CompositesWorld
Issue link: https://cw.epubxp.com/i/944580
NEWS CompositesWorld.com 31 Carbon Fiber Wheels 7Y. Both feature a woven carbon fiber rim with bolted-in aluminum spokes. e wheels were both launched into the after- market, but Shelley says Dymag also has secured several OEM contracts, all for relatively high-end performance cars, including the Fisker EMotion all-electric sportscar. Shelley notes the BOXSTROM wheels' price point starts at £10,000 (~US$13,550) per set, "but that will come down as the tech- nology matures and costs fall." To reach the next level of volume (10,000-25,000+ wheels per year), he says, the price point needs to drop into the £5,000-£10,000 range (as low as ~US $6,775), with an even split between aftermarket and OEM sales. Carbon fiber outside, aluminum inside Most of the know-how in Dymag's patented BOXSTROM is in the composite rim, which is made with preformed biaxial and triaxial carbon fiber fabrics (standard-modulus 12K and 50K tow) wrapped around a mandrel and then resin transfer molded (RTM). e epoxy resin system, Araldite XB 3292/Aradur 2954, is provided by Huntsman Advanced Materials (Switzerland) GmbH (Basel, Switzerland) and was initially chosen because it offered both high-temperature durability and high mechanicals, and met the processing requirements. is resin system, says Olivier de Verclos, Huntsman Advanced Materials' technical support manager, also has been on the market for several years and, therefore, offers a level of reliability important to Dymag. Much of the rim's (or barrel's) intellectual property rests in the BOXSTROM's patented wheel flange and fastening system, which Shelley would not discuss in detail except to say that they incor- porate a structural foam core and convey physical and mechanical performance not found in other composite or aluminum rims (see the graphic at top right). Overall, he argues, the design gives Dymag a technical edge on its competitors. "is is not just black aluminum," he points out, "but a structure optimized for composites to reduce weight, while improving durability and safety." "Dymag wheels are designed and tested to the highest OEM specifications," he adds, "as opposed to some other companies that aim to meet the minimum recommended aftermarket standards, but may fall well short — especially for high-performance tuned cars and tire packages that significantly exceed these aftermarket specifications in terms of load conditions." e question is often begged of carbon fiber/aluminum wheels, why use aluminum at all? Indeed, there are monobloc carbon wheels on the market that use carbon fiber in the rim and the spokes, thus there is no technological barrier to be cleared. Shelley says it is simple: Carbon fiber limits design options in the spokes. Aluminum can be more easily adapted to a variety of spoke shapes, styles and designs, allowing aftermarket and OEM customers to meet a variety of end-user preferences and tastes. Indeed, spoke design has become a significant differentiator in the marketplace, so the material that best facilitates spoke design variation wins out. Where the wheels are tested to full OEM speci- fications, the weight differences between hybrid vs. monobloc is minimal compared to the large weight savings both offer vs. all-aluminum wheels. Shelley also notes that use of carbon fiber in spokes in high-performance applications may necessitate the design of larger cross-sections or addi- tional carbon material to achieve the lateral spoke stiffness, because vehicle brake packages often limit the physical space available for a deep spoke design. With Dymag's BOXSTROM wheels, a typical 20-by-9.0-inch wheel designed and tested by an OEM checks in at about 9 kg, and Shelley says this will likely drop closer to 8 kg as spoke design is optimized. is compares to a traditional OEM cast-aluminum wheel, at 15 kg, or an OEM forged-aluminum wheel, at 12 kg. Such weight saving, Shelley notes, is very difficult for OEMs to ignore. "OEMs wouldn't touch a carbon fiber wheel five years ago," he argues. "But now they are all working on it for a combination of performance benefits and the drive for lower weight to reduce carbon emissions." Room for improvement Increased OEM interest in carbon fiber wheels, although good for companies like Dymag, also brings with it more scrutiny and more pressure to improve materials and manufacturing processes, and reduce costs. Still, once developed, this wheel now capable of higher-volume manufacture also must meet safety and perfor- mance standards. Wheels are every vehicle's touchpoints with the road, and as such, must perform flawlessly. "Wheels, tires Patented wheel flange and fastener system Much of Dymag's design innovation and expertise has gone into the Boxstrom wheel's resin transfer molded barrel and how it intersects with the spokes. This cross-section shows the fastener from the aluminum spoke engaging with the barrel's flange. Section 1 is the primary load path of the barrel. Section 2 is a biaxial layer that supports the primary load path and supports the fastening system, which carries 90% of the radial load. Wheels are every vehicle's touchpoints with the road, and as such, must perform flawlessly.