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Carbon and its fibre composites

In providing the physical contact points with the track surface, the ‘corners’ of a Formula One racecar –the wheels, brakes, uprights and suspension – could be argued to perform the most important function of all. Reacting forces induced by tyre traction, the generation of aerodynamic downforce, deceleration and acceleration make these elements among the most highly stressed in the overall car package.

Like every other aspect of a racecar design, these areas are subject to sustained and strenuous efforts to reduce mass and increase performance without compromising structural integrity and safety. This has led to the use of advanced composite materials such as carbon fibre in the manufacture of parts previously produced in metals.

Carbon brakes, originally developed for use on aircraft, were first tested on a Formula One car in the late 1970s, and since the mid-1980s all Formula One cars have been equipped with them. The manufacture of brake friction material begins with same basic fibre as that used in the production of the carbon fibre fabric from which racecar chassis are made – PAN (polyacrylonitrile). It is, however, processed in a wholly different way, being passed through carbon deposition and high-temperature oxidation stages lasting several weeks that transform its molecular structure and result in a material that as well as offering lower weight relative to metal brakes provides greater durability and higher thermal capacity.

This means drivers can rely on shorter braking distances throughout a race, which was not always the case with the previous generation of metal brake discs. These gains were not without cost though, and a long development path was necessary to provide sufficient cooling of the discs, pads and calipers and other parts in their vicinity before the use of carbon brakes became practicable.

Carbon fibre composite materials have been used for the manufacture of suspension components since the early 1990s. The primary advantages they offer over metal suspension parts are lower weight and higher stiffness per unit weight, but they can also fulfil an aerodynamic function. If the front suspension is correctly configured then the track rod, even when steered, can remain in the same plane and in concert with the top wishbone legs, allowing an aerofoil section to be formed by these two links.

This provides a slender frontal area with a large plan area that generates lift that can be used to redirect the upwash from the front wing and clean up the downstream flow to the top of the sidepods, the diffuser and the rear wing. The stiffness of composite suspension elements has also been a contributing factor in the reduction of the range of suspension movement on a Formula One car. Typically the vast majority of this is catered for by deformation of the sidewall of the tyre.

Carbon fibre composite suspension uprights and wheels have been produced and evaluated in the past in Formula One, but there are concerns over certain characteristics of these materials in such applications, including the difficulty in designing a true three-dimensional composite material capable of handling complex and multiple stress directions. Techniques like z-pinning can help but generally large, relatively lightly loaded structures are the best applications for composites.

On items such as a piece of bodywork or a wing, these loadings are more easily defined and the stresses are easier to predict, making it relatively easy to design a laminate that can handle the loads in the most efficient way. Such considerations would seem to make the future widespread use of carbon wheels and uprights in motorsport applications unlikely. Indeed, the current FIA Formula One regulations proscribe the use of carbon fibre wheels and instead require them to be made from magnesium alloy, and suspension uprights to be made from aluminium alloy.

Fig. 1 - A Formula One car’s carbon fibre suspension links perform mechanical and aerodynamic functions

Written by Alan Lis

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