| The "tire failure" issue with
Stans Sealant
OK. Before going on, let me make two points: (1) Stans is a good
product that fills a unique need in mountain biking. I use Stans sealant.
I like it. Our store sells it. (2) The company that sells Stans assures me
that their product does not weaken rubber in any way.
But from personal experience with dozens of Stans-filled tires, and
after hearing from many mountain bikers who use Stans, I'm convinced that
non-sealing tire failure is an occasional complication of sealant. It may
not be the fault of the sealant, but these failures occur in ways that the
riders say they've never seen in tires that didn't contain Stans. The two
most common types of failure are (1) sidewall bursts, which are more
common in non-UST tires filled with Stans, and (2) de-lamination (bubble
formation) leading to non-sealing blowouts, more common in UST tires.
So what's going on? I think it's a combination of tire-manufacture
defects (such as poor adherence of the outer rubber to the cords, or areas
of inadequate rubber coverage on the inside of the cords) interacting with
something in the sealant. Possibly ammonia gas, I don't know. |
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One bicycle's experience:
New front and rear UST tires of same model, both filled with an
identical quantity of Stans from the same bottle, at the same time.
At one year's time, the front tire is rock-solid and never loses a
molecule of air.
At age 2 months, the rear wheel occasionally showed these damp-looking
spots after riding. The tire would loose air, from 32 pounds down to 20
over about two weeks' time. |
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This was the first time I'd ever seen these wet spots with
Stans. Usually the first sign of a problem was sudden deflation, with
sealant spewing all over me and the bike.
With time, the air loss became more rapid. At age 10 months, the tire
could lose up to 5-10 pounds overnight.
Ten months after the original tire mounting, inspection revealed a damp
interior with white fibrous material but no free-flowing sealant. 60 ml of
fresh Stans was added. After riding 10 miles and sitting overnight, the
tire again again showed a couple of "wet spots" and developed
subtle bulges. |
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The tire was kept inflated to 32 PSI. Over the next 24
hours, many additional bulges rose up from the tire. Some were quite large
(see bulge 2) while others were small. In some areas of the tire, a
regular series of small bulges were seen along the row of tread between
the center tread and side tread (see bulge 3). |
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A cut-away of the tire shows NO accumulation of sealant on
the underside (such as I'm used to seeing in an area of puncture). And the
bubble contains only air.
Further, the tiny "ooze" spots (which are probably tiny
punctures) didn't correspond to the bulges. |
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Pulling the top rubber away from the cord rubber, we don't
see any sealant. But in this bubble we can see two cords that have
separated slightly, without splitting the rubber on the inside of the
tire.
What I think would have happened: When the weakened area fully ruptured
during a bike ride, this tire would have suddenly spewed sealant and
deflated completely. And that's something I've experienced before, maybe 8
times over 6 years of heavy riding and racing tubeless. |
Factors that make tire failure more likely, in my opinion:
(1) Age of tire. Older tires accumulate stresses (such as the flexing of
cords in the sidewall of an underinflated tire) that may expose the cords
to the liquid sealant or cause the outer rubber to separate from the cord
layer. Even a chemically inert substance such as water could contribute to
tire failure if it soaks into the cords.
(2) Length of time exposed to sealant. Using one brand of non-UST tires, I
typically experienced sidewall rupture after 3 month's use. But also, I've
had the experience of placing fresh sealant in a tire that's already had
Stans for several months and had the tire fail dramatically on the next
ride.
(3) Brand of tire. Certain brands, and I'm not naming them here because I
don't relish hearing from lawyers, have been reported again and again. But
even within a specific brand, manufacturing anomalies may affect only a
certain year's model, or tires coming from a specific factory.
(4) Non-UST. The thicker rubber in a UST tire tends to protect against all
types of failure, sealant or no sealant.
(5) Bum luck. As in the example above, two supposedly identical tires, of
a brand that's supposed to be sealant-friendly, with dramatically
different outcomes. Except perhaps one was manufactured on Friday, just
after lunch.
So what do I recommend? First, pick the right tire. I suggest USTs of a
brand recommended for use with Stans by your local bike shop. Buy Stans
and use it. And instead of just "refreshing" the sealant every
few months, replace the tires every 6-12 months. Seriously. It beats
walking off the mountain while your competition is racing on without you. |
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