Tire Pressure & relationship to punctures

caaron said:

I've noticed that when I keep my tires inflated to 80 or 90 psi I almost
never get flats (maybe 1 flat in 6 months), but when I keep them at
110-120psi I get flats very frequently (maybe one flat per week).

Can anyone explain why that is, and if there is any disadvantage to keeping
them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the
highway shoulders and city streets.

What are the tires supposed to be inflated to?

Chuck Aaron said:

I've noticed that when I keep my tires inflated to 80 or 90 psi I
almost never get flats (maybe 1 flat in 6 months), but when I keep
them at 110-120psi I get flats very frequently (maybe one flat per
week).

Quoted message said:

Can anyone explain why that is, and if there is any disadvantage to
keeping them at 80-90psi? I'm riding on 700x23c Michelins and I
ride mostly on the highway shoulders and city streets.

There is no logical explanation for what you observe. The firmer the
backing, the easier penetrating a tire casing becomes. Just visualize
trying to stab your tire with a jack knife with 20psi inflation
compared to 120psi. Penetrating the tread requires a certain force
for a given object and what you report does not follow.

I believe two things could be at work. One is that when riding with
more inflation pressure, you are riding faster and more attentive,
especially because riding over rough spots gives a rougher ride, which
should reduce flats. The other is that in an effort to make sure your
low pressure hypothesis is correct, you are inadvertently making are
avoiding debris in the road more than you were.

I don't think you have a random sample of sufficient size to be sure
of cause and effect. There are seasonal changes in puncture causes
such as how many beer bottles get tossed from cars and how much
puncture vine is along roadsides. You didn't say what was causing your
flats so I can only guess guess.

Often enough riders will report here that the new brand of tire they
are using greatly reduced the incidence of flats even when there is no
significant difference between old and new tire other than a puncture
resisting name lime Armadillo or the like.

Flats are cause by beer in winter and cigarettes in summer:

http://www.sheldonbrown.com/brandt/thorns.html

Jobst Brandt

caaron said:

I've noticed that when I keep my tires inflated to 80 or 90 psi I
almost never get flats (maybe 1 flat in 6 months), but when I keep
them at 110-120psi I get flats very frequently (maybe one flat per
week).
Can anyone explain why that is, and if there is any disadvantage to
keeping them at 80-90psi? I'm riding on 700x23c michelins and I ride
mostly on the highway shoulders and city streets.

You're being more careful when the tires are softer.

Problem solved; off you go... BS

Chuck Aaron wrote: I've noticed that when I keep my tires inflated to 80
or 90 psi I almost never get flats (maybe 1 flat in 6 months), but when I
keep them at 110-120psi I get flats very frequently (clip)
^^^^^^^^^^^^
Jobst Brandt wrote: There is no logical explanation for what you observe.
The firmer the backing, the easier penetrating a tire casing becomes. (clip)
^^^^^^^^^^^^^
Jobst, it looks to me like you may have misread what Chuck wrote. He gets
more flats when the tires are harder. That is consistent with: "The firmer
the backing, the easier penetrating a tire casing becomes."

It looks to me as though there must be a steep break in the curve between 90
psi and 110 psi, according to Chuck's experience. Is this compatible with
what you know from other sources?

caaron said:

I've noticed that when I keep my tires inflated to 80 or 90 psi I almost
never get flats (maybe 1 flat in 6 months), but when I keep them at
110-120psi I get flats very frequently (maybe one flat per week).

Can anyone explain why that is, and if there is any disadvantage to keeping
them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the
highway shoulders and city streets.

Chuck

Dear Chuck,

Theoretically . . .

Higher pressure makes a tire easier for a sharp object to
penetrate. A softer tire will roll over and "tent" over a
blunt enough piece of debris, while a harder tire will
concentrate pressure and impale itself on the blunt debris.

It's somewhat like lying on a bed of nails.

If a 200-lb person lies down on a single nail, the pressure
is 200-lbs/nail-point and drives the nail right through the
flesh.

If a 200-lb person lies down on 200 nails, the pressure is
only one pound/nail-point, and the circus audience is
disappointed.

Similarly, a small piece of debris must push up hard enough
against the tire to pierce it.

To do so, the debris must be tall enough and sharp enough.

Even a needle will not puncture your inner tube unless it's
tall enough to reach through the tread, the casing, and the
tube.

On the other hand, a blunter piece of debris has to be tall
enough to raise enough of the tire to concentrate enough
force to shove the debris through the tread--otherwise, the
tire just "tents" over the debris, which is why blunt
pebbles don't go through the tire at normal pressures.

(Squeeze the tire and blunt pebble hard enough in a vice and
the stone will make a nasty hole in the tire.)

At least two other factors confuse things.

First, higher pressure reduces the number of impact flats.
The greater the pressure, the greater the impact that the
tire can absorb before the tube is pinched between the
obstacle and the rim.

Second, higher pressure changes the shape of the contact
patch. At first, it would seem that the smaller contact
patch of a higher-pressure tire would reduce the area swept
by the tire and the amount of debris available to damage it.

But higher pressures in the range that you mention often
change the contact patch from a long, thin, oval to a
shorter (and wider) oval:

low high
pressure pressure

/XX\
<XXXXXX> \XX/

The ASCII diagram above is exaggerated, but illustrates the
idea. The higher pressure tire has less area, but sweeps a
wider path and therefore rolls over more debris.

The high-pressure tire sweeps a debris path 2X wide,
compared to the 1x of the lower pressure tire, and applies
more pressure to the point of the debris that it encounters.

So yes, higher pressure will encourage more punctures
(rather than pinch flats) where the debris is only modestly
sharp--great pressure will fall on more pointy objects.

(With needle-sharp goatheads, pressure doesn't make much
difference. It's quite common to see the filthy little
things sticking out of the curved side of a tire.)

Carl Fogel

Quoted message said:

But higher pressures in the range that you mention often
change the contact patch from a long, thin, oval to a
shorter (and wider) oval:

low high
pressure pressure

/XX\
<XXXXXX> \XX/

The ASCII diagram above is exaggerated, but illustrates the
idea. The higher pressure tire has less area, but sweeps a
wider path and therefore rolls over more debris.
The high-pressure tire sweeps a debris path 2X wide,
compared to the 1x of the lower pressure tire ...

Why do you think such an effect occurs? That would imply that
increasing the pressure would result in a more obvious bulge of the
sidewall in the vicinity of the contact patch. But the actual effect is
the opposite; at 120 psi there is much less sidewall deformation at the
bottom of the tire than there is with 80 psi. Assuming the same tire
and loading, the contact patch at 80 psi will be both longer and wider
than the one at 120 psi.

caaron said:

I've noticed that when I keep my tires inflated to 80 or 90 psi I almost
never get flats (maybe 1 flat in 6 months), but when I keep them at
110-120psi I get flats very frequently (maybe one flat per week).

Can anyone explain why that is, and if there is any disadvantage to keeping
them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the
highway shoulders and city streets.

Perhaps the tubes are stretched thinner at higher pressures? Tubes
that are sized the same on the box seem to have different thickness of
rubber and different cross section size. Less rubber between the road
and air space and more flats.

Leo Lichtman said:

Chuck Aaron wrote: I've noticed that when I keep my tires inflated
to 80 or 90 psi I almost never get flats (maybe 1 flat in 6 months),
but when I keep them at 110-120psi I get flats very frequently...

Quoted message said:

Jobst Brandt wrote: There is no logical explanation for what you
observe. The firmer the backing, the easier penetrating a tire
casing becomes...

Quoted message said:

Jobst, it looks to me like you may have misread what Chuck wrote.
He gets more flats when the tires are harder. That is consistent
with: "The firmer the backing, the easier penetrating a tire casing
becomes."

Oh, then that makes it all much clearer. But then again I want to
know what sort of flats these are to understand whether the assessment
is appropriate. Thorns would not be materially affected by this, they
penetrating equally well at a range of pressures due to their high
concentration of pressure, a sharp point.

Quoted message said:

It looks to me as though there must be a steep break in the curve
between 90 psi and 110 psi, according to Chuck's experience. Is
this compatible with what you know from other sources?

I can't substantiate that from my experience because my incidence of
flats is too random and has insufficient frequency.

Jobst Brandt

Paul Kopit said:

Quoted message said:

I've noticed that when I keep my tires inflated to 80 or 90 psi I
almost never get flats (maybe 1 flat in 6 months), but when I keep
them at 110-120psi I get flats very frequently (maybe one flat per
week).

Quoted message said:

Quoted message said:

Can anyone explain why that is, and if there is any disadvantage to
keeping them at 80-90psi? I'm riding on 700x23c michelins and I
ride mostly on the highway shoulders and city streets.

Quoted message said:

Perhaps the tubes are stretched thinner at higher pressures? Tubes
that are sized the same on the box seem to have different thickness
of rubber and different cross section size. Less rubber between the
road and air space and more flats.

Tube rubber is essentially incompressible and the inside volume of a
tire casing does not change with pressure. Therefore, inflation
pressure cannot alter inner tube shape in a tire.

Jobst Brandt

peter said:

Quoted message said:

But higher pressures in the range that you mention often
change the contact patch from a long, thin, oval to a
shorter (and wider) oval:

low high
pressure pressure

/XX\
<XXXXXX> \XX/

The ASCII diagram above is exaggerated, but illustrates the
idea. The higher pressure tire has less area, but sweeps a
wider path and therefore rolls over more debris.
The high-pressure tire sweeps a debris path 2X wide,
compared to the 1x of the lower pressure tire ...

Why do you think such an effect occurs? That would imply that
increasing the pressure would result in a more obvious bulge of the
sidewall in the vicinity of the contact patch. But the actual effect is
the opposite; at 120 psi there is much less sidewall deformation at the
bottom of the tire than there is with 80 psi. Assuming the same tire
and loading, the contact patch at 80 psi will be both longer and wider
than the one at 120 psi.

Dear Peter,

Hmmm . . . my first thought was that you're right, so I
assumed that I was thinking of smaller wheels with wider
tires versus larger wheels with narrower tires, or just
confused.

Then I decided to prove that I was wrong and amused my
neighbors by pouring a few glasses of water on my driveway
and repeatedly riding my bicycle through the puddle and into
the well-lit garage, with its smooth concrete floor.

After all, it's width that we're interested in, so a long,
wet front-tire track is easier than a stationary contact
patch.

My measurements didn't strike me as convincing, but it
didn't seem as if I was seeing the increase in width that I
expected when I dropped the pressure from 120 psi to 80 psi.

But the tracks were shrinking even as I looked at them, the
reduction is actually quite small, and I think that the
angle of the belts affects things.

Unfortunately, it's too hot and dry in Pueblo right now to
do this with much confidence. At 11 p.m., the damned wet
tire-tracks evaporated so fast that they shrank visibly
while I was kneeling on the cement floor and trying to get
my calipers on them.

The reduction in any case is going to be small. Taking an
idealized 2.0" x 0.5" rectangular front contact patch an
inch square supporting 80 pounds at 80 psi, pumping the tire
up to 120 psi would theoretically reduce it a touch less
than 20% in each direction to 1.61" x 0.41"--so I might have
been trying to measure a tenth of an inch of damp concrete.

As for the angled belt business, I rummaged around and found
a vaguely remembered passage in Sharp's 1896 "Bicycles &
Tricycles." Translating his section bb and aa, he says at
the end of section 336 that the tension on a tire the long
way (fore and aft) is twice as much as it is sideways. (This
is along the lines of the inflation of the correctly belted
tire shortening the tire's circumference and causing the
tire to constrict onto the rim.)

This is for what Sharp calls the Palmer-type tire, with
angled belts. I'm not sure if this is the same as modern
tires, nor how it affects the contact patch as pressure
rises or falls, but it suggests that the contact patch may
be trickier than I thought at first, or than I thought after
reading your reply, which struck me as plausible.

So I thought at first that I was mistaken, but now I'm not
even sure of that. If anyone has a link or some pictures
that show contact-patch width changes with pressure, I hope
that they'll be interested enough to post them.

I expect that there are more modern calculations than
Sharp's, but if anyone is interested, I'll copy them out.

Carl Fogel

Peter Rathman said:

Quoted message said:

But higher pressures in the range that you mention often change the
contact patch from a long, thin, oval to a shorter (and wider)
oval:

Quoted message said:

Quoted message said:

low high
pressure pressure

Quoted message said:

Quoted message said:

/XX\
<XXXXXX> \XX/

Quoted message said:

Quoted message said:

The ASCII diagram above is exaggerated, but illustrates the
idea. The higher pressure tire has less area, but sweeps a wider
path and therefore rolls over more debris. The high-pressure tire
sweeps a debris path 2X wide, compared to the 1x of the lower
pressure tire ...

Quoted message said:

Why do you think such an effect occurs? That would imply that
increasing the pressure would result in a more obvious bulge of the
sidewall in the vicinity of the contact patch. But the actual effect
is the opposite; at 120 psi there is much less sidewall deformation
at the bottom of the tire than there is with 80 psi. Assuming the
same tire and loading, the contact patch at 80 psi will be both
longer and wider than the one at 120 psi.

I think that boils down to this subject:

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

Jobst Brandt

Paul Kopit said:

caaron said:

I've noticed that when I keep my tires inflated to 80 or 90 psi I almost
never get flats (maybe 1 flat in 6 months), but when I keep them at
110-120psi I get flats very frequently (maybe one flat per week).

Can anyone explain why that is, and if there is any disadvantage to keeping
them at 80-90psi? I'm riding on 700x23c michelins and I ride mostly on the
highway shoulders and city streets.

Perhaps the tubes are stretched thinner at higher pressures? Tubes
that are sized the same on the box seem to have different thickness of
rubber and different cross section size. Less rubber between the road
and air space and more flats.

Dear Paul,

That's an idea, but . . .

I don't think that the tire (much less the rim) constraining
the inner tube actually expands even 1% when the pressure
increases from 80 to 120 psi.

The problem is that once the tube meets the inside of the
tire, there just isn't any place for it to go if it wants to
become thinner.

(Inner-tube rubber is pretty much incompressible--it remains
the same thickness unless there's some place for it to go.
Unless it can squish out somewhere else, solid rubber is
about as stubborn as water when it comes to compression.)

Thicker thorn-resistant inner tubes work in two ways.

First, piercing the extra-thick rubber on the outer curve of
the tube simply requires a slightly longer thorn. That's why
Kevlar tire belts actually help a bit in thorn country, even
though the thorns go through the Kevlar threads like needles
through steel wool--the tips on a goathead thorn are short
enough that just a little more padding can help.

Second, many thorn punctures are pin-[censored] that you find by
inflating the tube, holding it underwater in a sink, and
looking for slow, tiny bubbles. With a thicker
thorn-resistant tube pressed against the tire by 80 to 120
psi, the extra rubber tends to close the pin-[censored]--the
rubber under pressure can squish into the tiny void and fill
it if the layer of rubber is thick enough.

Carl Fogel

Wow, thanks for all of the replies to my question. Few of my flats are from
thorns. Most are from glass or small shards of metal. From my experience
the explanation that the tire is more compliant at lower pressure and forms
around the objects resisting punctures whereas at higher pressure the
objects have more ability to penetrate the tire seems correct. The tires do
seem faster at higher pressure (perhaps due to less road resistance). But
if I have to decide between several flats a week at high pressure or an
extra mile per hour at lower pressure I'll go for the lower pressure.

Thanks for all of the replies and comments.

Chuck

Carl Fogel said:

The calculator is based on some old metal-drum rolling-resistance
data from Jobst Brandt and doesn't address the question of how much
highly-inflated tires lose to increased bouncing on real-world
pavement.

Bounce losses are base on an invalid model, one that is not random but
rather like a cattle guard taken at the worst possible speed, one in
which the next bump is encountered at the steepest incline and never
landing on the back side to get that energy back.

First, that model only works with large (cattle guard) roughness that
causes tire lift-off and only with ones of regular spacing to act as a
saw tooth hindrance. Roads are made of random roughness in all
instances except washboard where the cattle guard syndrome is
guaranteed because the divots are made by exactly the action that
causes the dread bounce losses.

Quoted message said:

But it suggests that we're fooling ourselves when we think that we
can sense any speed difference in rolling resistance between 120 psi
and 90 psi--the extra 43 seconds is less than 1% of 5,000 seconds.

I'll agree with that fully.

Jobst Brandt

I find fewer flats at higher pressure. I have attributed this to the
opposite effect of what has been described here. That is, the softer
the tire, the easier it is for a piece of glass or metal to embed in
the rubber initially, so after a few hundred rotations of getting
worked in, the object pierces the tire and punctures the tube. With a
tire made harder by higher pressure, the pieces of glass or metal sluff
off the hard tire more often than the soft tire. That's my story and
I'm sticking to it. Glenn

On Fri, 19 Aug 2005 18:12:18 GMT,
[email hidden] wrote:

[snip]

Quoted message said:

Quoted message said:

1. I've felt them in the store and noticed how incredibly stiff and
hard to bend they are, like no other tire I've ever felt.

That would give them the highest rolling resistance of any similar
tire. I have not had one in hand or seen one on a bicycle.

[snip]

Dear Jobst,

If two tires are the same size, weigh as much, and are
inflated to the same pressure, which one should have less
rolling resistance--the harder and stiffer one, or the
softer and more flexible one?

I'm not arguing, just wondering what exactly goes on. I
expect that any explanation will have to go into some detail
about hardness and resilience.

Carl Fogel

Quoted message said:

I find fewer flats at higher pressure. I have attributed this to the
opposite effect of what has been described here. That is, the softer
the tire, the easier it is for a piece of glass or metal to embed in
the rubber initially, so after a few hundred rotations of getting
worked in, the object pierces the tire and punctures the tube. With a
tire made harder by higher pressure, the pieces of glass or metal sluff
off the hard tire more often than the soft tire.

I don't particulary buy that theory, but here's another possible
advantage of harder tires: The contact patch is going to be narrower so
the "stipe" of pavement the tire rolls over is also narrower. This
improves your chances of missing any given piece of glass or other sharp
object is near your path.

Sheldon "Don't Count On It" Brown
+---------------------------------+
| Is ambivalence a bad thing? |
| Well, yes and no. |
| -- Garrison Keillor |
+---------------------------------+
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
http://harriscyclery.com
Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

Carl Fogel said:

Quoted message said:

Quoted message said:

1. I've felt them in the store and noticed how incredibly stiff

and >>> hard to bend they are, like no other tire I've ever felt.

Quoted message said:

Quoted message said:

That would give them the highest rolling resistance of any similar
tire. I have not had one in hand or seen one on a bicycle.

Quoted message said:

If two tires are the same size, weigh as much, and are inflated to
the same pressure, which one should have less rolling
resistance--the harder and stiffer one, or the softer and more
flexible one?

You almost answer your own question. The tire that required more
force to deform is the more retardant one. Somehow that seems obvious
to me, especially when we consider how high performance tires look and
feel. They have thin pliable casings, thin lightweight tubes and a
similar tread with no voids for tread pattern.

Quoted message said:

I'm not arguing, just wondering what exactly goes on. I expect that
any explanation will have to go into some detail about hardness and
resilience.

You would have to assume a discontinuity in elastomers if suddenly one
of them displayed less internal friction as it got thicker and
stiffer. Consider what it is that makes these tires stiff. They are
made of chords of the same material as others that are bound in a
rubber matrix. Just bouncing such a wheel should make that apparent
as it doesn't rebound as high as a similar tire that is thinner and
lighter but inflated to the same pressure.

The tires in the RR charts are an example of that. The higher RR
tires that get substantially worse at lower pressure are such tires
with low TPI and stiff casings.

Jobst Brandt

Glenn Katon said:

I find fewer flats at higher pressure. I have attributed this to
the opposite effect of what has been described here. That is, the
softer the tire, the easier it is for a piece of glass or metal to
embed in the rubber initially, so after a few hundred rotations of
getting worked in, the object pierces the tire and punctures the
tube.

Please explain how this occurs. What is it about a tire at 60psi that
attracts glass that a tire at 120psi does not. Both tires fully engage
the piece of glass as the tread rubber conforms to the object while
rolling over it.

Quoted message said:

With a tire made harder by higher pressure, the pieces of glass or
metal sluff off the hard tire more often than the soft tire. That's
my story and I'm sticking to it.

Explain how the glass adheres to the tire better if it has lower
inflation pressure. This sluffing off paints a picture of a sticky
surface.

Jobst Brandt