Instructions for the tire I have specify max pressure 4 atm., but only 2.7 atm. if mounted tubeless. What can be the reason for so big difference? (Definitely not tube able to hold 1.3 atmoshpere!)
asked Dec 4 at 14:43
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3Are you certain it’s about tubeless, not hookless? Other than that, maybe the manufacturer doesn’t think their tyre can hold higher pressure tubeless (won’t burst, but the rubber won’t hold air) and they ignore sealants?– EyeBrownCommented Dec 4 at 20:48
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3Could you share a pointer to published specs?– Sam7919Commented Dec 5 at 0:27
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I wonder if the manufacturer is being annoying by not distinguishing clearly between the max rated pressure for safety and the max recommended pressure for ride quality; the latter being lower is common with tubeless. This is why a link to manufacturer's details is always good.– Chris HCommented Dec 5 at 6:38
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2 Answers
It is in fact because the tube holds the tyre to the rim, in a certain sense.
Tubeless tyres are prone to blow off the rim at too high pressure. I.e., the pressure forces the bead over the edge of the rim. The bead in tubeless tyres is generally made of strong polymers (aramid?). These aren't very stretchy compared to rubber, but significantly more stretchy than the steel beads in traditional tyres. At high enough pressure, the bead can stretch enough to pop out of the rim, even without placing the opposite side of the bead it into the channel in the middle of the rim like you would when changing tyres/tubes.
So far so obvious, but the interesting question is: why does this happen more easily in a tubeless setup, than with the same rim&tyre with an inner tube? As you said in the question, the tube itself can't withstand much pressure, more like 0.1 atm than 1.3 atm.
The reason is that blowouts happen asymmetrically: the bead doesn't pass over the entire rim wall simultaneously - that would require it to stretch twice as much as needed to pass only on one point and then unravel from there. In a tubeless (and hookless) setup, nothing prevents the bead from drifting in some direction from which it then unseats. A tube however does, if not prevent, then at least discourage this. In a fully seated tyre, the portion of the tube that's not in contact with the tyre lies flat on the rim tape. No force on the bead here. If some section of the bead starts to "lift off" however, the tube will stretch at that spot, which requires force. Not much force, but enough to convince the bead not to slip away even further at that particular spot - more pressure will rather cause the bead to slip upwards on the opposite side of the wheel.
tl;dr: a tube keeps the tyre radially centered, so that more bead-stretching is necessary before the tyre can unseat itself and blow out.
There is a risk of 'burping' running tubeless at higher tire pressures.
This is where the tire momentarily unseats during impacts or cornering, allowing air to escape.
This risk increases at higher pressures, especially when:
- hitting bumps
- sharp cornering
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8Isn’t it the other way around? Low pressure causes burping as there’s less pressing of the tyre to the rim and more deformation of the tyre.– EyeBrownCommented Dec 4 at 20:51
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2@EyeBrown Yeah I’m pretty sure burping is just when the lateral force from cornering is greater than the force pressing the tire laterally onto the rim, and so they separate a bit and air escapes. More pressure would help keep the tire pressed against the rim. Commented Dec 5 at 1:55
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2@KonstantinShemyak This a very fast process and the ooening is often very narrow and the tube would have no time get through and explode. Also, it most often happens at such low pressures that you couldn't ride safely at all with a tube, you would get pinch flats. Commented Dec 5 at 7:36
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2It's also (@KonstantinShemyak, following on from Vladimir's comment) only a very narrow gap. A bit of tube pushed through a narrow gap can survive surprisingly high pressures. I've seen one bulging through a sidewall slit at around 4 bar, though another exploded at 7 bar, not instantly but within a minute– Chris HCommented Dec 5 at 11:34
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2The tyre “rolling off” the rim in sharp corners is more likely to happen at very low pressures. Nothing to do with tubeless or not.– MichaelCommented Dec 5 at 19:40