Riding is Life


Question time with Zipp on Tubeless Straight Side Safety

Zipp offers some answers – but also more questions – in the debate over hookless road wheel systems.

Almost four weeks ago, Thomas De Gendt suffered a nasty accident at the UAE Tour, the result of which, thankfully, was only minor injuries for De Gendt. But a huge spotlight also shone on the use of Tubeless Straight Side (hookless) wheels in road races. The CPA, the de facto riders’ union, called for a ban; Escape Collective reported that De Gendt’s rim and tyre combination did not comply with the ISO standards; the UCI announced an investigation into the use of hookless’ and the manufacturers involved claimed De Gendt hit an object, which caused the catastrophic failure.

Last week, Zipp released photos of De Gendt’s broken rim, claiming that the damage confirms an impact was the root cause of the crash. As the saying goes, “a picture is worth a thousand words,” and nothing Zipp could have said would have painted a clearer picture of what caused De Gendt to crash as the cracked and battered carbon rim. 

But if an impact is to blame, those pictures also create more questions, most notably, is there an impact resistance issue with Zipp rims given De Gendt’s teammate Johannes Adamietz suffered a remarkably similar failure on the same wheelset and tyre model combination just nine days later at Strade Bianche? Further questions also arise: as whatever both riders hit isn’t firmly known, what magnitude were the two impacts to cause such destruction; what impact testing does Zipp do on its NSW rims; was the rim and tyre size combination a contributing factor, and why was Zipp approving the use of a rim and tyre-width combination that is no longer within ISO standard?

For many, these are interesting questions in response to a head-scratch-inducing incident, but for others, namely, those with hookless rims, the answers to these questions are paramount to regaining confidence in the wheels beneath them. 

We invited Zipp onto a dedicated episode of the Geek Warning Podcast to get answers to these questions and more. I am joined on the episode by David Morse, Category Manager of Wheels at Zipp, and Nathan Schickel, Product Manager.

Thomas De Gendt's bike sits against a railing at the UAE TOur. The front wheel has blown off the rim and the internal foam tire liner has come completely out, and looks like a very thin, lime green pool noodle.
Zipp has analysed De Gendt’s rim and shared some of its findings with Escape Collective.

Zipp investigates the failures

So what happened in De Gendt’s and Adamietz’s tire dismounts? Schickel explained Zipp’s investigation into De Gendt’s rim identified a failure consistent with an impact, the laminates (layup) were “as prescribed,” and the failure occurred where Zipp would expect, meaning the construction of the rim was in line with its design and engineering. Zipp also claims De Gendt’s team had inflated his tyres to “within range, well below 4.5 bar,” or less than 65 PSI, below the maximum permitted 5 bar/ 72.5 PSI for a 28 mm tyre on a hookless rim. 

Zipp could not identify the exact object nor the exact forces that would have caused such a failure, stating only that it must have been at the upper limits of its impact testing protocols. Zipp points to video of the incident to claim that an unknown object seen on the roadway apparently caused De Gendt’s crash.

Zipp has also identified what appears to be the missing section of carbon rim flying up and away from De Gendt as the crash happens. In video meetings with Escape Collective, Zipp showed screen grabs highlighting both the supposed object and the carbon rim section, but despite several requests, it has not yet shared these nor the images of De Gendt’s broken rim directly with us.

Zipp’s explanation for Johannes Adamietz’s remarkably similar incident nine days later is markedly in line with its findings in De Gendt’s case. Zipp explains that Adamietz, also racing with Lotto Dstny, reported hitting “something” at high speed. According to Zipp, Adamietz fortunately avoided crashing and didn’t notice anything wrong immediately after the impact and kept riding (as pros do) until shortly after when the tyre flatted, and he rode it until it dismounted the rim.

The image shows Lotto-Dstny rider Johannes Adamietz carrying his bike with the front rim clearly broken and tyre dismounted.
Zipp claims Johannes Adamietz also struck an unidentified object, causing a similar rim failure, but that he did not crash.

Schickel explains Zipp does not actually advise its teams to use the 353 NSW rims in Strade Bianche, a race it classifies like a cobble or gravel race where riders want a wider tyre. Instead, it recommends the 303 Firecrest, which it explains is designed for the roughest races. Schickel says while both rims pass the same impact test, “if we outline a different use case, we’ll see higher impact values for a wheel like the 303 Firecrest.” Nevertheless, pro riders have ridden the 353 NSW to success at Strade Bianche: Movistar’s Alejandro Valverde finished second in 2022 on the wheels, and other Lotto Dstny riders, including third-place finisher Maxim Van Gils, also ran the 353 NSW at this year’s race. The wheel seems largely to work fine in those more demanding conditions – until it doesn’t. Zipp, incidentally, is an official equipment partner to Movistar, but not Lotto Dstny.

Regardless, the damage to both rims across different races and incidents seemed remarkably catastrophic, considering neither the riders, nor others around De Gendt in his incident, seem to have spotted the offending objects. Zipp dispels other potential scenarios, like whether the object must have been at least as wide as the rim, or that perhaps a crack in the rim bed allowed air in the tyre to pressurise the rim cavity, creating an explosion similar to that which Specialized witnessed after Peter Sagan hit a kerb during development of their previous generation Roval Rapide SLX. 

In short, Schickel describes the episodes as just “racing incidents.” But two similar incidents, nine days apart, adding to images of incidents of Movistar and Q36.5 riders with tire dismounts on the same Zipp rims, raise the question of whether these incidents are all inevitable regardless of the rim or tyre system, as Zipp suggests, and if Zipp is just the unluckiest manufacturer in the peloton right now.

Not so standards

One of our central questions is around Lotto’s use of a 28 mm tire on a 25 mm internal-width rim in the De Gendt incident. Zipp insists that combination played no role in the incidents, and that wider tyres or even a crotchet-type (hooked) rim would not have mitigated the consequences. 

Right now, Zipp’s recommendations on minimum tyre width are somewhat contradictory. As reported previously by Escape Collective, under revised ISO-5775-1-2023 standards, the minimum stamped-width tyre size recommended for use on a 25 mm internal-width rim is 29 mm. Multiple sources have told us that the update to ISO standards to now specify 29 mm as the minimum nominal tyre width for use with a 25 mm internal rim width was in direct response to failures witnessed with the 28 mm tyre and 25 mm inner rim combination. 

According to recent media reports, Zipp is now recommending its pro team partners adhere to that ISO standard. Schickel also told us that Zipp is recommending, when possible, that teams use 30 mm nominal-width tyres on its rims with 25 mm internal widths. But Zipp’s published chart of compatible hookless tyre and rim combinations still lists many 28 mm tires as approved for use on its 25 mm rims.

Schickel points to Zipp’s history and success in using 28 mm tyres on 25 mm internal-width rims as rationale for why the charts still approve those combinations. But he says Zipp is discussing the issue both internally and with tyre manufacturers, although no final decision has yet been made on compatibility recommendations going forward.

Much of our hookless focus has been on the new ISO standards, which also dictate certain rim dimensions and tolerances. Morse states all Zipp rims adhere to ISO dimensional standards, and points out that “it is the rim manufacturer’s responsibility to put out product that meets the dimensional standards and beyond that, it is the tyre manufacturers to make sure their tyres function with an ISO-standard rim.” 

That raises a valid point: while there are specific ISO standards for hookless rims, there are none for hookless tyres; it’s essentially up to the tyre brands to make products that pass tests like blowoff pressure. The ISO standards clearly state the dimensions (and tolerances) to which manufacturers must adhere when producing TSS rims. They are: 

The image shows a section of the ISO standards specifying rim dimensions for tubeless straight-side (hookless) rims.

But when asked about the ISO’s rim standards, Zipp suggests some are less critical than others. Zipp claims that bead seat diameter (BSD) and outer diameter (OD) are the critical factors in tyre retention, and while outer diameter is not specified in ISO – while flange height (G) is – Zipp designs to its BSD and OD targets, and then checks to ensure the resultant G height is within tolerance. Zipp claims all its rims are compliant with ISO dimensional standards (which is a requirement).

While admittedly quite difficult to measure perfectly accurately at home, our measurement of a Zipp 858 NSW rim suggests a BSD, G, and H measurement within .01 mm of tolerance, but the L1 measurement was well beyond the stated 7 mm spec (again, there is no tolerance standard for this dimension). Escape Collective has asked Zipp to share the rim dimensions for its 353 and 454 NSW rims. It has not yet done so. 

As for the rim well depth and width, Zipp agrees tyre installation is influenced by the well depth and it “drives the well depth to the lower limit of the spec to make it (the tyre) as easy as possible to install.” Easier installation sounds great, but is the trade-off a tyre that also dismounts much easier? Morse claims that it depends on if the tyre is “prone to falling back into the well,” explaining this depends on the shape of the beads in the tyre. 

The dimensional conversation highlights one inherent design challenge with hookless road systems. For all the talk of the missing hook, rarely mentioned is the dimensional and customer-expectation hurdles manufacturers must scale in producing hookless road system that is both safe and easy to use from a tyre installation standpoint.

One could argue a truly safe hookless road system should consist of a steel-bead, stiff-walled, non-folding tyre. That tyre should be mounted on a rim with a BSD, flange height, and rim well depth all exactly to standard. 

The only issue with such a system is that it would prove all but impossible to install a tyre at home, let alone do any roadside repair that required more than a plug, some sealant, and air. The claimed and debated benefits of hookless road system may exist, but if they do, they do not come for free. The cost is either a thumb-busting process for changing tyres, or a system that allows for easier tyre mounting but is perhaps more prone to tyre dismounts. 

The weighty impact of testing

There are also some specifications not found in ISO standards, namely rim flange – or sidewall – thickness (think the area that once was the braking surface with rim-brake systems.) Sidewall thickness can be a factor in a rim’s performance in impact testing, which is relevant given the De Gendt and Adamietz rim failures.

Asked if there is a failure issue with NSW rims, Schickel told Escape Collective in a prior discussion and again in the podcast that no such issue exists. Schickel explained Zipp isn’t simply pulling material out of the NSW rims to reduce weight, but it’s in optimising the laminates that Zipp can maintain durability while reducing weight. 

He also pointed to Zipp’s in-house testing and explained in our previous discussions that since introducing its lifetime warranty in 2020, Zipp has a 1.4% warranty return rate for impact damage across all its wheels. While it is unclear how the inclusion of all Zipp rims affects that return rate compared to NSW rims specifically, when asked how this return rate compares to other rims or any kind of industry average, Zipp said this is not a number that is shared across the industry. 

Morse also explains that all Zipp’s rims must pass the same impact testing and that the “TSS rims of today are roughly 40% more impact resistant than the tubulars Zipp previously produced.” Zipp claims its impact testing is more stringent than that used by ISO or the UCI, but did not elaborate further on what Zipp’s in-house testing protocol involves. 

The NSW line did recently see an increase in claimed weights, which Zipp denies anything to do with failures or warranty rates. Schickel explains that the weight increase had two causes. The first followed what Schickel calls “an exercise to improve the robustness of the spoke holes,” after Zipp identified higher than desirable scrap rates of the NSW wheels in production. This process resulted in a weight increase and corresponding update to the claimed weight.

Secondly, Schickel explained that Zipp manufactures rims both in Taiwan and Indianapolis, and while it uses the same designs, processes, and testing in both facilities, it relies on different carbon vendors in each location. An audit on carbon rims at both facilities during the process to reduce scrap in the spoke-hole area highlighted differences in the weights of rims produced at each facility. Zipp uses the 90th percentile weight as its claimed weight and updated its claimed weights in line with the heavier of the two 90th percentile figures from each factory.

Under pressure

Unsurprisingly, Zipp conducts in-house max inflation tests for tyre retention testing. While ISO standards specify a tyre must withstand five minutes at 110% of the max 73 PSI inflation pressure for TSS rims, Schickel says Zipp’s testing extends the duration element of this test to 60 minutes and the max inflation to 150% of the lower of the tyre pressure limit or rim pressure limit in a given system. 

Topeak Transformer pump gauge
Inflation pressure matters … especially with hookless systems.

Following De Gendt’s crash, Zipp inflated a Vittoria Corsa Pro tyre to 50 PSI on a Zipp 353 NSW rim and inflated it 10 PSI every hour until failure. Schickel explains Zipp sees this failure occur “generally in the neighbourhood of 150%” of the inflation pressure limit, but the failure pressure is not exact as there are variances from different tyres or rims. 

As for pressure spikes due to impacts, Zipp claims that testing on impacts “up to failure” using the same rim and tyre combination as De Gendt had demonstrated a max pressure increase of 4 PSI. 

That’s all welcome information, if short of conclusive on whether there is an actual issue with road hookless and what might be causing it, and why Zipp has been at the center of recent incidents. Zipp is adamant that nothing more than two unlucky impacts with two unidentified objects was the catalyst for the hookless uncertainty over the past four weeks. It is also confident that all its manufacturing, dimensions, and testing are working as designed and warranty rates are well within acceptable limits. And for now, Zipp has not changed its official recommendation that 28 mm tyres are compatible with 25 mm internal-width hookless rims.

But many questions remain unanswered, not least of which is whether Zipp will hold to that tyre compatibility chart. While we may not get all those answers, only time will tell if Johannes Adamietz is the last rider we see carrying a Zipp rim sans tyre, and Zipp just had an incredibly unfortunate start to 2024.

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