Geek speak

Carbon Creations

by JEZ SPINKS

The complicated explained

Carbon Creations

Extremely light, extremely strong, extremely rigid

T’S VERY OBVIOUS why carbon fibre has been the go-to exotic material for the world’s most exotic cars. It was perhaps more surprising that the first Formula One team to pioneer a monocoque made from the magic black weave would be the first to transfer this advanced construction to the road.

The 1992 McLaren F1’s carbonchassis approach was exclusive for several years before the Pagani Zonda came along – a car created by the man who had envisioned the composites trend back in 1987 with his Lamborghini Countach Evoluzione concept.

Lamborghini eventually got there with the Aventador. Ferrari had already dabbled with composites for the F40 but didn’t stretch to the monocoque for 14 more years, with the 2002 Enzo.

Porsche joined the club with the Carrera GT and, more recently, 918 Spyder, while niche Swedish supercar maker Koenigsegg continues to be arguably the most obsessive user of carbon fibre.

Carbon fibre passenger cells have recently made it into relatively I affordable territory. There’s one at the heart of the $89,000 Alfa Romeo 4C and $64,000 BMW i3 city car. The Munich marque’s i8 also features a carbon cell, though there are no signs yet that M cars will feature much more than a carbon roof or bonnet in the near future.

Limitations continue to be set by carbon fibre’s other extreme – cost.

McLaren says it moved to new techniques with greater efficiencies to establish a business case for producing carbon tubs for every single supercar it produces.

The likes of Ferrari, as well as F1 teams, continue to use the traditional process, where carbon fibre parts are placed in a singlesided tool, pre-injected with resin to create a ‘pre-preg’, and then baked in an autoclave.

McLaren’s Austrian supplier, Mubea Carbo Tech, uses advanced techniques such as RTM (resin transfer moulding) and SMC (sheet moulding compound) to reduce production and labour time.

Where it took McLaren 4000 hours to build the F1’s monocoque, the Monocage II passenger cell of its latest car, the 720S, takes less than four hours.

Advances over the 25 years since the F1 are not necessarily in lightness, though. The 720S weighs about 150kg more than the F1 (and 18kg less than its 650S predecessor) though McLaren says comparing the two cars in this respect is not apples-versusapples, owing to tougher crash regs and advanced vehicle systems.

The 720S is still faster than the F1 in almost every measure.

After all, beyond the inherent safety of a carbon tub – they’ve been known to survive multiple high-speed crashes without incurring significant damage – the desired benefit for owners is performance. M

Carbon fibre passenger cells have made it into affordable territory

When F1 went tubs

THE FORMULA One paddock was initially sceptical of McLaren’s aerospace-industryinspired move to construct its 1981 MP4/1 race car’s monocoque out of carbon fibre rather than the conventional aluminium sheeting with steel reinforcement.

McLaren’s then designer, John Barnard, believes McLaren were the turning point was driver John Watson’s high-speed crash during the 1981 Italian Grand Prix, which initially had everyone fearing the worst. Despite an impact that ripped the engine off the back of his car and snapped the gearbox in half, Watson walked away unscathed.

“That set the tone for the carbon pion carbon monocoques thereafter,” Barnard would say of the crash.

“It became the beginning of the safety cell. It was a huge moment in the carbon composite chassis monocoque life.”

McLaren is recognised as the first team to race a carbon fibre car, though in the same season Lotus also produced its eers innovative twin-chassis Lotus 88. The car never raced, however – banned controversially on the grounds it contravened regulations.

The rest of the teams soon followed McLaren’s composite construction, though that didn’t prevent the team from being a major force in the 1980s and 1990s.

Fibre diet

How McLaren builds its tub

1 FIBRE COUNT

Layers of non-crimp and unidirectional carbon fabrics with high fibre counts are layered manually with the aid of machined shapes to create the complex carbon fibre preform.

2 MOULDED

The preform is loaded into a two-sided tool and transferred into the moulding area, where a secret process feeds epoxy resin into the mould via a single injection point under high pressure.

A few hours of curing later and a robotic arm removes the completed Monocell for a final finishing phase.

3 PLAYING WITH DOUGH

The 720S’s Monocage II is a development of the P1 hypercar’s monocoque, adding an upper structure to the Monocell (which itself continues to evolve). This section, which further lowers the centre of gravity, uses another major development in carbon-fibre construction – sheet moulding compound (or SMC). McLaren likens this composite material to Playdoh – a soft, uncured resin and carbon fibre ‘dough’ of chopped fibres that is squeezed in a two-sided tool under temperature and pressure to allow the material to cool.

4 ADDED STRENGTH

Carbon SMC is applied for the roof and rear structure that will adopt the tailgate. However, SMC is combined with ‘pre-preg’ – the more traditional weave already injected with resin – to add strength for the 720S’s strikingly thin windscreen pillars. The techniques also enabled McLaren to create wider door apertures and a lower sill for easier access.

5 TWO BECOME ONE

The passenger cell and upper structure are shipped from Austria to Woking, where they are bonded together as part of the body assembly process. The Monocell features embedded aluminium inserts, the largest of which feature the attachments for the aluminium front crash structure.