Windtunnel tyres: Pirelli's secret weapon

Pirelli already had multiple Formula 1 world titles from the 1950s and grand prix wins as recently as 1991 to its name when it decided to join the championship as its sole tyre supplier for 2011. In the intervening period it had also achieved great success in world rallying, sportscars, touring cars and superbikes, in addition to serving a year as GP3's rubber partner in 2010. Stepping back up to the big league would surely be easy, even if in-season testing was banned...

However, the rate of development in F1 is ferocious - particularly since the early 1990s - and none of the above categories really bear any resemblance to grand prix racing either, thanks to being devoid of aerodynamics as its primary performance driver, or even of having open wheels.

Thus the company came into an aero-driven formula without ever having developed tyres for windtunnel testing. In fact, during Pirelli's 1980s and '90s campaigns, windtunnel 'wheels' were turned from aluminium or formed from carbon fibre, with about the only resemblance to the real thing being that both were round. Since then aero has progressed to such a degree that even having insufficient clearance between model and windtunnel wall can make the difference between winning and being off the podium - as Ferrari has recently demonstrated.

Nelson Piquet was Pirelli's last F1 race-winner before its return this year © LAT

When Pirelli was first announced as F1's sole tyre supplier in place of the departing Bridgestone, Toro Rosso technical director Giorgio Ascanelli congratulated Pirelli motorsport director Paul Hembery, and added: "Paul, the biggest task you've got now isn't making Formula 1 tyres, it's making windtunnel tyres."

"And he was right," admits Hembery a year later. "That was one of the first conversations we had, because we obviously spoke to some friends when we were looking at going into the sport, you know, the pros, cons, difficulties and that, and Giorgio quite rightly pointed out very early on, almost the first conversation, first words: 'windtunnel tyre'."

The reason is easy to see: tyres make up 30-40 per cent of the frontal area of an F1 car, spinning at enormous speeds while doing so. The resultant wake affects the airflow and vortices all the way to the back of the car and beyond. In addition, they swivel according to steering input, and deflect and deform according to downforce and lateral/centrifugal forces, thus causing the sidewall and the contact patch to constantly change shape.

All this crucially impacts on the aerodynamic efficiency of a car that is primarily dependent upon aero (virtually all other performance drivers have, over the seasons, become spec units). As a result the major performance differentiator among current F1 cars is aero, with tyres being the major spoiler.

"Understanding where that wake goes is very important for computational fluid dynamics models and also the wind tunnel," explains Williams technical director Sam Michael. "So we try and replicate that tyre, and remember the tyres in the tunnel [are] in a squashed condition, so they're permanently deformed, to be like the tyre is in the middle of the corner.

"We're not really into straightline downforce anymore; things have moved on so much. Everything is all about cornering. We've been running rubber tyres in the windtunnel since we started with Michelin back in 2001; they produced a tyre for us in the first year; it's almost 10 years since we've run solid tyres. We used either machine aluminium or a carbon tyre before that."

Compounding the problem is that, as the sport's sole supplier, Pirelli needs to ensure every team is treated evenly. This is a problem because the windtunnel and CFD set-ups of those teams are chalk and cheese, with some budget-restricted teams not owning tunnels and instead renting spare capacity in various facilities.

"The problem for a windtunnel tyre is it's got to be adaptable to everyone's requirements. You might want to create different conditions or you might want to create different speeds. You've got different sizes with 50 and 60 [per cent scales], so they have to be very versatile. And that takes a lot of learning, because it's not a tyre, it's laboratory equipment. So what Pirelli have done in the time they've had, starting from scratch, has been really good," says Sauber's technical wizard James Key.

Ferrari's windtunnel is huge compared to that of some rival F1 squads

"The first challenge, of course, is to get the dynamic profile the same as the full-sized tyre, within scale," Hembery told this column. "So you have a very, very lightweight structure. You can see just by pressing it like that [illustrates with a soft finger push], that it is clearly different to what a normal road tyre would be."

Made of a softer rubber than used for race tyres - and with more pliable sidewalls - these 'tyres' run at extremely low pressures in the windtunnel, with Hembery admitting only to "less than 10 psi," while Ascanelli would not divulge pressures, saying only: "It's up to us to decide where we want to exercise it in the condition of the car which we want to reproduce in the tunnel. So in fact there is not a single pressure at which we run the wind tunnel tyres, depending whether you are low speed, high speed, rolling..."

When pushed further, he still does not budge.

"[I'm] Not saying that, it's strategic. Much lower than what the real tyre does."

The other challenge faced by Pirelli is that moving road tunnels - whereby a belt moves below the car at desired speeds to drive the wheels - are now de rigueur in F1, and so Pirelli faced not only the question of traction, but also wear of belt and tyre.

Ascanelli says: "The stability of the shoulder shape and its stiffness actually generate vertical forces - jacking forces - on the tyre, because they are going down at an angle. So we had large vibrations on the tyres which induced accelerated wear of the [fabric] belt. To give you a figure: a belt to us lasts about six or seven months. At the beginning of development [with Pirelli] our belt lasted a fortnight. The other thing is that the amount of vibration jeopardises the accuracy of aerodynamic measurements."

Although Williams runs a steel belt, it experienced wear issues, and while Michael will not divulge precisely how the team overcome this costly problem - belts run in at around £100,000 a throw - he was prepared to reveal that there are two 'tricks', namely "surface tension and coating of the belt".

"You've got to have a very versatile [tyre] surface where it doesn't wear or it doesn't wear the belt," emphasises Key. "So you've got that, you're trying to replicate a profile, so sidewall profile, contact patch shape and area.

"You're trying to do that at the speed a wind tunnel can run, and we're limited to 50m per second. The wheel spins obviously at 50mps, but you're not trying to replicate 50mps of speed necessarily on the car. You might want to replicate different speeds in your tunnel, you may want to replicate a fixed speed, which is typically higher than 50mps," he adds.

Hembery shows Rencken Pirelli's windtunnel tyres © FD Rencken/Racinglines

"So with the vertical load that you can place, which is variable depending on how you do it, if they have wheels on or wheels off - most people have wheels on I think now, so it's wheels on the model rather than on spindles - it's all manner of different approaches."

Since taking on the F1 project Pirelli has constantly developed its 'laboratory' tyres - in both 50 and 60 per cent scales - with the current ones being "Mk IV", as Hembery calls them. "They are the personal project of one guy who has been following it from start to finish, and we'll keep going until the teams are happy, because we're developing our know-how of course."

Key is extremely complimentary about the progress made by Pirelli since stepping in the Bridgestone breach just over a year ago: "[They have made] Huge steps and, to be honest, it was a very good process because Pirelli were very keen to have some advice from the teams, they need feedback to understand how good these things are.

"So for them it's difficult to test a wind tunnel tyre because of all the conditions that people perhaps run. So I think we definitely saw improvements. It's [the development] not 100 per cent there yet, but I don't think you'll ever find a single tyre that suits everyone's needs. Certainly where we are now is a massive step forward from where they started."

Ascanelli agrees, saying: "We started with a very difficult beginning, and yes, as it turns out we're getting better and better. From the first iteration where we couldn't really run them, now we are steadily running them. And we are starting [to] get also better correlation with what we find on the car."

However, Toro Rosso finds itself betwixt and between, for the team uses its 50 per cent tunnel Bicester, England, and with Pirelli not having released the 50 per cent Mk IV rear tyre at time of researching this column [during the German/Hungarian rounds], has been forced to run a Mk IV front and Mk III rear set-up, while Michael admits Williams earlier this year persevered with Bridgestone windtunnel tyres.

"We've been on Pirelli windtunnel tyres for about four or five months; we ran Bridgestone tyres up until then," he said, adding that the Japanese tyres provided better data compared to Pirelli's early attempts, despite having completely different profiles. He agrees that Pirelli's latest versions are a significant improvement: "Oh yes; they've changed a lot".

When developing the tyres, Pirelli's specialist starts with full sized tyres running on drums, which enables the full spectrum of deformation to be measured by laser, and then scales the results to the required size for mould making. This every time Pirelli upgrades its profiles, as will be the case next year.

"Not quite all over again," says Hembery in response to the obvious question. "But it's a modification. Part of the skill is obviously trying to get the mould shape correct on these small tyres, so when it is inflated and running it replicates real running; again, it is not just a 60 per cent scaling down of the profile, it's a complex package.

Testing tyres in the windtunnel has become a crucial science in F1 over the last decade

"Our R&D team is made up of various background individuals, from compounding and the string and metal bits, and then there's testing, of course. We have a lot of people working on testing methodologies to replicate what happens not just for these scale tyres but also on the full-scale tyres in a race situation. It's quite a large group of people, a lot of whom are specific obviously to F1."

There are plans for radical improvements of Pirelli's windtunnel tyre qualities, too. "We obviously don't know all the work they [the teams] do in the windtunnels, because it's very secretive. They are doing steering work, but we don't believe that they're doing cornering, so that doesn't come into the equation [yet].

"Possibly in the future - if we're able to do it - I guess as well, if you could create a dynamic mini tyre that mimics also in cornering the tyre performance, then the teams would love that. I suppose the easiest way to do it would be to build a full-scale windtunnel."

Arranging a photoshoot of Pirelli's 50 and 60 per cent tyres was no easy business due to their confidentiality. "These are the real secret in F1," says Hembery. "It's not what you see on the circuit, because these are almost the most difficult tyres to make.

"It's a small little area of the business, yet it's a fundamental area for the teams. It is our largest learning curve because we'd never made a product like this before. We know that the previous suppliers had spent a significant amount of time perfecting such items."

It's no wonder Hembery smiles when asked about the cost of a windtunnel tyre budget given that about 1000 such tyres will be produced, before allowing "I wouldn't like to calculate. They don't have a price, it's one of these things, it's so one-off."