On the 5th stage of the UAE Tour Thomas De Gendt (Lotto-Dstny) crashed in front of the camera. Fortunately, the crash was not serious, but the camera footage caused a lot of discussion: it shows that the tyre slipped off the rim and the green safety foam was torn. Was the hookless rim to blame for the crash? The team defended the material used. Adam Hansen from the drivers' union CPA explained that the union was "100 per cent" against hookless rims. There was soon speculation that the wrong combination of rim and tyre had been used.
Because the discussion has taken on such a large dimension, there is now also an official statement from Vittoria: "It is important to clarify that the failure of the rim was caused by the impact with an object, in this case a rock, and has nothing to do with the hookless rim design. The force of the impact caused the rim/wheel to break in such a way that the tyre no longer remained securely in place. The force of the impact was so great that the tyre insert was torn.
It is important to emphasise that the accident was not due to compatibility issues between the components used by the team. The Vittoria Corsa PRO 28mm TLR has been extensively tested on various hookless, off-the-shelf wheels, including Zipp models (e.g. 353NSW). Compatibility has been thoroughly validated through laboratory testing, outdoor field testing and racing, with official communication via the Sram website. As a member of the ETRTO, we develop our products in strict compliance with ETRTO standards, which include both the current ETRTO standards and the ETRTO's Previous Standard Data (PSD)."
The accident has made hookless rims a topic of conversation. We explain the technical background:
TOUR Background: Hookless rims
This feature was removed from mountain bike rims years ago; the road bike rim is pretty much the last one that still has a hook. Rims on motorbikes, cars and aeroplanes get by without it, because the tyre is not supported by the hook, but by the rim shoulder. The hook is a back-up to prevent a tyre that is too wide from coming off a rim that is too small, especially with the high tyre pressure typical of racing bikes. On hooked rims, tyres can withstand around 20 percent more pressure before they run the risk of coming off the rim, according to measurements by Specialized and other manufacturers.
However, manufacturers now claim that the hook is not necessary as a safety feature. It is more important that the dimensions of the rims and tyres are precisely matched - which has not always been the case with tubeless tyres and rims due to a lack of uniform standardisation. This is why manufacturers only approve tyres for their hookless rims that they have tested. And these are exclusively tubeless tyres, because they stretch less and are therefore more dimensionally stable than conventional tyres. They must also be used if you want to fit an inner tube.
Only approved combinations
So you can't mount the tyre of your choice and inflate it as usual, but have to follow the list of products approved by the manufacturer. In view of these many restrictions, the road cyclist naturally wonders what is the argument in favour of hookless rims? You guessed it: the manufacturing costs. First and foremost is the simpler production of carbon rims in particular. This is because, unlike with metal, moulding the small hook from carbon means a lot of additional work. As a rule, an inflatable core is used to form the horn in the mould. Straight flanks, on the other hand, can be moulded with metal cores; this allows the carbon to be compressed more and makes the manufacturing process safer. "We save around ten per cent of the working time," explains Product Manager Sebastien Donzé from Zipp. "Above all, however, we produce fewer rejects and can therefore manufacture more cheaply. We pass this price advantage on to our customers." There are also other advantageous properties: The rim flanges are more robust against puncture damage with the same weight - or as robust as before, but lighter.
Point two for the hookless rim: A smooth rim sidewall constricts the tyre less laterally; the rim and tyre merge almost straight into one another, the tyre does not inflate over the rim to form a pear-shaped cross-section. This is theoretically good for aerodynamics and also means that the tyre flexes less in bends. The principle follows the successful trend in mountain biking: the rim is effectively wider, the tyre is supported wider, which creates a better overall compromise between directional stability and suspension.
Advantages of hookless rims
- More stable rim flange or lighter rim
- Wider support of the tyre
- Cheaper production (carbon)
Disadvantages of hookless rims
- Currently only a few tyres compatible
- Incompatible tyres can come off
