Tech Talk: Composite Monocoques

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Pioneer aircraft were built using fabric and glue. But so is today’s high-tech racing machinery…

Since McLaren revealed the MP4/1 in 1981, a racing car that doesn’t have a carbon-composite monocoque has been just an also-ran. To the spectator this perfect melding of technique and material is only obvious in a crash, where the huge strength protects the driver, but its effect on chassis design has been enormous.

If you build a car around a traditional chassis you have to clad it in something, and apart from its streamlining effect that bodywork is dead weight. Pioneer French aeroplane builders learned to steam and glue thin plywood into the desired external shape to produce a strong, light ‘stressed skin’ which required little internal bracing – hence the French term monocoque, or ‘single hull’ – and that remains the goal.

In its purest form a monocoque is both structure and body, with no removable panels. But a car needs access openings, which compromise the integrity. For a simplistic example, think loo rolls. That tube inside is light yet relatively strong – but if you slit that tube up the side it reverts to just a floppy sheet of card. That’s how crucial shape can be in giving strength to a structure.

If that tube is your racing car you need an opening for a driver, but as soon as you cut into the tube you weaken it. It will both twist and bend easily. You need reinforcement around the opening, perhaps along the sides, certainly in front of and behind the driver in the form of bulkheads. Next you need suspension, but as soon as you apply a force to one end of your tube it will buckle inwards. Too much local stress – a monocoque is good at resisting evenly spread torsion and bending stresses, but not localised compression. A simple answer is to close both ends – more bulkheads – making the tube much stronger, but to ensure the suspension doesn’t punch through the thin skin we have to reinforce those areas with cross-braces or bulkheads, so the eight suspension mounts (if you’re using wishbones) become the corners of a box. Suddenly our ‘monocoque’ has become a hybrid form, an assemblage of tubes and boxes.

In fact the strong point loads – suspension, engine, gearbox – mean that there has probably never been a true monocoque vehicle; an F1 car comes closer in that its skin is broadly also the structure, whereas a sports-prototype conceals most of its monocoque under removable panels. Both, however, have the same aim – to rigidly connect the load points, as rigidity is crucial to making the suspension work predictably. This is and a few high-spec sports cars such as Bugatti Veyron, KTM X-Bow and McLaren F1.

Because of these constraints the design of the monocoque needs to be fixed well in advance, which means knowing which power units and gearbox you will be using and having an accurate picture of the suspension geometry. Anchorage points cannot be changed later, at least up front: since the Lotus 49 appeared F1 cars have thrown away the rear of the chassis and used the engine and gearbox as a stressed member to hang the rear wheels on. Making double use of this big, heavy unit reduces weight, although the rear bulkhead sees major stress concentrations where the engine bolts up to it. To handle these and other point loads such as suspension and harness fixings, metal mounts on ‘load spreaders’ are bonded in, which can also be ‘blind’ – ready to be drilled to suit. With these minor modifications a company such as Lola can offer a standard tub able to handle a range of running gear.

Like timber, carbon fibre is much stronger along the fibres than across, so these must run in different directions according to the stress paths, with extra layers at high-stress points. This means every piece of mat has to be individually planned before cutting, a task made easier by Finite Element Analysis. A powerful computer tool, FEA ‘breaks down’ the chassis to tiny cubes and triangles which are simpler to analyse individually but which add up to a good picture of stress and strain; this allows engineers to refine the structure for strength and also to see where weight can be cut from low-stress areas. The resulting digital model develops shapes for each piece of mat, controls their cutting and steers the milling machine that shapes the full-size buck or pattern from a resin-aggregate block. Once the mould, itself made of composites, is formed round this and cured, construction can begin.

A racing tub is typically stiffened by using sandwich construction – twin skins kept apart by a lightweight core, usually aluminium honeycomb, in places foam. First the outer skin is laid up, then the core, then the inner skin, with curing after each stage. It’s this length of process which hobbled Virgin Racing at the start of 2010: with the monocoque design fixed months before, it took an age to alter it once they knew the integral fuel tank was too small. Plus an FIA dispensation, as the monocoque should be homologated pre-season. The new tub only appeared in Spain, five races in.

The immense strength of carbon fibre protects drivers, but still it’s vital to build-in crumple zones outside the safety cell to absorb as much crash energy as possible. Composites don’t crumple like metal, they shred, but by tapering chassis elements and altering the fibre layup the structure can be made to deform progressively to absorb energy. Repairs are possible, but time-consuming if strength is to be maintained. Impacts also bring a risk of internal delamination which would diminish integrity; ultrasound can be used to check for this.

Despite earlier worries, composite hulls even seem to be ageing safely: Lola, which applies its composite technology in aerospace and military fields as well as racing, is monitoring its earliest tubs and reports that if they haven’t been crashed too often, they are retaining their rigidity. So far there seems little danger that the mounts of Rosberg, Senna and Prost will collapse into coal dust before our children get to see them race.

With thanks to the Lola Design Office for their consultation. For more information on Lola click on to www.lola-group.com

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