Porsche’s Group C Icons – 40 Years On: Part 2 – Shape And Chassis
Peter Falk became head of the newly established main motorsports department, acting as the head of race development and racing director. Norbert Singer took on responsibility for the 956 project.
Peter Falk: “This car was Norbert Singer’s baby. Much preferring the practical approach, he wasn’t one to stand at the front lecturing. He was one of the first in our office to own a computer, spending a great deal of time working on it right from the start. Singer headed up the project and designed the aerodynamics. It was quite a revolution to harness the ground effect from Formula One for a prototype.”
Norbert Singer on working with Peter Falk: “He also gave me a great deal of free rein in the development of the 956, made important decisions concerning the technology within the team and then took full responsibility for them. That is not an everyday occurrence in today’s world.”
The establishment of the new main department marked the organisational separation of the motorsport department from the press department, which had headed up motorsport operations until that point. This reorganisation also had geographical consequences. The entire motorsports department was given its own premises, about a kilometre away from the actual Weissach development centre and on the boundary with the district of Flacht. Peter Falk and his main department moved to less luxurious barracks and the former tank hall, which housed the motorsports workshops and the model workshop.
Walter Näher joined from the series development department, and Klaus Bischof moved to the newly formed team from the motorsport workshop. Horst Reitter was responsible for the monocoque chassis, with Eugen Kolb designing the bodywork and Norbert Singer taking over wind tunnel development. Manfred Wanner was responsible for building the 1:5 scale wind tunnel model. When the company procured its own wind tunnel in 1986, Norbert Singer and his colleagues Dr Reiner Müller, Rolf Junginger and Heiko Mikula pushed ahead with aerodynamic development. Valentin Schäffer, who had already been responsible for the previous turbo engines such as the 917/10, 917/30, 934, 935 and 936, was in charge of developing the turbo engine.
Aeronautical engineering used to construct the monocoque
In building the chassis, Norbert Singer’s team broke new ground. Instead of using a traditional tubular frame, a technique that had been established for decades, the 956 had a monocoque made of one-millimetre-thick aluminium sheeting. Helmuth Bott, Board Member for Development, initially favoured a tubular frame design for the 956 too, but the greater safety provided for the driver ultimately convinced him to opt for the more modern option.
The construction of a monocoque made of carbon laminate instead of aluminium, which was already being used in Formula One at the time, was also considered. However, Porsche did not have its own autoclave at the time, so Norbert Singer and Klaus Ziegler, foremen in the Composite Materials department, obtained a quote for buying in a carbon monocoque. “It came to over a million marks, which was a lot of money,” Singer recalls. In the end, the quote was declined. The proposal from Klaus Ziegler’s department to construct a synthetic monocoque in-house was also rejected for reasons of production capacity and budget, so a monocoque made of aluminium turned out to be a viable solution.
However, producing a monocoque from aluminium sheets also represented new territory for Porsche. “At the time, we had no experience whatsoever, so we approached Dornier in Friedrichshafen. They grinned a bit at first, but were then very helpful in telling us which rivets, rivet spacings, adhesive and tooling we needed to make a monocoque. Then we headed back and set about creating the first test specimens using this process. After that, we subjected the manufactured aluminium bodies to torsion to see what would happen,” recalls Norbert Singer, describing how the know-how for monocoque production was built up under high pressure. Finally, rigidity tests followed on the first experimental chassis, which achieved 80 per cent higher rigidity than the 936’s tubular spaceframe. “The second chassis we built was number 956 001, which was our first race car,” Singer continues.
The joining together of the individual pieces of sheet metal finally resulted in the monocoque. In terms of its dimensions, it extended roughly from the front axle to above the driver’s seat and, along with a welded-on roll cage made of circular aluminium tubing, formed a safety cell for the driver. The roll cage was also used to strengthen the substructure. The engine was bolted on directly behind the rear wall of the monocoque. Between the engine and the gearbox was the ‘ox horn’. This was a magnesium component that held two steel tubes on each of the two booms, one on the left and one on the right, which ran diagonally upwards to the rear wall to further secure the drivetrain. Directly behind the ‘ox horn’ was the gearbox, which also acted as a chassis support for the two tubular steel suspension bell cranks, which in turn linked to the magnesium wheel carriers. The bell cranks also linked to the compression-loaded suspension struts that were bolted to the gearbox at an angle. A double wishbone design was used on the front axle. The lower wishbone was made of tubular steel, while the upper wishbone was composed of an elaborately milled rear aluminium strut and a front adjustable aluminium round strut.
Complex rivet bonding technology – the ‘Porcupine’
The construction of the monocoque involved high-quality and elaborate craftsmanship. For example, the individual sheet metal parts were joined using a rivet-bonding technique, following principles used in the construction of metal aircraft. A tape soaked in epoxy resin is used for bonding, and the two sheets are then riveted together. The riveting, which must be carried out immediately after application of the adhesive tape, is performed by two people using a riveting hammer assisted by compressed air. This requires a second person to position the heavy counterweight for the riveting hammer.
Before the individual sheet metal parts are joined, however, the construction of the monocoque requires a great many preparatory steps. These range from absolutely precise cutting to size, as per the relevant design drawing, to the pressing of various openings, for example for hose guides or in the front side panel, to the exact drilling of the holes for the subsequent riveted joints. Not only do the openings pressed at certain points give the monocoque a handcrafted and pleasing appearance, but the way they are shaped – by flanging – also increases the rigidity of the component while reducing its weight. This design principle can still be found in many areas of aircraft construction today – for example in fuselage frames or wing ribs. The monocoque was therefore created sheet by sheet, and at this stage was still held together by screw staplers or tacking needles in place of the rivets that would later be used. Once complete, this resulted in a structure that, in line with Porsche’s tradition, was given a nickname in Norbert Singer’s team due to the countless screw staplers: The Porcupine.
Body made of glass fibre reinforced plastic (GRP)
The body of the 956 was built under the direction of Eugen Kolb using the well-known, tried-and-tested construction method of glass fibre reinforced plastic (GRP). First of all, this requires the construction of a 1:1 model of the body from rigid foam. Finally, negative moulds are made from this 1:1 model in GRP, which can then be used to manufacture the bodywork parts.
The body consisted of a removable front and rear section as well as a centre section that accommodated the two doors. These opened upward and were hinged from the leading edge of the door so that they opened towards the windscreen, too. This central body section was not designed to be removable, but was bonded to the aluminium monocoque, making the car more rigid overall.
The lightweight construction principle could be seen at various points on the body, for example on the front end, with laminated aramid honeycombs (Kevlar). These Kevlar elements were sandwiched between the outer and inner layers of the glass fibre laminate, forming a hollow body that gave the component enormous additional rigidity without increasing its weight. This method is also common in aircraft construction, for example in the production of gliders.
Both the short-tail and long-tail versions of the 956 measured 4,800 mm in length. According to the original technical regulations for Group C, the front and rear body overhangs were not permitted to measure more than 80 per cent of the wheelbase in total. At 2,650 mm, the 956 had the longest wheelbase of any Porsche racing car to date. FISA also stipulated that the difference between front and rear body overhang must not exceed 15 per cent. For this reason, the long-tail version of the 956 had a longer rear section, but with an attached rear wing, while the short-tail version had the rear wing protruding to the rear.
With thanks to the teams at Porsche Motorsport, Porsche Heritage and the Porsche Museum
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