Tire Pressure & relationship to punctures

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

caaron 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 (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 writes:

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.

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 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 (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?

On Thu, 18 Aug 2005 19:00:25 -0400, "caaron"
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 (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

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.

On Thu, 18 Aug 2005 19:00:25 -0400, "caaron"
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 (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 writes:

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...

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

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.

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 writes:

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.

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