Above: Porsche factory driver Patrick Long hustles the 918 Spyder around the Circuit of the Americas, a Formula 1 racetrack near Austin, Texas, when the automaker debuted the hyper hybrid for the press in 2014. Photo: Porsche Cars North America
Magnesium has had applications at Porsche for more than half a century
May 2016 - Magnesium, it seems, is a material whose breakthrough is perennially just ahead. Following World War II, when it was a strategic metal, magnesium was touted as the next aluminum. This magazine was founded in 1945 by Bill Griffin, who came from the magnesium refining industry, and Modern Metals’ mission was to promote peacetime applications for the light metal.
Some manufacturers kept magnesium in their metals repertoire, and there is no better example than auto manufacturer Porsche AG. Company founder Ferdinand Porsche specified magnesium (then called Elektron for its refining process) in his groundbreaking Volkswagen Beetle from the 1930s, and that tradition continues at Porsche to the present.
Some of Porsche’s notable magnesium components in the last 50 years include the two-piece crankcase for the 1966 906 Carrera race car (also called the Carrera 6) and the crankcase—also two-piece—for the 1968 911. The 1970 914-6 had cast mag wheels and, a year later, the all-conquering 917 race car was built on a magnesium frame. Mag was extended to both the frame and the body skin for the 962 race car, which won the 24-hour Le Mans race in 1972 and 1973. The groundbreaking 959 (1986 to 1989) was shod with magnesium wheels. In 2003 Porsche introduced its Carrera GT supercar, with both forged magnesium wheels and cast mag trim on the cockpit console.
But wait—there’s more. A cast magnesium cover encloses the camshaft chain drive for the current 911, and the four-door Panamera has mag window frames on the doors. The competition version of the 911, the RSR, rolls on magnesium wheels.
For this article, we’ll explore three current magnesium applications: the road wheels for the 918 Spyder, the roof of the 911 GT3 RS, and the tension plate for the convertible top on the 911 Cabriolet. We were granted liberal access to the engineers at Porsche’s research and development center in Weissach, Germany, and we are indebted to Frank Wiesmann, product experience manager for sports cars at Porsche Cars North America in Atlanta, for translating. Sources are quoted collectively as “the Weissach engineers.”
918 Spyder road wheels
If the 1980s-era 959 and the Carrera GT of a decade ago can be called supercars, the 2014 918 Spyder deserves a promotion to hypercar. With 200-plus mile-per-hour capability and nearly 900 horsepower from its all-wheel hybrid drive, the 918 can be considered state of the art in automotive technology. (For details on Porsche’s latest halo car, see the sidebar below.)
No effort has been spared to lower the 918’s weight and that includes the car’s forged magnesium wheels. The obvious benefit of magnesium for road wheels is reduced unsprung weight, and the lower density of mag makes the wheels about 30 pounds lighter per vehicle. Without providing too many details on how the wheels are fabricated, Porsche says the same standards applied to aluminum wheels are applied to magnesium.
The three-piece magnesium tension plate for the Porsche 911 Cabriolet is 9 pounds lighter than a comparable aluminum assembly. The articulated top retracts fully within the car body and, when raised, features the same unbroken roof line as the 911 Coupe. Illustration: Porsche Cars North America
The wheel spokes are forged; the rim is flow formed, then turned and milled. Their design, the Weissach engineers point out, meets all relevant racing expectations regarding structural rigidity, center-lock hub performance, unsprung mass, brake ventilation and aerodynamics.
Finishing the wheels is a multi-stage process. Five applications make up the wheel finish, including a plasma-ceramic coating, cathodic protection, primer, base paint coat and clear coat. “This ensures the same level of durability as an aluminum wheel,” the engineers say, “and requires no additional care or maintenance.”
911 GT3 RS roof
Carbon fiber has become the material of choice for many high-performance vehicles, but for the outer roof panel of Porsche’s 911 GT3 RS coupe, the choice was rolled magnesium sheet. Lest you miss the point, RS stands for Rennsport (racing sport); this car is a brilliant track performer.
Porsche considered carbon fiber, but rejected the material on a number of points. “Carbon fiber exterior body parts often cannot meet the theoretically possible level of weight savings. This is due to the fact that additional surface films and paint coats are necessary to achieve a Class A surface. The structure of carbon fiber is inherently inconsistent and needs to be smoothed out. These inconsistencies are not accepted by Porsche,” according to the Weissach engineers.
The engineers also cite the high cost of carbon fiber and the amount of labor required to form and paint the material. The magnesium roof is less costly than carbon fiber, and it is lighter by 700 grams (24.7 ounces) or 24 percent. And, yes, a reduction that small does make a difference.
The magnesium roof panel follows a circuitous supply chain that covers four countries on three continents. Mag sheet is first warm-rolled by a company in Korea, where Class A rolling blanks are strip cast.
The mag is formed at a temperature between 380 and 460 degrees Celsius (716 to 860 Fahrenheit) with a cycle time less than 20 minutes, significantly shorter than curing composite resins. The panel is trimmed by a laser, with forming radii less than 3 millimeters (0.118 inch). This is comparable, the engineers say, to the capabilities of an aluminum roof.
The thickness of the sheet varies from 4.5 millimeters (0.177 inch) to 1.1 millimeter (0.043 inch). The surface is refined by grinding, then it is cut to shape.
Phase two takes place in Canada. A single-stage male die forms the roof panel, with the blank clamped and heated. The mold clamp moves downward and air pressure is applied above the blank to complete the forming cycle.
“The advantages of this process include the [ability] to form complex geometries,” the engineers continue. “Contact between the forming die and the Class A surface is eliminated, and undercut forming is integrated into the first stage.”
The United States is the third stop for the roof panel, where it is coated. A deoxidation pretreatment removes impurities, and a protective ceramic layer is applied by plasma oxidation in a cycle of less than 9 minutes. This is followed by a thin layer of conversion coating and a thicker deposit of E-coating on both sides.
The final stop is Germany, where the panel is bonded to the roof pillars, primed and given a basecoat/clearcoat finish. Screws and adhesive bonding attach the roof to the unibody at all four corners, steps that isolate the roof from galvanic action.
911 Cabriolet convertible top tension plates
Porsche has been building open-top roadsters and cabriolets since the first Type 356 rolled out of a converted sawmill in Gmünd, Austria, in 1948. Ever since, the convertible tops have been supported by conventional bows—serviceable structures but aesthetically not ideal. Porsche designers envisioned a top that would be completely smooth, without bow bulges, and be comparable to the roof contour of the 911 Coupe.
They met their goal with the current model Cabriolet, which replaces multiple bows with a three magnesium tension plates that are 9 pounds lighter than comparable components made of aluminum. The plates provide both an unbroken appearance and reduce the car’s center of gravity by lowering weight at the highest point.
The revised top mechanism stows completely in the Cab’s body, and this fit was possible because development of the tension plates began two years before the design of the vehicle. “The package of the folded top, was able to be matched to the available stowage space very accurately,” the engineers note.
The reduced weight of the mag tension plates keep the overall weight of the top assembly the same as the previous generation at about 79 pounds, in spite of a larger top with more sound insulation and increased padding. The plates are thin-wall castings, with an average thickness of 2 millimeters (0.079 inch). Casting, the Weissach engineers conclude, “offers a high amount of freedom regarding the geometric design of these parts, which allows for a high integration of functionality.” MM
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This Porsche 918 Spyder at the 2015 Chicago Auto Show features “standard” aluminum wheels. The Weissach Edition replaces those with mag wheels. Photo: Neiland Pennington
THE FUTURE IS NOW
Originally the domain of ardent environmentalists, hybrid drives now power some of the fastest cars on the planet. Extraordinary machines like La Ferrari, the McLaren P1 and Porsche 918 Spyder dominate hybrid technology.
Porsche’s 918 is derived from its hybrid 919 Le Mans Prototype racer, which took both first and second overall in its debut race, the 2015 24-hour Le Mans classic. The concept 918 was first shown in 2010, and brought to market by designer Michael Mauer, project chief Frank Walliser and their team in just four years.
The 918 is a true hybrid, but with staggering performance. A 4.6-liter V8 rated at 608 horsepower and a 154-horsepower electric motor drive the rear wheels; a 127-horsepower electric motor powers the front wheels. The motors draw current from a 312-cell liquid-cooled lithium ion battery that provides 6.8 kilowatt hours of power. Porsche rates the engine/motor combination at 887 horsepower because the power curves of the propulsion units are not linear.
Performance of this gasoline/electric juggernaut is zero to 60 miles per hour in 2.8 seconds and a top speed of 211 miles per hour. As a fuel-sipper, the car is rated by the Environmental Protection Agency at 67 miles per gallon equivalent.
The 918 wasn’t operating in economy mode when factory driver Mark Lieb turned his stunning record lap around Germany’s immense Nürburgring race course. He blasted one round of the 12.944-mile Nordschleife (the north circuit) in 6 minutes, 57 seconds, the first sub-7 minute run by a street-legal production car.
Although Porsche’s 918 Spyder is the least costly of the three hyper hybrids, all of this automotive prowess comes at a daunting price: $845,000 for the base model (if there is such a thing) and $929,000 for the reduced-weight Weissach Edition. But if you want to order either version, you are too late; the entire production run of 918 cars (what else?) is sold out.
