The roll-cage

Technofile

It’s easy to ignore a safety product when it not only adds weight but also cuts performance. However, as Keith Howard explains, a roll-protection structure is one area where security can make your race or rally car tougher – and quicker

Photographs of 1960s rally cam and saloon racers tell their own chilling story. Find an image that allows you to see right through the car from windscreen to backlight, and you’ll see just what I mean. Nothing. No cocooning highback racing seat as we expect today, and no steel pipework that signals the presence of a roll-cage. Flipping one of these cars — having a shunt of any description, in fact — was not a particularly good idea. Photos from the early 1970s show the beginning of a change. B-post roll-hoops can often be spotted. Compared with a modem safety cage, they’re laughable: merely the hoop itself usually a pair of badcstays — not always terminating at a strong part of the bodywork — and often no diagonal to prevent the hoop lozenging when loaded. But it was a start.

John Aley, whose company Aleybars supplied BMC’s competition department, among others, for many years, was one of the roll-hoop pioneers. An employee of his, a New Zealander by the name of Brian Wilkinson, did what many in the motorsport business do: learn the ropes, establish contacts and then set themselves up in competition with their old employer. Wilkinson called his company Safety Devices. Based first in Cambridge and then in Newmarket, it went into receivership in 1993 but was bought out and moved to Soham, Cambridgeshire, where it now thrives under the direction of one of Britain’s most successful rally drivers of the ’60s and ’70s, Tony Fall. (He drove for BMC, Lancia, Ford, Porsche, Datsun, BMW and Opel, and later ran the official Dealer Opel Team.) Ironically, Safety Devices now owns, and still uses, the Aleybars trade name.

At the time Wilkinson set up his company in the mid-70s, the roll-hoop was a strictly bolt-in device. Competition regulations insisted that it be removable, to sustain the notion that race and rally cars were simply adapted road cars. For the hardnosed racing car designer, the roll-hoop was a pain. It contributed nothing but weight, and that weight penalty only increased as roll-hoops metamorphosed into more complex roll-cages, with additional A-pillar tubes, sill tubes and the like. To offset this weight penalty, designers turned to aluminium in place of steel. But a spate of accidents in which aluminium roll-cages failed to perform correctly alerted the FIA that regulations were being flouted.

After fabrication, any welded aluminium roll structure was supposed to be annealed to restore strength to the material that had been heated in the course of welding. The trouble was, this couldn’t easily be checked. Nobody then making roll-cages had the necessary ovens, and the suspicion was that claims to have had the annealing performed externally were not always true. So the FIA reacted in the only way it could: it banned aluminium roll-cages, insisting on cold drawn, seamless, unalloyed steel tube instead — a requirement that persists to this day.

With aluminium there had never been any prospect of the cage and the bodyshell being welded together; steel and aluminium won’t co-operate in this way. But with the reversion to steel and a relaxation of the bolt-in rule, there was an opportunity to convert the roll structure into a design asset. If it could be welded to the bodyshell, using largearea mounting plates and long gussets to join the tubes to the sills and pillars, it could serve a dual role, stiffening the bodyshell as well as improving its crash performance.

A stiffer bodyshell meant improved handling and, particularly for rally cam, enhanced ruggedness. Safety Devices claims to have been the first to weld in safety cages in this way. Brian Wilkinson recalls that the Rover SDls successfully campaigned by Tom Walkinshaw, starting in 1980, were some of the first cars to benefit; Fall remembers the Triumph TR7 rally cars also having a partly welded-in cage. In engineering terms, the justification for a weld-in cage boils down to increasing the crosssection of key structural elements of the bodyshell or, more properly, their second moment of area. Whether it is a sheet, a bar, a tube or something more complex, any structure resists deformation because sections of it are stretched and others compressed.

In between lies a so-called neutral plane, where no tensile or compressive stresses act. Material located near to the neutral plane does little to resist deformation; material far from it contributes much more because the degree of stretching or compression is proportionately greater.

This is why a tube is much harder to bend or twist than a solid rod of the same weight, and why composite sandwich materials are now so widely used in the top echelons of motorsport. It’s the thin skins of a sandwich material which provide the stiffness; the much thicker foam or honeycomb core contributes next to nothing. Its function is solely to keep the skins spaced apart, away from the neutral plane. Exactly how much stiffer a typical modern bodyshell becomes with a safety cage welded in (and the panels seamwelded) is not entirely clear. The last time Safety Devices did a comparison, on an old Volkswagen Golf bodyshell, the torsional stiffness was about doubled. But road car makers have made great strides in increasing body stiffness in the interim, so the improvement may now be less dramatic.

What isn’t in question is that a well-designed and -installed safety cage can still make the difference between a successful race car and an also-ran. I spoke recently to Peter Hignett of ABG Motorsport, the small privateer team that’s running a Locus IS200 in this year’s British Touring Car Championship, and he is adamant that, within the highly-specified regulations, the safety cage is one of the few remaining areas where a team can steal a march over its competitors. The added complication of installing a modern triangulated safety cage is considerable — a top-spec Safety Devices example comprises no fewer than 30 elements — but it’s no longer a case of driver safety compromising car performance. With a welded-in cage, you get a better race or rally car as well as a safer one.