The case of the mysterious un-punctured puncture

[Mark J.]

So, yesterday I was riding and heard the familiar
fwisssh-fwisssh-fwisssh of a puncture.

Veloflex Master 700x25 tire, unpatched Vittoria latex tube, originally
at 105 psi.

In the first few seconds, expecting the tire to bottom out immediately,
I looked down to see if it was the front or rear tire. Couldn't tell.
Stopped and examined the tires. Snapping a finger against the tire
showed the rear was a bit soft (lower frequency "ping" than front when
snapped), but definitely not flat.

"Fwisssh" is greatly reduced, but still slightly audible.

There is ***NO*** sealant in my tubes - unless Vittoria puts some in
without advertising, and it's undetectable when mounting.

Remounted and rode another several hundred feet to some shade (around
90F and sunny), assuming I'd need to change the tube. Rear tire is
/not/ sagging visibly; I have tan sidewalls, which makes sag easier to
see. In the shade, examine rear for protruding wire/tack/stone/glass,
see the tiniest bit of glass, but the tire is still mostly firm. Pick
out glass with Swiss army knife, it was so small and shallow, might not
have been the cause.

Then I do something I *never* do - top off the tire with frame pump.
This should have been futile - I was ~8 miles from home, but I thought
I'd see where it went. Pump's built-in gauge starts at ~60 psi, I top
it up to ~100 psi, head home. Weeks ago I had removed the inflator from
my bike bag, and it was really hot, so I appreciated not needing to do
the entire tube replacement and slow inflation with mini-pump.

On the way home, looking at the tire periodically, I see no sag.
Intentionally going over small bumps/rough pavement, front and rear feel
similar. Get all the way home. Gauge says tire is now ~50, but it
feels much firmer than that.

Next day, tire is flat as expected (and latex tubes leak a bit anyway).
Dismount tire, find generic pinhole in tube, matching tiny hole in tire.
Can't remember if this is where the glass was; there are 5-6 *tiny*
holes/blemishes in tire tread, the kind that often don't result in a
flat. The pinhole is *NOT* a *slow* leak; I've patched plenty of slow
leaks that require immersion to locate. This one shows up audibly right
away.

The mystery - on the road, the flat behaved as I would expect a tire
with sealant to behave. Fast, highly audible leak that should have had
me quickly riding on the rim slowly diminishes and stops at about 60
psi, or at least changes to a very slow leak. At home while patching,
the hole was clean and unclogged - no signs of sealant (and I'm 99% sure
there never was any.) (My entire experience with sealant is reading
about it, I've never used any).

Tube could have stuck to the tire casing and sealed somehow (I think
Jobst once speculated on this), but I talc liberally, and the tube
definitely didn't stick to the tire when dismounting.

Other details: Front tire is at 70 psi today, down from 90 yesterday,
that's normal latex leakage. I weigh about 170 lbs. Road debris around
here is mostly bearing-sized gravel from pavement deterioration, with
some larger bits mixed in.

What happened? Benevolent Gremlins?

Mark J.

[AMuzi]

Could be just a very small puncture, which will bleed faster
at higher pressures.
Once out of the tire, such a hole will distend when you
inflate to examine it so the original size of it is now unknown.

Regarding sealer- a latex tube is only about 60 grams. You
would have noticed right away if there was any significant
goop inside from the weight alone, not to mention sloshing
at the bottom.

--
Andrew Muzi
<www.yellowjersey.org/>
Open every day since 1 April, 1971

[Mark J.]

Yes, but when I reinflated back to 100 psi, the hissing didn't return.
-Mark\

[AMuzi]

Well, that leaves a demon infestation doesn't it?

[cycl...@gmail.com]

I've had this a couple of times within the last five years. I THINK that what is happening is that the tube was a little twisted in the tire. When you picked up the glass the deflation caused you to stop before the glass tore a larger hole. When the tire got soft enough and you picked the glass out the tube twisted a tiny bit so that there wasn't a hole in the tire where the tube puncture was. This allowed the tube to press solidly against an airtight surface. The more bouncing you do the faster these things will deflate. And overnight they will deflate completely. I've pulled the tube out and pumped it WAY up and used a pail of water and still been unable to find the leak. But after a ride the tire is soft again. A couple of these tubes I simply threw away rather than chase the leak more than with a pail of water.

[Mark J.]

Maxwell's demon, yeah. That's why I gave such lengthy detail - the
obvious answers don't seem to work.

Mark

[David Scheidt]

Mark J. <MarkU...@comcast.net> wrote:

I've seen some some holes that behave like this, too, but in car
tires. (I used to run a garage, I've dealt with thousands of flats)
My though was maybe that the puncture isn't normal to the surface of
Jthe tube, but at an angle, and so under pressure the tube can squeeze
it shut, at least some times. That's pretty common in car flats,
which have rather more depth in the innerliner than bike tubes do.

--
sig 127

[cycl...@gmail.com]

I think that might be part of the effect. Bicycle tires being completely round the tube is never at a "normal" angle. I do know that this type of flat occurs often enough that most active riders know about them.

[Doc O'Leary]

For your reference, records indicate that

David Scheidt <dsch...@panix.com> wrote:

> My though was maybe that the puncture isn't normal to the surface of
> Jthe tube, but at an angle, and so under pressure the tube can squeeze
> it shut, at least some times.

Yeah, I’d guess something along these lines. Even a straight slice
would exhibit similarly behavior. Like a piece a paper pressed against
a screen by the wind, a clean cut wouldn’t allow much air to pass
through. But if it were held open a bit by debris, the air flow could
actually help to push the opening even larger. A small 2D cut becomes
a large 3D hole, which goes back to 2D when the debris/flow is removed.
It’d be interesting to examine the puncture in question under a
microscope to see what it looked like.

--
"Also . . . I can kill you with my brain."
River Tam, Trash, Firefly

[Mark J.]

Sigh. Trust Muzi, he was right. Turns out I had a ~1mm casing cut.
Next ride, the latex tube (again) /slowly/ bulged out 10 miles into the
ride and blew, then seemingly re-sealed at around 40-60 psi. Stuck a
Rema patch over the casing cut, but haven't road-tested it yet.

Mark J.

[pgr...@gmail.com]

Superglue, if you get the ends to meet. I've fixed about a zillion pretty deep slices/cuts, over the years (and helped keep a ton of others' tires from going to the landfill prematurely).
I have yet to see one of my glued fixes come unglued, even on 100-115 tires, the glue is pretty darn tenacious with rubber, apparently.
Sometimes I've had success gluing a boot in like this, but the inside of the tire has that release junk on it, so I would say the rate on those is pretty darn low.