Two incidents in the pro peloton have sparked many debates about the safety of hookless rims. The UCI is looking into it, but what does all this mean for us, the end users? We set out to shed light on the background and asked industry insiders the crucial question on all our minds: hooked or hookless?
It looked bad. The insert torn, sealant everywhere, and the tire unseated from the rim. That’s the scene of Thomas De Gendt’s bike leaning against the guardrail on the 5th stage of the UAE Tour. Just last year, the European Tire and Rim Technical Organisation (ETRTO) made changes to the recommendations for compatible tire and rim sizes with hookless tubeless systems, which received little attention. It was only the drama of the images that rushed through the Internet after this live crash that really sparked the discussion about the safety of hookless rims. But the real question is: What are the advantages and disadvantages of hookless rims for the end users, without team vehicles and personal mechanics? And are they safe to use?
A good deal, theoretically
Let’s start from the beginning. Theoretically, the promise of the manufacturers of hookless rims goes like this: “we can save up to 20 percent on manufacturing costs through a simplified manufacturing process that reduces work steps, material requirements, and waste products. In return, you only ride tubeless tires and get improved aerodynamics, cornering grip, impact resistance, damping, and weight savings. All for less money.” Sounds like an offer you can’t refuse. So it’s not surprising that some brands now rely exclusively on hookless rims. These include industry giants like Zipp (SRAM) and CADEX (GIANT).
Unfortunately, it isn’t as simple as it sounds. Firstly, wind tunnel tests are yet to demonstrate any significant aerodynamic advantage of hookless rims, as Ralf Eggert of DT Swiss confirms: “We carried out aero tests in cooperation with Swiss Side and compared the performance of rims with and without hooks. We couldn’t find any aerodynamic advantage with hookless rims. On the contrary, our measurements showed that hooked rims are at a slight advantage” What remains decisive for aerodynamics is whether the tire and rim width match, hookless or not.
Secondly, there are rims that are on par with the weight of the top hookless models, despite the use of hooks. Thirdly, the claimed advantages in handling are due to an increased air volume in the tire and lower tire pressure. Both are equally relevant to hooked rims with wide internal measurements. “At low pressures, the sidewall stability on hookless rims improves slightly, since the casing isn’t pressed in by a hook. But these differences are marginal and difficult to quantify,” says Patrick Brown, a development engineer at HUNT, but explains that “The feedback from the riders and work with our pros speaks for a small advantage.”
What about impact resistance? Ralf Eggert says: “Rim profiles are generally robust. Ultimately, it all comes down to the tires and the pressure.” Indeed, you can lower the tire pressure far enough that any rim will break.
So, having mentioned those four provisos, what we’re left with is the promise of lower prices. At € 2,900, however, the Zipp 454 NSW aren’t exactly a bargain. The new Black Inc 48/58 are in the same price segment, weighing 100 g less than the 454 NSW, and have an internal width of 23 mm, just like the Zipp. However, Nathan Schickel, Product Manager at SRAM, points out, “With inflation rates factored in, a current Firecrest wheelset costs 35% less today than it did in 2019 before we made the move to hookless with this range.” And he adds: “For our NSW wheels, i.e. our top-end models, we’ve kept the prices the same.”
It turns out that if you question the marketing of hookless rims, it proves to be little more than hot air, and even the theoretically cheaper manufacturing process doesn’t hold true for every manufacturer: “Bigger manufacturers will probably have savings. But we’re still very small players by comparison. It doesn’t make much of a difference for us,” says Patrick Brown.
It would be interesting to see further tests of hooked and hookless rims with the same inner widths and identical tire set-ups, providing comparable data on grip and rolling resistance. However, none of the manufacturers we interviewed were able to offer reliable results. As such, the jury’s still out on whether hookless rims offer significant performance advantages.
Intuitive user experience? Nope!
On the contrary. Because tubeless tires only hold onto a hookless rim by means of specially shaped bead cores, the safety of this connection depends on the correct interaction of the tire width, internal rim width, and tire pressure. Users of hookless rims must check whether the rim and tire are compatible before fitting the tires, quickly finding themselves confronted with confusing or untransparent safety standards and compatibility tables. According to the ETRTO standard, you must never (!) exceed the maximum tire pressure of 5 bar or 72.5 psi when fitting the tire on a hookless rim, even if the tire states otherwise — so much for the intuitive user experience of hookless rims.
Throughout all this, tire and rim manufacturers provide different degrees of insight into their respective internal standards and test procedures before approving a tire for hookless rims. Moreover, there is no binding regulation about the jurisdiction between the companies involved. Therefore, it remains unclear who’s responsible for testing and communicating whether a particular tire fits on a particular rim, in both practical and legal terms. In the worst case, you’ll even find contradictory information. Nathan Schickel from SRAM says: “According to the ISO standard, the information should be provided by the tire manufacturers.” However, Zipp drew up their own compatibility table because they found that the exchange of information isn’t always what it should be, and they couldn’t rely on the tire manufacturers’ specs.
Untransparent standards
While there are ISO and ETRTO standards, they also raise further questions. The ETRTO changed their compatibility recommendations last year. Since then, they only recommend tires with a minimum width of 29 mm for hookless rims with an internal width of 25 mm. They haven’t bothered to give a reason for making this change, though we can assume that it’s got to do with tires popping off the rim.
Which brings us back to Thomas De Gendt’s accident. He had a 28 mm wide Vittoria Corsa Pro tire fitted on a Zipp 353 NSW rim. With an internal width of 25 mm. Note: both Zipp and Vittoria had approved this tire and rim combination before the accident and published statements after the crash in which they assured the public that the deviation from the ETRTO standard wasn’t what caused the tire to come off. Instead, it was De Gendt’s collision with an object on the road that was responsible, causing the rim to break and subsequently the tire to jump off. Shortly thereafter, Zipp changed their compatibility specifications for the 353 NSW — to at least 29 mm wide tires, thus putting them in line with the ETRTO standard. However, Nathan Schickel also says: “Due to some of our existing, close relationships with tires and tire brands, there were reasons to continue approving other sizes.” That’s one way to erode trust in a standard.
That aside, nobody wants to depend on a system where a difference of just one millimeter determines whether a tire will stay on the rim or not, especially considering the large tolerances inherent to tires.
Many unanswered questions
In addition to the ETRTO, the ISO also set safety standards for the use of hookless rims. According to ISO specifications, a tire must be capable of retaining 110% of the maximum permissible pressure on a hookless rim for five minutes without popping off to be considered compatible. On the other hand, the ISO state that a tire must be able to maintain 150% of the maximum permissible tire pressure on a hooked rim for the same time.
In concrete terms, this means that a folding tire with a maximum permissible tire pressure of 8.2 bar, or 120 psi could be pumped to 12.4 bar (180 psi) before it pops off a hooked rim. That’s a tolerance of 4.2 bar (61 psi). However, according to the ISO standard of 110% for hookless rims, it’s permissible if a tire with a maximum pressure of 5 bar (72.5 psi) comes off the rim at pressure of 5.5 bar (80 psi). That’s a tolerance range of just 0.5 bar. According to Ralf Eggert, a pressure difference like that can easily occur: “Pressure fluctuations of 0.5 to 0.6 bar can be achieved quite easily through an increase in ambient temperature or direct sunlight.”
Only the ISO know why they make this difference between the standards for hooked and hookless rims. Nathan Schickel also comments: “Everyone in our technical team would welcome it if the ISO standard were stricter. Even the tire manufacturers always tell us that they consider the ISO standards to be too low. Every company I know tests far beyond this standard.”
In line with this, Schwalbe state on their website that they rely on their own internal test procedures. Accordingly, they test all products up to 1.6 times the maximum permissible pressure of the tires or rims for a period of 60 minutes. That sounds a lot more reassuring. However, having to search for information on internal test procedures before deciding on rims and tires requires a lot of additional effort on the part of the consumer. That’s if you can find, understand, and rely on information provided by internal tests. Not all brands are as transparent as Schwalbe. And isn’t it enough to have to keep an eye on whether there is enough sealant in the tires, the brakes are bled, and the batteries of your drivetrain, head unit, power meter, lights, and radar are charged?
Statements from brands that publicly oppose hookless rims — due to safety concerns — lead to further uncertainty. Ralf Eggert of DT Swiss said in our interview that: “We make no compromises in terms of safety. That’s why we accept the higher complexity involved in the production of hooked rims for our high-pressure, i.e. road bike, tubeless rims.”
So, it can be incredibly dangerous if bikes come equipped with hookless rims as standard and end users with less technical know-how and experience aren’t even aware of it, let alone of the special requirements of hookless rims. It’s on the dealers to counter this by educating and advising their customers.
Stay hooked
Even though hookless rims don’t necessarily pose a safety risk when used correctly, many questions remain unanswered and it’s difficult to discern any major benefits to riders. Rather, we get the impression that certain brands are chasing greater profit margins at the cost of safety with the production of hookless rims, at least for their high-end products.
Riders who don’t want to limit their tire choice or spend an entire day researching tire pressures, tire widths, and internal rim widths should stick to hooked rims. But if you really want the whole package when it comes to state-of-the-art bike tech, you are always in search of even the smallest marginal gains, and aren’t afraid of putting in the time to educate yourself, you can happily go for hookless rims. “When used correctly, the system works,” says even Ralf Eggert.
Either way, you’d be well advised to educate yourself and rely on manufacturers that are transparent about which products are compatible with each other — so that you don’t eventually end up with your own bike leaning against a guardrail with the tire unseated and yourself lying next to it.
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Words: Dorian Steinhoff Photos: Jan Richter
