Tire Compound
History of the Pneumatic Tire
The first practical pneumatic tire was made by John Boyd Dunlop, born in Scotland, while working as a veterinarian in May Street, Belfast, in 1887, for his son’s bicycle, in an effort to prevent the headaches his son had while riding on rough roads. (Dunlop’s patent was later declared invalid because of prior art by fellow Scot Robert William Thomson.) Dunlop is credited with realising that rubber could withstand the wear and tear of being a tire while retaining its resilience.
Pneumatic tires are made of a flexible elastomer material, such as rubber, with reinforcing materials such as fabric and wire. Tire companies were first established in the early 20th century and grew in tandem with the automobile industry.
Racing Tire Specialisation
Racing tires are highly specialised according to vehicle and race track conditions. This classification includes tires for drag racing, drifting, road racing, as well as the large-market race tires for Formula One, IndyCar, NASCAR, rallying, MotoGP, and similar series. Tires are specially engineered for specific race tracks according to surface conditions, cornering loads, and track temperature. Racing tires are often engineered to minimum weight targets, so tires for a 500-mile race may last only 100 miles before a tire change, or a 300 km Formula 1 race may require tires that last only 150 km. Some tire makers invest heavily in race tire development as part of their marketing strategy.
Racing tires are often not legal for normal highway use.
Formula 1 Tire Compounds
In Formula 1, the type of rubber mix used in the construction of a tire ranges through several different compounds – from supersoft, soft, to hard and extra hard – with each offering different performance and wear characteristics.
In the era of two tire suppliers (Goodyear-Bridgestone, Bridgestone-Michelin), the number of compounds was not limited to four types. During the tire manufacturer war, compounds were mixed specially for each circuit, and there were always several choices a driver could make with the racing engineer, depending on track temperature and track conditions.
The FIA introduced a rule with only one supplier and a limited number of different compounds to be used during the year. The manufacturer must announce several months before a race what type of rubber will be available to the teams.
The FIA introduced the single-supplier rule because tires had become so competitive and compounds so sophisticated and grippy that car speeds during cornering and braking were becoming dangerously high. Cars were essentially glued to the track, but once grip was lost for any reason, it was almost impossible to control the resulting spin. Another reason for the single supplier was cost cutting, as the price of tire development had become extremely high.
Tire Assembly and Components
The tire is an assembly of numerous components that are built up on a drum layer by layer and then cured in a press under heat and pressure. Heat facilitates a polymerisation reaction that cross-links rubber monomers to create long elastic molecules. These polymers create the elastic quality that permits the tire to be compressed in the area where the tire contacts the road surface and spring back to its original shape under high-frequency cycles. Typical components used in tire assembly include:
Natural rubber, or polyisoprene, the basic elastomer used in tire making
Styrene-butadiene co-polymer (SBR), a synthetic rubber often substituted in part for natural rubber based on comparative raw materials cost
Polybutadiene, used in combination with other rubbers because of its low heat-buildup properties
Bromobutyl rubber, used for tubeless inner liner compounds because of its low air permeability. The halogen atoms provide a bond with the carcass compounds, which are mainly natural rubber
Carbon black, which forms a high percentage of the rubber compound, providing reinforcement and abrasion resistance
Silica, used together with carbon black in high-performance tires as a low heat-buildup reinforcement
Sulphur, which cross-links the rubber molecules in the vulcanisation process
Vulcanising accelerators, complex organic compounds that speed up the vulcanisation
Activators to assist the vulcanisation, with zinc oxide being the main one
Antioxidants and antiozonants to prevent sidewall cracking due to sunlight and ozone
Textile fabric (mainly Kevlar and carbon fibre threads in Formula 1) to reinforce the carcass of the tire
Compounding is the operation of bringing together all the ingredients required to mix a batch of rubber compound. Each component has a different mix of ingredients according to the properties required for that component.
Micro Dispersion
One of the most important parameters of a given rubber compound reinforced with carbon black is the micro dispersion of the filler. This micro dispersion governs the fundamental viscoelastic response of the compound and is responsible for the ratio of filler-filler to polymer-filler interactions. The balance between these two types of interactions dictates the most relevant properties of the finished tire.
Why Are F1 Tires Wide?
The first classical law of friction states: “The size of the contact surface does not influence the friction.” Why, then, are tires for Formula 1 racing cars so wide?
According to the “FIA Technical Commission”, the front and rear tires must not be wider than stated in the technical regulations.
The classical friction laws apply to materials that undergo mainly plastic deformation in the asperity summits. This is not the case for polymers and elastomers. Polymers display a lower friction coefficient at higher nominal contact pressure (narrow tires produce higher nominal contact pressure and vice versa). A high friction force can consequently only be achieved with a large surface area.
Other advantages of a wide surface are that wear is distributed over a larger area, heat development per unit of surface is lower, and the result is a larger heat-radiating surface.