Carbon Fiber Comes to Performance Wheels
Ford’s GT350R is the first series-built automobile to offer one-piece carbon fiber road wheels from the factory. However, owners of vehicles like the Porsche 911, Audi R8, BMW M3, Chevrolet Corvette, as well as various vehicles from Lamborghini and McLaren, have been able to plunk down their money for the privilege of reducing the unsprung weight of their high-performance automobiles since 2013. And while some buyers may have bought them for street cred—at $15,000 a set—most saw this as a performance upgrade on par with blueprinting the motor, upgrading the suspension and adding a turbocharger. “It’s not a case of ‘Do I like the wheels on my car?’,” says Jake Dingle, CEO of Australia’s Carbon Revolution (carbonrev.com). “It’s a case of bolting on a single technology that gives you better acceleration, better braking, higher cornering force, lower NVH levels, improved steering feel and handling, and increased fuel efficiency.” The things that attracted Ford as it put together the GT 350 program.
Carbon Revolution’s composite wheels didn’t happen overnight. They followed a long process that originally began as part of an independent R&D mentoring program for university teams involved in Formula SAE. Ashley Denmead, Carbon Revolution’s Design Director, was a member of one such team, and that participation led to the first composite wheels appearing in 2004 on a Formula SAE car. The design and idea behind it went through many iterations before Carbon Revolution contacted European OEMs in 2009 to see if they had any interest in lightweight wheel technology, and what they needed to see from a validation standpoint for such a safety critical part. Three years were spent collaborating with them to discern the necessary requirements for a composite wheel.
What caught the OEMs’ attention was that the Carbon Revolution wheel could provide a 40% to 50% weight savings versus a factory alloy wheel design. “For Porsche, our OEM-validated wheel is 15 lb. and the factory wheel is 26 to 28 lb.,” says Dingle. “That’s a big weight savings.” It took a lot of testing with the German testing agency TÜV, talking to the OEMs about their requirements, and developing a validation program that encapsulates both OEM and aftermarket standards. From a structural standpoint, the tests fall into two categories: (1) an impact test, and (2) fatigue tests. One is a bi-axial fatigue test done in Germany that Carbon Revolution now does in Detroit with Independent Test Services. “We baked all of this together to create our own test criteria that encompasses all of the most stringent checks, and added another 30% on top of that.”
In the test, a robotic arm presses the tire against a rotating drum, loading up the wheel in both the vertical and axial directions. The forces are based on laps of the Nurburgring, and take four days to complete. It covers 7,500 kilometers (4,660 miles), a duty cycle that, Denmead says, “is equivalent to what you’d see in a vehicle over 300,000 kilometers.” However, at 130% of standard, the requirement Carbon Revolution set for itself, was beyond the burst limit for the rear tires. “We had to run it to 125%,” says Dingle.
“Carbon fiber composites have an almost infinite fatigue life,” he says, “but there have been questions about how it would perform on impact as the material has a reputation for being brittle, especially when impacted the ‘wrong’ way.” They needn’t have worried. In destructive tests replicating a serious accident, designed to highlight whether or not the material has a safe failure mode, an aluminum wheel will split open circumferentially. “Ours are designed to fail on the outer bead so you can see it, and it gives a slow air loss,” says Dingle. “Certification bodies and OEMs favor failure modes that are readily apparent to the average consumer.”
When Ford called, it wanted to be certain the composite wheel would meet all of its requirements. That meant subjecting the wheels to a lifetime of sun exposure to see how UV rays would affect the carbon fiber resins, running them against curbs at speed, and exposing them to the extreme high temperatures produced by the GT 350R’s hybrid aluminum hat/cast iron disc Brembo brakes. According to Brett Gass, Carbon Revolution’s Executive Director, “The process is proprietary to Carbon Revolution, and plasma arc sprays a multi-layer coating on the wheel’s inner barrel and spokes.” This ceramic material protects these areas from the 1,652º F (900º C) heat that can be generated by the GT 350R’s brakes under hard use. The wheels also receive a special coating designed to shrug off road salt and UV degradation, though the idea of a GT 350R playing in the snow is a bit … ridiculous.
“The goal from the outset has been as much on manufacturability as on the product itself,” Dingle declares. That means the Carbon Revolution design doesn’t require the use of an autoclave or the $100/kilogram pre-preg process. Instead, it uses dry fibers and is closer in concept to a classic resin transfer molding (RTM) procedure, though it has been designed to be something that is “light industrial” in terms of energy, infrastructure and automation. The work with Ford has pulled Carbon Revolution along to its next step, a production setup that can build 250,000 wheels per year. “The process can be expanded relatively easily,” says Gass, “by adding lines in parallel. It’s a very low energy process with robots for the placement and cutting of the tailored fibers, and will allow us to achieve aerospace quality levels.” Yes, aerospace.
With weight a major determinant of fuel use and cargo capacity, pulling weight out of an airplane is profitable. “Aerospace is quoting $3,000 per kilogram per year in fuel cost,” says Denmead. Extrapolate that against the 14 to 16 wheels per aircraft, multiply that across a fleet of planes and then add in the retrofit fleet, and you have a market ripe and ready to pay for this technology. Heavy trucks are another. Painted truck wheels that don’t corrode, are built at low cost, and have a long life while conferring fuel savings are very attractive. As these projects move forward, another lurks in the background, and it may be the most significant yet.
According to Denmead, there is a plan for a value-engineered product in the “low hundreds of dollars per wheel that would be painted and with metal bits that are value engineered instead of hard anodized.” A 17-in. wheel built to these specs would weigh about 4.5 kg (10 lb.) with a weighted average material cost of less than $10/kg. Even with the labor, paint and fitting costs added on top, carbon fiber wheels would still be very competitive with alloy designs. “Now you can see why [alloy wheel maker] Ronal is an investor,” says Gass.
Helping to drive the move toward a significantly lighter wheel in volume production is the legislative landscape. Not only do automakers have to dramatically reduce the weight of their vehicles in the next decade to meet fuel economy regulations, legislation is pending that will require a full lifecycle analysis of the car and its components from cradle to grave. “We do very well in that analysis,” says Gass. And, as automakers increase their use of carbon fiber and drive the industry to create better fibers and resins at lower cost, Carbon Revolution benefits. “Our production process is independent of a particular fiber or resin,” says Gass. “As they improve, we improve.” Further, each $100-million invested to erect a major carbon fiber plant will create the capacity necessary to move to alloy wheel production levels (2.5 million units/year, which will require about 20,000 tons of carbon fiber) cost effectively. As to how long this next step will take, no one is saying. However, with CAFE standards tightening considerably in the 2020 to 2025 timeframe, the time for this revolution is near.
Forged Magnesium: The “Affordable” Choice
Bill Koenig is a self-described “lightweight freak” and long-time Porsche owner who has been chasing magnesium wheels for street cars for years. The executive vice president of California-based MKW Alloy Wheels (mkwalloy.com), Koenig—who consulted with Carbon Revolution on its composite design—was able to convince “a Japanese company that builds forged magnesium wheels for multiple Formula 1 teams” to build a forged magnesium design “optimized for both street and track.” Though Koenig is unwilling to divulge the identity of his wheel partner, it’s likely Rays Engineering Co. Ltd of Osaka, Japan stepped up to the plate.
Porsche’s 911 is the target vehicle for the first set of wheels as these owners are not only well-heeled, they also are rabid about using them at track days. Wheels for the 911 GT3, for example, come in a 19-in. diameter and widths of nine inches (front) and 12 in. (rear). However, unlike the $15,000 Carbon Revolution wheels, the forged magnesium rims—which sit between the factory alloys and composite wheels in terms of stiffness and heat dissipation—cost $8,800. That’s just $700 more, for example, than the carbon composite brake package on a BMW M4.
“The Japanese have metallurgists on staff working with suppliers to ensure the magnesium alloys are to spec.,” says Koenig, and the design works for “multiple applications without having to forge multiple blanks.” A good thing since each blank design costs about $100,000. “We developed profiles that work on multiple vehicles,” he says, “and the reduction in rotational mass is about three times less than a stock alloy wheel,” or just short of the reductions Carbon Revolution is quoting for its wheels.
MKW’s forged magnesium wheels are part of its RSR (“Road, Street, Race”) line. Road rims are exemplified by cast-aluminum wheels. Street wheels use flow-form casting that spins the heated form in order to get the aluminum to flow and bind together. This creates a wheel stronger than a cast design, but with a thinner cross-section. The Race units are the forged magnesium wheels. Eyeing the success of Carbon Revolution’s design with Ford, Koenig is quick to point out: “We are open to new projects and vehicles other than Porsche.”—CAS
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