Another tubeless tire coming off the rim in a major pro race

Also, Challenge TLR tubeless tire mounting video and other tubeless mounting tips

Dear Readers,

Well, the inevitable happened again, namely a tubeless tire coming off the rim at a critical point in a pro road race, this time in the Étoile de Bessèges stage 2 sprint finish, where Kern Pharma rider Marc Brustenga’s tire came off, sending him careening into the barriers and down hard onto the tarmac.

A statement from his team said it was due to hitting a hole in the road moments prior. In the same statement, Giant, whose Cadex 50 Ultra Disc Tubeless wheels he was riding, said it was, “not the result of a tyre or rim malfunction.”

Indeed, as you can see from the crack in the rim in the above photo, a tubeless tire was sure to deflate from that rim damage. In my opinion, it is further evidence of an insufficient margin of safety for road racing using the combination of tubeless tires on carbon rims.

As discussed last fall in an interchange with Zipp here, the only thing keeping air in a tubeless tire is the seal between the tire bead and the rim sidewall. When hitting something hard enough, a carbon rim will crack rather than bend, and that will also break the air seal.

Furthermore, the crack in the rim will cause the circumference of the rim bead seat to decrease, since tension in the spokes will pull the weakened rim inward.

Since the rim bead seat is now smaller than the tire bead (when new, they are matched in diameter at 622mm), the rider’s weight on the tire will push one or both tire beads into the rim valley. The rim valley’s diameter is smaller than the bead seat diameter to facilitate installing and removing the tire. That diameter will have become smaller yet due to the rim crack, making it easier yet for the tire bead to come over the rim wall. Add to that the side forces involved both in sprinting and in turning a bit in an attempt by Brustenga to pass Decathlon AG2R La Mondiale’s Sam Bennett in the sprint, and you have all of the necessary factors lined up to pull that tire off the rim.

One obvious takeaway when riding tubeless tires is to stop immediately after hitting an obstacle with enough force to crack the rim. However, it is not in the mindset of a sprinter fighting for position in the final meters of a race to analyze how hard he just hit a hole and whether it is prudent to continue racing for the line. The rider clearly did not think he had hit it hard enough to necessitate stopping; as his team said, “A few hundred meters before the puncture, Brustenga entered a roundabout and hit a hole, which slightly damaged the Cadex rim due to the speed. However, he felt that he could continue riding, and he did.”

I believe that an inner tube in that tire or a tubular tire glued onto a tubular rim would have had a much higher chance of maintaining the air pressure in the tire and hence keeping the tire on the rim and the rider on his bike. The margin of safety is less with a carbon rims paired with tubeless tires than with tubed clinchers or with tubulars, IMHO.

And as long as we’re on the subject of tubeless road tires, here is that video I promised to post back in the fall and forgot. It is a video of how I mount a Challenge TLR tire. As you can see by my struggles in the video, those tires are notoriously hard to mount due to both the tight bead and the “open tubular” construction, which results in a flat, horizontal cross section rather than a cross section like a “C” rotated downward. The flat “open tubular” profile also makes it next to impossible to install a tubeless insert inside the tire.

And below are some tubeless mounting suggestions from Tim Voegeli of Tubeless Solutions, who makes the tire levers that made it possible to mount that Challenge TLR tire, as well as the rotatable tire-mounting jig I use to hold the wheel