Burning rubber

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They might not be sexy, but tyres are poised to play a greater part than ever in defining how hard drivers can push over the course of a Formula 1 race

On the eve of the new F1 season, as the new pumped-up, bad-ass generation of cars is about to be let loose with vastly more downforce, there remains one very big question mark around whether they will deliver the performance gains envisaged and therefore whether the whole exercise has been an expensive waste of time and effort. 

Having for the first time legislated in more downforce, F1 is presenting a very different look of car in 2017: wider, lower, swept-back and fatter-wheeled, they look like they’re moving even when standing still. They are sexier, more menacing – and should be a whole lot faster. Yet actually by far the most significant thing about them – the single factor far outweighing all the others in determining their performance and the quality of the racing – will be the tyres. 

These cars will create more downforce than any in F1 history, are heavier than any previous F1 cars – and can deliver close on 1,000 horsepower. Which means the tyres are under more load than any in F1 history too, by a spectacular amount. Teams last year were estimating the downforce gains from 2016 would be in the order of 35-40 per cent, Brembo was saying the braking energy would increase by around 30 per cent. That would be challenge enough for Pirelli but, simultaneous with that, the tyre supplier has been tasked by the FIA to produce a tyre very different in its behaviour to what it has supplied to date: specifically one that does not ‘fry’ into permanent inflexible uselessness once taken past a certain temperature threshold and which thereby has defined the fastest way of completing a GP as driving slowly. For 2017, the FIA has asked that the tyre’s performance is recoverable if it becomes too hot, in order that drivers can push hard for most of the race rather than simply manage the rubber in between a couple of fast laps around each pit stop.

It’s far from a given that Pirelli will have been able to accommodate these demands – from both the increased performance and those of the governing body. Doubts have been expressed about the suitability to F1 of Pirelli’s whole technical philosophy and, furthermore, the tyre company has been stymied by the current era of testing restrictions. It has also been the victim of mixed messages from F1 about what is required. Bernie Ecclestone’s original brief to the company was ‘to mix up the field, make it less predictable’. It achieved that through its heat-degrading tyre concept. But teams quickly got their heads around the traits and the unpredictability had largely disappeared by 2013, leaving just the unsatisfying ‘driving to a time delta’ style of racing. Not only has this been philosophically the antithesis of F1’s traditional flat-out ethos, but it’s also imposed a style of racing where it’s impossible – in the dry at least – to perform the sort of ‘comeback against the odds’ drives that have in the past created the sport’s legends. 

When Ferrari had got out of sync with McLaren at Hungary in 1998 and Ross Brawn told Michael Schumacher he needed to reel off ‘qualifying laps’ to bridge the gap, the German produced exactly that – an incredible demonstration of a genius driver on the absolute outer edge of his ability, striving to recover an apparently impossible situation. In pulling it off he created a golden sporting moment. 

At Monza last year, after pole man Lewis Hamilton had made a bad start and been bundled down to fifth place while team-mate Nico Rosberg ran away at the front, he was 15sec behind Rosberg by the time he’d clawed his way back up to second. There were 35 laps to go and in qualifying he’d been 0.5sec faster. If he could maintain that pace advantage for 35 laps, he could in theory have been fighting out the win with Rosberg in the race’s closing stages and each lap that the gap closed, so the tension and anticipation would have intensified towards a thrilling crescendo. But of course it could no longer happen that way; the tyres didn’t allow it. They had to be driven to a delta, around 2sec off the ultimate pace in order to not fry after a couple of hard laps. So Hamilton was still 15sec behind 35 laps later. This is just one example of thwarted races in the Pirelli era. There are many others.     

* * *

For 2017, the brief to Pirelli has been unambiguous: a more traditional racing tyre that can be driven hard throughout.    

Pirelli has been helped of course by the regulated increase in tyre width – up from 245 to 305mm at the front and from 325mm to 405mm at the back. This increases the contact patch – as does another, more subtle, change: the tyres’ diameters have been increased slightly too. The bigger the contact patch, the less stressed the compound is, as the load is spread over more area. 

This will help in controlling the temperatures, giving a better chance of equilibrium between the load frequencies and the optimum temperature of both surface tread and the tyre’s core. The ideal racing tyre is at that point of equilibrium even as it is driven as fast as the car will go, and the grip will progressively degrade only as the tread wears away. By contrast, the heat-degrading concept of tyre favoured by Pirelli to date typically (but not always) reaches equilibrium when the car is being driven whole seconds off its potential. Beyond that pace, certain materials within the tyre’s compound permanently change their viscosity and harden the whole tyre, drastically reducing its elasticity and grip, regardless of how much tread is left. Backing off does not bring the tyre back to life – it is from that point on ‘fried’ and useless. Pre-Pirelli era F1 tyres could also suffer heat degradation of course – ie the grip falling off when the surface becomes too hot and can no longer support the loads – but the tyre could be brought back to life by backing away from the ragged edge for a lap or so, because the compounds did not contain plastics that permanently hardened when overheated.    

To fully grasp the mechanism at play here requires delving a little further into the chemistry and the dynamics involved. The demands of a circuit’s layout (its corner radiuses and duration, the gap between them), its surface and its temperature, together with the traits of the car, impose an ideal viscosity of compound. As the tread temperature rises, the compound becomes softer. But as the contact frequencies rise (ie how much stress the cornering or braking or accelerating is putting the tyre through) the compound becomes harder. There is always therefore an ideal compound that best resolves those two opposing pulls on a given track on a given day on a given car. When these are in harmony, the tyre is said to be in its ideal state, that of ‘vitreous transition’.    

The tread temperature needs then to be in harmony with the tyre’s core temperature. If the loads generated by the tread do not put enough movement in it, the tyre’s inner core will remain too cool and inflexible. If the construction itself is not flexible enough, it forces the tread to take up more of the load – causing it then to overheat. Which in turn reduces grip further, a vicious circle of degrading grip. 

Creating a construction that flexes enough not to overstrain the compounds requires that it be strong. Repeated flexing can create a failure point if the flexing loads are not fairly equally distributed. If the construction design spreads the loads in a way that induces a weak spot from repeated flexing at the loads the tread is capable of generating, then either the construction needs to be stiffened or the tread grip reduced. Several shoulder failures of the Pirellis over the years have suggested that this indeed has been a weakness of that particular construction. 

The heat-degrading mechanism of the Pirelli compounds tended to place a natural ceiling upon the construction. Because drivers were not flat-out for extended periods, it was enough to contain the loads within the construction’s tolerance. But as the hybrid cars became faster through aero and engine development, so they began to edge the construction into an area that revealed the repeat stress point to be the inner shoulder of the rear tyre. This became apparent at Spa 2015 with the blow-outs of Rosberg and Vettel. In response, Pirelli increased the minimum permitted pressures and reduced the minimum camber angles, thereby limiting the grip the tread was capable of generating. 

The more downforce created by the car, the harder a compound it can run without overheating the tread or underheating the core. Because rubber is visco-elastic (between a solid and a liquid), the way it reacts to loads is not consistent. Up to a point it will accept incoming energy and react against it, trying to spring back in the opposite direction to the load and thereby creating grip. But increase the loads beyond a certain point and the rubber cannot regain shape quickly enough to absorb the incoming load and it stiffens and hardens, breaking the process down and causing the tyre to slide. Downforce gives the rubber help in opposing the load. So at a given speed, a car with more downforce imposes less stress on the rubber than one with less downforce. Or, expressed another way, at a given level of rubber stress the car with more downforce will be going faster than the one with less. 

So although Pirelli’s heat-degrading compounds place a false ceiling on how hard the driver can push for extended periods, the car with more downforce is still faster at that false ceiling than the car with less downforce. (There is however a point at which the downforce can ‘saturate’ the tyre and it simply cannot absorb any more load. Mercedes was nudging this limit at Spa last year. Red Bull used to regularly find itself in the same situation in the blown-diffuser era). 

So into 2017 the question is how well matched the hugely increased downforce generated by the new regulations will be with the latest Pirellis, enhanced by greater width and diameter, improved compounds and new constructions. The enhanced dimensions allow a bigger contact patch that should give better temperature control, but the details of the changes in construction and compound have not yet been publically revealed. Pirelli says its compounds, “Make use of entirely new materials and a completely fresh design philosophy.”

Which suggests it just might have responded to the FIA’s brief with a move away from the designed-in heat-degrading philosophy. But there remain doubts about how far it can do this. Pirelli has always insisted it included heat-degrading composites within the rubber compound in order to meet the original randomising brief. Others have suggested those chemical constituents are there only to facilitate the automated tyre manufacture process at the Istanbul factory where they are made, that they are needed in that process to get the base material into a sufficiently liquid state that it can be extruded effectively. If true, this means that those plastics are intrinsic to the concept of tyre. 

Also Pirelli stops short of committing itself to the new compounds beyond the first few races, saying, “After a very positive testing programme with the new sizes, Pirelli has decided to additionally homologate a back-up compound alongside each of the five new base compounds. These extra compounds are formulated using more traditional criteria compared to the new base compounds. For the first part of the championship, only the new-generation base compounds have been selected. The back-up compounds could be introduced later in the season to respond to any particular requirements once the real performance levels of the 2017 cars have been identified.’

All of which sounds less than confident.

The concern of the teams is that the aero overwhelms even the new tyre’s construction and that Pirelli’s answer will be – for the sake of safety – to increase the pressures and reduce the cambers again, thereby negating much of the potential performance increase.

That would also reduce again the contact patch and that might – depending upon how the new compounds behave – take us right back to Lewis Hamilton and co having to ‘drive to a delta lap time’ for whole stretches of the race. By the time you’re reading this the first tests of the new cars and tyres will have been conducted at Barcelona and we might have the first indications of just how successful – or otherwise – this whole regulation change might be.