Road tubeless has been in the wind for over a decade and in the last couple of years the technology has shown real signs of going mainstream, with numerous wheel and tyre brands jumping on board.
Pro cyclists continue to dabble in the technology, and it’s becoming increasingly common, particularly at the higher end of the market, for road bikes to ship with tubeless wheels and tyres, or at least ones that claim some level of tubeless-readiness.
Despite all of this, tyre and rim compatibility remains a thorny issue. Right now, there’s no guarantee that a given tubeless road tyre will work with a given tubeless rim, and that’s a problem.
In some cases, the incompatibility means tyres are simply too tight to fit. In others, there’s a risk of tyre blow-off, a dangerous scenario that could lead to real injuries or worse.
Recently, the situation has been further confused by the appearance of hookless rims for the road.
Hookless comes with its own set of purported benefits, but also raises important questions about safety, and highlights the lack of universal standards.
I’ve been looking into hookless technology as well as talking to key players in the industry about where the tech is headed, and how we unpick the mess that is road tubeless.
Thanks go to Continental, Enve, Giant, Schwalbe, Stan’s NoTubes and Zipp for their help in researching this feature, with a particular shout-out to Mike Bush of Stan’s whose help in dissecting the standards was invaluable.
What is a hookless rim?
Hookless rims have a number of claimed advantages that we’ll get into in more detail below.
A hookless rim is simply one with a profile that lacks bead hooks, the protruding edges that help retain conventional clincher tyres under pressure.
Instead, a hookless rim has straight, vertical sides, and indeed in wheel maker circles the technology is often referred to as “tubeless straight side” or TSS (not to be confused with “Training Stress Score”, a performance metric developed by TrainingPeaks).
Hookless rims have been around for mountain bikes for years, but in the road world, where tyre pressures are typically much higher, they’re relatively new.
Hookless rims are usually tubeless-only, i.e. you must run a tubeless-specific tyre, which will have a stiffer bead than a conventional tubed clincher.
It is possible to run a tube (e.g. in the event of a puncture that sealant won’t fix) but only in that same tyre – it’s not usually considered safe to swap to a standard clincher tyre.
On the face of it, you might assume a hookless rim is less safe than a hooked one, but the reality appears to be much more nuanced.
The safety of a rim–tyre interface is heavily dependent on the tolerances of the two components and proponents of hookless claim it’s perfectly safe (or indeed safer than hooked) and offers real performance benefits.
Detractors suggest the move to hookless is driven primarily by manufacturing concerns and not for the benefit of riders.
The state of play in road tubeless: it’s all happening
Right now, new tubeless wheelsets and tyres are appearing right, left and centre.
At one end of the spectrum, the likes of Zipp and Enve are coming out with hookless carbon rims, and their large internal widths would have placed them squarely in mountain bike territory just a few years ago.
These are wheels aimed at tyres wider than old-school road cycling norms would have demanded.
Where 23mm was the go-to size for road, these are designed for 28s or larger, and they straddle the road and gravel riding categories.
At the same time, many manufacturers are launching explicitly road-oriented wheels with internal widths in the 18 to 21mm range.
It’s a sign of how much things have changed that 18mm doesn’t even count as wide these days.
Road rims used to be around 13 to 15mm internally and tyres larger than 25mm were regarded with suspicion by those with even modest performance ambitions.
Ten years ago, race bikes shipped with 23mm tyres, while comfort-oriented endurance models typically got barely-bigger 25s.
On the tyre front, virtually all the major players now have a flagship road tubeless offering. Notable examples include Schwalbe’s Pro One TLE and the Continental Grand Prix 5000 TL, the tubeless version of Conti’s update to one of our favourite all-rounders, the all-conquering Grand Prix 4000 S II.
28mm tyres are showing signs of becoming the go-to for general road riding and many endurance bikes now accept 32s or even larger.
This is a trend that goes hand-in-hand with the rise of disc brakes. Few rim brake road bikes accepted a tyre larger than 28mm, or maybe 30mm at a squeeze.
Designing a road frame for larger rubber meant moving to non-standard long-drop calipers, cantilevers (as used on cyclocross bikes) or mini v-brakes.
Disc frames inherently have fewer restrictions on clearance, and bike designers have been capitalising on this to produce ever more versatile road bikes that overlap with the gravel category.
Where are the road tubeless standards?
We keep being told that a new road tubeless standard is coming or that one has already arrived. Again, the reality is more complicated.
Wheel and tyre standards are overseen by two bodies: the ETRTO (European Tyre and Rim Technical Organisation) and the ISO (International Organization for Standardization).
The ETRTO publishes its standards manual annually, and some years there are major changes while others see only minor tweaks.
The ISO in principle reviews its standards every five years, but, in practice, this timetable appears more elastic. The current standards governing bicycle wheels and tyres were published in 2014, and the next update appears to be some way off.
As the name implies, the ETRTO is fundamentally European but, in reality, it exercises considerable influence and works with major international players in the industry.
Historically, ETRTO standards have been baked into the international norms agreed by the ISO.
The problem we now have appears to be two-fold: not only is the current ETRTO standard now out of step with some of the products on the market, but the infrequently-updated ISO standard is still pending and liable to undergo changes before it’s finalised, meaning that even wheels and tyres that conform to current ETRTO norms may not conform to the global ISO road tubeless standard if and when it is agreed.
The only real solution to the current situation is a globally agreed standard covering both rims and tyres and, critically, how they interact.
According to the industry players I’ve consulted, the discussions are happening and a global standard is coming, but it’s probably not going to arrive for around 12 to 18 months and there’s plenty of wrangling to be done in the meantime. For now, it remains a wild west out there.
The great hookless compatibility question
Right now, if you take a Continental GP5000 TL out of its box and examine its sidewalls, you’ll find the following phrase written in capital letters: “mount only on hooked rims”. This goes to the heart of the road tubeless mess.
As things stand, tubeless wheel and tyre makers are having to devise their own rules to inform their customers.
Brands take differing approaches to this, with some wheel makers publishing lists of tyres they’ve tested and deemed safe, and others making more general statements about compatibility that place the onus on tyre makers to guarantee safety.
Fundamentally, it’s a question of liability. No wheel or tyre maker wants to be in the position of promising compatibility with another brand’s products only for a rider to suffer injury as a result of it all going wrong.
Enve, for example, has a very detailed section on its website listing tyres that are known to be compatible with its hookless rims.
The brand, an early advocate of hookless for the road, has undertaken extensive tyre testing.
Jake Pantone, VP of product and consumer experience at Enve, says: “the reality is that hookless just brings the skeletons of poor tire construction out of the closet. Hooked-beads conceal the problem.
“Today, and due to good collaboration between Enve and most all tyre manufacturers, you are seeing nearly all tubeless road tires that are introduced over the past 12 months, pass our hookless compatibility test.”
Enve’s testing protocol demands that a tyre must achieve 1.5 times the maximum pressure of either the rim or the tyre, whichever is the lower number, without blowing off.
For Enve’s SES 4.5 AR, for example, which is rated for 80psi, that means a tyre must hold 120psi without coming off the rim.
Testing is carried out using actual rims and also a machined aluminium test rim that’s used to measure bead stiffness and diameter.
Tim Ward, from tyre maker Schwalbe, says: “there are no particular restrictions for using Schwalbe tubeless road and gravel tyres on hookless rim as long as they are explicitly designated as tubeless compatible.
“However, it should be borne in mind that hookless rims tend to have a more restricted pressure range and, without the bead-lock lip of many tubeless clincher rims, the tyre can de-mount at very low pressures or when flat.”
It’s also worth noting that the “optimal” rim shown on Schwalbe’s own website does feature a hooked bead.
Zipp, another key hookless advocate that recently launched two hookless variants of its 303 aero all-rounder wheelset, has purposely not produced compatibility lists, with product manager Bastien Donzé saying: “Our statement regarding tire compatibility is clear: we believe that our rims are compatible with all tubeless or tubeless-ready tires on the market, EXCEPT those where the tyre manufacturer specifically prohibits the use of hookless rims.
“We have tested a large variety of tires on the market and we have come to realize that our new 303 wheels are just as safe as the previous generation when it comes to tubeless tire retention.
“However we cannot issue a statement or create a compatibility list that would overwrite the position of tire manufacturers on their own products. In the case of Continental, their position regarding the GP5000 TL is clear and we respect it.”
Donzé also revealed that in Zipp’s own testing, the blow-off pressures for its hookless rims were close to those of the hooked designs.
The figures are confidential, but I can tell you they’re much higher than you might imagine, far in excess of the ETRTO’s 5-bar number and well beyond any normal inflation pressure for a road tyre.
Stan’s NoTubes – arguably one of the best-known names in tubeless, but one that’s more established in the mountain bike world than the road one – doesn’t produce fully hookless road rims, instead favouring a design that’s jokingly referred to in-house as “damn near hookless”, where the bead hook is quite minimal.
Stan’s is another brand that doesn’t publish compatibility lists. “We haven’t gone down that road”, says company president Mike Bush, “we haven’t had blow-off issues.”
Some manufacturers test tyres on rigid steel rims that mimic their wheels in a lab setting. By contrast, Bush says: “we do everything on a real, built wheel, and that’s to 150 per cent of our rated pressure.”
Bush doesn’t rule out a move to a fully hookless design for the road, but says one is not currently “in active development”.
Bike maker Giant bears a mention here too, because the higher-end models of the new 2021 TCR race bike feature hookless carbon rims as standard.
At the time of writing, Giant has a list of compatible tyres, but it only includes those made by Giant (and in-house brand Cadex).
That’s set to change, though, and Giant’s David Ward says: “we’re working with the tyre brands to come up with a list and a test [i.e. a safety test] that both us and the tyre brand is happy with.”
Talking to the brands, there’s a consistent theme of cooperation – ultimately it’s in everyone’s interests if riders have confidence in their equipment, and there are no compatibility pitfalls.
In the absence of concrete standards, they’re doing the best they can, but consensus on the best approach remains elusive.
Is the future hookless? That remains to be seen
Continental stands out among tyre manufacturers by explicitly forbidding the use of hookless rims with its flagship road tyre.
According to Continental product manager, Jan-Niklas Jünger: “[hookless] does not work in our favour and we would certainly prefer to stay on hooked rims as it allows more safety to the end user, however we understand the wheelbuilding industry will push towards hookless-only as it decreases weight, carbon-scrap rate, production costs etc.
“As for now, we would like to recommend all our customers not to use our road tires with any hookless rims, if you are choosing gravel tires we’ve got you covered. Please do not exceed 5 bar / 73psi on any hookless rim and tire combination!”
If you’ve been following recent wheel launches, you might have noticed repeated mentions of that 5-bar pressure limit on hookless rims.
This number comes from the existing ETRTO handbook, which includes some spec details for hookless rims, but seemingly not for hookless road rims specifically.
Historically, 73psi would have been a very low limit for road riding, where pressures as high as 120psi are not unheard of.
If you ask the likes of Zipp, the number doesn’t really matter anyway because its testing consistently points to wider tyres and lower pressures being both faster and more comfortable in the real world, with no need to exceed that number.
Zipp’s Bastien Donzé is emphatic in his belief that this new normal of wide, squishy tyres for the road (and gravel) is the way forward and that it will be a case of “a mental transformation in the market” to get riders to accept this.
He notes that, with the latest Zipp wheels and 28mm tyres, he pumps his tyres up to 57psi front / 61psi rear at a rider weight of 77kg.
This is in line with our own experiences at BikeRadar. I, for instance, have been running Schwalbe’s latest 28mm Pro One TLE at around 50/58psi on my Specialized Roubaix long-termer with 21mm internal Roval rims (hooked, incidentally), and I’m absolutely a convert to this approach.
Zipp is all-in on hookless, claiming numerous advantages for the technology compared to hooked bead tubeless. Key among them from a performance point view is that “no hooks means cleaner transition between the rim and the tire”, i.e. better aerodynamics.
On the face of it, it also produces a more natural tyre profile, with the casing adopting a far less convoluted shape.
Enve anticipates a hookless future too, with the brand’s Jake Pantone acknowledging that it doesn’t “have a single rim in the pipeline with hooked beads”.
Zipp’s Donzé is very keen to counter the narrative that hookless is solely for manufacturers’ benefit: “There’s a perception that the bicycle industry is cutting corners with hookless; the truth is simply that hookless makes a better rim.
“Here’s why: by replacing the silicon mandrel used to create the tire bed cavity [with] a hard tool made of steel, we gain more control over the manufacturing process: better compaction of the carbon, better resin distribution through the rim, tighter tolerances in the tire bead seat, less waste and less scrap.
“In summary, hookless rims are lighter, faster, stronger, more affordable and better for the environment. The end-user wins at every level, really.”
Giant’s David Ward echoes these sentiments, and it’s notable that the brand’s hookless wheels don’t carry a warning about exceeding 5 bar – in fact, Ward says you’re fine as long as you stay within the limits marked on the rims and tyres.
We have a 2021 Giant TCR Advanced Pro 2 Disc (pictured above) in for testing (my colleague Simon Bromley is running it as a long-termer) fitted with Giant SLR-1 42 Disc hookless wheels and 25mm Giant Gavia Course 1 25mm tyres, and both the rim and tyre permit pressures up to 125psi / 8.6-bar.
Whether you’d actually ever need to run tubeless tyres that hard is another matter, but there you have it.
It’s worth noting that Giant’s alloy rims remain hooked and indeed I’m not aware of any hookless alloy road rims on the market right now.
From a manufacturer’s perspective, hookless is a lot less appealing for alloy rims. While carbon rims are moulded as a single unit, alloy ones are extruded, and then the ends of the extrusion are joined. Watch this delightfully cheesy How It’s Made clip for a demonstration of the process:
This means making a bead hook doesn’t add any particular complexity to the process, and that controlling tolerances for bead seat diameter (the critical dimension that determines how tightly a tyre fits) is inherently more difficult than it is with carbon rims.
It’s conceivable that we could see a divergence in tubeless technology between the high- and low-end as a result.
BikeRadar’s take | Do your homework, double-check everything, pray for the standards to come
Frustratingly, road tubeless always seems to be just on the verge of becoming truly mainstream.
We know it can work really well, with fast tyres that mount easily, stay on the rim when you’re riding, and run happily at pressures lower than you could get away with using standard clinchers.
We’re still some way from that being a given, though, and the hookless issue further muddies already murky waters.
Proponents of hookless make a compelling case, particularly when it comes to carbon rims.
At the same time, there are numerous very good hooked rims on the market right now and while the standards are still fluid, and are likely to be for quite some time, they do come with fewer unknowns for riders.
If we reach a point where the standards are truly resolved and across-the-board compatibility is guaranteed, the balance could well tip in hookless’s favour. Right now, it’s too early to pass judgment.
For the time being, we advise checking and double-checking your rim-tyre combo and following manufacturer recommendations to the letter.
Don’t exceed recommended pressures, don’t mount non-hookless approved tyres on hookless rims, and consider very carefully whether you want to shoulder the risk of trying unproven wheel and tyre pairings.