Flints don't puncture bicycle tires?

[[email protected]]

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

 

[LSMike]

[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

Flints? I wouldn't know whether they are flints, but small pieces of
sharp flat stone have caused the last few p***t**"s I've had.
Sometimes the odd piece of glass too.

 

[Don Whybrow]

[email protected] wrote:
> 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.

Certainly flints have been in the UK since primitive times, The area
round where I used to live was a well known stone age industrial site
exporting flints around Europe [1]. In fact at the local Rugby [2] club
both teams had to walk the field prior to each game to pick up flints
that had worked their way to the surface, even if you were playing
immediately after anther match.


[1] http://en.wikipedia.org/wiki/Grimes_Graves
[2] For the leftpondians, a superior version of your "football", not to
be confused with football, that you call "soccer".

--
Don Whybrow

Sequi Bonum Non Time

"Lord, please make me the kind of person my dog thinks I am."

 

[Paul Boyd]

On 26/10/2006 20:47, [email protected] said,
> Over in rec.bicycles.tech, Jobst Brandt argues that flint flats are
> just a myth:

Really? The last puncture I had was on a newly surfaced Sustrans
cyclepath in Lincolnshire. Two punctures in fact on the same section of
path. Surfaced with small sharp flinty stones, several of which were
sticking out of the tyre.

(For the US readers, Sustrans masquerades as a national cycling
organisation, so really should know better)

--
Paul Boyd
http://www.paul-boyd.co.uk/

 

[peter]

[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."

I've never had a flat from a flint or other sharp stone, but a couple
years ago I had a simple twig go right through the tread of my tire.
It was a cheap Hutchinson that Nashbar had on sale for about $3 - after
several flats culminating in the one above I gave up on this tire even
though it had plenty of rubber left. If a stick can make it through a
tire tread and casing it seems reasonable to me that a properly
oriented sharp rock could as well.

 

[Tim McNamara]

This is relying on my memory, which is known to have lacunae so this
should be seasoned with a grain of salt.

I can remember many flat tires from small shards of glass. I really
don't know if these are picked up and work their way through the tread,
through the casing and into the tube to cause the flat or if they just
go straight through. I've heard arguments both ways. Not sure that it
matters.

I've had sharp somethings cut my tires without causing flats. That's
probably not germaine.

I've had thumbtacks, screws, roofing nails, construction staples, box
staples, stationery staples, large shards of glass, sharp pieces of
metal all cause flats. I've ever seen the seed of a puncture vine
except in person, since it doesn't seem to grow here in Minnesota or at
least in the parts I've lived in.

I've had flats from odd little pieces of wire about 1/4" long. Some
call them "Michelin" wires and allege that they come from blown-out
Michelin car tires. I don't know if that's the source. Not sure it
matters. I've also had flats from what are obviously little bits of
brake or derailleur cable, from cutting cables in my workshop and not
sweeping adequately.

I've had pinch flats from running into large rocks, potholes, etc. Once
I pinch flatted in a road race when someone had laid large square metal
poles across the lane in the middle of the race between laps, and I
couldn't see one for all the riders packed in front of me who were
yelling incomprehensibly and jinking around every which way.

I've had a number of flats from valve stems separating from the tube,
almost all of them with Specialized brand tubes. I've had mysterious
flats when my bike has been in my car on hot sunny days, sometimes
not-so-mysteriously resulting in the aforementioned valve stem failure.

I've found small stones imbedded in cuts in the tire tread. The vast
majority were just noticed and removed, and were not associated with
flats. I can't unequivocally recall ever having had a flat that was
definitely caused by a small sharp piece of rick (a.k.a., a "flint").
But then I wouldn't take an oath on pain of death that it had never
happened. I just don't remember ever having it happen for sure. If it
has, it's been a very very small percentage of the flat tires I have had
in the past 40 years.

 

[Andrew F Martin]

[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

Sorry, no pictures - but when I was working in Copenhagen I averaged a
flat a ride until I put Tuffy's in. the locals swear by riding lower
pressures, but it didn't seem to help me (Vittoria Rubino Pros).

 

[Simon Proven]

Tim McNamara wrote:
> I've found small stones imbedded in cuts in the tire tread. The vast
> majority were just noticed and removed, and were not associated with
> flats. I can't unequivocally recall ever having had a flat that was
> definitely caused by a small sharp piece of rick (a.k.a., a "flint").
> But then I wouldn't take an oath on pain of death that it had never
> happened. I just don't remember ever having it happen for sure. If it
> has, it's been a very very small percentage of the flat tires I have had
> in the past 40 years.

I tend to find lots of small stones in the tyre tread; when I'm out on
a group
and someone's got a puncture I amuse myself by picking them out their
tread (as well as the one that cause the puncture) while they repair
the
tube. I find 2-3 embedded when I do that, typically. They're not
flints,
IME because they're too light in colour, and are probably cherts.

My touring bike has 11,500 miles behind it, and has had /one/ puncture,
and IIRC (it was 2004 that it happened) it was a chert. It was right
by
the front door when I noticed it, having just got home. Convenient.

Other punctures since 1998:

- folding bike, drill bit through bottom of tread and out of sidewall
- folding bike, small nail in bottom of tread

- racer, tread puncture, small fragment of glass
- racer, puncture due to poor rim tape supplied with bike, at spoke
hole

- tourer, aforementioned chert

- audax bike, no punctures

I think those bikes have about 16K miles and 5 punctures, so about
3.2K miles per puncture. The tourer with the TT2000 tyres seems to
be the best, the audax bike's not done enough miles yet to know.

Simon

 

[peter]

Tim McNamara wrote:
> I've had flats from odd little pieces of wire about 1/4" long. Some
> call them "Michelin" wires and allege that they come from blown-out
> Michelin car tires. I don't know if that's the source.

They're not specific to any particular tire brand anymore, but were
originally given the name "Michelin thorn" back when steel-belted
radial car tires were relatively rare and Michelin was the brand most
associated with that technology.

 

[Sue White]

[email protected] whizzed past me shouting
>
>Primitive superstitions about flints still flourish in the UK and
>Denmark, so I'm asking for your views on the matter, somewhat like a

I live in Reading, which is a very good area for flints.
You can pick up Stone Age flint tools in the fields round here.

You can also pick small sharp chips of flint out of your punctured
tyres, a few times a year depending on your mileage.

Of course there are millions of small sharp flint chips on our roads and
bridleways, and it's only a tiny antisocial minority that stick into
your tyres. Most often the back tyre: your front wheel disturbs a chip
that was lying flat on the ground, and your rear one rolls onto it while
its pointy end is still sticking up.

--
Sue ]

 

[Andrew F Martin]

Sue White wrote:
> [email protected] whizzed past me shouting
> >
> >Primitive superstitions about flints still flourish in the UK and
> >Denmark, so I'm asking for your views on the matter, somewhat like a
>
> I live in Reading, which is a very good area for flints.
> You can pick up Stone Age flint tools in the fields round here.
>
> You can also pick small sharp chips of flint out of your punctured
> tyres, a few times a year depending on your mileage.
>
> Of course there are millions of small sharp flint chips on our roads and
> bridleways, and it's only a tiny antisocial minority that stick into
> your tyres. Most often the back tyre: your front wheel disturbs a chip
> that was lying flat on the ground, and your rear one rolls onto it while
> its pointy end is still sticking up.
>
> --
> Sue ]

Strange - when I lived in Maidenhead, Berkshire - I never got a flat
from flints. Actually - I can't remember getting any flats at all
while there.

 

[[email protected]]

Andrew F Martin writes:


>> 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.)

> Sorry, no pictures - but when I was working in Copenhagen I averaged
> a flat a ride until I put Tuffy's in. the locals swear by riding
> lower pressures, but it didn't seem to help me (Vittoria Rubino
> Pros).

Oops! That's another piece of myth and lore. That lower tire
pressure (and we're talking about -20% or so) is a visualization that
assumes a tire casing and tread of extreme flexibility by which the
tire casing could form an umbrella of a few mm diameter around a sharp
object. That is about as valid as the tire wiping gesture:

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

Jobst Brandt

 

[[email protected]]

Peter Rathman writes:

>> I've had flats from odd little pieces of wire about 1/4" long.
>> Some call them "Michelin" wires and allege that they come from
>> blown-out Michelin car tires. I don't know if that's the source.

> They're not specific to any particular tire brand anymore, but were
> originally given the name "Michelin thorn" back when steel-belted
> radial car tires were relatively rare and Michelin was the brand
> most associated with that technology.

I beg to differ. I introduced the term "Michelin wire" in this forum
when it was just one group rec.bicycle, and i was describing that type
of puncture. As you say, it was at a time when steel belted tires
were primarily Michelin, the tire that was developed primarily as a
rubber tired railway wheel before its other benefits were accepted by
the auto industry. The Paris metro still runs pneumatic rubber tires.

Jobst Brandt

 

[[email protected]]

On 27 Oct 2006 01:35:24 GMT, [email protected] wrote:

>Andrew F Martin writes:
>
>
>>> 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.)
>
>> Sorry, no pictures - but when I was working in Copenhagen I averaged
>> a flat a ride until I put Tuffy's in. the locals swear by riding
>> lower pressures, but it didn't seem to help me (Vittoria Rubino
>> Pros).
>
>Oops! That's another piece of myth and lore. That lower tire
>pressure (and we're talking about -20% or so) is a visualization that
>assumes a tire casing and tread of extreme flexibility by which the
>tire casing could form an umbrella of a few mm diameter around a sharp
>object. That is about as valid as the tire wiping gesture:
>
>http://www.sheldonbrown.com/brandt/wiping.html
>
>Jobst Brandt

Dear Jobst,

Why wouldn't sharp objects be less likely to penetrate tough materials
like tires if the resistance is lowered to 60-90 psi instead of 90-120
psi?

There is, after all, a lower limit of pressure below which the sharp
object will not penetrate the tire.

Incidentally, what we call "tire savers" in the US are often called
"flint catchers" in the UK, where riders appear to be unaware of your
theory. (You can google the archives for the quaint phrase, which
appeared in posts to which you replied.)

Cheers,

Carl Fogel

 

[Werehatrack]

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

May the gods of perversity visit him for that one. The mere fact that
it has not happened to him is not proof that it does not occur.

If I had been prescient enough to find it for him back in 1976, I
possibly could have brought him the stone that poked a hole in the
tire of my '67 VW Van on a road in Montana, but he thoughtlessly
failed to let me know that he wanted it. The road was freshly graded
and gravelled, and I was so certain that there must have been
something other than a rock involved that I spent the better part of
half an hour searching the road for the offending object. The search
was fruitless, of course; under the assumption that a rock couldn't do
the deed, I looked for anything else...but rocks were the only
possible cause. The majority of the rocks were 35 to 70mm in size,
and they were freshly crushed rather than rounded river rock. Sharp
edges abounded, but I ignored them. Finally, I gave up the quest for
the puncture's cause, mounted the spare, and carefully made my way
back to civilization to get the sanctified tire cursed again.

The guy at the tire store knew just which road section was involved.
He'd had several flats to fix that day due to that road and another
which had received similar treatment by the DOT.

Perhaps more to the point, however, in a conversation I had years ago
with one of the engineers at Armstrong (are they still in business?)
he noted that stone-induced holes are exceedingly rare on car tires
now that steel belts are the norm under the tread, and even in those
cases where they still occur, they seldom leave a rock in the breach.
Sometimes, that's because the stone causes a rupture rather than a
puncture; the stone never actually penetrates the surface, but deforms
it so severely that it tears from the inside. When that occurs, the
cords on the inside show a burst pattern rather than a cut. Tire
punctures due to an outright cut from a rock do occur, though, but the
rock is seldom still in the cut when the tire is removed; such rocks
are almost never shaped regularly enough, with a small enough
cross-section with nearly parallel sides, that they get trapped by the
tire material. They poke a hole and get blown back out.

>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.

The Loch Ness Monster, on the other hand, is a known hoax, while the
stone-cut tire is not.

>Any pictures of flints in tires and tubes would be interesting, even
>though they must be obvious fakes.

I suspect that it's just such a matter-of-course occurrence in such
areas that they don't bother taking pictures. To them, I'm sure that
it would be like making photo documentation of dryer lint; why bother?
(Nessie, on the other hand, is a subject whose photo is worth real
money; I can just imagine the melee that would ensue should that
mythical beast surface before a crowd of Scots with cameras in hand.)

>(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.)

And I live in an area where the odd rock is more often a shard of
oyster shell, with less probability of puncturing a tire than the far
more common (on the road, at any rate) shards of broken glass and
plastic remaining from the most recent auto collisions. Still, for
me, the puncturing object is still in the tire only about half the
time with street flats, and less than that when off-road.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

 

[[email protected]]

On Fri, 27 Oct 2006 01:45:26 GMT, Werehatrack
<[email protected]> wrote:

>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
>
>May the gods of perversity visit him for that one. The mere fact that
>it has not happened to him is not proof that it does not occur.
>
>If I had been prescient enough to find it for him back in 1976, I
>possibly could have brought him the stone that poked a hole in the
>tire of my '67 VW Van on a road in Montana, but he thoughtlessly
>failed to let me know that he wanted it. The road was freshly graded
>and gravelled, and I was so certain that there must have been
>something other than a rock involved that I spent the better part of
>half an hour searching the road for the offending object. The search
>was fruitless, of course; under the assumption that a rock couldn't do
>the deed, I looked for anything else...but rocks were the only
>possible cause. The majority of the rocks were 35 to 70mm in size,
>and they were freshly crushed rather than rounded river rock. Sharp
>edges abounded, but I ignored them. Finally, I gave up the quest for
>the puncture's cause, mounted the spare, and carefully made my way
>back to civilization to get the sanctified tire cursed again.
>
>The guy at the tire store knew just which road section was involved.
>He'd had several flats to fix that day due to that road and another
>which had received similar treatment by the DOT.
>
>Perhaps more to the point, however, in a conversation I had years ago
>with one of the engineers at Armstrong (are they still in business?)
>he noted that stone-induced holes are exceedingly rare on car tires
>now that steel belts are the norm under the tread, and even in those
>cases where they still occur, they seldom leave a rock in the breach.
>Sometimes, that's because the stone causes a rupture rather than a
>puncture; the stone never actually penetrates the surface, but deforms
>it so severely that it tears from the inside. When that occurs, the
>cords on the inside show a burst pattern rather than a cut. Tire
>punctures due to an outright cut from a rock do occur, though, but the
>rock is seldom still in the cut when the tire is removed; such rocks
>are almost never shaped regularly enough, with a small enough
>cross-section with nearly parallel sides, that they get trapped by the
>tire material. They poke a hole and get blown back out.
>
>>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.
>
>The Loch Ness Monster, on the other hand, is a known hoax, while the
>stone-cut tire is not.
>
>>Any pictures of flints in tires and tubes would be interesting, even
>>though they must be obvious fakes.
>
>I suspect that it's just such a matter-of-course occurrence in such
>areas that they don't bother taking pictures. To them, I'm sure that
>it would be like making photo documentation of dryer lint; why bother?
>(Nessie, on the other hand, is a subject whose photo is worth real
>money; I can just imagine the melee that would ensue should that
>mythical beast surface before a crowd of Scots with cameras in hand.)
>
>>(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.)
>
>And I live in an area where the odd rock is more often a shard of
>oyster shell, with less probability of puncturing a tire than the far
>more common (on the road, at any rate) shards of broken glass and
>plastic remaining from the most recent auto collisions. Still, for
>me, the puncturing object is still in the tire only about half the
>time with street flats, and less than that when off-road.

Dear Werehatrack,

Yes, my impression is that flats from what the UK crowd and the Danes
call flints are so common that they'll be amused by this thread.

I don't doubt that Jobst's tires are unsullied by such thoughtless
rock chips, but his habit of denying the existence of anything outside
his experience grows tiresome. Right now, he's evading the issue in
several posts, with red herring quibbles about who first used the term
"Michelin wire" and blather about how terrible I am to ride where I
live and take pictures that don't support his theories about
goatheads.

What's actually interesting is how the stone-chip flats seem to be
much more common in England and Denmark. I wondered if it was just
some local road-building custom, but a post in this thread mentions
that the rugby teams in one area search the grass for flints before
games, which suggests that the evil little things are part of the
general landscape.

For what it's worth, here's the shortest and sharpest (sorry, can't
resist it) comment that I've found in the archives about the
supposedly non-existent flints:

Andrew Muzi of yellowjersey.org in Wisconsin wrote skeptically about
the danger of checking for sharp objects inside flat tires:

"Myth. Having run my fingers inside literally many thousands of
tires, often with nicely mounted glass shards, staples, wire, nails
and evey imaginable penetrating object, I cannot recall ever having
cut my finger while wiping the inside of a tire."

David Damerell of the UK replied:

"I have once, on a flint. However, I don't think it's worth buggering
about with cotton balls to prevent one trivial injury in many years of
riding."

http://groups.google.com/group/rec.bicycles.tech/msg/64c1101610ba1d6e

As a sidelight, a friend of mine startled me recently by casually
mentioning that he expects two or three flat tires per year just
driving his car. He started ticking off flats in the last year in his
car, his wife's car, his parents' car, and rental cars, all just
driving around paved streets all over the western US. The tires were
new, not worn out.

After I decided that my friend wasn't joking, I told him that he
certainly won't be borrowing my car. Tthis seemed like a more
practical approach than the Jobstian tactic of telling him that he was
mistaken.

Around here, we actually have a phrase to describe a certain
personality: "He'd argue with a flat tire."

Cheers,

Carl Fogel

 

[[email protected]]

Carl Fogel writes:

>>> Sorry, no pictures - but when I was working in Copenhagen I
>>> averaged a flat a ride until I put Tuffy's in. the locals swear
>>> by riding lower pressures, but it didn't seem to help me (Vittoria
>>> Rubino Pros).

>> Oops! That's another piece of myth and lore. That lower tire
>> pressure (and we're talking about -20% or so) is a visualization
>> that assumes a tire casing and tread of extreme flexibility by
>> which the tire casing could form an umbrella of a few mm diameter
>> around a sharp object. That is about as valid as the tire wiping
>> gesture:

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

> Why wouldn't sharp objects be less likely to penetrate tough
> materials like tires if the resistance is lowered to 60-90 psi
> instead of 90-120 psi?

As I mentioned above. For this scenario to succeed, the tire would
need enough flexibility to allow a dimple several mm in diameter to be
supported on a thorn or sharp (knife like) object not more than 2mm in
length. Tires are not that flexible. Try that manually and you'll
see that even at 30psi there is enough pressure to puncture through to
the air chamber.

> There is, after all, a lower limit of pressure below which the sharp
> object will not penetrate the tire.

Describe that for an average tire. Just try that with any of yours.

> Incidentally, what we call "tire savers" in the US are often called
> "flint catchers" in the UK, where riders appear to be unaware of your
> theory. (You can Google the archives for the quaint phrase, which
> appeared in posts to which you replied.)

Tire savers are those things that do no good except that, when roads
are moist, cover the rider and bicycle with road debris, much like the
back of automobile vans. I haven't seen one used in years.

Jobst Brandt

 

[[email protected]]

[email protected] wrote:

> Yes, my impression is that flats from what the UK crowd and the Danes
> call flints are so common that they'll be amused by this thread.
>
> I don't doubt that Jobst's tires are unsullied by such thoughtless
> rock chips, but his habit of denying the existence of anything outside
> his experience grows tiresome.

Dear Carl,

This may be true, but right now you want to bust him so bad it is
interfering with your judgement. Calm down and regroup -- if you are
correct, he will soon will afford you another opportunity.

Cheers,
Doug


Doug

 

[[email protected]]

On 27 Oct 2006 02:25:24 GMT, [email protected] wrote:

[snip]

>> Why wouldn't sharp objects be less likely to penetrate tough
>> materials like tires if the resistance is lowered to 60-90 psi
>> instead of 90-120 psi?
>
>As I mentioned above. For this scenario to succeed, the tire would
>need enough flexibility to allow a dimple several mm in diameter to be
>supported on a thorn or sharp (knife like) object not more than 2mm in
>length. Tires are not that flexible. Try that manually and you'll
>see that even at 30psi there is enough pressure to puncture through to
>the air chamber.
>
>> There is, after all, a lower limit of pressure below which the sharp
>> object will not penetrate the tire.
>
>Describe that for an average tire. Just try that with any of yours.

[snip]

Dear Jobst,

Okay, let's try it with an average tire, one of mine.

Here's about 4mm of small nail sticking up in a vise, with its twin
brother lying beside it and spoke ruler. The nail point, considerably
enlarged, appears to be less than half a millimeter wide:

http://server5.theimagehosting.com/image.php?img=325a_30psi_setup.jpg
or http://tinyurl.com/whahk

Here's a 700c x 26 tire inflated to 30 psi and about to test your
theory, again considerably enlarged:

http://server5.theimagehosting.com/image.php?img=326a_30psi_and_nail.jpg
or http://tinyurl.com/y6g36w

And here's the result:

http://server5.theimagehosting.com/image.php?img=327a_30psi_no_puncture.jpg
or http://tinyurl.com/y74wc8

As you can see, the tire is holding pressure, having flattened out and
tented over the nail point quite flexibly--no puncture, not even a
mark on the tread.

Experience suggests that this is not the result of running over a
small nail point at normal touring tire pressures.

Obviously, the sharper the object, the lower the crucial pressure. And
equally obviously, the lower the pressure, the less the likelihood of
a puncture.

Feel free to do more testing on your own to confirm this.

But you can save time by pondering how Indian fakirs lie on beds of
nails.

I hear it has something to do with keeping the pressure below the
crucial level by spreading out the body weight over enough nails,
which allows the skin to tent and dimple over the the nail points
instead of puncturing (like certain theories).

Cheers,

Carl Fogel

 

[[email protected]]

On 26 Oct 2006 20:03:45 -0700, [email protected] wrote:

>
>[email protected] wrote:
>
>> Yes, my impression is that flats from what the UK crowd and the Danes
>> call flints are so common that they'll be amused by this thread.
>>
>> I don't doubt that Jobst's tires are unsullied by such thoughtless
>> rock chips, but his habit of denying the existence of anything outside
>> his experience grows tiresome.
>
>Dear Carl,
>
>This may be true, but right now you want to bust him so bad it is
>interfering with your judgement. Calm down and regroup -- if you are
>correct, he will soon will afford you another opportunity.
>
>Cheers,
>Doug
>
>
>Doug

Dear Doug,

Right now, I'm waiting for the predictable explanations that my
pictures of a nail failing to penetrate a tire at 30 psi are mistaken,
since they fail to confirm Jobst's latest theory:

http://groups.google.com/group/rec.bicycles.tech/msg/6955c93a5f6a06a0

Cheers,

Carl Fogel