Each year, in addition to our production tyres, Pirelli develops thirty or so development models for the Superbike World Championship alone, with the most important changes in the tyres being found in the compounds. But why is the compound used in a tyre—and especially in a motorcycle tyre—so crucial?
Since 2004, Pirelli has been the official sole tyre provider for all classes of the FIM Superbike World Championship, which includes Superbike, Supersport, Superstock 1000, Superstock 600, and European Junior Cup. The tyres used in world championship racing, as well as in the various national championships in which Pirelli is involved, are production tyres available to the public. Specifically, DIABLO™ Supercorsa tyres are used in all categories except Superbike, where the DIABLO™ Superbike slick tyres are used. Throughout the year, Pirelli then presents a number of other development solutions.
To what end? At Pirelli, the goal is to develop a range of tyres that are as versatile as possible and suitable for use on any bike and to meet the needs of every racer and every track. There is no single tyre or compound that can meet all of these needs, which is why our range of tyres is updated and improved each year with the help of our efforts for World Superbike and for the various national championships.
While it’s true that there’s no such thing as an all-purpose compound, Pirelli does have a fairly limited range in terms of number of solutions because each type is highly versatile and has a wide array of applications. Specifically, we have two compounds for the front tyre, the SC1 and SC2, and three for the back, the SC0, SC1 and SC2. The rear-tyre SC2, which is the hardest compound, is finding increasingly limited use in world championship races, but it is still constantly updated for local championships, such as for British SBK, the German IDM, and the other championship of northern Europe, where they need a tyre that can stand up to the colder temperatures.
Conversely, the SC0 is the softest rear tyre and is the most popular in World Superbike because, in recent years, Pirelli has significantly broadened its range of application, so now this tyre can be used at lower temperatures than in the past. When the SC0 can no longer be used, the SC1 then comes into play. This mid-range compound can be used on most types of tarmac and temperatures, but because it is harder, it offers less grip than the SC0.
And how do riders pick the best compound to be used during a race?
Normally, the choice of which compound to be used for a motorcycle race is fairly simple because their a certain basic rules to be followed; however, it is still possible for the uninitiated to make a mistake because the reasoning behind tyre selection for a motorcycle is not the same as for a four-wheeled vehicle.
Without getting overly technical, let’s just say that heat softens a tyre, which is why a harder compound is preferred for a car as temperatures rise, and a softer compound is chosen as temperatures fall. When it comes to cars, the tyres have a very wide contact patch, and the vehicle's power is best transferred to the road when the tyre is able to penetrate as much as possible into the tarmac, which is achieved as the weight of the car presses down on the tyre. However, at the same time, this contact patch has to remain firm in order to lend stability to the vehicle. A soft tyre may not provide adequate support, thereby compromising steering precision and resulting in oversteer or understeer depending on the characteristics of the tyre. This is why a harder compound is preferred on a car as temperatures rise.
But let’s see what happens when we only have two wheels. The main difference between a car and a motorcycle lies in the tyre’s contact patch. On a motorcycle, the area of contact is much smaller—roughly the size of a credit card—and the load applied to the tyre is nowhere near that of a car. For this reason, in order to achieve grip, we need a tyre that is soft enough to penetrate the tarmac, particularly when the tarmac itself provides little grip either because it has been worn down with age or because the surface is particularly hot. Although the tarmac itself is made of a material that is nearly insensitive to changes in temperature, as temperatures rise, the efficiency of the compound declines because it is made of a thermoplastic material that changes significantly in terms of stiffness and grip as temperatures rise and fall. Under higher temperatures, a hard compound loses grip, so we need a softer compound, such as the SC0, which can penetrate into the tarmac and provide the traction we need during the race. This is why a hard compound deteriorates faster than a soft one as temperatures rise. In other words, with a car the primary objective is driving precision, whereas with a motorcycle it's grip, which leads to the opposite choice of compound compared to a car.
Easy, right? Actually, it doesn’t end there, because what we’ve just seen above only applies to a motorcycle's rear tyre. When it comes to the front tyre, Pirelli uses the same strategy as for cars. The semi-soft SC1 compound for the front tyre is generally used at lower temperatures, while the harder SC2 is used at warmer temperatures.
In order to understand the logic behind the choice of front tyre, we need to look at what happens to a soft tyre and at high temperatures during braking. When braking before a curve, the front tyre compresses, and as the racer enters the curve, he releases the break, and the tyre regains its elasticity, which can cause vibrations, imprecision and understeer. This effect is amplified with a softer compound, which is why Pirelli follows the same strategy as a car for a motorcycle's front tyre, opting for harder compounds, such as the SC2, which ensure the right precision. Conversely, as temperatures fall, we can go with softer compounds, like the SC1, which, at these lower temperatures, are still able to provide the precision and support needed to guide the motorcycle. However, while riders nearly always agree on using a softer rear tyre in order to have the most grip, the selection of the front tyre is more complicated and often depends a great deal on the rider's racing style and personal preferences.