Technofile - wobbly web wheel

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In the first of a series on innovations which changed the course of racing, Keith Howard looks at one of Colin Chapman’s less famous strokes of genius.

It was fitted to every racing Lotus from the 12 to the Formula One championship winning 25 and, particularly during the early years when customer cars raced alongside those of the works team, it was the instantly recognisable trademark of Cohn Chapman’s cars. It also took Vanwall to the ’58 F1 Constructor’s Championship. It was an unwitting centre of attention at Le Mans, where the intransigence of the stewards prompted Chapman to shake the dust of La Sarthe from his feet forever. Most importantly, it consigned the traditional wire racing wheel to the dustbin of technological history, where it assuredly belonged by the mid-50s. It was a Chapman engineering coup and a watershed in race history. But when talk turns to ACBC’s contributions to racing, the wobbly web wheel is usually overwhelmed by advocacy of the Chapman strut, rnonocoque tub and ground effect.

Chapman, as everybody knows, was obsessed with saving weight, particularly unsprung weight. His suspension philosophy – taken for granted today but poorly understood by many at the time – was to maximise grip by keeping the tyre at the optimum angle to the road surface, and in firm contact with it for the maximum possible time. The latter requires paring unsprung weight to a minimum, and it was inevitable that Chapman’s engineering gaze should eventually fall on the wheel’s contribution.

He didn’t much like what he saw. The 4x15in 48-spoke wire wheel used on the Six, for instance, weighed about 11 pounds, and the 4.5x15in 60-spoke variant around 16Ibs. Chapman realised he could slash these figures by casting a wheel in magnesium, in line with aircraft practice, and so the wobbly web concept was born. The end result first appeared on the 12 at the car’s Fads Court Show launch in 1956. Drawings were also prepared for a 12 equipped with wire wheels, an insurance policy in case the mag wheel suffered teething problems, but very soon the logic of a cast disc wheel became obvious to most on the grid except Ferrari, which clung to the wire wheel while others happily binned them.

According to Motor Racing, December 1956, the 12’s 4x15in front wheel weighed 6.51b, its 4.5x15in rear just 7lb, so the savings were considerable and enhanced by Chapman further defying convention by ditching the heavy spinner of a knock-off wheel. Reasoning that tyres didn’t normally need changing during a race, and that a quick-release mechanism was therefore unnecessary, he used studs instead, which for the 12 were borrowed from the Austin 7 -a hark back to Chapman’s early racing career. Lotus road cars, meanwhile, perversely continued to use knock-off wheels for years. The brass spinners used on SE versions of the Elan – one of which serves as Keith Duckworth’s ashtray in the opening shots of the film Nine Days in Summer – weigh 1.4lb, to which has to be added the weight of a threaded hub extension. Clearly, the weight saving was worth having.

The 12 used cast magnesium transmission casings, so the obvious assumption is that the first wobbly webs were sand-cast by the same company – Percy J Smith of London EC1, whose clients included Malcolm Campbell and Henry Segrave. On the 12’s blueprints the material is unhelpfully specified as ‘magnesium alloy’ or ‘Elektron ‘; a rumour at the time had it that it was obtained by melting down scrap transmission casings from the magnesium-rich VW Beetle, and that the quality of the resulting wheel depended critically on how thoroughly the oil had been removed. The story is probably apocryphal, though, a more likely source being the many warplanes scrapped following WW2.

To understand why the wobbly web looks the way it does, you have to set aside all preconceptions about wheel design engendered by the aluminium wheels which adorn modem road cars. These are not, as a rule, engineering forms at all, being shaped more by stylists than stress engineers. They rarely represent anything like an optimum use of material, which explains why they commonly weigh as much – sometimes more – than the low-rent but efficiently designed Steel wheels they are intended to supplant.

Colin Chapman wasn’t in the jewellery business, so it’s no surprise to find a lack of gaping holes in the wobbly web through which to admire the brakes. No holes meant less risk of creating stress raisers from which cracking might originate, and permitted a purer, lighter structure. The wheel may lack what would today be considered sufficient brake ventilation but that’s not to be taken for granted: more than one car designer has been surprised to discover that airflow around his car’s wheels was nothing like he’d imagined. What we can say for certain is that the wobbly web, by eschewing holes in the wheel disc, required no unnecessary reinforcement of the rim section to bridge gaps between spokes.

This is good news not only for the mass of the wheel but also its distribution. It’s well known that a reduction in unsprung mass ( strictly, an increase in the ratio of sprung to unsprung mass) is desirable to improve roadholding. Less well appreciated is how it also affects performance. The rotational inertia of the wheels in effect adds to the weight of the car, so that reducing wheel inertia – by keeping as much of the wheel’s mass as close to the rotational axis as possible – improves acceleration. Also it reduces precessional gyroscopic forces at the front wheels and their corruption of steering feel.

Of course, it wasn’t only weight reduction that Chapman wanted: he wanted added wheel stiffness too. He did this by adopting an idea seen on many a shed roof: of increasing the stiffness of a thin sheet by corrugating it. In the wobbly the corrugations are radial and deeper at the centre, reflecting the distribution of bending stresses imposed by cornering loads, but the principle is the same. How stiff the resulting wheel was has never been quantified to my knowledge, but the Vanwalls provide anecdotal evidence. Their combination of wire wheels at the front and wobbly webs at the rear was arrived at after it was found that wobbly web fronts, because of their stiffness, robbed the car of sufficient understeer.

For the wobbly web the beginning of the end came in the mid-1960s with the development of a new breed of racing tyre. From being rather spindly items on 15-inch rims, F1 tyres suddenly morphed into much broader, 13-inch covers, and wheels were forced to change with them. Almost overnight, the rim design which had stood Lotus in such good stead for seven such formative years became yesterday’s technology. But magnesium remains the material of choice for F1 rims, albeit today forged and spun rather than cast. The finest examples, like those from BBS, tip the scales at under 9lb front and rear yet are tough enough, provided they’re not subject to impact damage, to last two gruelling seasons. In them, the spirit of the wobbly web lives on.

Our thank to Mike Bennett and Bill Colson of the Historic Lotus Register for their help with this feature.