Innertube Porosity?

In article ,
Steve Sr. writes:
I was wondering if anyone has any information on the porosity of
various kinds / brands of innertubes. By porosity I mean the very slow
leakdown rate that occurs with sound tubes over days and weeks. You
know... The reason you are always topping up your tires to prevent
pinch flats.

My current interest is any differences between the regular and
"lightweight" road tubes of the same size. I am also wondering if
"brand" makes any difference as well.

Y'know, I've had tubes of the same brand with different seepage rates.
So IMO, the particular brand may not be a useful quantifier.
There may be a variation from batch to batch with tubes from
the same manufacturer (or factory.) There may even be a variation
within batches.

I believe getting an especially air-retaining or an especially
air-losing tube boils down to the luck of the draw.

In general, tubes made from natural latex are lighter than butyl
tubes, and are somewhat more porous[*]. Perhaps the weather
conditions at the rubber plantations might cause some variation
in the quality of batches of latex tubes? I dunno -- maybe how
closely latex or butyl tubes are stored to the ship's generators
(or other cargo whose out-gassings might react with "rubber")
en route from China might affect their quality too.


cheers,
Tom
[*] http://www.sheldonbrown.com/tires.html

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