Technical FAQ: Why so many flats at ‘cross worlds?

Cyclocross world championships were marred by tons of flat tires. Lennard explains why the top riders didn't see the course's hidden danger.

Dear Lennard,
I read a few articles about the men’s CX worlds race, where punctures played a huge role. I read Lars Boom, for example, ran out of gear and had to DNF. Laurens Sweeck only was able to finish because his mechanics had his brother’s bikes/wheels nearby. And Gianni Vermeersch also ran out of wheels after 7 or 8 flats, and had his dad yelling at him on the last lap through the pit that he just had to keep riding on the flat.

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Is there a reason the mechanics wouldn’t use sealant once they start running out of wheels? If they don’t want the sealant in the tires during a race for weight or other considerations, fine. But once all the backup wheels are used I would think adding some sealant is better than a DNF!
— David

Dear David,
Well, putting sealant in them would have made sense to me, too, if those punctures had been from thorns or industrial garbage like sharp wires or metal shards. But those were from pinch flats, and sealant doesn’t help with pinch flats unless you completely fill the tire with sealant so that it also fills the hole on the rim side. And nobody would ride such a heavy, sloshy tire.

The bottom line is that, even though this challenging world championships course in Bieles (Belvaux in French) was in southwestern Luxembourg right on the French border and just a stone’s throw from Belgium, it was not a home course for the Dutch and Belgian riders. The adjoining part of Belgium is French-speaking, and the majority of the top cyclocross racers in the world instead hail from a band along the border of northern Belgium and southern Holland where Dutch (a.k.a. Flemish if in Belgium) is spoken. In that area, the courses are almost entirely on river-delta land resulting in very high sand content in the dirt, and riders grow accustomed to running very low tire pressures.

This course, by contrast, was a manmade course built from scratch on a temporary park at the site of former steel factories and iron-ore mines. On its eastern edge is the University of Luxembourg and Belval, a new high-tech scientific and cultural development whose name may intentionally conjure up images of Silicon Valley rather than of a steel mine. The course lies on “brownfield” land being reclaimed from its prior industrial use and has the potential for hazardous waste in the soil. Perhaps for this reason as much as because race organizers wanted to build a super-challenging course, much of the dirt to build the course was hauled in. In fact, workers were still moving dirt around on the course and packing it down last Wednesday, just before racing commenced.

Had the Belgian and Dutch team mechanics dug down into that recently placed dirt making up the course, they would have found that it is not all clay, as it appeared on the surface. Rather, there was lots of fractured hard rock in 2-3cm-width chunks mixed into it. This might have informed their tire-pressure decisions.

Prior to the elite men’s race, there was not much warning about the carnage to tubulars that was to ensue on Sunday afternoon. On Saturday morning, the junior men raced on a packed, frozen coursef where some of the biggest crashes were on the paved sections due to being coated with ice. The three British rides who swept the medals were all riding file-tread Challenge Dune tubulars meant for riding in sand. The course had softened up for the elite women Saturday afternoon, but the mud was not deep enough to necessitate that everyone ride mud tires; riders were on everything from file-tread-with-side-knob tubulars like Dugast Pipisquallos and Challenge Chicanes, to general wet-condition tires like Challenge Baby Limus, to full-on mud tires like Dugast Rhinos and Challenge Limus. Because the mud was not deep, riders had not moved it around enough to expose those jagged rocks mixed in with the fill dirt.

Early on Sunday, Dutchman Joris Nieuwenhuis won the U23 race with a perfectly-executed solo ride with no mechanicals, so even though some of the riders behind him suffered multiple flats, the alarm bells had not properly sounded for the elite men’s mechanics to take the notice that they should have about tire pressure.

The temperature had increased yet more by the time the elite men took the line, and the ruts were already getting deep down into that fill dirt. Having seen how Nieuwenhuis had never been caught once he got away, the pace was fierce from the gun to prevent Van der Poel or anybody else from doing the same thing. This meant that riders were hitting the muddy ruts very hard and in such close quarters that they were not getting a good look at the track as their aggressive riding continually exposed more of the sharp rocks. The tubular carnage was extreme. In addition to the riders you mention, I heard that the BKCP team of the Van der Poel brothers, Philipp Walsleben, etc. had around 100 flats for its riders alone on Sunday; that may have included some in the U23 ranks.

One mechanic, however, was paying attention, and that may have made all of the difference. Wout Van Aert’s father, after his son had flatted one of his Michelin-Mud-tread-on-Dugast-carcass tubulars, reportedly increased his son’s tire pressure, and the rest is history.

Another issue is the change in rim width and its divergence from the width of the tubular base tape. In my December 13, 2016 column, I discussed why a tubular needs base tape, and this race provided a perfect laboratory to study the issue. Tubular base tapes have generally been 24mm wide, because rims were 21mm wide or so. Challenge recently went to 26mm base tape, but that still does not keep pace with today’s super-wide rims. You can bet that the rim companies didn’t consult with the tubular tire makers before making this change in design, either.

If there is no base tape between the tubular’s sidewall and the edge of the rim, and a big Belgian slams it at low pressure into a sharp rock, there is very little protection for the tire. Not only can the latex tube get a big snake bite in it, but, especially with some of today’s wide rims with sharp edges, the casing can even get cut.

The solution is not just blunter rim edges and wider base tape. If tubulars all had 30mm-wide base tapes, when used on narrow rims, they would gain unnecessary weight and lose performance. Maybe we’ll see a day coming where the same tire model comes with two different options on base-tape width.

As for your original question about sealant, even if the riders had instead been digging up buried thorns or pieces of wire and metal shards in the dirt, I can see lots of reasons for why mechanics still might not have reacted properly by injecting sealant into their tires. The answer could be as simple as that mechanics generally don’t bring sealant with them in the pits.

Imagine the situation in the heat of the battle on a tightly-crowd-controlled course at the biggest race of the year. After his rider had dumped off a bike with a flat tire, the mechanic would have made a quick wheel change and then would have run to the power washer to clean the bike up and give it a quick lube and a once-over check in time for the rider to be back a half lap later (under four minutes) in case they need that bike. Then if the rider comes in with another flat, it’s the same routine for the next frenzied four minutes.

There is generally not emotional space in those conditions to make a cool-headed assessment and say, “Hey, this guy’s getting a lot of punctures from sharp objects; I ought to put some sealant in the tires I have left.” And even if the mechanic had done so, there is likely not time or materials at hand to remove the valve cores on the remaining spare wheels, squirt a measured amount of sealant in, replace the valve core, and re-inflate the tires to the precise pressures the rider has demanded, while still being at the ready every half lap with a clean bike for the rider to grab. Furthermore, there may not be extra personnel around with UCI credentials allowing them to go back and forth to the pit to fetch a bottle of sealant from the rider’s truck, for instance.

And to fix tires that had already come in would have required some non-traditional thinking ahead. Rather than heaping flatted wheels in a pile or sending them off to a truck with a runner to get them out of the way in the pits, they would have to think, “Hey, I might need that tire again.” This is not something mechanics in the middle of a race tend to do. When it’s failed, it’s failed, so it’s pushed aside and out of mind. And it takes some time and undivided attention to reseal a punctured tire to the level of certainty of its ride-worthiness required to be willing to hand it up to a rider in the world championships. Nobody wants to be taken to task later for making an out-of-the-box decision that might not work out and could be second-guessed later with the benefit of hindsight.
― Lennard