A Rebuttal from Zipp

More on Tubeless Tire Retention and Safety

Hello Lennard,

After reading some of your recent Q&A I wanted to offer some information based on firsthand observation that may clarify your understanding of how tubeless systems retain air. Tubeless tires do not form their air seal between the tire bead and rim bead seat. It occurs between the tire bead and the rim sidewall. After inflation, the rim bead seat does not interact with the tire much after air pressure stretches the tire bead and lifts it off the rim bead seat. You can show this with a $30 WiFi camera stuck inside a tire to watch the bead while you pump it full of air. Tubeless tires form their air seal against the vertical walls of the tire bed. We have verified this with three separate methods of observation using several tires. Other cycling brands have verified the same effect with CT scan equipment. 

Your statement here takes a matter-of-fact tone of causation; “The crux of the issue is that tubeless tires remain mounted and inflated only when the diameters of the rim bead seat and the tire bead match. If the diameter of the rim bead seat is less than that of the tire bead, the tire immediately loses all its inflation pressure, the beads consequently dislodge and drop into the center valley of the rim, and the tire often comes completely off the rim.” None of those events are inevitable serial consequences. In fact, every single tubeless rim bead seat is less than that of the tire bead during inflated operation, tires do not lose any air when this happens, and beads only dislodge and drop into the center valley of the rim when tire bead geometry and rim geometry allow it. I have yet to witness a tubeless tire coming off a rim after a flat tire in person, yet I have several experiences of riding flats for many miles with the tire remaining on the wheel.

In the article you link in your answer, I was surprised to see you claim that Joseba Beloki’s crash was not caused by rolling his tubular. It was evident from the original footage that his tubular tire rolled, and clearly visible in this photo:

The running commentary about this crash centered around the heat of that day, and the hot tarmac playing a factor in softening Beloki’s tubular glue. The takeaway seared in my head being – if you value your pelvic bones, don’t ride tubulars aggressively in hot conditions. Regardless of this accident, logic would tell you not to judge safety of a system based on perfect installation conditions by the top echelon of bicycle mechanics, but on actual installation conditions by the expected installer base (i.e. amateurs). Tubulars are notorious for rolling under challenging environmental conditions or poor execution of a complicated and multi-day installation process. Not to mention, tubular rims are more fragile than other rim types. An easy example to point to is from 2023 Paris Roubaix, where you can see a Movistar rider on a Zipp tubeless wheel pedaling past a Jumbo Visma rider with a detonated Shimano tubular wheel. The irony here is we had direct reports from that Movistar rider that they passed by while on a flat tire and made it to the end of the cobbled sector with tire in place and wheel intact for an eventless wheel exchange. It is clear by looking at the photos that the tubular tire, while still attached to the rim, was not involved in helping the Jumbo Visma rider come to controlled stop.

Your readers put a lot of trust in you as a balanced source of cycling wisdom. My note here is an earnest effort to help you retain that function.

Sincerely,

David Morse

Zipp Category Manager, SRAM LLC

Dear David,

I appreciate very much your efforts for see to it that I disseminate accurate information here.

Thanks for clarifying the sealing of the tire against the sidewalls of the rim and of the inflated tire’s beads lifting off their bead seats. I especially appreciate the internal photo of the bead when inflated and not. What a cool way to view what is happening inside a tire!

My statement you quoted was poorly worded, and I appreciate you pointing it out. What I was thinking about when I wrote it was a broken rim, where the bead seat diameter is no longer as large as it is supposed to be, but also the rim sidewall can no longer make airtight contact with the tire sidewall. It would have been better for me to have eliminated that first sentence and just said: “If the diameter of the rim bead seat becomes less than that of the tire bead due to a hard impact that cracks the rim, the tire immediately loses all its inflation pressure, the beads consequently dislodge and drop into the center valley of the rim, and the tire often comes completely off the rim.” 

Even if the bead seat no longer contributes to the air seal once the tire is inflated, my fundamental premise remains unchanged, namely that if its rim cracks in a hard impact, a tubeless road clincher will deflate, and, if ridden further, will come off the rim due to the reduction in rim diameter caused by cracking and collapsing in of its circumference. The crack itself or the resulting deviation in shape of the rim wall will be a pathway for the air to escape the tubeless tire. And the high pressure and low volume of a road tubeless tire means that this deflation will be rapid. Once the tire is deflated, the only thing keeping its beads from dropping into the rim valley if ridden further is the tire bead fitting tightly on the rim bead seat. Due to the rim’s crack, that bead-seat diameter will have decreased, and the rider’s weight on the rolling tire will push the bead into the rim valley. The resulting looseness of the tire around the rim’s circumference will allow the bead to come over the rim sidewall, especially if any side force is applied.

Yes, a simple flat tire on a tubeless clincher can often be ridden for a while without it coming off the rim, as long as the rim is undamaged and it is ridden in a straight line. I once rode a flat tubeless clincher a kilometer or so on a curvy descent without it coming off the rim, but that was a stiffer, UST tubeless tire (much stiffer than a tubeless-ready tire and built with square, carbon-fiber beads to fit a UST hook-bead rim) locked into a UST Dura-Ace aluminum rim (I kept riding it to see how well that bead-lock system worked). Though I have ridden the last half mile home on a flat clincher with a hole in its inner tube rather than change it so close to home, I generally stop right away when I get a flat out of concern for the tire coming off as well as for damaging the rim. When you say, “I have several experiences of riding flats for many miles with the tire remaining on the wheel,” I wonder if these were flat tubeless clinchers and you were cornering on them, because even at low speed, a flat tubeless road tire lubricated with sealant inside often cannot resist much cornering force, as can be seen in the video of Tadej Pogacar’s crash in the 2024 Giro d’Italia.

Thanks for the better recall on Joseba Beloki’s tubular from 21 years ago. Due to the high heat of the day and particularly due to hard braking on the softening pavement when trying to catch a flying Alexander Vinokourov, his rim glue clearly melted enough for the side force of his fishtailing to push it off the rim. Beloki’s Campagnolo Neutron tubular rim was aluminum and became hotter due to the rim brakes. Had he had a carbon rim with his rim brake, that heating would have been higher due to carbon’s reduced ability to conduct heat away relative to aluminum and to a carbon rim’s lower mass. Thank the component wizards who have made disc brakes so good; they are safer for both tubulars and clinchers, especially with carbon rims.

I don’t think I ever knew that Beloki’s tire rolled off. Thanks for that. I don’t remember if I even noticed that the tire was partially off the rim in the shots of him and the bike lying on the ground. If I did, I imagine I thought it was a side effect of the crash and not the cause of it. I remember cringing so much at that crash that I could hardly watch the details and focused much more on Armstrong’s amazing threading of the needle into the field and ride down it and then jump the ditch to not only avoid crashing but also to regain the group immediately. 

Knowing that he rolled his tire does temper my conviction about relative safety of one tire type vs. the other in pro racing, at least with rim brakes, and it was a poor choice for me to use it to illustrate a greater level of security for racers to ride tubulars relative than road tubeless clinchers. Maybe it’s fortunate that I screwed that up and inspired your comments from which we can all learn. I had only remembered Beloki locking up his rear wheel and getting sideways at high speed, first one way and then the other, and had attributed his crashing to that, not to a rolled tire. I wonder whether he still would have crashed if his tire had stayed on the rim. Once he got that far sideways at that speed with his rear wheel locked up, I thought he had lost the chance to ride out of it. Unlike the takeaway seared in your head from that, the one seared in my head was, “don’t enter a turn so hot that you lock up your rear wheel and start fishtailing on hot, grippy pavement.”

There are a couple of details to note about this when comparing to the present day. First, Beloki was riding with rim brakes, which caused most of the rim heating on that fast descent; this doesn’t happen today in pro racing thanks to disc brakes. Secondly, while Beloki's tubulars were professionally mounted by Faustino Muñoz, ONCE’s legendary team mechanic, they were held on with classic tubular cement, which has a lower melting point than the melting point of the adhesive on Effetto Mariposa Carogna tubular gluing tape (Carogna tape can still provide a safe adhesion at 150°C/302°F, for hours at that temperature) as well as that of at least one other brand.

I find your statement that, “tubular rims are more fragile than other rim types” to be surprising, given the fact that Zipp worked hard to make a carbon tubular wheel to win Paris-Roubaix. The resulting wheel did indeed win The Hell of the North — for five straight years! I remember Zipp stating quite proudly that from 2010-2014, Fabian Cancellara, Johan Vansummeren, Tom Boonen, Cancellara again, and Niki Terpstra won Paris-Roubaix aboard that 303 wheel. James Huang’s review of Cancellara’s 2010 race-winning bike acknowledges Zipp’s work in improving the 303 for this race, saying, “Interestingly, Cancellara bucked the trend of traditional alloy box-section tubular wheels and instead relied on Zipp's new 303 carbon aero rims. True, he did break one rear rim coming through the Arenberg forest, but to be fair, alloy rims have failed there as well. Moreover, Cancellara's wasn't a catastrophic failure, and he was able to continue until a more convenient time for a bike change presented itself – a significant improvement considering how many Zipp carbon rims then-Slipstream-Chipotle rider Magnus Backstedt broke at the 2008 Paris-Roubaix.”

I doubt that Cancellara would have been “able to continue until a more convenient time for a bike change presented itself” if he’d been riding tubeless clinchers when he broke his rim in the Arenberg Forest. I think the tire would have come off and he would have been left standing there.

I would like to know why you say that tubular rims are more fragile than other rim types, given that the box section (or triangular section, in the case of aero rims) of the rim is essentially the same on a tubular rim and a clincher rim. Where that is all there is to a tubular rim, the clincher rim has additional sidewalls sticking up from the box (or triangular) section that are only supported along their bottom edges. Assuming two rims made of the same materials with identical construction methods, it would seem that the box (or triangular) cross-section common to both tubular and clincher rims would be similarly robust, and that the edges of the clincher rim, being unsupported by a top crosspiece, would be more fragile than the edges of a tubular rim.

That is not to say that the broken Shimano tubular rim under the Jumbo-Visma rider in the photo you sent was up to the task; it clearly was not. However, I believe that a carbon clincher rim made of the same type and layup of carbon fabric with identical molding and curing methods would also have broken in a similar way when subjected to the identical impact. And, unlike the tubular tire that stayed on the Jumbo-Visma rider’s rim, its tubeless clincher tire would have come off the rim. 

Obviously, a fully-tacoed rim like that Jumbo-Visma one will not keep rolling, no matter what tire it has on it. My point remains that if a carbon rim gets a crack in it after an impact that still allows it to rotate through the frame or fork, as long as the tire stays on the rim, there is a decent chance on a disc-brake bike that the rider can safely roll to a stop without crashing. Conversely, I believe that the chance of not crashing in that circumstance is low if the rider is going fast and the tire comes off the rim.

Your point is well-taken “not to judge safety of a system based on perfect installation conditions by the top echelon of bicycle mechanics, but on actual installation conditions by the expected installer base (i.e. amateurs).” Having witnessed plenty of rolled tubulars in races I participated in, I know, that it is/was common for amateurs to improperly glue their tubulars to their rims. My intention is to compare the two systems in Pro Tour road racing and to discuss rider safety at that level, where top bike mechanics can be counted on to properly install tires, be they tubeless clinchers or tubulars. I used to believe and write that tubeless clinchers were safer than tubulars; now I believe pro riders would have a higher margin of safety with tubulars than with tubeless clinchers in the event of an impact hard enough to crack their rim.

As for amateur riders installing their own tires—that’s a different question entirely. Particularly for a rider who does not ride so aggressively that there is significant chance of cracking a carbon rim, I believe they would be safer on tubeless road clinchers than on tubulars, given the lower incidence of punctures with tubeless and the possibility of them improperly gluing the tubulars to their rims. This is more nuanced since the advent of Carogna tubular gluing tape, which I think makes it less likely to insufficiently adhere the tire to the rim. And comparing tubed clinchers to tubeless ones for riders who do not descend fast and don’t corner aggressively, I would also come down on the side of tubeless clinchers due to reduced punctures and pinch flats with them.

Thanks again for your feedback and corrections. I greatly appreciate it. We all benefit from the knowledge and experience you bring to it.

― Lennard

Subscribers can send brief technical questions to Lennard at: veloqna@comcast.net.

As a frame builder, Lennard Zinn has been designing and building custom bicycles for over 42 years; he founded Zinn Cycles in 1982. His Tech Q&A column on Substack follows his 35-year stint as a technical writer for VeloNews (from 1987 through 2022). He is a former U.S. National Cycling Team member and author of many bicycle books including Zinn and the Art of Mountain Bike Maintenance, Zinn and the Art of Road Bike Maintenance, and The Haywire Heart. He holds a bachelor’s degree in physics from Colorado College.

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