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.
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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”.
“When there is no graining, the tyre can last at least two stints or even three here,” explains Alves. “But if you have graining the tyre will wear very quickly and last for just one stint. The graining does not go away as the tyre wears: it will remain for the rest of the tyre’s life.”
All nine manufacturers competing in the Hypercar class this year appear to have been hit to a greater or lesser extent by graining since the start of the pre-season Prologue test on the Losail circuit on Monday. Worse afflicted appears to be Toyota. It probably explains Kamui Kobayashi’s comment that he was “driving like an amateur” and “driving differently every lap” in the wake of the two-day test.
It is no coincidence that the Toyota GR010 HYBRID Le Mans Hypercar is the heaviest car in the field. It has been given a 1089kg minimum weight under the Balance of Performance announced ahead of the Prologue. That makes it 19kg heavier than anything else in the field baring the Tipo 6 LMH Competizione fielded by garagiste Isotta Fraschini.
New Toyota Gazoo Racing Europe technical David Floury insisted that he wasn’t going to go into detail on the reigning champion team’s woes over the course of the two-day Prologue, but he did admit “with the graining we have seen weight has an effect”. Put simply, a heavier car slides more than a lighter one on cold rubber.
Cadillac, on the other hand, has stated that it isn’t experiencing any graining. That’s at least on the Michelin tyre known as the hard, as opposed to the medium, the other spec available for this race. It is probably no coincidence that the Cadillac V-Series.R is the lightest car in the field along with the Peugeot 9X8 LMH at the class minimum of 1030kg.
Porsche, which is somewhere in the middle on weight with its 963 LMDh at 1048kg, did experience graining when it tested in Qatar back in November. But it reckons it has largely overcome the problem, says Porsche Penske Motorsport boss Jonathan Diuguid.
“We’ve made some set-up changes and it seems to have gone,” says Diuguid, who hinted that the Porsche was also looking good on the medium tyre. “You either go slower on the first laps on the tyre or you try to control the way the temperature builds with set-up.
“There’s definitely more rubber down now, because when we tested there were just four cars on track. That could have an effect. It’s a very complicated problem to solve and then to understand when it has gone away. We’ve tried not to overthink it.”
The message from Michelin is simple. “The solution is to manage the warm-up of the tyre properly,” says Alves. “Then you don’t have this micro sliding and therefore no graining.”
Michelin might be urging calm, but there is time to be gained on initials laps out of the pits on fresh rubber. And then lost somewhere down the line if graining occurs. There’s a fine line to tread.
