BARCELONA, Spain (AP) — Tires shredded like paper, rubber flew everywhere and cars flitted in and out of the pit lane like supermarket shoppers in a car park.

On a frustrating day for drivers and fans alike, nearly 80 pit stops were made during Sunday's Spanish Grand Prix, again highlighting what has become a major concerns for Formula One teams five races into the season — the vulnerability of Pirelli's tires.

And with less than two weeks to go before the showcase Monaco Grand Prix, Pirelli director Paul Hembery acknowledged the company is struggling to find the root cause of the problem.

"At this rate, F1 is going to become a pit-stop contest with a few race laps thrown in," 1997 F1 champion Jacques Villeneuve said during television commentary.

Pirelli has been unable to resolve the problem and does not have much time left for modifications before Monaco.

"At the moment it is very much a stab in the dark," Hembery said, when asked what could be done to improve things.

Most drivers had hoped for a three-stop strategy on Sunday but instead had to settle for four.

"I'm not here to defend four pit stops. We know it's too complicated," Hembery said. "It wasn't what we wanted. However, there's also many factors behind that and we have to make sure we get it back in line with what we've been asked to do: which is two or three (stops)."

It was a confusing sight at the Circuit de Catalunya with leading drivers pitting as early as the ninth lap. And with so many stops, it's hard for spectators to keep track of which driver is actually in the lead.

"With this year's degradation and this year's tires we see the races keep changing all the time. Whoever keeps the tire alive normally is on the podium at least," said Fernando Alonso, who won the race. "If it's too much confusion for the spectators? There is no doubt. I think it's impossible to follow one race now. ... If I'm sitting in the grandstand, without any radio, telephone or something, you only see cars passing."

Strips of rubber were shooting off Jean-Eric Vergne's rear tire less than 40 laps into the race, and championship leader Sebastian Vettel, who was fourth, fears races are being more influenced by tires than driving skills.

"We are not going to the pace of the car, we are going to the pace of the tire," Vettel said, adding that Red Bull got its strategy wrong. "I think we tried to hang on to the three-stop for too long in the race and had to admit toward the end we wouldn't make it."

Red Bull's team principal Christian Horner echoed Vettel's concerns.

"They are black and round and called Pirelli," he said. "And whoever masters and understands these tires best and most consistently will emerge (victorious) at the end of the year."

Pirelli made modifications to its hard compound before the Spanish GP, and gave drivers an extra set of hard tires to practice on, but this made little difference on race day, where orders barked out over race radio were usually about sparing the tires.

"There's something wrong. This is the pinnacle of motor sport," former champion Jenson Button said. "We shouldn't be driving round so slowly to look after the tires."

Hembery, however, thinks some of the criticism has been unfair, especially as Pirelli has had to do testing on older F1 cars.

"It's all right to sit there and criticize. We're not exactly given the tools to do a precise job," he said. "We have absolutely no in-season testing. We don't have access to these cars that are going around now. We have to run around on a 2010 car."

Although Kimi Raikkonen chose a three-stop strategy on Sunday, finishing second, the sight of top drivers changing tires early seemed to cause a panic reaction.

"Sometimes it's driven by the strategy of the first people that pit. If anyone was thinking of doing three, it might have been influenced by everybody else coming in and already deciding to go on a four," Hembery said. "Having seen it though, four was probably the right call. Kimi was hanging on. We were missing four laps."

Even before Sunday's race itself, there were clear signs of degradation when Force India driver Paul di Resta lost the tread on his left-rear tire in second practice.

"Completely unexpected, in the middle of a high-fuel run," technical director Andrew Green lamented.

Pirelli believes debris was also to blame for both of Felipe Massa's punctures in the Bahrain GP three weeks ago. Fears that the tire is not resistant enough were also highlighted by Lewis Hamilton's left-rear tire failure during Bahrain practice.

"As long as it doesn't blow up at 300 kilometers per hour, that's OK. Touch wood, we've never had a problem with them for Lotus," Frenchman Romain Grosjean said. "But it's sure that you have to be careful, and it doesn't make things easy."