Was Vettel right to attack Pirelli?

For 41 of the 43 laps that made up the 2015 Belgian Grand Prix, Ferrari looked to have pulled off a masterstroke in damage limitation. Sebastian Vettel was clinging on for dear life in third place, on a weekend where the Prancing Horse had otherwise looked a little lame.

Then the horse did go lame. Vettel's heavily worn right-rear tyre exploded on the Kemmel Straight on lap 42, robbing him and his Ferrari team of a podium finish behind the dominant Mercedes of Lewis Hamilton and Nico Rosberg, and leading Vettel to launch a furious rant against the integrity of tyre supplier Pirelli.

It wasn't quite Silverstone 2013, but Vettel's outburst recalled a time when Formula 1 became obsessed with the structural rigidity of Pirelli's tyres, and whether they were so weak as to be exposing drivers to unnecessary risk following a spate of spectacular failures during that year's British GP.

Back then, Pirelli concluded a combination of tyre-swapping (the process of running tyres on the opposite side of the car they were designed for, which placed undue pressure on the wrong sidewall), extreme camber settings and low tyre pressures used by some teams, and drivers clattering across some of Silverstone's rougher kerbs, swelled into a perfect storm of destruction.

So it altered the construction of the tyres (replacing steel belts with Kevlar ones) and the FIA banned tyre swapping and began enforcing Pirelli's limits on camber settings and tyre pressures, though Pirelli says a further request to cap the total number of laps permitted on each type of tyre at a given grand prix was ignored.

Rosberg wasn't satisfied with Pirelli's explanation for his Friday failure © XPB

Since then, things have calmed down considerably, and Pirelli has suffered no repeat of those kinds of failures. Instead, complaints from drivers have generally related to the compounds, usually accusing the tyre supplier of being too conservative with its choices.

The problem at Spa this year was that two drivers suffered two separate failures of their right-rear tyres at different points in the weekend, and each had no clear, definitive explanation.

First, Nico Rosberg had a blowout approaching the high-speed left-hander at Blanchimont during the second free practice session on Friday. Pirelli's detailed investigation concluded that something external cut Rosberg's tyre and that it had not failed because of some structural integrity problem.

Trouble was, Pirelli couldn't explain where this 'external cut' came from exactly. Rosberg hadn't obviously gone wide anywhere, or run over any debris. This failure does not relate to that suffered by Vettel in the race, except that Pirelli's conclusions did nothing to reassure Rosberg, and clearly this also played on Vettel's mind too.

After he suffered his own spectacular blowout on lap 42 of the grand prix, Vettel was forthright in labelling a situation where tyres could suddenly explode, without warning, at speeds approaching 200mph "unacceptable".

WAS FERRARI PUSHING THE LIMITS?

Pirelli motorsport boss Paul Hembery said immediately after the race that Vettel's failure was the result of excessive wear.

Ferrari said one stop was its plan from the outset © XPB

Vettel started the Belgian GP down in eighth place, after making a mistake on his best lap in qualifying, and Ferrari team principal Maurizio Arrivabene confirmed the Scuderia entered the race with a "plan" - decided at "11 o'clock this morning" - to try to get Vettel to the end using an unconventional one-stop strategy.

The quadruple world champion made that sole pitstop on lap 14 of 43, which meant he had to make a set of Pirelli's medium-compound tyres last for 29 laps.

Pirelli's projections suggested that sort of stint was easily possible, and Arrivabene insisted Ferrari's strategy hadn't been "stupid or crazy", and that Ferrari's own Pirelli engineer had not indicated any potential problem from attempting it.

Hembery suggested Vettel's tyre failure was clearly the result of wear, despite the stint length being within Pirelli guidelines. The problem, he said, was one of real world experience not matching up precisely with simulation.

"Wear life was indicated at around 40 laps, but it's an indication, as race conditions can change, and some factors involved in racing mean it's not a precise data," Hembery explained.

"We give indications and the teams have to make decisions based on the data they have. Before the race we were obviously talking two stops, and people were saying if they needed more margin, three. It was never discussed about a one-stop strategy, so that was always going to be on the limit."

But Lotus head of trackside operations Alan Permane said he would be "very surprised" if wear was the cause of the failure, because the performance of Pirelli rubber usually drops off massively when it wears out. Vettel was lapping at similar pace to other medium runners when his tyre gave out.

Pirelli personnel take Vettel's damaged tyre away for investigation © XPB

Hembery conceded Spa is a circuit that "creates enormous loads into the structure of the tyre" but he insisted "in this case that wasn't the issue, it was more to do with wear. If you look at the images, the carcass was still intact. It was a wear issue".

The implication was that Ferrari simply gambled on a strategy that was more aggressive than the norm, seemed perfectly feasible based on simulation, but turned out to be just beyond possible in reality.

You could argue Pirelli needs to beef up its simulations, but the tyre supplier will rightly complain that a lack of testing in modern F1 hampers efforts to make accurate simulations, and thus predictions.

Because Ferrari's failure happened at a high-speed part of a high-speed track, on the same weekend as another failure, while using tyres from the same supplier that provided rubber when tyres were exploding too often a couple of years ago, Vettel lost his rag. Understandable in the circumstances.

So if it wasn't fully the fault of the tyre supplier, the team or the demands of the track, what else could have been at fault here?

A raised 'sausage kerb' was installed at the top of Eau Rouge before the weekend began, but was removed on Friday evening after a sequence of high-flying incidents in the GP3 support series and complaints from some F1 drivers. This allowed them to again cut the final left-hander at the top of the hill (Raidillon) before joining the Kemmel Straight.

Vettel insisted he hadn't left the track during the race, but replays showed clearly that he ran all four wheels outside the white line that defines the edge of the circuit to 'straightline' Raidillon. Perhaps traversing this (slightly) rougher surface on heavily worn rubber contributed to his downfall?

Sebastian Vettel, Ferrari, leads Romain Grosjean, Lotus, Belgian GP 2015, Spa © LAT

It's a view supported by AUTOSPORT technical expert Gary Anderson.

"To me it looked like delamination of the tread starting from the outside shoulder - we saw this a few years ago when the treads started to come off," he says.

"I don't know what warning the team would have had - the pressure would have been fine, probably even for a few seconds after the tread came off, and again the temperature in that area probably wouldn't have shown anything.

"The question is: did they go far enough to wear out the tyre tread, exposing the carcass? I personally don't think so, because that's something that gets monitored very closely by both the team and Pirelli, and also - because of the camber - the highest wear should be on the inner shoulder.

"Vettel was fairly annoyed by it all, and rightly so, but he did say that neither him nor Rosberg on Friday went off the track. I think they should both sit down and have a look at the replay.

"If I was involved in the decisions I would instigate that 'track limits' means the complete car has to be within the white lines at all times."

ALL ABOUT THE BATTLE FOR THIRD

Perez edged into a very slight lead for a moment on lap one © XPB

Whether it was a failing of team, tyre supplier, driver, circuit, the FIA in not enforcing track limits, or a combination of all of the above, Vettel's tyre explosion was the final act in a thrilling fight for third place that lit up an otherwise routine race at the front.

Any one of Vettel, Romain Grosjean (who eventually claimed that spot to record his and Lotus's first podium since the 2013 United States GP), Force India's Sergio Perez, Red Bull's Daniel Ricciardo, or the Williams of Valtteri Bottas could have staked a genuine claim to the place, but all bar Grosjean ran into trouble of one sort or another that extinguished their hopes.

Perez (fourth on the grid) made the early running, and even led the race very briefly on the first lap, edging ahead of Hamilton's Mercedes before they braked for Les Combes.

But Perez ultimately lacked the pace to keep up with the other frontrunners and slipped back as the race wore on, eventually finishing fifth at the head of a train comprising Felipe Massa's Williams, Kimi Raikkonen's Ferrari (recovering after a gearbox failure in qualifying) and Max Verstappen's Toro Rosso.

Ricciardo (sixth on the grid) jumped Perez with an early pitstop on lap seven, but was easily repassed by his Mercedes-engined rival on the Kemmel Straight five laps later.

The Australian remained within a few seconds of a podium spot until lap 19, when what Red Bull team boss Christian Horner described as a "complete failure" of Renault's ERS system stranded Ricciardo on the inside of the circuit at the exit of the Bus Stop chicane.

By that time, the hopes of Bottas and Williams of scoring a podium finish were also in tatters. The Finn was favourite to be best-of-the-rest after qualifying third, but he made a bad start and struggled to get his soft tyres 'switched on' early in the race.

Bottas found himself on mismatched tyres after a pitstop © LAT

He was still in the mix until the first round of pitstops, when his team fitted a rogue medium tyre to the right rear of his car while putting the intended new softs on the other three corners. The resultant drive-through penalty put him out of contention too, though Bottas reckoned he wasn't fast enough anyway.

"The first stint we were really struggling with the tyre," he explained. "People were passing us. The problem at the pitstop cost us a lot - looking at the time without penalty would have been fifth or something - but the pace was not enough for top three, which was our goal. Definitely not our day."

Apart from Hamilton, the day actually belonged to Grosjean, who produced a faultless drive to claim that final podium position.

The Franco-Swiss qualified his underdeveloped and underfunded E23, which worked well with the low-downforce demands of Spa, fourth fastest, but a penalty for an unscheduled gearbox change after Friday practice relegated him to ninth on the grid.

He made no progress on the first lap, but thereafter picked his way expertly through the frontrunning group with a succession of DRS-assisted passes along the Kemmel Straight and into Les Combes, and was challenging Vettel's Ferrari when the SF15-T suffered its spectacular fate late in the race.

"It has been an incredible weekend for us," said a delighted Grosjean. "I still can't believe we're on the podium. Those guys have been working hard to give us that car to be able to be here.

Grosjean's podium was a huge morale boost for Lotus © LAT

"Of course going into Turn 1, every time I make a start at Spa I remember 2012 [when he was banned for causing a huge first-corner accident], but it has made me stronger and [able to] be on the podium 10 times.

"Being here today is special and has the prize of a race win."

The actual win belonged to Mercedes, and specifically to Hamilton, who was equally faultless in recording the 39th win of his grand prix career.

The race was pretty much in Hamilton's pocket once Rosberg "completely messed up the start", which was nevertheless not dramatically affected by new rules demanding less assistance for the drivers from the pitwall, or the need to complete a second formation lap when Nico Hulkenberg's Force India broke down and caused an aborted getaway.

"Nico had good pace, but I was able to answer most of the time so I was fairly relaxed at the front," said Hamilton, who was more comfortable in leading Mercedes' 18th one-two in the last 30 F1 races held than his final 2.058s winning margin suggested.

"At the end, when I saw a tyre had blown on another car, I was being very cautious, so the last two laps Nico was allowed to close the gap, but it felt really under control the whole way.

"I felt 100 per cent all weekend."

Ferrari felt similarly about its tyre strategy, until the penultimate lap...