Unexplained tubeless tyre blowout causes heavy crash in sprint finish at Étoile de Bessèges

In the sprint finish on stage 2 of the Étoile de Bessèges, Marc Brustenga (Equipo Kern Pharma) was sitting in 10th wheel with 100m to go when he inexplicably veered left, lost control of his bike, and crashed into the barriers.

At first, it seemed as though the Spaniard had crossed wheels with Sam Bennett (Decathlon AG2R La Mondiale) as he tried to pass but the overhead shots told a different story. 

In the video below from Eurosport, at 43 seconds, a small white cloud emanates from Brustenga's rear wheel just as he leans left to pass Bennett. 

The reason for this is that his tubeless tyre had punctured, with the white tubeless sealant inside exploding out in an instant, failing to seal whatever damage had been done to the tyre. 

Images captured of the sprint finish show Brustenga in the background, midway through his incident with the bike bouncing through the air, his rear tyre ejected from the rim and instead entangled around the frame of the bike.

Luckily, despite the swirling peloton powering toward the finish line behind him, he was the only rider to go down, and importantly, a subsequent tweet from his Equipo Kern Pharma team's X account has confirmed the Catalan rider came away with "many abrasions and injuries" but otherwise "fine and able to cross the finish line." 

The cause for the blowout is currently unknown, and it is unclear from the footage of the incident as to whether the tyre came off the rim immediately, leading to Brustenga going down, or whether it was shod in the aftermath as a result of hitting the barriers, as was the case in Annemiek van Vleuten's not-dissimilar crash at the 2022 World Championships. 

It's also unclear what caused the initial puncture, but a likely scenario is that Brustenga's tyre hit something sharp in the road, such as a stone, that the sprinters ahead of him managed to avoid.

Most often in such a case, assuming the puncture itself is small enough, the tubeless sealant within the tyre would clog up as it was forced out through the hole, in turn sealing the puncture and allowing the rider to continue safely. 

That doesn't always work as intended, of course, but even in the event of a complete loss of air, tyres are designed to stay on the rim in order to allow the rider to remain in control the bike and come to a stop safely. 

But as we've seen in incidents such as Thomas de Gendt's tyre blowout in spring 2024, the tyre can sometimes be shod from the rim. That incident, which featured a Zipp wheel, led to a heated debate on the safety of hookless rims, prompting the UCI to announce an urgent investigation. However, it was later proven that the rim in question had suffered structural damage when De Gendt hit what he claimed was a rock in the road. 

It's unclear whether Brustenga's rim - in this case what appears to be a Cadex 50 Ultra tubeless wheel, also hookless in design - suffered structural damage or not.