From: jbrandt@hpl.hp.com (Jobst Brandt )
Newsgroups: rec.bicycles.tech
Subject: Re: slow leak?
Date: 16 Aug 1996 18:41:46 GMT

Scott (who) writes:

> Does anyone else need to put air in their tires every couple of weeks?
> Seems my tires need about five pounds or so every now and then. Is
> this normal or do both my tubes have slow leaks?

There are too many variables here but the fact that you have two tires
doing the same thing gives a little help.  The first guess is that you
have a leaky valve or a leaky patch, but since you didn't mention
patching, I assume these are new tubes, and because they both do this,
it suggests you have super light weight tubes or even latex tubes.  If
you don't like to pump, aren't planning on setting world speed
records, and want a little more flat resistance, get some larger cross
section, thicker tubes.

I say larger, because most tubes are substantially undersized in their
minor diameter, to facilitate installation for novices.  These tubes
are lighter because the are so small and on top of that, some are thin
walled.  When inflated, they are stretched even thinner and air loss
gets annoyingly high.  Latex racing tubes require daily inflation.  I
pump my 28-700c tires every couple of months if I don't get a flat first.

Jobst Brandt      <jbrandt@hpl.hp.com> 

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Latex vx. butyl tubes--horror story
Date: 3 Sep 1998 01:10:43 GMT

Tim McNamara writes:

>> Tim Marsh writes:

>>> Ditto here - three successive tubes all failed with less than 100kms
>>> on them. Each blew at the join, in such a way that they couldn't be
>>> patched.  The Vittoria rep claimed that my rim tape must have been
>>> faulty - ie the flats were my fault. Surprisingly enough, I don't
>>> buy Vittoria any more.

>> I don't understand what you are describing here.  You use the term
>> "blew" as in past tense of blow(out) implying a noisy report.  A tube
>> failing inside a tire casing cannot make much noise if any, so the
>> alternative is that your tire came of the rim at least temporarily.
>> That is, if you heard it fail.  Can you describe the nature of this
>> event and what the tube showed afterward.  I can't picture a tube with
>> a leaky joint that cannot be patched.

> In my case, each of the tubes failed where the ends of the tube were
> overlapped to form a circle.  Each of the tubes had a "crimped" spot in
> the overlapping which was where the leak occurred.  The thickness of the
> overlapped layer made it impossible to get a good seal with a patch (see
> bad ASCII art below).

>   | | <- Valve
>   | |
>   | |                            Leak
> __| |_________________________  /
>                               |____________________________
>                               |
>                               |
>                               |
>                               |____________________________
> ______________________________

All you had to do is shove some rubber cement into the joint with a
toothpick or the like.  This is a leaky joint, not a fault of latex in
particular.  I can't imagine that all the tubes in the store were like
this.  Get a replacement.

> None of my tubes "blew out," they just deflated rapidly.  My first clue
> was that wallowy feeling followed within 30 seconds by the unmistakable
> sensation of being on the rim.  I said, "Merde!  Je suis sur le jante" )
> or words to that effect).

As you note above, my response about a blow-out is to another reader,
not you.

> Oddly enough, each of the failures like this occurred in the rear wheel.
> The tube in the front tire was bought at the same time and has been fine.
> I inspected the rim and tire for sharp projections, pieces of debris,
> misaligned Velox rim tape, etc.  In two cases the leak occurred away from
> the rim bed, partly around the tube but not in the same places, in one
> case in the section of tube in contact with the rim bed.

How many of these did you have?  I think I would have tested the next
tube before putting in in the tire if I suspected a trend.  The point
you post this under the heading that latex tubes are no good for xxxx
reasons.  I don't buy it.  They leak down over night.  They are so
thin that they easily get caught in small voids in the tire/rim
volume, but the blame they are getting here is off the mark.

> Basically, I think the ends of the tubes just weren't glued together
> properly and they failed.  Inconvenient and annoying but fortunately no
> harm done.  I'd hate to have it happen on a fast descent, though.

Stem separation on butyl tubes is no less threatening and it occurs
often.  It is something that cannot be anticipated because it develops
with use.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.tech
Subject: Re: Fixing a Loose Presta Valve Stem
Date: 11 Aug 1999 16:04:31 GMT

Steve Freides writes:

> My wife was unscrewing the whatever-you-call-it on her Presta-valved
> tired this morning, getting ready to pump them up, and the whole
> business came out.

> Do we need a special tool to fix this and, if so, what?  Otherwise,
> how do you fix this?

What do you mean by "the whole business came out"?  Do you mean the
valve core came out of the stem?  There are presta valves with
removable cores and these merely screw in and can be tightened with an
adjustable wrench by the flats on the coarse cap thread.  Tightened,
they will no longer come out when opening the valve release nut.  On
the other hand, if the whole stem came out, and this occurs with some
tubes, there is no way of putting the stem back in except to vulcanize
it, something that can only be done by a tube manufacturer.

But there is hope.  If you know someone with old racing tubular tires
that use latex tubes, these have removable stems that are secured with
a nut because stems cannot be vulcanized to a latex tube.  These stems
can be inserted into a butyl (black) tube whose stem has separated.  I
have a had few of these and they work like new.

Jobst Brandt      <jbrandt@hpl.hp.com>

From: jbrandt@hpl.hp.com (Jobst Brandt)
Newsgroups: rec.bicycles.misc,rec.bicycles.tech
Subject: Re: tubular or clincher for race wheels?
Date: 23 Aug 1999 18:26:21 GMT

Jim Quinn writes:

> Your wrong about that David.  Although they perform the same
> function, clincher and sew-up tires are completely different in
> their construction.  Because of the difference clincher tires are
> much more prone to pinch flats than sew-ups.  I use top of the line
> tires and tubes on my clinchers and I still get much more flats on
> these than I do on my sew-ups.

The difference is not the construction but rather the type of tube in
the tire.  Latex tubes stretch several times as far before tearing, the
mode by which tubes perforate in a pinch.  Compressing rubber between
two surfaces extrudes it from the gap with a large lateral stress that
increases the area of the rubber in the compressed gap.  It is this
stretch that causes perforation.  It is not any mysterious structural
difference.

On the other hand, latex tubes do not work well in clinchers because
the interface between casing and rim is not protected with a soft
cloth as it is on tubulars and the thin tube can chafe at this
juncture to cause a leak.  Besides, latex tubes do not hold air well
and must be pumped daily.

> And this is not just me this is everybody I have ever talked to.

So this is a consensus thing.  Everybody's doing it so it must be
the thing to do... etc.  There are real and concise reasons for
tube failures and they can be analyzed by means other than hear-say.

Jobst Brandt      <jbrandt@hpl.hp.com>