Because weather conditions can change in the middle of a stage, and drivers have to legally drive their cars on public roads, a rally tyre cannot be a true slick like in circuit racing. The tread pattern is asymmetric, with two longitudinal grooves running around the internal part of the tyre to help with water drainage on wet stages, while the flatter outer part of the tread maximises grip and lateral stability through the corners.
Finding the right compromise between rubber and drainage on the tread pattern, as well as the correct compound, is the fundamental key to speed and therefore success.This is what Pirelli has been working hard on throughout its WRC development campaign, but the whole point of rallying is that conditions are unpredictable and constantly varying.
For extreme wet conditions there is also a dedicated rain tyre: the Cinturato RW. This has a symmetric tread pattern with deeper grooves to clear standing water and reduce aquaplaning – and vital for really heavy rain. A rally car can carry up to six tyres: four on the car, and two spares in the boot. So sometimes drivers choose to take a very wide range of tyres to cover all potential conditions: even though two slicks and two wets, for example, would hardly be an optimal choice under normal circumstances. Their thinking is that even just two wet tyres, in the event of an incredibly heavy downpour, is better than no wets at all. Rallying is about improvisation.
As well as Croatia, Pirelli’s latest range of asphalt tyres will additionally be available on two other new WRC events this season in Belgium and in Japan, not to mention Spain – which returns as an all-asphalt event with its popular circuit-style super-smooth stages, having been a mixed-surface rally for the previous decade. Together, these rallies could well be among the most-action packed of the season. Gravel may be the predominant surface on the WRC, but on asphalt the margins are always so much finer…