Reigning world champion Lewis Hamilton says his pre-season testing is being held back by Pirelli's 'rock solid' Formula 1 tyres not working as he would like in cold conditions.
The Mercedes driver set the third fastest time at Barcelona on Saturday on the medium compound, but he was far from happy about the way the tyres were behaving.
"It was a hard day," said Hamilton, who has been battling illness this week. "I was glad I got 100 laps and glad I got time in the car.
"The guys are working hard here and back at the factory, and we are trying to get mileage.
"But I would say the biggest issue is the tyres. They don't work, they are so hard. They are not spectacular."
Hamilton said that he struggled for most of the day to get the tyres in to the right operating window.
"If you are not weaving - you come in [the pits], and the tyres might be 80-degrees so it is hot but they don't work and they are not in the operating window hardly ever.
"Then one lap you might feel it is not so bad, but then it drops off.
"I also ran the hard yesterday and it was even harder.
"My first impression of the tyres is not spectacular, but maybe when we get to hotter conditions it will be better."
Hamilton believes that the fact that he was having to concentrate so much on managing the tyres meant that there was still plenty to be learned about the new Mercedes W06.
"It feels like we could know more," he said. "There is always more.
"We could do with more days, lots more tyres, good tyres, better tyres.
"[Tyres] that work all the time when they go out, so you can actually go out and pound around and make set-up changes. That is what testing is all about.
"Testing is a lot different these days. You are trying to balance it with mileage.
"But the tyres are rock solid and they don't really work very often. Well, they haven't been for me at least."
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