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I would welcome knowledgeable answers rather than opinions please. I have been cycling 50+ years and now work in an LBS.

I have seen numerous examples of so-called “thorn-proof” TP tubes, of different makes, on customers’ bikes, where the Schrader valve stem has ripped out from the tube. This is particularly so with wider tyres, like 48 mm and up.

Initially we put it down to people using a slightly under-width tube due to what being all that was readily available, and also under inflation. However, even with TP tubes that are bang on the numbers, and inflated close to the max. recommended pressure, we see a common excessive failure rate where the valve stem parts from the tube, sometimes repeatedly, where the standard tubes do not fail this way. Customers request TP because we are prone to goathead thorns (aka bindi, doublegee etc.) around here.

The thick rubber TP tubes are clearly less stretchy than thinner tubes, and thus possibly more prone to tearing.

Does anyone have any real factual technical insight into what I’m describing? Many thanks.

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    Short of finding someone that works for a tube manufacturing company it might be hard to find a factual answer. I don't have facts, or even an opinion, but I do have at least a theory. Is that acceptable as an answer or would you rather i didnt?
    – Andy P
    Jan 14 at 10:13
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    @AndyP a hypothesis is better than nothing :)
    – Criggie
    Jan 14 at 10:20
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    @henry Are the rims drilled poorly? Even minor sharp burrs can eat a tube, exacerbated by stems cocked over at an angle.
    – Criggie
    Jan 14 at 10:22
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    AndyP your theory is welcome thanks. I do intend asking the suppliers’ representative but this route can be rather tortuous. Criggie definitely not due to poor valve hole drilling or burrs. Many instances on different wheels. Valve tears out from tube at or next to the bonded joint between stem and tube. Seems to be the hard-ish material tears rather too easily.
    – Henry S
    Jan 14 at 10:51
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    How long are they lasting? If the tube is really tough, it could be in there for a long time until the tube is rather old
    – Chris H
    Jan 14 at 11:12
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The following is purely a hypothesis based on other things that we know (or at least highly suspect) about the interactions between tyres and tubes.

In an ideal world, a tube is thin and well inflated. It pushes firmly against the tyre and deforms easily with the tyre when passing over the surface.

It's almost impossible to prove, because you can't see inside a tyre in use, but evidence suggests that an under-inflated tube behaves a little differently. When a tube is under-inflated it is not uncommon to later discover it has strange wear patterns on it caused by friction against the tyre. This means that the tyre/tube are not always moving as one and that a force will be trying to rotate the inner tube around the rim. Since the valve stem is anchored at the rim, that's the place that ends up taking the strain. This is almost certainly why torn valve stems on 'normal' tubes are often associated with under-inflation.

In the case of a thorn proof tube we have two possible cases that could potentially replicate this effect to some extent.

  1. They are generally made of a much thicker material - it's guaranteed that this will not deform as easily as a thin tube - it's at least possible this could result in some friction between tyre/tube much like in the case where a tube is under-inflated.
  2. If the tyre/tube actually has thorns embedded in it. Thorns are certainly not highly flexible and again may cause the tyre/tube to no longer act totally in unison causing friction which again puts strain on the valve stem.

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