Under the Skin: Bentley’s Incredible Carbon Fiber Wheel Car News

Reducing a significant amount of a car’s unsprung weight has a real effect on its handling, handling, braking performance, steering and agility. Unsprung weight means exactly that: the weight of a car that is not supported by the suspension springs. A big part of that on any car is the wheels and tires, especially if they are larger sizes.

Bentley’s bespoke division Mulliner developed a carbon fiber Bentayga wheel which, with a diameter of 22 inches, would be the largest composite wheel in the world. The weight saving is 6 kg per wheel – a reduction of 17.4 kg for the original aluminum alloy to 11.4 kg. Making large diameter carbon fiber wheels for large, high-performance cars is no small feat, and Bentley engineers have been working on the project with its supplier, Italian specialist Bucci Composites, since 2015.

Bucci holds its own patent on the manufacture of carbon fiber wheels and has the know-how to manufacture them by high pressure resin transfer molding (HP-RTM). The carbon fiber fabric is cut and placed in a heavy steel mold, which is sealed before the resin is injected at high pressure.

The direction of the weave in the different layers of carbon fiber contributes significantly to how the wheels hold their shape during hard braking and cornering. For example, a conventional alloy wheel can deflect (bend) in tight corners as negative camber is reduced by up to 1 degree – a lot in terms of camber. The carbon fiber wheel is so stiff that it doesn’t flex at all, which has a positive impact on grip, contact patch stability and wear of the tire it is shod with.

The biggest challenge was coping with the heat from the brakes. The wheels are basically big heat sinks, absorbing the immense heat generated during hard braking. In the center of the wheel, a forged aluminum hub molded into carbon fiber absorbs and dissipates some of the heat, but much of it is transferred to the carbon fiber spokes as well. The way the carbon fiber is laid out in the mold and the directions of the weave help dissipate heat into the air flow.

The main thing is that the heat is not dispersed by the hub and the brake disc before reaching the wheel: the wheel must face it. One of the tests the engineers did was to check the torque of the bolts after the wheel got hot. On early versions of the design, they found it was falling off, so the wheel structure was changed to fix the problem.