On 26 Oct 2006 12:47:13 -0700,
[email protected] wrote:
>Over in rec.bicycles.tech, Jobst Brandt argues that flint flats are
>just a myth:
>
>"I keep hearing about these mysterious cherts [flints] yet have never
>seen one on the road or in a tire. I ride many miles of rocky roads
>here and in Europe and have not had a flat from these mysterious sharp
>rocks that don't seem to cut car tires or we could find examples of
>them embedded in the surface of car tires."
>
>http://groups.google.com/group/rec.bicycles.tech/msg/97f538aa81f0f1ed
>
>Primitive superstitions about flints still flourish in the UK and
>Denmark, so I'm asking for your views on the matter, somewhat like a
>sociologist asking about the Loch Ness monster.
>
>Any pictures of flints in tires and tubes would be interesting, even
>though they must be obvious fakes.
>
>(Alas, the nearby sand and gravel pit closed a few years ago, so I'm no
>longer able to photograph the flint flats whose existence Jobst
>denies.)
>
>Cheers,
>
>Carl Fogel
Prompted by Jobst's shoot-from-the-hip claims elsewhere in this
thread, I pumped a smooth, worn Kevlar-belt 700x26 tire up from 30 psi
to 110 psi at 10 psi increments and rolled it over a nail sticking up
from a vise.
(Pictures below.)
Nothing happened until the tire pressure reached 100 psi.
At that pressure, the nail audibly penetrated into the tread, leaving
a hole and making a little popping noise--but the tire didn't go flat.
I stopped rolling various sections of the tire over the nail at 100
psi after only half a dozen popping noises, which showed considerable
maturity and restraint.
(Try to stop popping after only six bubble-wrap bubbles.)
At 110 psi, ten psi higher, the pop seemed louder when I rolled the
tire over the nail--but the tire still stubbornly held air.
Suspicious, I took the tire off the rim and looked at the inside of
the tire and the inner tube.
Aha! No penetration at 100 psi, but a hole all the way through the
tire at 110 psi, plus a faint little circular scrape and tiny dent on
the inner tube.
As the pressure increased, the nail dimpled the tire and inner tube
until it broke through the tread at 100 psi and then through the
hidden Kevlar belt at 110 psi.
After it broke through, the nail pushed the stretchy inner tube off
the inside of the tire, and started making a new dimple in the inner
tube.
Two points are illustrated.
First, tires at lower pressures obviously resist punctures by reducing
how hard the debris can push against the tire. Lower tire pressures
have been recommended by Danish riders, whose pavement seems to
provide a lot of rock chip or flint flats:
http://groups.google.com/group/uk.rec.cycling/msg/5f7c729c68b72204
A drop from 110 psi to 100 psi stopped a short section of an ordinary
nail, roughly similar to a small rock chip or flint, from penetrating.
Of course, some debris is so sharp and so tall that it will focus
enough force in a small enough area to penetrate a tire at any
reasonable pressure. For example, the needle-like tip of a goathead
thorn will go through the same tire at only ten psi. But my fairly
sharp nail tip failed to break through the tire until the air pressure
reached 110 psi over the dimple that the nail made in the tire.
Second, some objects can puncture tires without puncturing the inner
tube.
A shorter but similarly sharp object that stopped without puncturing
the tube, such as a rock chip, would still have eventually worn a hole
in the inner tube and caused a flat.
This unexpected behavior makes an interesting case for the
often-maligned tire-saver theory that some kinds of embedded debris
can be removed before they cause a flat. Most debris goes right
through the tire and tube immediately, so there may be little
practical use for scraping debris off tires or digging it out of the
tread, but some kinds of debris must end up behaving like the nail.
Four pictures follow.
Here's the nail sticking up about 6 mm from the vise:
http://server5.theimagehosting.com/image.php?img=334a_110psi_setup.jpg
or
http://tinyurl.com/wrm78
Here's the outside of the tire and the exposed inner tube. Blunt
toothpicks are stuck in the holes made by the nail at 110 psi on the
left and 100 psi on the right. A third toothpick points to where the
inner tube shows a faint mark from the nail that penetrated the tire
at 110 psi:
http://server5.theimagehosting.com/image.php?img=331a_110psi_penetrates.jpg
or
http://tinyurl.com/ssc8k
Here's the inside of the tire. One toothpick is visible where the nail
penetrated at 110 psi. The second toothpick can't be seen, since the
100 psi hole in the tread didn't go all the way through the Kevlar
belt. The inner tube mark is out-of-focus, but shown by a third
toothpick:
http://server5.theimagehosting.com/image.php?img=332a_110psi_penetrates.jpg
or
http://tinyurl.com/y7bmnx
Here's a slightly better picture of the inner tube mark. It's just a
very faint circular scraped area around a tiny dent that shows up as a
black shadowed area, not quite as wide as the end of the blunt
toothpick:
http://server5.theimagehosting.com/image.php?img=333a_110psi_mark_on_tube.jpg
or
http://tinyurl.com/y2ngj3
For those curious about such simple tests, far more time was spent
fiddling with the camera and bashing the keyboard than clamping a nail
in a vise, pumping an old tire up, and imitating an Iron Maiden. The
actual testing took about fifteen minutes.
It never occurred to me that the inner tube would behave like this,
which is why I like fooling around with such simple tests--I was
annoyed when the nail seemed to puncture the tire, but the tire
refused to go flat.
Cheers,
Carl Fogel
