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From: (Jobst Brandt)
Subject: Re: No Flats Below Freezing
Date: 12 Jan 1999 21:34:37 GMT

Sheldon Brown writes:

>>> For the specific issue of small glass slivers, the usual mechanism
>>> is for the sliver to get stuck to the tire, and to roll 'round and
>>> 'round, getting pressed in a bit farther on each revolution, until
>>> it finally punctures the tube.

>> This theory of punctures appears to be derived from the experience
>> of driving nails, stroke at a time.  The only part of it that has
>> merit is that the tube often makes a tent over the intruding sharp
>> object and is penetrated after several rotations, that work the
>> object with tire casing flex.  The intrusion into the tire is
>> essentially complete when first contact is made.

> Do you have evidence to support this theory?

Yes.  Glass chips are wedges, usually in a minor and major axis and
penetrate until friction limits their progress in an operation that is
not inertia dependent, therefore, has no time dependence with respect
to wheel movement.  It is also not an impact phenomenon, but rather a
pressure insertion.  The action is complete when the greatest pressure
has been exerted and maintained under the tire contact.  Subsequent
passes on hard surfaces serve mainly to scrub off any protruding
glass, leaving a flush blunt end on the surface of the tire.  The
penetration process proceeds the same in slow motion as at 30mph, that
being an insignificant speed for the stab into the tire.  This
sequence can be deduced from mechanical observations of similar
phenomena and by observing the results on the tire carefully, whereas
the nail driving mode finds no logical support in a physical model.

> It seems to me to fly in the face of common sense.  Glass shards come in
> varying degrees of sharpness; some of them are too blunt to penetrate at
> all.  Others are sharp enough (and long enough) to be driven into the
> tire instantly.  In between, there are those which are able to grab a
> foothold in the tire on the first encounter.  I believe it is reasonable
> to conclude that some of these shards are able to stick in just a bit
> the first time 'round.  I also believe it reasonable to conclude that
> they can, indeed, work their way in.

If the shard is too blunt to penetrate, it will not get a foothold in
the tire.  It may however be too small to reach through to the air
volume, but that is another case.  I don't see the chip of glass sharp
enough to partially penetrate the tire doing any better the second
time.  As I said, this is not an impact event.  In the scale of the
displacements involved, elasticity makes this a contact pressure
phenomenon that is less likely on the second pass than the first.  The
limit of penetration is reached on the first attempt and does not get
better with repeated attempts.  This can be simulated by pushing a
sharp object into a tire with a calibrated force.  Repeatedly pushing
with the same force does not advance the probe farther.  It takes a
greater force than the first time to advance.

> I've seen my share of glass punctures in my time.  I have seen larger,
> more dangerous-looking shards stuck into, but not yet through, tire
> tread; I've seen smaller shards buried in the tread and through the
> tube, causing a puncture.

I suspect that seeing large protruding pieces of glass in a tire must
be by stopping in the minefield, so to speak, because these get broken
off by further riding.  Both of your observations do not show that the
depth of penetration progressed by repeated turns of the wheel.  As I
said, the tube makes a tent over some intrusions and the squirm of
tire casing and sharp object can belatedly cause a perforation.  This
does not support tire wiping by hand or a dragging piece of wire.

>>> Anything that increases the "stickiness" of the tire will increase
>>> this type of puncture.

>> If you try to visualize this scenario, it doesn't hold up.  I think
>> it comes from riding over freshly tarred and graveled road where
>> bits of gravel are picked up and fly in all directions.  Tires,
>> when dry, do not pick up sharp objects by adhesion, rotate them
>> into a penetrating position and drive them into the casing on
>> subsequent rotations.

>>> On dry winter days, the rubber is at its least sticky, and this
>>> problem is minimized.

>> Temperature in the range of interest has no affect on the adhesion
>> of particles to a bicycle tire.

> Except for my comment about fresh asphalt, I wasn't really speaking
> of adhesive properties, and the word "stickiness" was ill-chosen.
> Better to say that the rubber is softer when it's warm, harder when
> it's cold.  [snip]

The harder rubber, if any, may even enhance penetration because it
does not flex with the sharp object.  However, I don;t think you'll
find much difference in durometer between hot and cold tire tread.
You can test this by tossing a tire section into the freezer and
leaving another part out.  It's easier to see with a piece of inner
tube because the casing of a tire may confuse the issue.

>>> This is the reason many cyclists wipe their tires after riding
>>> through glass patches.

>> Tire wiping is akin to throwing salt over ones shoulder except that
>> it has more ill effects.  Tire wipers hands are dirty and if they
>> ride hard they wipe this grime onto their faces.  Today's short
>> bicycle frames make wiping rear tires dangerous because the fingers
>> can get jammed between seat tube and tire that nearly touch.

> I don't ride bicycles with this silly design.

You mean the tire save, of course.  Actually, the selection of high
performance frames is almost exclusively Short-short-short.

>> Quite aside from the hazards, wiping tires is completely
>> ineffective in preventing flats other than possibly feeling that
>> something is stuck in the tire, then dismounting and removing the
>> offending item (thereby letting the air out).  A tire wiper usually
>> wipes because he noticed that he rode through glass.  Had he seen
>> the glass in advance, he wouldn't have ridden through.

> Sometimes one is forced by traffic considerations to hold one's line
> even through a glass patch.

And at such a time, riders are even less likely to wipe tires while
traversing a glass strewn area.

>> Having noticed it after the fact, the wheel has made three to four
>> revolutions before he can wipe.  As I mentioned, any penetrant is
>> completely embedded by the first revolution, but for doubters,
>> certainly by the second or fourth.

> This assertion is un-proven and, in my opinion, implausible, as
> stated above.  I'll also mention that, when I used to make a habit
> of tire wiping, I would frequently succeed in dislodging glass
> fragments before they could do any real harm. [snip]

That is your assumption.  I propose that you did not retain the
dislodged piece so you do not know that it was glass that you felt,
and I am not convinced that had you not wiped, you would have gotten a
puncture.  This is an old tread that has been disputed similarly for a
long time.  If nothing else, few experienced riders consider tire
wiping or tire-saves more than tossing sale over the shoulder.

Jobst Brandt      <>

From: (Jobst Brandt)
Subject: Re: No Flats Below Freezing
Date: 13 Jan 1999 00:47:48 GMT

Greg Thomas writes:

> Is the subject of goathead thorns and knobby tires off-topic here?
> Because it seems to me that it is a similar situation.  Many, many
> times I've stopped and pulled out thorns after a revolution or two
> and saved a puncture.  If the thorn is penetrating in the non-knob
> portion of the tire then you have little chance.  But I've seen
> several times thorns that easily could have penetrated a small knob
> in a few more revolutions.  The next time I have time I'll even test
> it, we've got plenty of thorns around here.

How about testing first before you write and then tell us about it.  I
think you'll find as I have, that thorns also make full penetration
when first encountered.  If the seed pod is still attached, you'll
find that it is snugly against the tread, even pressed into the tread
slightly.  When the seed pod breaks off, the next revolution could
push the thorn a bit farther in if there is any overstand after
separation.  I have not observed any and even riding over rough
pavement does not seem to push the thorn deeper because the thorn end
has always been flush with the slick surface of the tread.

tribulus terrestris

Jobst Brandt      <>

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