BARCELONA, Spain -- Understanding the behaviour of Formula One tyres is never easy, but the basics are pretty simple: the softer the compound, the quicker the tyre over a single lap. Yet in Friday practice for the Spanish Grand Prix, four of the top six drivers set their quickest lap on the soft compound and not the super-soft.
That partly down to mistakes on flying laps, which weren't helped by windy conditions and the drivers getting used to Barcelona's recently laid tarmac, but for six of the world's best drivers all to struggle on the same compound hints at a common issue.
"I didn't really feel any gain in grip with the super-soft compared to the soft or the medium compounds," said Valtteri Bottas, who set his fastest time of the day using the soft in the first practice session. "I think this will be the key issue for us tomorrow."
Vettel, who set his fastest time on the super-soft in the second session but seemed out of sorts throughout the day, added: "Everything that we put on the car seemed to work, so that's important. I think the biggest change to be honest came from the tyres. They seem to be quite a bit different."
Pirelli has reduced the tread on all compounds by 0.4mm this weekend in order to combat the overheating and blistering some teams suffered in testing on the circuit's new smooth track surface. But that shouldn't impact one compound over the others and Pirelli's racing manager Mario Isola suspects it is down to the characteristics of the super-soft compound.
"For me it will be very interesting why we have this situation," he said. "The super-soft at the moment is giving a very small [performance] delta [over the soft] -- OK degradation is very low -- but it seems drivers are preferring the soft.
"One explanation, but again I need to check the data, is that maybe the super-soft is a little bit like jelly -- moving around -- and this is a feeling the drivers don't like. They prefer a tyre that is not moving, so it's more solid and more precise and they know better how to manage the tyre a little bit. They are Formula One drivers so they feel everything, so to have a tyre that is more solid and consistent is important."
Asked if that jelly-like feeling could be a result of the thinner tread, Isola added: "I don't think it is the thickness that is changing the performance. The reduced thickness should help as it reduces the movement. And it is the same for all three compounds."
Given that there is only 0.4s performance difference between the super-soft and soft, expect to see the top six attempt to make it through Q3 using the soft tyre. That would then set up a one-stop race using the soft and medium compounds and eliminate using the super-soft at any point other than Q3. However, Friday practice can be misleading and changes to track conditions over the remaining two days of the race weekend could have a big impact on performance.
"If we have cooler conditions, this will help the super-soft because the super-soft has a lower working range," Isola added. "Being so soft and with the lower working range, it is quite easy to overheat the super-soft.
"During the weekend we have two possible conditions that are helping the super-soft. One is the rubber on track, which is generating less sliding so you generate less temperature in the tyre and the other one is the weather. If the weather is cooler, this helps the super-soft."
Final practice on Saturday morning should give the teams a better indication of how the tyres are performing. But don't be surprised if teams attempt to limit their time on Pirelli's fastest compound this weekend.