Blowout mystery to remain at Belgian GP start

Aug.23 - Drivers will start the Belgian grand prix unsure as to what caused Nico Rosberg's horror 300km/h tyre blowout of Friday.

Pirelli said on Saturday that it had ruled out a structural failure, while the FIA made tweaks to the kerbs at the fabled Eau Rouge corner ahead of qualifying.

As to the exact cause of the blowout, however, Mercedes' Rosberg said on Saturday: "It is still not understood.

"What is assumed is that it must have been something from the outside -- that is the theory. But nobody actually knows," he is quoted by Speed Week.

"So that is still worrying for everybody. We have done a few things to help it, but for sure we need to keep a close eye on it so that it doesn't happen again," the German added.

Another theory is that a part of the Mercedes bodywork flexed at speed and made contact with Rosberg's tyre, but - again - the German driver admitted there is "no evidence" of that.

Ferrari's Sebastian Vettel admitted he also is concerned.

"I wanted to know and so I asked," he revealed, "but I didn't really get an answer. So at the moment we have to live with that.

"Of course, we are all hoping that everything is ok in the race."

Mercedes chief Toto Wolff said his team worked closely with Pirelli to identify the cause.

"We did everything humanly possible so that we are leaving nothing out on the safety side, but we could find no problem," he said.

"So I am pretty sure when I agree that it must have been a case of an external source.

"There were already certain limits that are set by Pirelli in terms of how we can work with the tyres, but we were within those limits. And now we have gone a step further," Wolff added.

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