Carbon fiber reinforced polymer (CFRP) is one of the top-performing composite materials. Its light weight and high tensile strength combine to create higher strength per density than metal. Add to that its corrosion resistance and damage tolerance and you have the formula for a high-performance, lightweight automotive wheel. Plus, the distinct beauty of a perfect carbon fiber weave cannot be denied.
ESE Carbon has been working to craft and refine a carbon fiber automotive wheel for almost a decade. Using innovative tailored fiber placement (TFP) and the latest in high-pressure resin infusion technology, ESE Carbon has developed the first fully carbon fiber automotive wheel manufactured in North America.
Although the performance, efficiency and innovation are undeniable, can carbon fiber wheels withstand impact and avoid failure? And can they do so under various environmental factors and temperatures?
With our extensive engineering team and a motto of safety first, ESE Carbon put the E2 carbon fiber wheel to the test. Download the Impact Resistance whitepaper for the full report. Not only has ESE been leading the charge on setting the standards for carbon fiber wheels, but we also continually strive to surpass them.
SAE Recommended Practices
The Society of Automotive Engineers (SAE) creates Recommended Practices for wheels, providing numerous fatigue and impact test methods which manufacturers can use to test the strength and durability of their wheels.
Fatigue tests assess the durability of a loaded wheel during turning/cornering, during straight line driving, and after curb impact. Success in these tests requires completion without significant cracking or gross failure and—in the case of radial fatigue—without air loss.
The E2 wheels pass all four SAE tests up to a heavy sedan service load, the Tesla Model S. The real-world durability of the design was demonstrated on the road and track, including proving grounds trials in 2020.
Unique Requirements for Composite Wheels
Up until 2020, the SAE Recommended Practices only addressed metallic wheels. ESE was forced to adapt the test requirements for aluminum alloy wheels to the E2 composite wheel. But carbon fiber reinforced polymer (CFRP) composite materials are unique in their mechanical behavior and their sensitivity to environmental factors.
For example, composite materials do not typically fail due the formation and growth of a single crack, as metals do. Rather, the fibers act to blunt microcracks such that many such cracks may accumulate before large scale failure occurs. This superior damage tolerance provides increased strength and fatigue life.
Composites also do not rust or corrode like metals. However, they are sensitive to temperature, moisture, and UV light. To account for these differences, ESE Carbon internally imposed requirements more rigorous than SAE to ensure a robust and conservative design.
In September 2020, SAE published SAE J3204 “Aftermarket Composite Wheels Made of Matrix Material and Fiber Reinforcement Intended for Normal Highway Use – Test Procedures and Performance Requirements”, a new Recommended Practice specific to composite wheels. ESE Carbon’s Head of Engineering, Dr. Michael Hayes, served on the committee, lending his expertise in the mechanics and durability of composite materials. SAE J3204 imposes new requirements for carbon fiber wheels including:
- Strength reduction factors (SRFs) to account for environmental effects
- Maximum operating temperature test
- Electrical conductivity test
To complete the testing, ESE Carbon partnered with several laboratories, including Oak Ridge National Laboratory (ORNL) through the DOE-sponsored LightMAT program [read more]. As this testing concludes and suitable SRFs are determined, ESE will be submitting wheels for SAE J3204 testing.
Tests for Impact Resistance
In addition to addressing the fatigue performance of composite wheels, SAE J3204 also includes a test for impact resistance. In fact, it is the same curb impact test required of metal wheels (SAE J175), and the E2 wheel has passed this test at a heavy SUV load rating (Tesla Model X). However, SAE J3204 does not yet include a radial (“pothole”) impact test, so ESE Carbon has looked to Germany’s TÜV regulations, which include additional requirements for composite wheels, including the AK-LH 09 radial impact test.
In addition, ESE Carbon’s proving ground testing of the E2 wheel included various pothole impacts on a profile road. E2 wheels were mounted on a Tesla Model S test mule and repeatedly driven across 1.5” deep and 4” deep chuckholes at speeds up to 50 mph [read more]. These wheels not only survived the impacts, but they also exhibited no visible damage and proceeded to road testing without incident.
For more details and the full report, download the whitepaper.
Going Wheel-to-Wheel with Alloy
Impact resistance is often cited as a concern or a weakness of composite wheels. Skeptics of carbon fiber wheels may argue that they are more likely to shatter as compared with aluminum alloy or steel wheels. However, this perception is likely based on outdated, historical experience prior to the advent of the SAE and TÜV test methods, as well as recent technological progress in the field of composites.
In fact, if designed properly, composite parts and structures can provide superior impact resistance. Due to the multiple damage mechanisms available within composites to dissipate energy, a composite part such as a wheel can withstand higher impact energies before experiencing ultimate failure. And at lower impact energies associated with more typical potholes, composite wheels may hold up better than their alloy counterparts.
ESE Carbon has benchmarked the E2 wheel against several aftermarket alloy wheels of similar size and found that at radial impact energies large enough to cause dents in the alloy wheel – damage that is likely beyond the limit of safe repair – the E2 composite wheel demonstrates no visible damage.
Cold Temperature Testing
Another common concern with composites is the effect of cold temperatures on impact resistance. A decrease in strain to failure and fracture toughness below freezing may reduce impact resistance. While neither the SAE nor TÜV test methods consider the effect of cold temperatures directly, ESE Carbon nonetheless performed coupon testing AND sent E2 wheels to an independent test lab to repeat the SAE J175 curb impact and AKR pothole impact tests at -40°C. The lab used dry ice to chill the wheels prior to testing and then monitored the wheel temperature during test setup using an infrared camera.
The E2 wheel passed both tests at the same impact energies as the room temperature tests, demonstrating no adverse effect of the cold temperature. This exercise speaks to the thoroughness of ESE Carbon’s development process as the only composite wheel manufacturer running such extensive tests.
Setting the Bar High & Then Surpassing It
ESE Carbon’s due diligence continues today as we seek continuous improvement of the current E2 wheel and future wheel designs. Our goal is to not only match the impact resistance of metal wheels but to surpass it. We have implemented several innovative technologies to help accomplish this. Our R&D group has identified material and processes changes that have boosted the strength of the E2 wheels.
For example, the plies in certain portions of the E2 wheel are manufacturing using Tailored Fiber Placement (TFP). This process allows for custom-tailored plies with arbitrary fiber direction, minimal waste, and vertically integrated production. Several TFP parameters are available for process optimization, including stitch settings such as tow spacing and stitch density. Alternate materials such as backing veils, higher strength fibers, and higher toughness fibers are also available.
ESE Carbon’s product development team also pushes the performance envelope by optimizing design variables at its disposal. Stress analysts are busy running hundreds of finite element simulations to optimize the designs for strength, fatigue life, impact resistance, stiffness, and weight.
For more details on the research and testing performed, see the full E2 Wheels Impact Resistance Report.