Hookless rim - the most important facts in brief
"Hookless" is a rim design without hooks in the rim flank. Carbon rims without hooks are easier and cheaper to manufacture, but less safe against tyre jumps, which is why the technology is controversial, especially with high tyre pressure.
Thomas De Gendt's crash causes a stir
Pictures speak louder than words. The most memorable image of Thomas De Gendt's crash at the UAE Tour in February is the photo of the bike with the front tyre that has come off, with the splashed tubeless milk and the torn green puncture protection hanging limply from the wheel, which illustrates even more than the tyre itself that something has gone wrong here. De Gendt crashed on the fifth stage for no apparent reason. Fortunately, he did not suffer any serious injuries. The cause was immediately suspected to be the front wheel, a Zipp NSW 353 with a hookless rim and 28 millimetre wide Vittoria Corsa Pro. Had the tyre jumped off and caused the crash? The television pictures did not reveal anything. De Gendt later said that he had hit something with the front wheel and lost control as a result. Zipp examined the wheel and showed pictures of the broken rim, which showed massive damage that looked like it had been hit. Vittoria also agreed with the theory of a run-over stone.
A short time later, a photo of De Gendt's team-mate Johannes Adamietz was published, showing him with a front tyre that had come off at Strade Bianche - the defect with the same material constellation as De Gendt's happened before the start of the television broadcast. The incidents alarmed the riders' union CPA. Its chairman Adam Hansen immediately called for Hookless to be banned and reported that he had received numerous reports from the peloton of tyres coming off hookless rims. The UCI promised to tackle the issue as a matter of urgency, then published a statement at the end of March stating that it was working on the issue and wanted to create a safe solution for 2025. The UCI also pointed out that the current standards must be adhered to.
Hookless rims: Of horns and hooks
Before we go into the standard, a brief explanation of what it's all about. "Hookless" stands for a rim design with straight sidewalls which, unlike classic rims, do not have a pronounced rim flange (= hook). The horn does not actually have a load-bearing function, but it increases the bounce pressure at which the tyre jumps off the rim and is therefore a safety feature. Carbon rims are easier to manufacture without this hook, as this requires an undercut in the mould, which requires multi-piece or inflatable cores. From the manufacturer's point of view, the simpler production, better compression of the laminate and better dimensional stability of the rim speak in favour of hookless. On the other hand, the tyre is less secure against bouncing.
And this is where the standard comes into play. ISO 5775 regulates in detail what a hookless rim should look like - with dimensions and tolerances. The standard also states that the tyres - all of them, i.e. for both hookless and normal rims - must be able to withstand at least 110 percent of the maximum nominal pressure for five minutes. The ETRTO, an association of tyre and wheel manufacturers that deals with compatibility issues, recommends which tyres can be combined with which rims.
The width recommendations of the ETRTO were changed in 2023. 28 mm tyres - such as those from De Gendt - are no longer approved for 25 mm rims. This contradicts what the hookless pioneers Enve and Zipp have announced. Tyre manufacturer Schwalbe also still considers its 2022 recommendations to be safe. Width is therefore a controversial topic, although it is generally recognised that wider tyres are safer for a given rim inner dimension.
Interestingly, all the experts we spoke to found the standard's 110 per cent rule for pressure completely inadequate. According to our calculations, a temperature increase of 40 degrees leads to 0.5 bar more pressure in the tyre; this corresponds to 110 percent at 5 bar maximum pressure, which the ETRTO recommends for hookless tyres, unless the tyre or rim has a lower limit pressure. The standard therefore offers no significant safety margin here.
Safety concerns with hookless rims
Internally, manufacturers therefore always test higher - at 150 to 160 per cent of the valid maximum pressure, i.e. at 7.5 to 8 bar, Giant even higher. That doesn't sound like much, but in the world of wide tyres and rims, where pressures below 4 bar are predominantly used, it's quite a lot.
However, this is not safe enough for many rim manufacturers. DT Swiss, for example, the largest in the industry, does not build hookless rims for tyres less than 33 millimetres wide. Philipp Lüdi, head of the DT test laboratory, explains this as follows: "We have to allow for obvious misuse. These include pressures that are too high or tyres that are not approved for hookless."
And this approval is indeed a major problem in the tinkering and swapping racing bike scene, where people like to combine materials to suit their own taste. This is because every manufacturer does their own thing and the labelling of the products is not clear. On the websites of the wheel manufacturers there are compatibility lists showing which tyres are approved for which rims. These lists are based on static overpressure tests, but as random samples they can only give an indication of which pairing works. Missing tyres in the lists raise questions: Have they not been tested, or have they failed?
This is where the decisive weakness of the standard becomes apparent: the tyre is not defined at all - except that it should fit the rim - and the pressure safety is specified far too loosely.
Better standards are possible
From a technical point of view, this would be much better. If the tyre's characteristic values were also given - nominal dimension and maximum elongation of the core with a pressure safety margin of 160 percent, for example - then the issue of tyre bounce would presumably not exist at all, and every tyre clearly marked as hookless-capable would be safe on a dimensionally stable hookless rim.
The fact that the standard is so lax is probably mainly due to economic reasons. Light and very stiff tyre cores made of Zylon, which are stiffer than the usual aramid cores, are so expensive that manufacturers do not use them across the board.
However, the required properties of the tyre core are measurable. The wheel manufacturer Enve has a test machine for this in its laboratory and speaks of significant differences in tyre core stiffness. Vice President Jake Pantone reports on Enve's experience: "Essentially, we have found that the right recipe for success is an optimum range for tyre bead stiffness and diameter. Basically, a bead that is stiff and, more importantly, one that doesn't stretch over time is beneficial. Secondly, a diameter that allows fitting but is also tight enough to achieve a seal and press fit on the rim, but again not so tight that it compresses the rim too much. Some tyres with small beads compress the rim a lot, which leads to other problems such as loss of spoke tension."
So is hookless actually safe after all? If the rim conforms to standards and the pressures are maintained, this obviously depends primarily on the tyre. If the tyre bead hardly yields even under high forces - due to the tyre pressure or dynamic load peaks when driving - the tyre is always securely in place.
Unfortunately, this is not always the case in practice. According to our research, tyres jumping off hookless bikes are not a mass phenomenon, but there are examples - also in letters from readers. The risk of misuse is real. An insider from the tyre industry told us: "Many people don't even know that they are driving hookless. The products of small manufacturers are more widespread than you might think. If the wrong tyres are fitted, it's a bang."
However, if the rims and tyres come from a single source, as is the case with Giant (brand name Cadex), safety should be relatively high. Giant also allows tyre pressures above 5 bar for hookless and tests relatively rigorously. We did not come across any tyres coming off Cadex rims during our research. However, if you combine Cadex wheels with tyres from other manufacturers, there are restrictions. On its website, Giant also lists tyres from other manufacturers that failed the pressure test.
Hookless increases the risk
To summarise: The situation is unsatisfactory because it is confusing. The best solution would be a binding and sensible standard that sets the safety level so high, even without hooks on the rim flange, that tyre jumps are ruled out. In addition, there should be clear and easy-to-read labelling of the rim and tyre. No tiny lettering, black on black on the tyre sidewall.
Hookless already works well with low tyre pressure - on gravel and mountain bikes - as long as you don't overdo it. If you pump 2.4-inch-wide tyres on a current mountain bike rim to 3.5 bar, i.e. around 2 bar more than is sensible, you will exceed the pressure limits of the rim and risk it exploding.
The fact that tyre pressure has been reduced on road bikes thanks to wider tyres and rims is a benefit. After all, these combinations ride better than the narrow high-pressure tyres of the past. This progress is perhaps also indirectly due to the precisely specified upper limits for the hookless system: a maximum of 5 bar on road bikes is a striking signal that something has changed. But floor pumps can easily deliver much more, and 7 to 8 bar in road bike tyres were still relatively normal until recently.
It will probably take some time before hookless becomes established as a foolproof technology. Industry veteran and technology pioneer Josh Poertner, head of Silca, is among those who believe that the system will prevail, as he said in an interview with Dan Empfield from Slowtwitch that every advantageous production method has so far prevailed in the long term. Until then, the motto is: Keep your eyes open when choosing tyres and keep the pressure nice and low. Or continue to use conventional rims, as there are plenty on offer.
Hookless requires harmonised components
One of the challenges is the perfect coordination of rims and tyres, as they have to function as a unit. The rim dimensions are standardised, but the tyres are not. It is the responsibility of the tyre manufacturers to design tyres that are safe for hookless: resistant to stretching, a perfect fit and therefore resistant to bouncing.
Compatibility is tested reciprocally by tyre and wheel manufacturers, but not according to a uniform standard. Each has its own procedure. Naturally, pairing is easiest when both components come from the same manufacturer, as is the case with Giant under the Cadex brand.
Hookless facts
How to recognise the wheels and what to look out for
- Hookless wheels are often not labelled as such. Only after the tyre has been removed can it be determined beyond doubt whether the rim flank has a horn or not. If no horn is visible, it is a hookless rim.
- The following applies to hookless rims: recommended tyre widths (nominal = imprint) depend on the inner rim width (rim width). Up to 21 mm inner dimension: min. 25 mm tyre width. If the rim measures 22-24 mm, the tyre should be at least 28 mm wide. If the rim measures 25 mm inside, the tyre should have a nominal width of at least 29 mm.
- Pressure limits according to the current ETRTO: For tyre widths (nominal = imprint) up to 29 mm, 5 bar is recommended as the upper limit, up to 34 mm 4.5 bar - unless tyres or rims have a lower pressure limit.
- Only tubeless tyres with hookless labelling or a printed 5 bar pressure limit are suitable for hookless. Older tubeless tyres are not all hookless-compatible.
- On the websites of Enve, Giant and Zipp you will find compatibility lists showing which tyres have been successfully tested on which rims. These give an indication of what works - but sometimes contradict the current ETRTO recommendation. If in doubt, choose the safer combination - i.e. wider tyres with less pressure.
Check the pressure!
Excessive tyre pressure costs performance on the road - the tyre rolls less smoothly and less comfortably. In conjunction with hookless tyres, over-inflation is the main risk. Modern tyre/rim combinations are run at significantly lower pressures than traditional tyre sizes.
It is therefore essential to use the relevant pressure calculator to set a sensible tyre pressure. With 28 mm tyres, this is usually less than 5 bar - the hookless pressure limit. If the recommended pressure is 5 bar or more: choose wider tyres with a nominal size of 30-34 millimetres and a correspondingly lower tyre pressure.
Tip: check the pressure gauges on the pump with a pressure tester.
Print calculator
- Giant https://www.cadex-cycling.com/de/tire-pressure
- Silca https://silca.cc/pages/pro-tire-pressure-calculator
- Zipp https://axs.sram.com/guides/tire/pressure
Compatibility tables
- Enve https://enve.com/pages/tire-compatibility
- Giant https://www.cadex-cycling.com/de/hookless-rim-technology
- Zipp ttps://www.sram.com/de/zipp/campaigns/hookless-tire-compatibility
28 mm tyres are no longer permitted on rims with an inner dimension of 25 mm following an update to the standard in 2023. 29 millimetres is now the minimum dimension.
Safety requirements
5 bar is the maximum permissible pressure for hookless rims with suitable tyres. But what are the safety factors - and are they sufficient?
Whether hookless tyres are safe depends on a number of factors. Key factors are pressure, dimensional accuracy of the rim and elasticity of the tyre. This is because the take-off pressure is lower than for rims with hooks. When used as intended, the pressure at which hookless tyres are ridden is well below the 5 bar recommended by the ETRTO as the maximum limit, and thus far from the manufacturers' test values, which generally test at 150 to 165 percent of the maximum permissible pressure, some even higher. That sounds like a lot at first, but the true safety margin is not that large, as it can shrink due to temperature (minus 0.5 bar for 40° C), expansion (approx. minus 1 bar) and mounting fluid (minus 1 bar). A very stiff tyre bead is essential - only then is safety guaranteed. 10 bar, as in our static pressure test, corresponds to 200 per cent safety. The greatest uncertainty arises from the combination of hookless rims with unsuitable tyres.
- Tyre manufacturers test with 150-165 percent of the maximum permissible pressure
- ISO standard 5775 requires 110 per cent of the maximum permissible pressure
Factors for safety
- Pressure
- Temperature
- Assembly fluid
- Dynamic forces (driving manoeuvres, impacts)
Problems
- the standard is inadequate and does not guarantee safety
- manufacturers test harder, but not uniformly
- Hookless products are not uniformly labelled
How TOUR tests
Static pressure test
We tested several combinations of tyres and rims with static overpressure tests in the DT Swiss laboratory and measured the wheels for dimensional accuracy. Result: All wheels are within the norm, all tyres and rim combinations passed the bounce test. Only at double the nominal pressure - i.e. 10 bar - and using mounting fluid did the tyre on the Zipp 303/Vittoria Corsa Pro 28 mm pairing pop off.
Dynamic impact test
We subjected some tyre/wheel combinations to a puncture test with rolling tyres - using a test setup that we designed specifically for this purpose. This dynamic test should clarify whether tyres can be pulled off the rim by impact loads. The result: the dynamic impacts could not pull the tyres off the rims; even critical rim-tyre combinations were able to withstand oblique impacts at a pressure just below the static breakaway pressure without the tyres jumping off. However, this does not mean that impacts per se are not a problem. Presumably, the conditions of our test are not sharp enough - our impact speed is only 17.5 km/h due to the limited run-up.
