Technical FAQ: Tubeless road tire failure

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Dear Lennard,
I saw the recent article on a Pirelli tire exploding off of a Stan’s Alpha 340 rim, and one specific detail stood out — that Tom inflated the tire to 100 PSI, and it’s a (stated) 26mm tire.

The Stan’s Alpha 340 rim is very lightweight and, like many Stan’s rims, there’s a pressure limit on the rim. According to Stan’s website, the Alpha 340 has a max PSI of only 116 for a 23mm tire, and this limit is reduced to just 45 psi for a 32mm tire.

Although I’m sure volume has as much of an impact as stated width here, I’d imagine that 100PSI might be more than the rim could handle for Tom’s 26 mm tire, and I’d assume it’s possible the rim is permanently damaged as well.

Of note, I’ve also seen this happen when a rider tries to install too narrow of a tire on one of the modern gravel- or cx-specific rims. Without checking, a gravel bike might have wheels that are too wide for a 23mm or even sometimes 28mm tire. Stan’s Grail rims, for example, at 20.3mm internal width are only rated for 25mm or larger tires, and the Stan’s Crest is marketed as a gravel wheelset or light cross country, and its 23mm internal width recommends tires of 38mm.
–Tyler

Dear Tyler,
Very good point. The stress on the rim and tire is higher when using a 700 X 26C tire at 100 psi than when using a 700 X 23C tire of the same construction at 100 psi. I discuss this, using the concept of hoop stress. Assuming both tires have the same casing thickness, the article explains that, to have the same stress (σ) on the rim and tire walls of a wider tire as a narrower tire, the pressure in the wider tire must be related to the pressure in the narrower tire in the following way, where D = tire diameter, P = tire pressure, and the “w” and “n” subscripts denote “wide” and “narrow”:

In other words, the pressure in the wider tire must equal the pressure in the narrower tire times the diameter of the narrower tire, divided by the diameter of the wider tire, or:

In the case of the Stan’s NoTubes ZTR Alpha 340 rim, if we relate the max pressure of 116PSI for a 23mm tire to a 26mm tire, we get:

So, the rim and the 26mm tire casing experience the same stress with 103PSI in it as a 23mm tire at 116PSI. By that measure, Tom did not exceed the rim’s maximum pressure by pumping his 26mm-wide tire up to 100PSI.

However, if we instead relate Alpha 340’s other max pressure of 45PSI for a 32mm tire to a 26mm tire, we get:

By that measure, Tom did exceed the max pressure of the rim by running a 26mm tire on it at 100PSI.

I cannot say why NoTubes picked the max pressures that it did. It might have to do with experimentation with blowoffs of various tires rather than with the stress experienced by the rim walls from the pressure inside. Because, for the rim’s hoop stress to be the same with a 32mm tire as with a 23mm tire at 116PSI, the pressure in the 32mm tire would have to be:

Or, conversely, for the rim’s hoop stress to be the same with a 23mm tire as with a 32mm tire at 45PSI, the pressure in the 23mm tire would have to be:

So, the jury is out on whether Tom overinflated his tire and went beyond the recommended capacity of a Stan’s NoTubes ZTR Alpha 340 rim.
–Lennard

Dear Lennard,
I saw your VeloNews article on tubeless tires. I have an interesting story.

I road ride about 5,000 miles per year, and have for about 12 yrs. In Aug 2018, I bought a Giant road bike that came with Giant SLR 1 tubeless tires. May of 2019, I got a piece of glass in the front tire that went flat immediately. I replaced both tires with new Continental 5000 TL tires. Every 3-4 months I added Stan’s Sealant.

On Jan 8, 2020, I was on a casual ride when I took a 90-degree turn at 24 mph. My next memory was in the ER. My front tire had suddenly deflated with ZERO warning. Two days later I looked at the tire and found no cut on the bead, but there were stains where sealant has leaked out the right side of the tire in two places. I pumped air back into the tire, and it is still holding. After research, I’ve come to the conclusion the tire “burped”. When I recover, I’m removing those and going to put tubes in a new set of clinchers. I cannot ride on something I do not trust.

On my other bike, a Trek I installed a set of HED wheels, I’ve not had a single flat in four years!

I do have a quick question: when putting tubes in the Giant tubeless wheels, is the tubeless rim tape present sufficient for tubes?
–Jim

Dear Jim,
I commiserate with you on the consequences of your burp. Yes, the tubeless rim strip is generally fine to use with an inner tube.
–Lennard

Dear Lennard,
I just read your VeloNews article about the mystery flat on road tubeless and the guy who woke up in a helicopter. I just wanted to relay my 2¢.

I recently received a new set of Zipp 404 NSWs that I ordered from Colorado Cyclist. These took about 4 weeks to get to me due to being built and the blue graphics. I initially ordered them as they were listed as rim brake tubed clinchers. I was fine with that and have a bunch of tubes, patches, levers, etc. so nothing new for me. However, they were only available as the new tubeless ones. Since that is supposed to be the “next big thing” and supposedly pretty much eliminate flats, I went along and left the order standing.

I also bought a pair of the Zipp RT 25 tubeless tires elsewhere. I had to take them to my local shop to get seated as they would not seat using a floor pump. They were filled with 2 oz. of Stan’s sealer each.

On Sunday, August 11th, it was my very first ride on the new wheels and tires. Due to my work schedule, I can only ride on Wednesday, Sunday, and every other Saturday, and that assumes the weather cooperates, and I don’t have something else to take care of. So, my riding time is quite precious.

The whole point of the ride was duplicating the loop I did the previous Sunday to see if these new $3,000.00 wheels plus a couple hundred dollars in tires, sealer, and other required tubeless accessories were any improvement in speed and road feel to my Mavics.

I made it a whole 13 miles from home when I hit a small rock in the road. I didn’t see it until I was pretty much on top of it. Missed it with the front, hit it off the left side of my rear. This was instantly followed by a cyclical blast of air against the inside of my left calf and a sinking feeling. Sure enough, a flat. There was a very small puncture on the left sidewall. I could see the sealant dripping out of the hole, but it did not seal it. I tried pumping it up several times with my frame pump and this only led to more sealer and air coming out.

As soon as I tried to ride, it would just deflate again. The final time I tried pumping it up, I was rewarded with sealant spraying the side of my shoe, my face, and my glasses. Reluctantly admitting defeat, I began walking the 13 miles back home, knowing it would take several hours. I have not had to walk a bike home in 30 years. That was due to not having any tools or patches at the time and luckily was only 5-6 miles. Luckily, a nice old gentleman in a Subaru Outback was kind enough to stop and give me a ride home after about 2 miles of walking.

So, I would caution anyone to seriously question the supposed benefits are of three thousand dollar fast aero wheels, tubeless tires, and sealant are when YOU GET STUCK ON THE SIDE OF THE ROAD ON YOUR VERY FIRST RIDE. Zipp’s site says ” These tires feature low rolling resistance and high durability.” Stan’s site says, “Seals punctures up to 1/4″ (6.5mm) quickly”. I have newfound evidence to the contrary.

I considered pulling the tire off and patching the inside surface with one of the stick-on Park Tools patches (assuming it would even stick), but that still leaves me with no way to seat the bead on the side of the road using a frame pump.

I can count on both hands how many flats I have had on this bike in the past 11 years and over 20,000 miles running Continental GP 4000s. Only two have involved sidewall punctures/cuts and one of those I found while checking over the bike and changed it at home (partial thickness slice in sidewall).

Had these been conventional tubed clinchers, I could have either patched the tube, swapped in a new one (I always carry a spare), maybe thrown in a tire boot piece of Tyvek, and either finished my ride or limped back home on two wheels, not two feet.

I am very angry, disgusted, and dejected that something that is sold for top dollar and trouble-free cannot even complete one ride.

Now I have what are either three-thousand-dollar carbon paperweights or tremendous liabilities that will be a never-ending source of anxiety and quite likely real trouble if this keeps happening.

The following day I sent 3 very angry emails to Zipp, Colorado Cyclist, and Stan’s. The only company who did anything was Stan’s. They had the smallest, cheapest part of this whole package by far and they sent a quart of their regular sealant and a quart of the race sealant. The other two? Nada.
–Anthony

Dear Anthony,
Thing is, what happened to you could have just as easily happened with a tube-type clincher tire. You got a sidewall cut.

If you get a sidewall cut on a tire with an inner tube, you get a blowout 100 percent of the time. To fix it enough to ride 13 miles on it, you have to boot the tire and put a new tube in. If you don’t put a boot inside the tire casing to reinforce it at the sidewall cut, the new tube will bulge out through it. It will also blow in short order, either just from the pressure, or from the lightest brush against something when it is up to pressure. Inner tubes cannot withstand burst pressures; they need a tire casing to constrain them.

The emergency boot you line the tire with at the hole in the sidewall can be a dollar bill, or an energy bar wrapper.

The mistake you made was riding without your spare tube, tire levers, and other tools you formerly brought with you on rides. Though they tend to get fewer flats from thorns and glass than tube-type tires, and are almost impervious to pinch flats, tubeless tires definitely do not guarantee you against flats, and certainly not against sidewall cuts. In general, they are no more or less durable against sidewall cuts than similar quality tube-type tires.

You can always put an inner tube inside of a tubeless tire, and that’s what you should have done, after booting the tire casing. That’s what you could have used the patch you mentioned for. Or, as I said, a dollar bill or an energy bar wrapper can serve as a tire boot.
–Lennard

Dear Lennard,
I totally agree about bead retention and burping on road tubeless tires and rims. Bontrager makes one of the best engineered tubeless setups in my opinion. For each rim, they make a specific molded rim strip that span hook-to-hook, includes distinct bead retention ridges on each side, and increases the bead seat diameter slightly. In addition, it has a molded boss that mates with a specific valve stem that seals with an O-ring, rather than a conical rubber base as on generic tubeless valves. This setup has great retention and doesn’t leak, and of course is free from the tape related headaches of the usual tubeless setups. I’ve used this on their CF Aeolus road rims as well as a number of their mtb rims. The only downside is it’s a little heavier than tape. ENVE has something similar for some of their mtb rims.
–Lou

Dear Lennard,
In case you are looking for other opinions about road tubeless tires to Jim from your most recent Q&A:

You mention about having about a dozen flats per year which seems like a lot of time roadside swapping tubes, but don’t want to “fuss with sealant”…

I switched to road tubeless 6 years/40,000+ miles ago and have had ONE flat when I rode a rear tire down to threads, and even then I made it home without adding a tube. I have used Hutchinsons, Schwalbes, and Continentals, always using their racing versions on all our NorCal roads without problem. I simply mount the tires, pump them up to make sure they seat themselves, deflate, and add 30-40mls sealant through the valve stem and reinflate. Every three to four months I add the same amount. I have gone through about a dozen pairs of tires and that is the sum total of my need to “fuss with sealant”.
—Bob

Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is a custom frame builder and purveyor of non-custom huge bikes, a former U.S. national team rider, co-author of The Haywire Heart, and author of many bicycle books including Zinn and the Art of Road Bike Maintenance (DVD), as well as Zinn and the Art of Triathlon Bikes and Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists. He holds a bachelor’s in physics from Colorado College.

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