The UCI has a new weapon in the war on tyres that are a little bit too wide

At the start of the men's Paris-Roubaix in Compiègne, the UCI was spotted with yet another tool to ensure the riders' bikes stay within its equipment rules.

It's a small, 3D-printed hand-held unit that commissaires were seen slotting over riders' tyres, seemingly to check their size.

At first glance, it appears very similar to the tool used at cyclo-cross races to check racers' tyres don't exceed the enforced width limit of 33mm.

But in road racing, there isn't a width limit per se.

There is, however, a rule limiting the maximum diameter of the wheel-tyre system, which means by proxy, given a wider tyre is also taller, a limit that works out at approximately 39mm. It does depend on rim width, though.

Checking the total diameter of a wheel tyre system at the buses at the start of a WorldTour race would likely be a much more time-consuming process, which would make it impossibly difficult to do across 25 teams quickly.

Given that road wheels are all made to an equal diameter of 622mm, and the total system is simply this plus the tyre top and bottom, it's a logical way to measure the total diameter more easily by proxy.

Cyclingnews understands this is what's being measured by the commissaires, although it's unclear whether the tool is measuring the tyre height itself, or simply using a nominal width (perhaps 40mm) as a proxy for an illegal height.

Witnessing the checks in action, Cyclingnews tech reporter Will Jones said the commissaires didn't appear to be checking diameter at all. "They just popped it onto the tyre and then, if it didn't get stuck, moved onto the next bike," he said.

Paris-Roubaix is likely the only race we'll see this tool in action each year. At most road races, teams will run tyres between 28 and 30mm, but the boneshaking nature of the Roubaix cobbles means teams are motivated to go wider, and push up against this limit.

When we caught up with Tadej Pogačar's bike ahead of the race, we spotted a 35mm tyre at the front that came up far wider, in one of a horde of tech updates as he 'threw the kitchen sink' at his setup to chase his third monument of the season.