Monocoque materials
While the monocoque of a modern Formula One car performs multiple functions, including aerodynamic performance and the provision of sufficient stiffness (combined with the powertrain and suspension) to ensure adequate traction and handling, its primary purpose is to protect the driver.
To this end the Formula One technical regulations require chassis to pass a series of impact and intrusion tests before they can be used in competition. This requirement was first introduced in the mid-1980s, and it is no coincidence that the evolution of this testing has led to a situation where accidents involving Formula One cars that result in serious injury or loss of life are nowadays extremely rare.
The path to the current ‘safety capsule’ Formula One monocoque began in the early 1980s with the adoption of carbon fibre composite materials for chassis manufacture, although in the strict definition of the word, composites had already been in use in motorsport since the 1950s in the form of glass fibre moulded body panels.
The first Formula One car to be raced that incorporated composite material in its chassis was the 1966 McLaren M2A, which featured panels of Mallite – a composite formed of aluminium sheets over a core of end-grain balsa wood. Although the McLaren benefited from a chassis rigidity advantage over its rivals, the technology was not pursued in later models.
Carbon fibre was first used in motorsport in the late 1960s as reinforcing strands bonded to the large glass fibre body panels of sports racecars, and was first seen in similar applications in Formula One in the early 1970s. Its first use as a structural material came when the McLaren and Lotus F1 teams both introduced carbon chassis in 1981. McLaren subcontracted the manufacture of this first model to Hercules Aerospace in the US, while Lotus opted to build theirs in-house. The McLaren was built using moulding and layout methods that were the forerunners of the techniques still used today. Lotus opted for using folded sheets of composite material in a similar manner to the way chassis had previously been fabricated using sheet aluminium and aluminium honeycomb.
Pioneered in aerospace applications, carbon fibre materials are produced by the application of a thermal decomposition process on a fibre precursor. The materials most commonly used in Formula One are based on a polyacrylonitrile (PAN) precursor, other precursors include Rayon and pitch fibres. According to the desired molecular structure of the finished material, the PAN precursor is co-polymerised with one or more of a range of monomers such as acrylic acid or methyl acrylate. Decomposition of the precursor allows it to be extruded into a filament via a winding process before it is subject to an oxidation phase in which heat is applied over a period of time with the filament held in tension. The resultant thread can then be woven into carbon fibre fabric.
A Formula One monocoque is manufactured by hand, the low volume-high performance requirements of motorsport making automation of the process impractical. Plies of carbon fibre fabric – pre-impregnated with resin – are laid up in mould tooling to form the outer skin of the structure. A hollow-cell core material is added before further plies of carbon fibre fabric are introduced as an inner skin. According to the anticipated loads on varying areas of the structure, fabrics with differing weave patterns and in varying orientations are deployed.
The basic materials and methods used to manufacture a Formula One monocoque have remained largely unchanged for more than 30 years. However, many scientific research groups are engaged these days in the development of nano-materials which are widely seen to be the future of composites. Currently the only mention of nano-materials in the FIA Formula One technical regulations is the banning of the use of hollow carbon nano-tubes in the chassis structure. Other nano-materials such as graphene are effectively outlawed by the simple fact that none can be supplied in sufficient volume at the present time to make their use viable. Nano filled resins could prove to be an interesting area of development, with the promise of increasing resin toughness and thus dynamic crash structure behaviour.
Basalt fibres are still not widely available but could prove an interesting alternative to glass and carbon fibres in terms of both cost and performance. Whether such a material would penetrate the Formula One market is unclear, as typically the teams opt for performance over cost.
Fig. 1 - A Formula One monocoque cocoons the driver in a carbon fibre safety cell.
Written by Alan Lis