Bikes and Tech
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Technical FAQ: A tubeless tire blowout in the Pyrénées

After a reader suffers a tubeless blowout on a Pyrénées descent, Lennard digs into the potential causes for the catastrophe.

Dear Lennard,
Thanks for your article on tubeless road tires. I also had an unfortunate experience(s) this summer. I have been riding Continental GP 5000 tubeless tires this spring and summer. I am riding 28mm tires on Reynolds AR41 rims. Initially, they worked very well, so I decided to use them in France this summer. The first blowout happened near the top of the Col de Macuègne in the Hautes-Alpes region of France. I heard a bang, and the rear tire went flat very fast. Since I was headed uphill, stopping wasn’t a problem; however, about 2–3 cm of the tire bead separated from the casing, making a repair impossible. I had to call someone for a ride in.

I wasn’t as fortunate the second time. I was headed downhill in the Pyrénées when the front tire went flat very quickly. The last thing that I remembered before the crash was that steering the bike was very difficult. I woke up on the pavement with several people around me telling me not to get up. This was followed by an ambulance ride to the emergency room in Lourdes. When I finally got the bike back, the front tire had a 4–5 cm rip in the sidewall near the bead.

My question is, what could cause the sidewall to fail like that? I have been riding for a long time and have never seen that type of tire failure. Both tires were fairly new and have never been inflated over 6.5 BAR (Continental’s recommendation). I have generally had good luck with Continental tires, but I doubt that I will ever ride tubeless tires on the road again.
— Tom

Dear Tom,
I think tubeless is a red herring here. I believe what happened to you could have also happened with a lightweight clincher tire with an inner tube. It sounds like the rim’s bead hooks cut enough of the threads in your tire casing that it blew. I had never seen this with aluminum rims. However, it has happened to me on multiple occasions on carbon clincher rims with handmade open tubular clinchers (explained here, or here as “handmade clinchers”) with inner tubes. Having such fine treads and a casing that is not vulcanized, handmade open tubular clincher tires are the canary in the coal mine of this issue I’m about to discuss with carbon clincher rims.

We discussed pros and cons of hookless carbon clinchers a few weeks ago, and now we get to talk about some issues with the hooks on some carbon clincher rims. While disc brakes and higher-temperature resins have greatly reduced the issue of carbon clincher failure due to rim heating while braking, not all of the bugs have been eliminated from this relatively recent technology.

While the ETRTO (European Tire and Rim Technical Organization) specifies rounded edges to the rim hooks, there are no minions out there checking that rims comply. All of the edges of an aluminum rim are rounded, and they must be, due to how it is produced; that is not the case with carbon rims. An aluminum rim is extruded; it is formed by pushing aluminum through a hole shaped like the rim cross-section in a steel die. The hole is cut by a milling machine, and it is inevitable that all of the edges of the hole are round, since the bit that cut the rim shape into the block of steel is spinning.

A carbon rim, on the other hand, is molded. When the mold is opened, there is hardened resin that has pushed out into the seams between the mold pieces. These resin edges are sharp and are called “mold flashing.” Unlike with a tubular rim, whose box cross-section doesn’t capture mold parts, a clincher rim has hooks that do. So, the mold pieces generally release along the edges of the hooks in order that the mold pieces can separate, leaving flashing along the bead hook when the mold is opened. This flashing must be removed or at least smoothed, or it can cut a tire.

Some rims, on the other hand, are not trying to achieve a net-molded shape to the bead hook and deal with its shape, and with the flashing, in another machining step after coming out of the mold to cut the bead hooks to their final shape. Since the spinning bits that cut under the bead hook and that cut straight along the face of the bead hook are moving tangentially around the rim, they will tend to cut a square edge on the bead hook face and a square cut under its edge, rather than rounded ones. It obviously takes at least one more machining pass with this type of rim manufacturing process to round the bead hooks, which costs time and money and may expose fiber ends. But a sharp edge can cut the thin threads of a supple tire.

An engineer who is now tire guy and who used to be a wheel guy, Morgan Nicol has been active on this tire/rim interface issue for a long time, attending all of the ETRTO meetings to come up with industrywide standards for both. Rim manufacturers know all about rims, and tire manufacturers know all about tires, and there can be a glaring lack of communication and understanding between them, possibly leading to situations like what you have experienced. It is natural that the consumer will tend to point the finger at the tire in a situation like yours, when perhaps the rim should be getting the scrutiny here.

Bicycle rim manufacturers and tire manufacturers don’t tend to send engineers to ETRTO conferences to ensure that the details of the standards affecting where the rim and tire meet are completely fleshed out; they instead tend to send a secretary to take notes to report back to the engineers. However boring these conferences can be, this mundane ETRTO standards work is critical.

Nicol is a standards wonk and lives and breathes this stuff. Here is what he has to say about your situation:

Unlike with the ENVE SES, that were specifically designed with square cut, then taper cut, to expose the carbon fiber ends on the hooks – instead of the ETRTO-required (soon to be ISO) “smooth, 0.7mm minimum radius” hooks – Reynolds periodically suffers a different problem with their carbon rims.

As with many carbon rim companies, the mold lines release on the rim hook, and many times, this leaves a resin flashing on the hook edge running tangentially around the wheel. This flashing is routinely sanded off during the finishing process, but, humans being what they are, sometimes the operator misses a section of this flashing, leaving a sharp edge that can quickly cut through high quality casing materials, and even through the aramid-fiber Shafers strip created to protect bead areas from just this type of cutting.

High performance handmade clinchers are not new technology; they have been appreciated back well into the last millennium. Vulcanized tubeless tires, like Continental makes, are a proven construction from the bike world’s largest and possibly highest quality producer. Carbon clincher rims are seven or eight years old, tough to make right and still being perfected (obviously). Now, even handmade Tubeless Ready tires are being released with Shafers strips (see Challenge Handmade Tires at the EuroBike show), but nothing will help the tires if the carbon clincher beads are not properly designed and finished following long-established international standards.

The casing being cut just above the beads by carbon rim hooks, like Thomas experienced twice, are especially problematic for high performance cyclists who corner, descend at speed, and climb while standing. Stressing while rolling the tires from side-to-side (as they are designed to be worked and have worked for years) is exactly the side-to-side action that is actively sawing the casing if the rim is not designed or finished properly. Wheel companies need to be aware and address these problems promptly or many athletes are going to be seriously injured and wheel company liability problems are going to continue to grow.

In my experience, Reynolds (and ENVE, DT Swiss, Zipp and many other reputable wheel companies) have quickly replaced wheels, or, at a minimum, walked through the process with the dealer or consumer of gently sanding the sharp section with emery paper to remove the flashing without damaging the resin or fiber of the hooks.

Final note: Respected brands like those mentioned above take care of these types of problems, for rider safety and their future business. Please warn consumers to be very careful with cheap, “no name” wheel brands or counterfeiters who do not care about your safety and can disappear from the global marketplace only to rise under another name and injure other athletes.

P.S. This is just one other reason the professionals ride the tubulars that the young ones have forgotten. When your life is on the line, you may want to reach back to technology finely tuned since 1888. The tubular system is still lighter, stronger, more resilient, safer, and cheaper to make. And further improvements await just around the corner.

Morgan Nicol
Challenge Handmade Tires

Long answer, Tom. It’s not always the tire, or the type of tire (tubeless vs. tubed) to blame when a tire fails. I’d check those bead hooks on your rims if I were you. I’d be interested to know what you find.
― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), 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.

Follow @lennardzinn