Ferrari's Tire Gas and Variable Brake System Explained

http://www.speedtv.com/articles/auto/formulaone/41398/?page=1

ON THE GAS...

One of the most talked-about elements of the McLaren spy case was a special gas used by Ferrari in its tyres – in particular, an incriminating email from McLaren test driver Pedro De La Rosa to lead driver Fernando Alonso that revealed that the gas reduced the internal temperature and blistering.

In the email De La Rosa is quoted as saying ‘we’ll have to try it, it’s easy!’ Racing tyres are typically filled with air or, in more professional series like Formula 1, Nitrogen. But in an email from Alonso to De La Rosa he states it is ‘very important’ that McLaren test the gas that Ferrari uses in its tyres as ‘they have something different from the rest.’ He follows this up with ‘not only this year.

There is something else and this may be the key. Let’s hope we can test it during this test, and that we can make it a priority!’ It has been widely reported that the gas used by Ferrari was carbon dioxide and, whilst this is partially correct, it’s not the whole story.
Contrary to those that refused to believe alternate tire gasses could have a genuine impact on performance, Ferrari indeed developed an innovative solution. (For a revealing of all of the test results, purchase the December issue of Racecar Engineering.)

In fact, the gas used by Ferrari is a hydrofluorocarbon (HFC)-based mixture designed specifically for use in racecar tyres, though not dissimilar in composition to gasses used in refrigerators, which are comprised entirely of hydrogen, carbon and fluorine.

A team headed by Andrea Seghezzi of Monza, Italy, in association with Gruppo Sapio developed the gas and subjected it to extensive track testing.

It was discovered that the HFCs were able to effectively conduct the heat generated during the rotation of the tyre to the wheel rim at a more or less constant pressure. The wheel rim then acts as a radiator, exchanging the heat with the outside air, maintaining a lower internal temperature and preventing it from overheating. This is particularly effective on aluminium or magnesium wheels.

Racing rubber inflated with air also suffers from the effects of some internal chemical interactions, which damage the structure of the tyre, and can result in a sudden drop in performance. Due to the high capacity for heat transfer, tyres inflated with the new gas mixture achieve excellent longevity, since the temperature of the tyre is kept low and the pressure is constant.

After extensive tyre testing the best blend of HFCs was found to be 52 per cent Tetrafluoroethane, 44 per cent Pentafluoroethane and four per cent Trifluoroethane. This mixture, known as HFC R404 A, was found to be most effective in a racing tyre when it was inflated with a blend of 50 per cent HFC R404 A and 50 per cent CO2.
The use of refrigerant gas to cool the tires and radiate heat away from the carcass through the wheel is but one of many secrets McLaren was privy to.

The use of this new gas mix for tyres has implications far beyond Formula 1, of course, as using it allows teams to run softer compounds for longer, or to work the tyres harder.

It could also be that there will be worthwhile benefits in races run over longer distances, like NASCAR or Le Mans.

The court case continued without revealing the precise details of HFC R404 A, but it transpires that, despite the encouragement of the drivers, a Bridgestone engineer doubted whether the gas mixture would be effective if used on the McLaren MP4/22. However, it is thought that the McLaren drivers were not fully aware of the gas mix, rather they only knew of the CO2 element.

http://www.speedtv.com/articles/auto/formulaone/41398/?page=2


During the hearing another email was produced that gave a fascinating insight into Ferrari’s use of variable brake bias on its cars. It was again sent to Alonso by De La Rosa on March 25 2007 and says, ‘They [Ferrari] have this system which delays the rear braking initially then proceeds to increase it gradually.’

The benefits of this are not as obvious as they might first appear. In the first instance it is easy to assume that because an F1 car generates much higher braking forces with the aid of downforce at high speed than at lower ones, the weight transfer is also much greater at high speeds. That would suggest the need for a greater forward brake bias, gradually migrating rearward as speed and downforce reduce. However, even at 5.0g deceleration, the long wheelbase and low centre of gravity of an F1 car mean the weight transfer is dwarfed by the aerodynamic downforce that enables such high deceleration. So the relative loads on the front and rear tyres do not necessarily correlate directly with braking force and weight transfer.

Why, then, would the Ferrari be attempting to migrate its brake bias rearward during the braking period? Perhaps the most likely explanation is the aerodynamic balance is also shifting greatly under braking. If the team has been successfully shedding downforce at high speed to reduce drag then this may well be from the rear of the car. For instance, despite the organisers’ attempts to prevent it, teams have been finding ever more sophisticated ways of allowing their cars’ rear wings to drop efficiency at high speed. This will both reduce drag and also cause a significant forward shift in the centre of lift.

Obviously a brake bias set to prevent the rear wheels locking at these speeds would have a heavy forward bias, but this would seriously compromise the braking efficiency at lower speeds, with the rear brakes only working at a fraction of their potential. So, to bias the braking effort to the front then move it rearward during the braking event would seem desirable, but how could it be achieved? Mention was made at the hearing of a spring that Ferrari uses to achieve the effect (see figure 1). Obviously a spring on its own cannot effect a change over time but, combined with input from the driver, it could.

Under the highest downforce, driver brake effort is at the maximum, as is displacement of the pedal. But as downforce reduces, so does the pedal effort and displacement and this can be used to vary the brake bias during the brake event. A spring could be placed in the system to resist the force on the rear master cylinder, reducing the rear braking effort at maximum force. However, as downforce diminishes and the driver modulates his braking effort to prevent wheel locking, pedal displacement would reduce gradually, taking the spring out of action and allowing the rear cylinder a greater proportion of the total force relative to the front cylinder.

In fact Ferrari has been observed to employ a three-position lever on the right-hand side of its cockpit that the drivers are seen to change several times during a lap. This could well be changing the bias profile for different corner scenarios. It could give different settings for high and low-speed corners or different levels of stability under high-speed braking to aid turn in. Put simply, it could insert a wedge between the pedal linkage and the spring, altering the pressure at which it starts to have an influence.

With the information from this email, McLaren installed and tested its own version of the system. But unlike the Ferrari version, it is said to use a valve instead of a spring and is a direct development of a system the team was using in 2001 and 2002. This may have been part of the brake steer system subsequently banned by the FIA, leading F1 regulations to now state: ‘The brake system must be designed in order that the force exerted on the brake pads within each circuit are the same at all times.’ In its current form the valve is most likely an interconnect between the front and rear circuits that, via a connecting piston, allows the pressure in one circuit to affect the other. To what degree will be determined by the fl uid pressure in the system ie how hard the driver is braking.

To get the second half of this world exclusive story, visit www.racecar-engineering.com to purchase a copy, or visit your local book store.
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