Pirelli says the tyre failures seen at the Belgian Grand Prix were due to an unusually high number of cuts from debris on track.
Nico Rosberg suffered a spectacular failure approaching Blanchimont during Friday practice which Pirelli at the time blamed on an external influence. In the race, Sebastian Vettel was also hit by a tyre failure on the penultimate lap of the race, leading to him firing an expletive-ridden attack at the tyre manufacturer.
Pirelli initially said Vettel's failure was due to the tyre reaching its end of wear life as Ferrari attempted a one-stop strategy. However, following a number of investigations which Pirelli says "have confirmed the absence of any structural problems", it now points to how dirty the track was as a factor in a high number of cuts.
"The events of Spa can therefore be put down to external factors, linked with the prolonged use of the tyres on one of the most severe tracks of the championship," Pirelli said in a statement.
"The external factors are demonstrated by a total of 63 cuts found in the tread of the Formula One tyres used over the course of the Spa weekend, following numerous incidents that took place during the support races before the Formula One grand prix. In the previous 15 events (10 races and five test sessions) an average of only 1.2 cuts per event were noted.
"All this indicates an anomalous amount of detritus on the track in Spa, with a consequent increased risk of encountering a foreign object.
If even a small piece of debris – made of carbon or any other particularly sharp material – penetrates and cuts the various structural parts of a tyre (which is obviously subject to high-speed use, and more susceptible if used for a prolonged period) without penetrating the actual structure, this can cause a failure that is different to that found in the event of a normal puncture, which is characterised by a loss of tyre pressure. And the former was the type of event seen on Sebastian Vettel’s tyre at Spa.
"As for Nico Rosberg, in whose case the tyre usage was less, the tyre held up – as the footage clearly shows – and the failure was not instantaneous. For four corners previously, an element of the internal structure of the tyre was visible, coming out of the tread pattern. This highlighted the existence of the damage and the consequent start of the tyre’s attrition.
"Throughout the Spa weekend (including practice, qualifying and the race) cuts caused by debris were found on the tyres of other drivers, which damaged the construction but did not cause any failures."
The FIA also issued a statement after Pirelli presented it with the information, revealing there was one other case at Spa of a cut reaching the belt - as it had on Rosberg's car - but without causing a tyre failure.
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