scott. Welcome to r.b.t.