F1 alchemists - carbon fibre composites in motorsport

Materials World magazine
,
1 Jun 2008

B3 Technologies in Guildford, UK, is at the leading edge of design and manufacture in Formula 1 (F1). The company developed around a core of personnel responsible for pioneering many of the technologies synonymous with modern F1 racing and currently supplies the majority of the teams on the grid.

B3 grew out of a company set up by Ferrari as a UK-based ‘skunkworks’ under their then technical director, John Barnard. The aim was to gain access to the advanced design, materials and manufacturing processes in the UK.

The plan was that the business would run for five years, during which time facilities and knowledge would be transferred to Italy. At the end of the exercise, the company became independent from Ferrari, trading as B3 Technologies from 1997.

John Barnard is credited with much innovation in F1. At McLaren he created the first carbon fibre monocoque, the basic building block of the racing car and critical to driver safety. This commitment to new technology remains important.

Exploiting CFC composites

Many innovations developed by B3 Technologies rely on carbon fibre composites (CFCs) to provide specific properties that are unobtainable with traditional metallic materials and manufacturing techniques. But it is not enough to take an existing metallic component and simply change it to a CFC.

While this approach can yield some gains, greater advantages can be achieved by designing components to exploit the properties of CFCs. Essentially, CFCs are not black metals. A detailed understanding of composite properties, manufacturing techniques and limitations are required to realise their full potential.

Formula 1 typically employs composites of continuous aligned carbon fibres in an epoxy resin matrix. This creates an anisotropic material, one with different properties in different directions, compared with metals which are isotropic, with properties constant in all directions. It is this directionality, combined with superior physical properties and formability, that enables the production of a more efficient component than an equivalent part made from metal.

By tailoring the direction, volume and distribution of the CFC, material and properties can be positioned where they are required and removed from where they are not. The minimum quantity of material is used to obtain the desired strength or stiffness and, therefore, the most efficient way to carry the required loads.

With CFC’s low density this produces a lighter component, but there are other benefits offered by manufacturing in carbon composites. The evolution of the wishbone is a good example to illustrate the development of a component from metal to composite, and highlights the advantages gained.

Wishbone design

On a F1 car there are eight wishbones, an upper and lower one at each corner, which essentially attach the wheels to the chassis. Before the evolution to composites these were fabricated from two elliptical section steel tube ‘legs’ welded together and then to machined ends. These ends housed spherical joints to allow for the vertical travel of the wheel relative to the chassis.

Aside from the obvious mass saving (the density of CFCs is roughly a quarter that of steel with equivalent or superior mechanical properties) CFCs allow greater flexibility in shape. In steel there is a limited choice of cross-sections in which the legs can be formed. Carbon wishbones, however, can be manufactured in one piece with full airfoil sections which can vary along the length of the leg. This yields a significant aerodynamic gain as well as removing the joint between the two legs. In fact, the greatest restrictions to the design are the requirements set out in the Federation Internationale de L’Automobile (FIA) Technical Regulations.

Another development in wishbone design was the introduction of flexures. As the spherical joints wear during a race, the ‘feel’ of the car is affected and handling becomes inconsistent. The solution is thin plates to replace the bearings at the wishbone-chassis interface. These flexures are designed to ‘bend’ as the wheel bumps. They work as the limited vertical wheel travel on a F1 car creates only a small bend angle in the order of two degrees.

The first flexures were manufactured in steel, however, they could not be made to work on the rear wishbones. This was due to the stiffness/thickness ratio required for higher loads at the rear, which buckled the flexures. The next move was to switch to titanium, which has just over half the stiffness of steel, with the flexures bonded to the carbon legs.

But this approach was still a compromise. To allow the flexure to function it has to be strong enough to withstand the suspension axial loads and, at the same time, be flexible enough to allow for wheel movement without inducing large bending stresses. With titanium, the combined stress levels were above the material’s fatigue limit and therefore the components had to be ‘lifed’ and replaced during the season after a predetermined mileage.

This was addressed by making the flexures in CFCs integral with the wishbone legs. For this application the CFCs have an effectively infinite fatigue life, allowing the wishbone to be run for an entire season. The fibres act as crack arrestors, ‘blocking’ the crack propagation path.

The full carbon wishbone is also faster to manufacture. The end fittings no longer require machining from titanium and by removing two bond interfaces, the bonding operation is removed.

The use of CFCs does, however, introduce some new challenges. One related to the wishbone is the retention of the wheel during an accident. Unlike steel, carbon has a catastrophic failure mode, being linear-elastic to failure with no region of plastic deformation.

This greatly increases the risk of a wheel becoming detached from the car in the event of a collision. This is reduced by running a tether inside the wishbone, between the chassis and upright. The tethers are made from polybenzoxazole (PBO) Zylon fibre, such as those employed in rigging on yachts, and restrain the wheel should the wishbones break.

Benefits of CFCs

Just by considering a single component on a F1 car it is possible to identify a series of advantages to using CFCs. Aside from the improved specific properties and tailoring of the material’s placement, orientation and type, the formability offers significant opportunities.

The ability to produce complex shapes aids aerodynamic performance and reduces the part count, improving overall efficiency. High fatigue resistance increases component life, ultimately improving reliability, which is critical to a modern F1 car with the current regulations restricting access to the vehicles over a race weekend.

But it is also apparent that certain CFC characteristics, including failure mode and operational temperature limitations, require careful consideration when designing a part.

 

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