Will Gray

Tech Talk: How tyre changes might turn the F1 season

Will Gray

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Pirelli are set to make significant changes to their tyres for the rest of the year following the dangerous blow-outs at Silverstone last weekend – but how could the likely changes affect the pecking order?

Last weekend saw six explosive demonstrations of why Pirelli needs to change its tyres immediately.

There have been concerns over the construction of this year’s tyres after failures in Bahrain and Spain, but in those cases the problem was in the form of delaminating tyres – where a cut in the rubber leads to the tread stripping away from the tyre surface, but the structure remains intact and the driver can still limp back to the pits.

The British Grand Prix, however, delivered a series of far more dangerous explosive deconstructions.

In final practice, the left rear on Sergio Perez’s McLaren exploded as he exited Copse corner. Following an initial investigation, a Pirelli spokesperson said: "Basically, it's a cut to the sidewall. It's an external cause - we don't know what exactly - but you can see that something entered in the sidewall, which is obviously the weakest point. It's got nothing to do with the previous issues. It's not a delamination issue - the tread belt is intact...."

In the race, however, no less than eight drivers suffered potentially related issues with their tyres.

L8: Lewis Hamilton - left rear explosion

L10: Fernando Alonso – right rear deflation but pitted before explosion

L11: Felipe Massa – left rear explosion

L13: Sebastian Vettel – cuts to left rear but pitted before deflation

L15: Jean-Eric Vergne – left rear explosion

L29: Esteban Gutierrez – cuts that led to left front failure

L42: Nico Rosberg – left rear vibration but pitted before failure

L46: Sergio Perez – left rear explosion

THE CAUSE

There has been much debate about the cause of these issues, with several key theories suggested:

1) Post-race site reports suggest the inside of the kerb at Aintree was heavily serrated, while small concrete blocks at Becketts could have pinched the tyres. At both these points, the cornering forces cause the inside tyre to become stretched and under extreme load, so both could have caused tyre cutting which could lead to explosive deconstruction.

2) The tyres used at Silverstone were different from those used so far this year, with a new bonding process to fuse the tread more firmly to the tyre structure. When the tyre suffered a cut, instead of shaking off the outer tread as in the past, the tread stayed on and the cut may have caused spot overheating that resulted in a tyre explosion.

3) A report in Spanish newspaper Marca claims the new bonding process also involved the use of a Kevlar layer, something that images of the incidents appear to support. If this is the case, that could also have contributed to the cut leading to an explosion.

4) The cars are faster this year – Mark Webber’s fastest race lap was 1.2s faster than Kimi Raikkonen set last year at a similar point in the race – and the new tyres may have simply fatigued under heavy loads.

5) The kerbs and high speeds could have contributed to the creation of oscillations within the tyre that were not considered in the design calculations and that could potentially have shaken the tyre apart.

On Tuesday evening, Pirelli claimed the bonding process had nothing to do with the incidents but added their own additional explanations, which put the blame squarely on the teams.

They confirmed that teams have been mounting tyres the wrong way round – the rear tyres are specific for left and right sides, but some teams have discovered performance benefits from fitting them on the wrong sides. This, they said, was the situation on all the failed tyres.

The firm said in a statement: “The sidewalls are designed in such a way to deal with specific loads on the internal and external sides of the tyre. So swapping the tyres round has an effect on how they work in certain conditions.”

It seems this situation was exacerbated by the fast left turns and the aggressive kerbs, but also by the fact that some teams were running under-inflated tyres for performance gain and also using extreme camber angles.

THE SOLUTION

So what will happen now?

Pirelli has been trying to change their tyres for some time, but Ferrari, Lotus and Force India have blocked the move because they claim they have deliberately designed their cars to suit the tyres better and have therefore claimed a justifiable gain on their rivals.

However, now the eye-opening failures have led to this being deemed a safety issue, Pirelli no longer needs unanimity to make a chance.

Tyres consist of a sidewall and a solid belt that runs around the circumference of the tyre to give it its strength, with the synthesised rubber tread bonded to this to create the grip. The belt is constructed of steel this year, but last year it was Kevlar.

Pirelli have now confirmed that for this weekend’s German Grand Prix they will use the Kevlar belt rear tyres tested in Canada, which had previously been vetoed by the three teams mentioned above. From Hungary, they will then change to 2012 construction and geometries with 2013 compounds, which will be used for the rest of the year.

Kevlar has a much better strength to weight ratio compared to steel and also has higher ductility, which reduces stress concentration and should also dampen internal vibrations.

However, switching to this rear tyre construction will dramatically change the way the tyres perform.

Firstly, the weight of the rear tyre increased by 700g with the steel belt, so the rear end weight will now be reduced by 1.4kg with this change.

More importantly, Kevlar is thought to dissipate less energy and give a wider operating window - and its use causes the tyre to run at a much lower temperature compared to the steel belt tyre, something up to 10 degrees Celsius lower.

This simple temperature shift will have dramatic consequences for the competitive order, as it will make life easier for tyre-eaters like Red Bull and will potentially put the more forgiving cars of Lotus, Ferrari and Force India further away from the optimum performance levels.

Beyond that, if construction and compound returns back to 2012 design, it could have significant knock-on effect on aerodynamics.

The 2013 tyre has a much more angular tyre shoulder shape, softer and more deformable sidewalls and stronger more rigid shoulders. In braking and cornering, the tyre deforms far more than the 2012 tyre – and because the teams consider that deformed shape in aero tests, it could mean a lot of work will now need to be re-done.

So it may seem a simple solution to change tyres for safety’s sake – but if it happens it could have some serious knock-on effects on the destination of this year’s world title...

This blog was updated at 19:30 on July 2 to reflect Pirelli's statement

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