Formula 1 tyre supplier Pirelli has presented its findings following an investigation into the causes of two tyre failures across the Belgian Grand Prix weekend.
Nico Rosberg suffered a failure during Friday practice - which was initially attributed to external cuts - while Sebastian Vettel's incident came on the penultimate lap of the race when he was attempting a one-stop strategy.
Ahead of the Italian Grand Prix, Formula 1's governing body the FIA issued a statement containing Pirelli's conclusions:
- Tyres used by different teams during the race were examined by cutting sections through the tyre, none showed any signs of internal delamination (hence fatigue)
- Tyres used by different teams during the race were run on a test rig and were subjected to a representative race duty cycle, again none showed any signs of internal delamination (hence fatigue)
- A highly unusual number of cuts were reported by Pirelli engineers during the Spa weekend, one of which reached the belt and caused tyre failure on Nico Rosberg’s car
- Microscopic examination was conducted on sections taken through some of the cuts found in tyres used by different teams in the race, the results showed one other case where the cut had reached the belt without causing failure
- The remaining tread thickness on Sebastian Vettel’s rear tyres was approximately 30% at the time of the failure making the tyre more susceptible to damage from even small piece of debris.
Pirelli issued a further statement shortly after the FIA's announcement and the Italian company suggested that excessive debris, combined with prolonged tyre use, led to problems.
It confirmed that it suffered 63 cuts in the tyres across the Belgian Grand Prix weekend, compared to an average of 1.2 across the previous 15 races and tests.
"The external factors are demonstrated by a total of 63 cuts found in the tread of the Formula 1 tyres used over the course of the Spa weekend, following numerous incidents that took place during the support races before the Formula 1 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 Vettel’s tyre at Spa.
"As for 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 said it is satisfied with Pirelli's findings, while Formula 1 teams have been told that minimum tyre pressures will be raised for this weekend's Italian Grand Prix at the high-speed Monza circuit.
Pirelli added that it advised the FIA to clean the circuit following Saturday's qualifying session - which it duly did - and that teams were also told.
Pirelli it intends to work with the FIA to find a way in which circuits can be cleaned more effectively in the future.