Composite wheels boost impact resistance

A UK company has developed a carbon/magnesium composite wheel for performance vehicles that is not only lighter than conventional wheels, but can also offer increases in speed, can take much greater impacts than conventional wheels and provides considerable potential for fuel savings into the bargain. The wheels, which have been developed over a five-year period, are the creation of NRG Wheels, a UK company, whose research and development director Bevis Musk admits ruefully that, while the first wheel was produced from concept in just eight months, development overall has taken five years to get to this point. Based in the UK, NRG Wheels specialises in designing, manufacture and marketing structural composites for the automotive industry. The carbon/magnesium car wheel was originally invented to create a better driving experience based on improved handling and better impact resistance as well as reduced fuel and engine emissions. This has been achieved by focusing on the two main components of the wheel; the hub and the rim. The forged, magnesium alloy based hub of the wheel has been designed to reduce the normal cast magnesium porosity and corrosion potential of the wheel to zero. The other major feature influencing the wheel's design and performance is the carbon composite wheel rim. NRG Wheels developed and produced the wheel rim with the support of Huntsman Advanced Materials who selected and provided an Araldite resin system especially adapted for Resin Transfer Moulding (RTM). The resin was also designed to achieve the highest level of targeted performance, defined by challenging criteria to deliver increased toughness and better impact resistance. During the RTM process to produce the carbon rims, aerospace type carbon fabrics are put into a mould and injected with epoxy resin. Specially-coated titanium fasteners working within specially-bonded bushes fasten the hub to the epoxy carbon fibre rim, so no additional auxiliary component bonding or finishing is required. Thanks to the quality of both the resin and the processing conditions, the visible outer side of the rim has an aesthetically pleasing surface finish, showing the carbon fibre pattern. The surface is protected from UV light and other environmental elements with a tough, high-gloss lacquer paint finish. Combining properties of toughness and high temperature resistance, the Araldite RTM system assists in creating a carbon wheel that offers better impact resistance than metal wheels and has high fatigue resistance. The carbon/magnesium wheel is 40% lighter than even the lightest aluminium or magnesium wheels. Substituting conventional materials with composites creates a direct primary weight reduction, allowing the carbon rims on the wheel to have a noticeable effect on a car's fuel economy, reducing fuel consumption and emissions while enhancing performance and handling. When tested on a Porsche with a 380 bhp engine, the carbon/magnesium wheels helped to achieve power savings of around 43 bhp, equating to an approximate fuel saving of 10%. Says Musk: "We tested them with Porsche and they were amazed by the increase in speed. "And, because the wheel is lighter at the rim, it means that there is less fuel used in turning it. These wheel types reduce gyroscopic effects and the moment of inertia. This results in improved acceleration and braking with reduced stopping distances, better grip, lighter, sharper steering, improved wheel and tyre balancing, more stable tyre temperatures and pressures, as well as reduced fuel consumption and engine emissions. "The high-toughness resin makes the carbon matrix work to its best possible extent," says Musk. "The special Araldite RTM resin was ideal for this application in meeting the high strength to weight ratios required for improved impact resistance, allowing the carbon/magnesium wheel to take over two times the impact of metal wheels." This impact resistance is probably the single most spectacular benefit offered by the composite wheels, in fact. Says Musk: "The wheel demonstrates resistance up to an impressive 1600 joules. In contrast to metal wheels, cracks do not increase and the wheel reverts to its round shape while retaining great strength, rigidity and the flexibility required for both road and track. This makes the wheel safer in road impacts as the tyre retains its air and the car can be driven safely, even following a major impact." In fact, video footage shows that the wheels pull back to shape from as much as six inches of deformation and, despite applying pressures "equivalent to dropping tonne weights on it", Musk says "I've never had a wheel break in a test yet". This resilience, claims Musk, means that the wheels could be of use in military, as well as civilian applications. For military applications, not only could carbon composite wheels prove useful in improving overall vehicle performance, it could also prove beneficial in ballistic response settings. When compared to metal wheels, in extreme blasts less shrapnel penetrates the vehicle equipped with carbon composite wheels. Says Musk: "These wheels could be used on military vehicles as their impact resistance gives them a chance of driving away from an IED, while the shrapnel from the composite wheel is non-injurious when compared to other materials." The overall potential for carbon composite wheels is large, but specific. The wheels are suited to higher-value and high-performance vehicles. The instant performance advantage and simplicity of fitment makes the NRG carbon wheels a natural choice for highly tuned vehicles, either OEM or aftermarket. The reduced moment of inertia generated by using composites also makes the wheel suitable for use on delivery vehicles and buses, where stopping and starting is frequent. RTM with suitable epoxy resin systems allow the production of reproducible high quality parts within acceptable cure times. The flexibility of this process also authorises new design ideas, such as the carbon wheel rim, which would not be possible with metal constructions. Musk concedes that, as things stand, the wheels are still "at the high end in terms of cost due to the cost of the raw materials", as he puts it. However, he anticipates further developments of the technology. NRG Wheels and Huntsman recently won the Sports and Leisure category of the JEC Innovation Awards 2011 for the carbon/magnesium wheels, in recognition of the innovative use of composites in high performance vehicle applications.