Tubeless tire maximum pressure

[MikeDee]

In this article, Jan Heine says not to exceed 60 psi in a tubeless tire: https://janheine.wordpress.com/2017/05/ ... -tubeless/ This is regardless of tire size. Panaracer also says the same thing for their GravelKing tires. (Panaracer makes Compass tires too.) This makes no sense to me. In the days of tubed tires, narrower tires always had maximum pressure ratings higher than wider tires. I think this is because, at a given pressure, there's more force acting on the tire casing the larger the surface area of the tire, which acts to stretch the tire bead and cause a blow off. I was looking to buy a 32mm tubeless tire, but according to Frank Berto's 15% pressure drop pressure recommendations, 60 psi would give me an under inflated tire at my weight. Should I just ignore this? My Mavic Yksilon 28mm UST road tires say 87 psi max, as a comparison.

[Brucey]

I think that panaracer may not have fully grasped the tubeless nettle in that their beads are not 100% proof against stretching and letting the tyre come off the rim. Fits vary; remember there is as yet no ISO standard for tubeless, so it is the wild west at present, with just proprietary standards in place, which may not even be published I full detail. I agree that on the face of it the blanket ban on >60psi doesn't seem sensible; it may just be a simple rule to avoid confusion.

However I would also note that as well as variations in fit, very small variations in the bias angle in the carcass will change the forces in the tyre considerably, and such variations may occur randomly in production and/or when the tyre is fitted onto different width rims.


A fundamental problem is that tubeless tyres are not safely retained on the rim by hook beads; the reason is that whether the bead is pushed into the hook or not varies with the exact location of the seal. If the leak rate past the rim lip is less than the leak rate past the tyre lip sooner or later the tyre edge sees the same pressure both sides and there is no benefit from having a hook bead (by contrast when you have a tube fitted the tyre is always pushed against the hook bead ). It doesn't help when folk marketing and using tyres don't seem to understand this.

Pressure limits on rims are normally set by mechanical considerations (e.g. the rim may split) but in the case of a rim that is designed to be used with 'special tyres' it may be to do with tyres as well. For example ISTR that the pressure limit for running tubed tyres on mavic new open pro (UST) rims is slightly higher than 87psi, but do check this.

cheers

[MikeDee]

Someone in that article discussion said to pump the tire up to 150% of the pressure you intend to use (no sealant and wear ear and eye protection). If the tire doesn't blow off, you're good to go. Seems that the tire bead could be damaged if it does blow off. What then?

I think what's happening is that a tubeless tire seals at the inner rim lip. When the bead stretches enough, air gets under the bead and between the rim wall and the bead. This lifts the bead away from the hook and off the tire, similar to an inner tube being pinched under the bead. As such, rim hooks may not do much for tubeless tires to retain the tire. I think that's what Brucey said.

[Brucey]

with sealant there, the tyre could seal or not seal in different places, and that could change over time, altering the chances of the tyre staying on the rim.

If the tyre blows off the rim when overinflated you have certainly proved that it wasn't safe to use that combination of tyre and rim. However if it stays on the rim for a period of time when overinflated, IMHO you certainly haven't proven that it is definitely safe; all that needs to happen is a little sealant to migrate nearer the rim lip and improve the seal there, and the tyre is no longer pushed into the hook bead, altering its chances of staying on the rim if the fit/stretchiness is marginal.

In other words;

……. If the leak rate past the rim lip is less than the leak rate past the tyre lip sooner or later the tyre edge sees the same pressure both sides and there is no benefit from having a hook beads.....

There is no way to know if the seal is really at the tyre lip or nearer the rim lip, or if it will stay that way. Even if you do an overpressure test without sealant present, there is no way to be sure that the rim lip won't seal 'perfectly' (or at least better than the tyre lip) once the sealant is added.

Unfortunately there is also no way of knowing just how stretchy the tyre beads are (in any given tyre), or how much load they really see. One of the few clues that you do get is if the tyres 'pop' under pressure when seating; this gives you some confidence that the fit is reasonably tight, even if it doesn't tell you for sure where the seal is or if the tyre is going to stay on the rim under full pressure.... Pretty much the only thing it tells you for certain is that the tyre is liable to be a complete so-and-so to remove later on....

Years ago I bought some folding tyres (in the 'open tubular' vein) that were simply too tight to fit on any rim and (in desperation) I built a frame with a screw jack to see if I could stretch them a bit. The result was that the beads in these tyres were indeed very slightly elastic, but wouldn't take a 'set' so were permanently damaged when strained beyond a certain point. In the end I gave up and the tyres went in the bin. I should have cut one up to see what the bead was made of, but I was so gutted at the time I probably wasn't thinking straight.

UST tyres seem quite a bit heavier than equivalents that are meant for use with tubes; part of the extra weight comes from extra rubber lining to make the thing tolerably airtight and some of the extra weight comes from the stronger/stiffer beads which are required to keep a tubeless tyre on the rim. My understanding is that in most such tyres the bead is made from carbon fibre, but there is no way to know if every manufacturer's tubeless tyres are made equal in this respect or not: I suspect 'not'.

cheers

[MikeDee]

Many rims have bead retention humps in the rim bed. I think Mavic pioneered them in the original UST. That is what causes the snapping noise as the beads get pushed over the humps and snap into place. My Conti nontubeless tires make the snapping sound on my Shimano tubeless rims. A lot of rims don't have these humps (like DT and Velocity). I would think they help prevent the tire from blowing off as they act to keep the sidewall pushed against the rim. Don't know for sure.

[MikeDee]

I've got Mavic Yksilon Pro UST tires on my new bike. They came with tubes in them. I went to run them tubeless without sealant and they leaked. Dunked them in water and bubbles were coming out of the sidewalls so they require sealant. They are not the UST tires of yesteryear with the heavy butyl liner. They are now tubeless ready tires that need sealant.

[NickJP]

Many rims have bead retention humps in the rim bed. I think Mavic pioneered them in the original UST. That is what causes the snapping noise as the beads get pushed over the humps and snap into place. My Conti nontubeless tires make the snapping sound on my Shimano tubeless rims. A lot of rims don't have these humps (like DT and Velocity). I would think they help prevent the tire from blowing off as they act to keep the sidewall pushed against the rim. Don't know for sure.

Yes, on rims with the retention hump, the tyres give a loud crack on initial installation as the bead snaps into place, and when removing them, it requires considerable hand pressure once they are completely deflated to get the bead back into the well in the centre of the rim so that tyre levers can be used to lever the bead over the rim wall. I wouldn't run tubeless on rims that didn't have this feature.

Here's the cross section of the MCFK rims I'm using, which have this hump. I've been running Schwalbe S-One 700x30 on them for the last 18 months or so, using about 70psi rear and 50psi front.

[Brucey]

tubeless rims and tyres for other vehicles always have retaining lips in the rim well.

Needless to say if you have an interest in minor conspiracy theories, you might like to consider that tubeless tyres would be nigh-on impossible in rims with an internal width of 15mm or less, as was commonly found until quite recently.

cheers