Making racing car bodies out of aluminium was not only difficult but expensive too. Keith Howard reports on the invention of a cheap alternative

It’s easy to regard the Air Marshall who makes a cameo in JE Gordon’s book The New Science of Strong Materials as a figure of fun, but I have sympathy for the man. The event took place during WW2 when Gordon — later to become Professor of Materials Technology at Reading University as well as the author of popular paperbacks on the subjects of materials and structures — was working at the Royal Aircraft Establishment in Famborough.

Showing said senior officer around an aircraft, he was asked of what material the radar antenna cover was made. Knowing it to be what today we would call GRP or GFRP — glassfibre reinforced plastic — and understanding equally well that his audience would never have heard of it, Gordon simply replied “Glass”. At which the blue tunic exploded, thinking the boffins were exposing his lads to the hazard of fast-moving shards of shattered glazing.

From a half-century’s remove, during which we’ve become used to seeing GRP used widely in both race and road cars as well as boats and aircraft, it’s easy to snigger. But why it is that glass, of all materials — weak, brittle glass — should ever have been thought suitable as a reinforcement for plastic, and the resulting material as ideal for racing cars?

The early history of glass fibres is dominated by the manufacture of glass wool, intended primarily for insulation purposes. The fibres produced were short and of inconsistent thickness — not the properties required of an effective reinforcement material, even had the plastics then been available to exploit it. So far as structural use of glass fibre is concerned, the important breakthrough came in November 1935 when the Owens-Illinois glass company began experimental manufacture of continuous glass fibres in its Ohio plant, which in addition to being long were of much more closely controlled diameter.

The following January the company first used the trade name Fiberglas, with a single S, for its glass fibre products — a name which rapidly became a generic for the material — and in 1942 Owens Coming Fiberglas, the joint company set up by Owens-thinois and Coming in 1938 to manufacture glassfibre, began its first experiments with GRP at the behest of the US Army Air Force. It proved to have far superior mechanical properties to those of the raw plastic, and thus the composites revolution began.

Why such an apparently uncompromising material should be so effective a reinforcement was a conundrum that had been answered two decades earlier in Britain by AA Griffith, a man who revolutionised our understanding of material properties. What Griffith discovered is that each material has a critical crack length below which the crack won’t spontaneously elongate, leading to failure. Bulk glass is covered with these tiny, sub-critical-length cracks which have the effect of substantially weakening it, masking the material’s underlying strength.

To realise glass’s full potential what you have to do is draw it into a thin fibre, one whose thickness is considerably less than the critical crack length. Ultimately, if you get it extremely thin, the fibre must then be either perfect or else broken, and the true strength of the material becomes exploitable. To put some figures to this, the ultimate tensile strength of thin fibres of E-glass — a glass originally developed for electrical insulation purposes — is typically 20 times that of a pane of ordinary window glass, over seven times that of mild steel and six times that of the strongest heat-treated aluminium alloys. Draw it thin and glass becomes a true engineering material.

One of the first racing car designers to appreciate GRP’s practical advantages was Colin Chapman. Although the material was no better than aluminium in respect of its physical properties — the strength and stiffness of the raw fibres are compromised when combined with a resin matrix — it was both a much easier and a much cheaper material to use within small-scale manufacturing. Initially Chapman had followed established racing practice and used all-aluminium bodywork for his cars, but the renowned penny-pincher increasingly baulked at both the time and costs involved. Eventually he struck on the perfect solution: prototypes were clothed in aluminium, and their aluminium panels then used to make moulds from which GRP equivalents could be harvested for production. Tooling for the moulds was cheap, and the GRP lay-up process required a much lower level of skill than hand-forming metal.

In conceiving an all-GRP monocoque structure for the Type 15 Elite — which was conceived from the outset as a racing as well as a road car — Chapman famously went too far, almost bankrupting Lotus with the stream of production difficulties it presented. Although the Elite was to prove a highly successful racing car, it was a near disastrous business proposition. Not until the Imp-engined Clan Crusader of the early ’70s did anyone feel brave enough to try a GRP monocoque again, although it was only the fortuitous development of the backbone chassis as a suspension test bed that rescued Lotus from repeating the folly with the Elan.

In fact GRP was never destined to become a successful structural material for racing cars as carbon fibre composite later was, but for cost-effectively clothing a metal chassis it very quickly became the material of choice. You can trace the process through the Lotus family tree from the all-aluminium bodied Type 16, via the aluminium/GRP Type 17, to the Type 18, which used aluminium for the Formula Junior prototype and GRP for all FJ and F1 production cars.

It would be wrong, though, to regard GRP as mere carrier of team colours and, later, sponsor’s logos. As aerodynamics became an increasingly important aspect of race car design, so the role of the bodywork became more active than passive, a process that reached its zenith in the late 1970s with the development of ground effect aerodynamics. Look underneath the Lotus 78 and 79 and you will see an aluminium chassis made as narrow as possible to allow the largest possible venturi areas beneath the sidepods.

Fabricating this complex sidepod structure, with its carefully contrived underside form, would have been a nightmare in aluminium. GRP made the task relatively easy. In the process it fostered both the ground effect era and, shortly thereafter, the no less significant move to higher-performance composites based on Kevlar and carbon fibre.