From wood to CFRP – a century of evolution at the 24 Hours of Le Mans

From wood to CFRP – a century of evolution at the 24 Hours of Le Mans

24 HOURS CENTENARY – PERPETUAL INNOVATION⎮For some, the race begins on the drawing board or in the workshop, well before the cars gather on the track. Especially for Le Mans. It is the designers’ and engineers’ job to find materials that are lighter, stronger and easier to shape to attain the highest level of performance for a full 24 hours.

A racing car basically consists of two distinct parts: the chassis and body on the one hand, and the engine on the other.

In the very early years of the 24 Hours of Le Mans, the body was made of sheet steel, wood or canvas and bolted on to a steel chassis. Canvas being lighter than steel or wood, it was often used for the side panels and roof. It is worth noting that the Mini Marcos driven by Jean-Louis Marnat and Claude Ballot-Léna in 1966 was the last to be built on a largely wooden chassis. Remarkably, it finished in 15th place.

In 1923, the chassis of the race-winning Chenard & Walcker consisted of pressed steel siderails and crossmembers, with thin steel plate used for the body. The engine was in cast iron, as were the gearbox and differential housing. Most pre-war racing cars were thus built, although manufacturers were starting to introduce aluminium for chassis and engine parts.

Shedding those surplus pounds

The architecture and materials had evolved steadily when the outbreak of the Second World War halted proceedings. In the 1950s, however, all designers adopted the same mantra: “weight is the enemy”. The challenge was then to find new materials to replace the cast iron used in engines and brake drums, for instance, and especially the steel used for chassis.

Aluminium, much lighter than cast iron, became the material of choice for many components, such as brake drums, and, subsequently, cylinder blocks and even the engines themselves. Aluminium is relatively easy to cast for mechanical components and in sheet form is sufficiently malleable to shape into body panels. However, it remains relatively flexible making it unsuitable for the tubular chassis that became the norm on fifties’ and sixties’ prototypes.

For a while, magnesium was used in the chassis tubes but it has the drawback of being difficult to extinguish in the event of a fire. This was tragically illustrated in 1955 when Pierre Levegh’s Mercedes ignited on impact with a wall, resulting in the material being banned.

During the 1960s, Lotus developed a solution in the shape of a “tub” made of riveted aluminium panels. The “unibody” construction and the reinforcements spaced along the entire length offered greater rigidity than welded tubular steel trellis frames. Matra adopted a similar construction for the 650 in 1969, albeit with a tubular steel chassis. In 1971, the French manufacturer opted for a riveted aluminium monocoque for the 660. Alfa Romeo and Porsche, on the other hand, continued for quite some time with the tubular chassis – until the 1972 33 TT and the 1977 936 respectively.

Chassis panel composition evolved quickly in the 1970s with honeycomb bonded to thin aluminium sheet. This technique improved rigidity considerably without affecting the weight.

Since the 1960s, manufacturers had gradually turned to fibreglass for their bodywork. The technique involved placing one or more layers of fibreglass bonded with resin in large moulds to produce sleek, aerodynamic bodies much more easily than with aluminium, which was not only heavier, but more labour-intensive too. Racing teams could therefore manage a stock of replacement parts at the track and change full panels, such as a front or rear hood, rather an attempt a repair during the race.

The use of aluminium, on the other hand, became more widespread for engines and gearboxes. Progress in casting technology meant that sufficiently fine walls could be produced in this material, while maintaining ample rigidity for the engine to be fully stressed (i.e. act as a chassis at the rear of the car) and avoid an excessively heavy chassis.

McLaren and the carbon revolution

The 1990s saw the advent of carbon fibre shells. This composite material consisting of resin and carbon fibres is known as carbon-fibre-reinforced polymer, or CFRP. McLaren, who had been honing the technology in Formula One since the MP4/1 appeared in 1981, introduced it to Le Mans in 1995. It was a great success as the F1 GTR won the race on its first appearance in the hands of Yannick Dalmas, Masanori Sekiya and JJ Lehto. The benefit of CFRP is two-fold: very high torsional strength, absorbing stress without damage, and a very light weight in comparison with aluminium. It does have one drawback, however. A carbon shell takes a long time to make as it must be placed in an autoclave (high temperature under pressure) to ensure that the various components are suitably homogenised.

Carbon has also found an application in braking systems since the late 1980s. Discs in this material withstand extremely high temperatures and offer constant braking performance throughout their life. Carbon brake discs have in fact become something of a symbol at the 24 Hours of Le Mans. Race fans and photographers alike delight in seeing them glow red-hot due to the heavy braking required at iconic spots, such as Mulsanne, Indianapolis, Arnage, and the Ford and Dunlop chicanes. Technological innovation certainly knows how to put on a show!

PHOTOS: LE MANS (SARTHE, FRANCE), CIRCUIT DES 24 HEURES, 24 HOURS OF LE MANS – FROM TOP TO BOTTOM (© ACO ARCHIVES): A three-shot illustration of the evolution in chassis and body materials, with the sheet steel used by Chenard & Walcker (here, the 1923 Raoul Bachmann/Christian Dauvergne entry, second behind the other car of the French marque that won the inaugural 24 Hours), tubular chassis and fibreglass for the Matra 650 (fourth in 1969 with Jean-Pierre Beltoise/Piers Courage) and carbon fibre for the McLaren F1 GTR (here, the fifth-placed car driven by David Brabham/Pierre-Henri Raphanel/Lindsay Owen-Jones in 1996).


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