Magnesium versus Carbon Fiber: Lightweight Wheels Reality Check

Porsche has announced that it’s creating braided carbon-fiber wheels, being offered as an option on the 911 Turbo S Exclusive Series.  

Cost (in Germany): EUR 15,232, including VAT.  

Granted, the 911 Turbo S Exclusive Series already has a price tag of “from $257,500” (the model’s trunk lid, roof, and side skirts are made of carbon fiber).  But at an equivalent in USD price of $18,000 per set of wheels, while certainly cool-looking and a terrific status symbol conversation-starter, it still might be asked if they’re considered a questionable investment.  In terms of performance are they “better” (are they worth it?) than competitive, best-in-class forged magnesium wheels?

   The Turbo S Exclusive lists 607 hp, 2.8 sec, 0-60 with Sport Chrono, and all-wheel drive among its features.  Now, Porsche says it has become the world’s first vehicle manufacturer to offer lightweight wheels with braided carbon fiber. For the Turbo S and other supercars across the board, in the vehicle mass/performance equation, lighter is better.  Porsche notes in a press release, “Together, the innovative wheels weigh around 8.5 kilograms less than standard alloy wheels, which is a reduction of 20 per cent.” (Porsche is quoting dimensions of 9 J x 20 for the front axle and 11.5 J x 20 for the rear axle.)  “They are also 20 per cent stronger,” the press release continues. “With a reduced unsprung weight, the tires trail the surface of the road better and are perfectly optimized for absorbing longitudinal and lateral forces. Lower rotating masses mean more spontaneity both in acceleration and braking. The result is increased driving dynamics and driving pleasure.”
But, according to Marks Lisnanskis of SMW Wheels, “The weight of forged magnesium wheels for this model is almost 10% lower.  And because of its hexagonal crystal structure, magnesium possesses special characteristics such as very high form stability and damping capacity. So magnesium wheels are decisively superior at vibration absorbance because the
damping factor is many times better than in carbon fiber.”

Beyond potential loss of kinetic energy that racing professionals may be concerned with, absorbing shocks and vibrations is critical for a comfortable ride.  As Mr. Lisnanskis notes, “A driver feels them in his spine.”  Because magnesium absorbs shocks and vibrations so well, automakers are increasingly designing the crossbar beam in many cars (the “backbone of the dashboard,” the component is sometimes called the steering hanger beam) to be produced in magnesium. Magnesium helps avoid the trembling/quivering of the steering wheel from occurring.

In the case of both passenger cars and high-performance racing vehicles the impact of significant weight reduction is especially important because wheels, along with the brake system, comprise unsprung weight. The ratio of sprung to unsprung weight is essential since the force exerted by the unsprung components on the car directed upward must be offset by the sprung weight–otherwise the car loses its grip with the asphalt making it harder to steer, especially at cornering.  (Tires, wheels, wheel bearings and hubs, and brake assemblies comprise a car’s unsprung weight.  Another way of considering the difference is “unsprung weight” moves when the wheel moves, “sprung weight” moves when the chassis moves.)

Reducing unsprung rotating weight can help improve the dynamics of a vehicle (as noted by Porsche), since the heavier the wheels, the more energy and time are required to alter their speed of rotation. Lightweight wheels help achieve faster acceleration and may reduce stopping distance. The aggregate effect on a vehicle when replacing typical wheels with forged magnesium versions can reach 5% and more.  And magnesium alloys have greater capacity for heat dissipation, which helps prevent brakes from overheating.

Actual head-to-head comparative analysis between quality, true-forged magnesium wheels and carbon fiber wheels of the type Porsche is now offering can be performed by conducting thorough comparative testing at various road conditions. But there’s another factor that might prove to be a deciding one for environmentally-minded consumers: magnesium is an incomparably earth-friendlier material than carbon fiber, which is quite difficult to recycle at end-of-life, and also leaves a higher carbon footprint when manufactured.