Rounding Up: Do Carbon Fiber Wheels Improve EV Efficiency?

After range and charging time, a concerning factor of electric vehicles, like the 2022 BMW iX, is weight. For the same size vehicle, an EV will be heavier than its ICE counterpart due to the weight of the battery pack. While carbon fiber has been used extensively in vehicles like the outgoing i3, i8 and going to be used in the unibody and body panels of the iX M60, it's never been extensively used as a material for the wheels, beyond some performance applications. Now, Carbon Revolution—the makers of the Z06 Z07 carbon fiber wheels—are looking to change that.

Why It Was Never Considered Earlier

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Up until a few years ago, producing anything in carbon fiber was a laborious task. Woven sheets needed to be hand laid, vacuum sealed, and heated in an autoclave to create their parts. It was this time intensive process that made the material expensive to work with. Wheelmakers needed to find a way to make their products in the woven stuff without that costly —in both time and labor—human factor by introducing automation.

It was that goal of automated carbon fiber work that is the heart of Carbon Revolution since it was founded back in 2007. What started 10 years ago as a bespoke process that resulted in just a few wheels a year is now done nearly entirely by automation and allows Carbon Revolution to produce up to 25,000 wheels per year. This is why OEMs like Ford and GM have begun to rely on the carbon wheel company to produce OE, fully carbon fiber wheels for their sports cars. Now, Carbon Revolution has their sights set on the EV market and using an economy-of-scale to bring carbon wheel production costs down even further.

How Carbon Fiber Wheels Compare to Aluminum Alloys

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We spoke to Ash Denmead, Engineering and Design Director at Carbon Revolution, about some of the benefits of using carbon fiber as a manufacturing material for wheels. Turns out, it's more than just the reduced mass of this lightweight supermaterial, but also how it can be shaped and sized over the standard cast and forged manufacturing of lightweight aluminum wheels. First, we're going to get you up to speed on how aluminum wheels are made, as this is important to know before we step into why carbon might be better.

Aluminum wheels are made in one of three ways, at the time of this writing. First there is casting, then there is forging, and finally there is a process that uses a bit of casting and forging that's called flow forming. What all three have in common is that finishing work like the bolt holes, center bore, and detail touches are then cut or ground out on CNC machines, including the "machined face" that many OEMs like to use nowadays.

Cast Wheel Manufacturing

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Casting is kind of like putting a cake into a mold to make a shape, but instead of flour, eggs, sugar, and a pinch of salt you're pouring in a molten aluminum alloy. Some manufacturers will also spin that mold to ensure that the alloy is spread evenly to prevent voids in the metal and try to get some consistent metal grain structure, but its grain usually never entirely perfect. That's the downfall and weakness of casting.

Hammering Out A Wheel

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Forging is the process you'll be familiar with if you watched a show like Master of Arms or Big Giant Swords, where you saw people slamming down hammers on metal held against an anvil. Rather than hammers and anvils, aluminum billets are pressed and hammered into shape by machines. This forging allows the metal's natural grain structure to remain, but be compacted as it's brought into shape and this is the strength of a forged wheel. It remains rigid and strong without becoming brittle because that grain is preserved.

Little of Column A, Little of Column B

Flow forming is a mix of both casting and forging, as mentioned earlier. First, the wheel is cast in its preformed shape, where the final shape of the wheel face is created by the casting but the wheel barrel is not fully formed. Once cooled to a sufficient temperature, the casting is spun on a machine as a forming drawbar—controlled by a robot—forms the retaining humps for the tire bead, the well that allows a tire to be mounted, and the final width of the barrel. This causes the random grain structure of the cast wheel to become more uniform in the barrel. This gives you the low cost of casting with the added rigidity that comes in drawing out the metal for the barrel. It's still not as strong and rigid as a fully forged wheel, but it is superior to a fully cast wheel.

What Makes Carbon Fiber Better In Forming A Wheel?

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You probably noticed a common theme of "rigidity" over simply saying that the wheel is stronger and can hold more weight. That's mostly because "stronger" isn't quite what you're looking for in a wheel. When it comes to it, you want a wheel that can flex under impact without breaking apart—its rigidity—while also holding the constant load it's rated for—its stiffness. What you don't see as you drive in your daily life is just how much your wheel flexes and moves as the tires impact potholes, speed bumps, curbs, and other daily hits it goes through. If it didn't, your wheel would break due to stress, as some "strong" wheels made inexpensively are prone to developing microcracks—or, more dramatically, failing entirely.

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High rigidity and strength without becoming hard and brittle is a key "strength" of carbon fiber. This comes from laying out the threads of a carbon fiber weave to influence how impacts are transferred through the wheel and how stiff it is under a sustained load. This allows a carbon fiber manufacturer to use less material for the same rigidity and load capability of an equivalent metal, which is why a carbon fiber part weighs much less than the metal one.

As Denmead points out, "The great thing about carbon fiber is its strength—it's so much lighter than aluminum at the same strength—and the fact that it's an anisotropic material." What he means is that carbon is strongest in the direction of its fiber rather than isotropic, where a material is indifferent to the direction you're measuring. Steel and aluminum are isotropic materials, up to certain criterias.

Denmead continues, "This means we have a lot of freedom to design wheels with different characteristics—we can add or remove material in certain places to make the wheel lighter, stronger or quieter. We can make a wheel that is 45 percent lighter than the aluminum equivalent, while meeting the same durability criteria set by an OEM. Alternatively, we can reinforce it with additional material and make it still 35 percent lighter, but 50 percent stronger than the aluminum equivalent."

Making Wheels More Aerodynamic

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There is also an advantage in designing a carbon fiber wheel that steel or aluminum can't match when it comes to carbon fiber manufacturing. Aerodynamic wheels typically require small openings or even be totally closed off, which would sound like a disadvantage in dissipating heat caused by braking. But EVs' use of regenerative braking means that braking heat isn't as much of an issue with EVs as their gasoline counterparts.

The issue, unfortunately for metal wheels, is that this is more material that remains and that means more weight. "Carbon fiber materials and manufacturing processes allow us to design and produce very efficient thin aerodynamic structures without any significant weight penalty," said Denmead, "which opens up fantastic design flexibility for EV OEMs."

This also applies to making wheels larger with the addition of being able to cope with the additional stresses of an EV and road conditions. "OEMs are equally excited about the benefits that Carbon Revolution wheels can offer for upcoming EVs," he continued, "For example, we are well progressed with the development of 24-inch carbon fiber wheels for an EV pickup truck, which will be around 45-50 percent lighter than an aluminum wheel of an equivalent size."

According to Denmead, even as the size increases, the strength of the wheel isn't compromised to meet a certain weight requirement that a manufacturer might have for their wheels. "The 24-inch EV pickup wheel will weigh the same as an 18-inch alloy wheel," said Denmead, "yet it will be strong enough to withstand the vehicle's high load requirements." Being able to create larger wheels also satisfies customers who like the look of larger wheels.

Reducing Rolling Resistance Increases EV Range

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Aside from the looks of larger wheels, there is another advantage to increasing wheel diameter: rolling resistance. Increasing the diameter helps in reducing rolling resistance by reducing the sidewall size, thereby reducing the flex and squirm in the tire that takes energy away from just allowing it to roll and increases an EVs range.

"The aerodynamic performance of wheels also becomes more important as wheels get bigger," stated Denmead, "and carbon fiber offers design teams far more flexibility to create aerodynamic forms without a weight penalty."

Why Not Work With 3D Metal Printing Or Metal Matrix Composites?

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If you're familiar with 3D metal printing, you're probably sitting there thinking about using additive manufacturing which would allow for strong, yet lightweight metal components by using only the amount of metal needed by design. You've potentially seen this before in load bearing structures that look "organic" and is indeed an interesting possibility.

However, the limiting factors for that are the cost, speed and quality of 3D printing metal at this time. The cost and speed are potentially equivalent to early carbon fiber manufacturing and the quality of a print is still questionable with certain types of 3D metal printers, particularly those that use direct metal laser sintering. While not an issue for prototyping and non-structural parts, there are places where the heat of sintering the individual grains of metal clump incorrectly or develop voids from where some of the grains fling away as it's being melted into form.

A Metal Composite

Metal Matrix Composite (MMC) is also an interesting manufacturing process in which metals are formed using strengthening materials of an opposite material, such as aluminum with embedded carbon fiber strands. This allows a manufacturer to utilize a lighter material by using a stronger material where needed for load strength and rigidity. Again, the issue of using MMC comes down to cost, tooling and time.

It also requires materials that will not negatively interact with each other, or to apply coatings to materials that do. One common example of an MMC is aluminum with embedded carbon fiber strands, but these two materials interact with each other to form aluminum-carbide (Al4C3), which is water-soluble and forms a brittle surface. This is prevented by coating the carbon fibers with nickel or titanium boride.

Carbon Fiber Is Older Than You Think

The evolutionary age of manufacturing is also an issue. While MMCs have been around since the 1950s, carbon fiber has been around long before that. Technically speaking, its first use dates back to the 1860s with lamp filaments and the modern version of this wondrous material was developed in the late 1950s to early 1960s.

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"It's well accepted that carbon fiber composites are the highest performing materials available at considerable scale in the world today," said Denmead. "This is a new, disruptive application for carbon fiber, carbon fiber itself is not a new technology which is very important because its performance now is well understood in many other applications." Much of that understanding has led to an evolution of new carbon fiber production technologies, bringing costs down while increasing the strength and quality to the point of producing wheels for OEMs.

Reducing The Cost of Manufacturing

Carbon Revolution is leaning heavily into automation to solve some of the labor/cost issues associated with the material. "As the first company to produce and industrialize carbon fiber automotive wheels," said Denmead, "we have learned a lot in the process. Our company was founded 15 years ago and our earliest wheels were made in a highly bespoke and laborious manner, which was reflected in the price tag."

By incorporating more automation and new processes in production of their fully carbon fiber wheels, Carbon Revolution wheels are nearly in line with the cost of high-end and custom forged aluminum wheels and the price will continue to drop. While a set of $1,000 carbon fiber wheels are not on the horizon yet, a set that costs less than $10,000 is.

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"Economies of scale will help achieve that," explained Denmead, "both in reducing raw material costs in our supply chain as demand for carbon fiber products increases, as well as spreading those and other costs over a larger number of wheels produced in our factory. We're also getting better at making high quality wheels on time, with new wheel designs better optimized for production, and the hiring of some key personnel with extensive OEM and OEM supply manufacturing experience to ensure our processes become even more consistent."

A huge improvement for Carbon Revolution is the start of their first "Mega-Line" factory and implementing technologies like machine learning, VR, virtual validation, and advanced robotics.

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To really hammer the point home, Denmead left us with this, "This has allowed us to serve a far greater range of OEM customers. But we aren't stopping there, because we have a much larger market to crack." We're excited to see what the future holds for both EVs and carbon fiber technology—from body parts to vehicle structures to wheels—as we continue into the 2020s. Both were once pie-in-the-sky dreams of Art Deco era engineers and are now becoming a part of our modern day reality.

What Do The OEMs Think? They Only Say, "We're Aware. "

We spoke to a few manufacturers who produce electric vehicles to get their input on using carbon fiber wheels for the advantages laid out in this article. None were willing to comment directly on the them except to generally say "we're aware."

Ford, for example, was only willing to go as far as to say, "Ford is aware of the benefits of carbon fiber wheels. We continually monitor relevant technologies for applications in our products, however we do not discuss future products." We even pressed them to potentially get an engineer's perspective, just to see if they even agree with the points laid out by Carbon Revolution, but they were unwilling to comment further than that. While that is a disappointing close to this story, we look at it another way: maybe this is their way of saying something is coming in the near future.