What could be the reason for flatting a tubeless tire

Question

I am running a tubeless setup since September and the switching process has not been easy(lots of small problems) but Yesterday I had the most bizarre occurence of all while riding with a friend. Out of nowhere, my front tire went completely flat. I stopped, looked for a leak, found nothing, reinflated the tire and tried to find a leak. Nothing. I rode another 5k without any hiccups when it suddenly happened again. No leaks, reinflate, ride. Only this time, 500m further it happened again. So I popped a tube in there, finished my ride and went to my bike shop.

The tech over there kindly refitted my wheel tubeless and cleaned everything to inspect my tire and wheel. Nothing out of the ordinary, my tire wasn't punctured and my wheel is good.

What on earth might have caused this? I've been riding these wheels for at least three weeks with new tape and valves without any hiccups (unexplained air loss at the back, but that's a story for another time).

Thanks

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More details on the setup:

• Bike is a Devinci Hatchet gravel bike
• Wheels should be considered brand new (around 1500km) ENVE SES 3.4AR (25mm internal width, hookless)
• Front tire is a Continental Terra Trail 40c @ 40psi.
• Rear tire is a Continental Terra speed 40c @ 40psi.
• The wheels were setup by my local bike shop after a few back and forth with the initial setup

I was riding easy pavement with an average of maybe 250w at the moment of the event. So no hard rocks, bumps, twists or turns. It was a beautiful sunny and coldish november day (maybe around 4-5celsius).

It's also worth noting that I tried this tire/wheels combo for a 650km gravel bikepacking trip with some bumpy sections without any issue of this kind.

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I was riding on road, with my gravel bike. Pretty clean bike path. Tubeless setup was done by the book by the bike shop. So all tubeless parts, no hacks or shortcuts here :)

The rim is "brand new" and the technician at my bike shop confirmed my setup was not damaged and tire not punctured. 40c tires @ 40psi.

Answer 1

In my experience, this type of sudden unexplained deflation is usually valve related.

Check the valve core is tightened correctly (applies to both tubed/tubeless). And for a tubeless system check the valve nut is tight (no need for tools, fingers are fine)

Answer 2

This answer is premised on the assertion that the tire did deflate with no puncture or other input during normal riding.

It’s possible that the tire bead got dislodged from its seat and that you lost pressure, I.e. you burped the tire. Generally this happens with some sort of lateral force on the tire, e.g. you hit an obstacle on the side of the tire. I believe that in cyclocross or MTB, burping doesn’t necessarily take the whole tire flat, but they run at much lower pressures than road bikes - assuming this is a road bike.

In theory, we would not expect tubeless tires on rims conforming to accepted dimensions to burp. The issue is that the industry only just agreed on a set of dimensions for tubeless tires. The Slowtwitch article I linked said that the ETRTO clarified that the bead seat diameter for tubeless tires is 621.95mm +/- 0.5mm. There may be other parameters involved as well, e.g. the G-height, or the vertical distance between the top of the rim and bottom of the bead seat groove, or possibly others.

My understanding is that at least to BSD, while there was no universally accepted standard for tubeless tires, some rim manufacturers might have built rims slightly oversized from the normal BSDs to give a tight fit. Or some manufacturers may have had higher variances in production (in common speech we might say they had poor tolerances), so they had a number of undersized rims. And then we might need to consider the situation from the tire manufacturers’s side. They have to make their tires to conform to the rims out there, but they did not necessarily know what dimensions the rims were targeting.

Basically, a possible explanation is that your set of tires and rims did not match in some way. If the tires mounted very easily, then possibly the rims were on the low side of the BSD tolerance and/or the tires were on the high side of their diameter tolerance. Or some other set of parameters is at play.

With the rim standards now set and the tire standards forthcoming, figment issues should generally disappear for new tires and rims. Unfortunately, that doesn’t tell us what to do about older tires and rims. If this is a road bike, then honestly, one option is not to run them tubeless. A lot of people aren’t sold on road tubeless over alternatives. Latex tubes produce the same rolling resistance as the same tire set up tubeless, and latex tubes are also fairly puncture resistant. TPU tubes are expensive and probably have a bit more rolling resistance than latex, but they retain air better. Alternatively, if the bike store has experience with your rims, you could ask what tire models they’ve had good experience with.

For what it’s worth, I had a similar experience on a DT Swiss CR1800 wheel set with Panaracer Gravelking SKs, I.e. loss of pressure while riding with no puncture. I didn’t lose all pressure, and not finding any issue, I tried reinflating the tire. Unlike you, I didn’t seem to have that problem again.

Answer 3

Broadly speaking, the larger the size of the puncture hole the lower the pressure the sealant will be able to effectively cope with.

So first time the tire went down; almost certainly a large puncture where pressure dropped sufficiently that by the time of inspection the sealant had been able to plug. Any sealant that might have been visible, got rubbed clean.

Then - I would guess - the sealant had a bit of a chance to set while reinflating, faffing etc. Which enabled it to cope with a higher pressure and made it appear okay - at least initially - to ride on. However, once underway the combination of tire flex and extra load caused the sealant plug to get blown out and the second catastropic failure to occur.

Subsequently the tech missed the puncture. Easy to do. These type of punctures are almost as hard to find in the workshop as they are in field. Particularly if using, treaded, knobbly tires, where they can lurk at the base of the knobblies. No problems since, could be down to the sealant having enough time set properly. Or could be different sealant - Stans Race or similar will seal most things.

Good luck with it.