Crashes, controversy, compatibility issues: What is the hookless rim debate, and how do we solve it?

It's an important moment not only for hookless rims, but also for the bicycle industry as a whole, which has long struggled to provide narrative clarity on such issues. That's partly because these technologies are, well, quite complicated. But it also comes down to responsibility — and cooperation. We have seen such dissonance with bottom bracket standards time and time again (remember the creaks that came along with early Press-Fit bottom brackets?), headset standards (new sizes, new integrations, proprietary tools, etc), and even hub spacing (Boost, Boost 148, Super Boost, and many more hub spacings necessitating new frame designs and a lack of backward compatibility). These new technologies are not without benefits or sacrifices. And often they create heated debates as various segments of the bike industry struggle to catch up with new technology.

So when it comes to hookless rims, history repeats itself. "Nobody is addressing the real issues," said Jeff Schneider, Head of Product & Business Development at Cadex Cycling. "They are just pointing fingers."

And the real issue, Schneider believes, has less to do with a bicycle rim's retention system and more with system precision. For any wheel system to work reliably, tolerances between these two distinct products must be closely monitored and controlled.

Such precision requires extremely close collaboration between wheel and tyre manufacturers, or vertical product integration – one brand creating both the wheel and tyre – both of which are difficult business propositions. So what does the future hold for hookless rims? Let's take a closer look.

What is a hookless rim?

Simply put, a hooked rim features a protrusion on the inside of both rim walls that acts as a stop or retainer to keep the tyre affixed to the rim at full pressure. A hookless rim, as the name suggests, doesn't have this protrusion and instead features flat-faced rim walls. In theory, if the tyre bead and the hookless rim wall are both manufactured to precise specifications, the tyre should stay affixed to the rim using the air pressure alone, just as it would on a hooked rim - or more broadly in the automotive industry - but without the need for the extra material inherent in a bead hook.

More recently, micro-hooked rims have entered the market. These are essentially hooked rims, but the hook itself is much smaller. This is supposed to hit the middle ground between hookless and hooked rims (even though technically, a micro-hooked rim is just a hooked rim).

To ensure that tyres and rims are built to size with some consistency, the European Tyre and Rim Technical Organisation (ETRTO) defines specific dimensions in millimetres. Most tyre and rim manufacturers operate off ETRTO specifications, and those manufacturers that have taken tubeless and hookless systems very seriously often exceed ETRTO specifications to ensure tight tolerances that prevent tyre blow-offs.

In the 1970s, hooked rims became the standard to accommodate folding bead tyres, which were lighter and allowed riders to run higher pressures. To ensure these types of tyres stayed on the rim at higher pressures, manufacturers integrated hooks into the rim wall.

These hooks provided a mechanical support for the tyre beads so they didn't disconnect from the rim. The hook essentially protruded inward, 'hooking' the tyre bead and preventing it from sliding up and off the rim. This was necessary because the beads of these new tyre types could stretch, whereas older wire-bead tyres did not.

This was very new territory in those days. But it was also a welcome change from glueing tyres onto the rim, such was the necessity with tubular tyres. These are essentially self-contained units; to put it simply, the inner tube and the tyre itself are one unit. There’s no tyre bead on a tubular tyre; it is simply a circular tube, with the rubber tread on the outer face, and cotton (or nylon/polyester) on the inner face, and it must be glued to a tubular-specific rim with several layers of glue or mastic.

For consumers, the switch meant less time prepping and maintaining the bicycle, and more time riding it. Tyre construction also advanced, allowing new materials to be used in compounds, beads, and layers. Pro cyclists, however, stuck with glue-on tubular tyres for a variety of reasons. Primary among them: the rider could still ride the bike even if a tubular went flat. The tyre was likely to stay affixed to the rim longer than a hooked tyre would.

Fast-forward about 40 years, and a chain of events was coming to a head thanks to another set of trends: lower tyre pressures in road cycling and tubeless tyre adoption, as well as the advancement of tyre technology that meant their beads resisted stretching more successfully.

Hookless rims weren't brand new at this point. They had long been working well on the mountain bike side, where lower tyre pressures have long been the trend and tubeless tyres are more or less standard.

Tubeless tyres rely on an airtight seal between the rim and tyre, and they can mate more precisely with hookless rims. So, as study after study showed the benefits of lower tyre pressures in road cycling, and tubeless systems became more reliable and viable, hookless rims became worthy of exploration.

On top of that, eliminating the hooks from rims meant a simpler manufacturing process, which not only reduced cost but also weight in the rim. Simpler tooling also meant a more refined finished product, as carbon wheels had, at that point, become the wheel material of choice for a massive swathe of the cycling population.

"Several years ago, the hook of hooked rims was formed in the manufacturing process using a silicone mandrel, as it was flexible enough to be removed post-curing," Hunt Bike Wheels engineering tandem Robert Fields and Paddy Brown explained. "By contrast, hookless rims could be formed with a one-piece steel mandrel. The steel mandrel allowed for more accurate rim internals due to its rigidity, as the silicone could expand and move in curing. This accuracy and consistency allowed for higher strength-to-weight ratios in layups used to make the rims, as well as a lower discard rate.

"We now use three-piece steel mandrels to make the tyre channel of all our hooked rims. This tooling is more complicated and expensive, but allows for the same, more accurate rim internals as the hookless one-piece steel mandrel, and delivers the same accuracy and consistency. That means there isn't any strength and weight difference between two similar hooked and hookless rims."

And there was precedent for the move. Mountain bikers had been using hookless rims for years, as the transition to tubeless tyres had come to that segment of the sport much earlier than their road counterparts. The move to road tubeless, and the peloton’s (eventual) embrace of it, ushered in the opportunity for hookless rims to take hold on the road. The growing pains had hit the mountain bike world much earlier, too, allowing wheel manufacturers to solve the problem of 'burping', or the tyre momentarily dislodging from the rim and allowing air to escape. It all seemed poised for hookless success.

Hookless rims, it turned out, needed to work in close conjunction with high-quality tyres with beads that could form an airtight seal with the rim wall. And that's what happened: wheels and tyres worked together to create airtight seals that served mountain bikers well for years. But road adoption has been a bumpier ride.

A transition to pavement sparks controversy

The notion of implementing hookless rims in road wheels came about as tyre sizes started trending larger and air pressures lower. Test after test hit the media, extolling the rolling resistance advantages of going wider, at lower tyre pressures. Then came road tubeless, and the timing seemed perfect for hookless rims to make their pavement debut.

All of those trends played perfectly into the hookless paradigm. Improved efficiency, comfort, grip, puncture resistance, and control all come as part and parcel with hookless systems.

That, coupled with the growth of gravel racing, provided the spark wheel manufacturers needed to make hookless road wheels a reality. Since gravel tyres are generally wider and run at a lower pressure, the impacts of hookless rims weren't really felt widely one way or another. But on the pavement, in the peloton, hookless rims caused worry among racers.

Crashes are inevitable. But as the industry had seen when disc brakes were introduced to the pro peloton, it's easy to fixate on a scapegoat. Back then, any cut in a crash was attributed to the spinning blades of death, AKA disc brake rotors. Hookless rims took a beating in a similar fashion as a few high-profile crashes led to media coverage and rider complaints.

But crashes are complex things. "With any crash, there tend to be several variables that are in play," the engineering duo from Hunt Wheels highlighted. "So pinning a crash on one piece of the puzzle is always a bit dubious."

Lennard Zinn, framebuilder, author, and longtime VeloNews Magazine tech writer, recently wrote a series of newsletters on the subject of crashes that may or may not have been caused by hookless rims. And he noted a few key points:

"Where hookless versus hooked comes into play is a secondary effect from the reduced tyre pressure required on hookless wheels for safety, due to lower friction between the flat rim wall and the tyre sidewall (versus a hook-bead rim wall)," said Zinn.

"With lower tyre pressure, the cushioning of the rim from hard impacts is reduced, making cracking of the rim more likely. This is somewhat counterbalanced by hookless rim walls often being stronger than hook-bead ones due to being thicker, and having continuous fibres running up to and back down from the top of the rim wall, rather than ending at the edge of the rim hook. And we also now understand that lower tyre pressure often reduces rolling resistance, so riders might use lower tyre pressures on hook-bead rims for that reason, rather than being mandated to do so by the rim brand."

In other words, hookless rims work best in low-tyre-pressure setups, which can lead to harder direct impacts to the rim. But hookless rims are also built to be stronger than hooked rims in those high-impact areas. That said, a rider of a hooked rim can also run lower tyre pressures, leading to the same types of impacts. While brands are now starting to strengthen layups to match hookless rims, that's not always the case. And whenever a rim is subject to a large enough impact to take the wheel out of round, the tolerances are no longer consistent, which can lead to failures regardless of whether or not a hook is present.

"The problem in these crashes in which the tyre comes off of the rim hinges primarily on a carbon rim and a road tubeless tyre," said Zinn. "A hook-bead rim won't make it less likely for the tyre to come off than a hookless rim after an impact that cracks the rim. However, higher tyre pressure allowed on the hook-bead rim might prevent the rim from cracking in the first place."

Unfortunately, that runs counter to trends, in which riders are running lower tyre pressures to increase grip in corners and lower rolling resistance. It's one trade-off for another.

Cadex's Schneider agreed that the issue is far more complex than simply saying one type of rim is better than the other. "Rider concerns are understandable, but the discussion often oversimplifies what is, in fact, a systemic issue. The most credible position is not that concerns are imaginary, but that many incidents likely involve multiple contributing factors, such as tyre compatibility, bead stretch, pressure, installation, or use outside recommended limits, rather than hookless architecture alone. That is why blaming 'hookless' as a single root cause can be misleading."

There's a keyword there – systems – that perhaps owns an outsized heap of responsibility for the hookless debate we find ourselves in today.

The solution: compatibility and wheel systems

It's long been an inside joke in the bike industry: A new 'standard' comes along that's not standard at all. Company A develops a new technology, but companies B through Z don't offer complementary products that make adoption of the new tech easy for the consumer. It's happened over and over again, from bottom brackets to stems to headsets and even wheels and tyres.

Such is the saga with hookless rims. Wheel manufacturers were relatively quick to find a new way to make the wheel lighter, stronger, and more useful at low tyre pressures. The problem was that tyre development hadn't advanced just as quickly.

"The broader point is that hookless, hooked, and micro-hooked systems all rely on the same basic requirement: a precise rim–tyre interface and a tyre bead that remains within specification," said Schneider. "If the bead stretches beyond tolerance, retention can be compromised on any rim style. In that sense, this is more of a broader compatibility and standards conversation than a simple 'hookless is unsafe' conclusion."

It's no surprise then that Cadex has long promoted wheel 'systems' in which the tyres are developed to work specifically with the rims. That has never been as vital as it is now, with hookless systems. By controlling the tolerances and interface between both the rim and the tyre, a manufacturer can ensure the hookless system works reliably, and we don't find ourselves wondering if that tyre that blew off the rim at the most recent race somewhere in the mountains was due to a hookless rim, a pothole knocking a wheel out of round, or just pure dumb luck.

The problem is solved, right?

A few key problems are preventing widely adopted system integrations. First, few brands are set up for vertical integration – that is, wheel brands make wheels, and tyre brands make tyres. Few brands make both. That means various brands need to communicate with each other well to ensure tolerances are met or exceeded, often well beyond what ETRTO standards dictate. And while wheel company A may have the funds and desire to produce hookless rims, tyre company B may not be similarly enthused – or even able to produce a new-specification tyre to work with a system that may or may not end up sticking around long enough to provide a healthy return on investment.

The second problem is consumer sentiment. Interchangeable components have long been a part of cycling builds. Over the last decade or so, that has certainly changed, with the likes of integrated cockpits, internal cable routing, and other complexities making non-OEM parts less compatible with high-end bikes. Still, wheels and tyres have largely remained interchangeable.

Arguably, the debate around hookless rims might also be applied to hooked and micro-hooked rims. Tolerances must be tight for any of those systems to work. Hookless rims have brought that notion to the forefront.

Schneider summed it up nicely: "The industry has spent a lot of time debating hookless versus hooked, but the more important conversation is how precisely the rim, tyre, and pressure are engineered to work together as a system."

Systems require collaboration between wheel brands and tyre brands, or, for maximum consistency, vertical integration in which one brand controls the design and manufacture of both wheel and tyre. Easy peasy, right?