Weighing up the advantages

A technique for creating carbon fibre body panels at a quarter of the cost, used on the new MG X-Power SV sports car, could lead to the material being used more widely in vehicle production.

A technique for creating carbon fibre body panels at a quarter of the cost, used on the new MG X-Power SV sports car, could lead to the material being used more widely in vehicle production.

The recently launched X-Power SV, designed to compete with other high performance sports cars such as Ferraris and Porsches but at the much lower cost of £75,000, weighs just 1,540kg. The panels were developed by the UK-based SP Group.MG is to make 250 of the cars each year for the next four years.

Carbon fibre is used in performance vehicles as it is tough but lightweight, improving speed and fuel efficiency. However, it is difficult and costly to manufacture, and as a result is only used in vehicles costing around £200,000 or over, such as the McLaren F1. Mass produced cars continue to be made from steel and aluminum.

But with the continuing trend for mass customisation, the potential for using a lower cost carbon fibre to build cars is likely to increase, said Martin Starkey, automotive projects manager at SP. Tooling for composite parts is cheaper, making the material more cost effective when producing a smaller number of cars.

The company claims to have reduced the cost of a carbon body shell from around £40,000 to around £10,000, a reduction of 75 per cent.

‘For mass-producing 20,000 cars, using metals such as steel and aluminium is still likely to be the cheapest option, but if you are producing from five up to around 2,000, carbon fibre could be the answer. These would still be top-end vehicles, but they would be the OEM’s top-end vehicles.

‘The technology means £75,000 carbon fibre cars are now realistic,’ he said.

The composite material typically used by luxury car makers is pre-preg, which consists of separate layers of pre-impregnated carbon fibre and glue film surrounding a honeycomb core.

This is lightweight and strong, but pre-impregnated carbon fibre is extremely expensive, and producing composite panels from this material is highly labour intensive. Also, during the process of layering the different materials together air often gets trapped inside, requiring an expensive autoclaving process to remove it.

SP, which has a background in producing composites for racing boats and wind turbine blades, has developed a ‘breathable’ material, in which the fibre is separate from the resin. ‘It is a very dry, porous material, so you can extract the air very easily, and you no longer need expensive claving, you can simply set out the layers and have a single cure at the end,’ said Starkey.

‘We have created a material that weighs 20 per cent that of steel, and 35 per cent that of aluminium, for the equivalent stiffness,’ he said.

Existing composites also suffer from surface problems, which can affect the aesthetics of the car’s body. pinhead-size imperfections in the surface of the material can be caused by air becoming trapped during the production process. When the large paint molecules hit these holes they compress the air trapped inside and spring back out again, forming ‘fish-eyes’ around the imperfections.

To prevent this effect, the company has produced a barrier layer over the carbon fibre, consisting of a fine fibre structure sandwiching a catalysed resin film, which forms a uniform surface on curing.

The barrier layer also prevents carbon fibre read-through in hot weather, the fish-scale effect that is caused by the fibre and resin expanding at different rates in the heat. As a result there is no need for the thick layers of paint used on other performance cars to prevent this effect showing through, which is expensive and can lead to paint chips.

‘Fibre read-through is very hard to see in the painting process, so you just keep layering on the paint, but the effect returns with time, temperature and humidity,’ said Starkey.

The panels can simply be painted in the same way as conventional steel panels.