Graining. It’s a buzzword in the paddock at the Qatar World Endurance Championship event so far, at least when it comes to the Hypercar class. It refers to a particularly extreme form of tyre wear that is more than subtly different to more linear degradation seen over the course of a stint on a set of Michelins and then on through a double. But what is graining?
The phenomenon that is sometimes called cold graining rather than hot graining - the latter is not a term not used by Michelin - is caused by the tyre sliding across the track surface when it lacks grip because it is not yet up to temperature. It has been a regular topic of conversation since the ban on tyre warmers at the start of the 2023 season, which was temporarily reversed for last June’s Le Mans 24 Hours. But it has moved front and centre on the Hypercar agenda in Qatar because of the characteristics of the 3.37-mile Losail International Circuit that hosts the 2024 WEC season-opener on Saturday.
A track resurfaced for last season as part of a major upgrade of the facility is largely devoid of bumps, but more pertinent to the graining problem is the nature of the surface. It is ultra fine with none of the big lumps of aggregate that you can see embedded in the asphalt at Spa or Bahrain, but at the same time it is high grip.
“Here we have a very smooth surface that has a lot of grip,” says Pierre Alves, Michelin sportscar operations manager, who characterises the asphalt in Qatar as low energy, which is why more conventional deg levels are low. “What creates the graining is that the tyre wants to grip, but the tyre is cold and you end up with what we call micro sliding, a kind of skipping. This hurts the tyre and creates the lines or waves that we call graining.”
That contrasts with what happens on a coarser surface. “It activates the tyre,” explains Alves. That means more heat is induced into the tyre, less sliding and therefore a reduced chance of graining.
Once the graining has occurred, it compromises rubber life. The irregular surface created as little strips of rubber are torn off by the skipping effect results in a vicious circle: it causes the tyre to wear exponentially and with it performance, as one team principal put it, “to fall off a cliff”.
Source: Autosport