Wobbly-web wheel


The wobbly-web wheel is a form of metal disk wheel where the disk is 'wobbled' into spokes. This provides a stiffer, lightweight wheel.

Wobbly-web wheels are best known through their iconic use on Lotus racing cars of the late 1950s and 1960s.[1]

Theory

The theory of a disk wheel depends on their behaviour as a stressed skin structure. Their materials are assumed to be inextensible, but flexible. Such a skin is stiff against tensile loads in the plane of the skin, but flexible for loads perpendicular to it. A flat disk is thus strong for radial loads in the plane of the disk and also for torques. It is flexible for axial loads, perpendicular to the disk plane and also for nutating, wobble of the axle at an angle to the main axis.

A coned disk also becomes stiff against axial loads, as the web is no longer purely perpendicular to the axis of the wheel. Unlike a flat disk, there is no direction where the whole of a coned disk would be perpendicular (and thus flexible) to it. A drawback of a simple cone is that the wheel now becomes wider, along the axis. The wobbly web effectively "folds" a coned disk wheel into pleats, making it narrower again, but retaining the axial stiffness. This pleated disk is also stiffer against nutating forces. A similar feature, a 'sine wave wall', is sometimes encountered in architecture where a straight wall is made stronger against sideways forces by corrugating it.[2]

Key features of the wobbly web wheel are that they are designed as a modification of the disk wheel with additional shaping, rather than a spoked wheel with merged spokes. They can be manufactured by either pressing from sheet steel, or by casting in light alloys. To avoid stress risers, the webs are formed into smooth curves, rather than sharply defined spokes.

When cast, foundries prefer a disk that has a consistent wall thickness, as this makes their shrinkage behaviour simpler to control. The Lotus wheel's peculiar shape was arrived at deliberately, by keeping this consistent wall thickness, for ease of manufacture, and folding it to achieve the varying stiffness distribution required across the radius, to meet the performance needs.[3]

Fedden and the Cosmos

The first wobbly web wheel was a design for a pulley centre, circa 1884. A straight strip of steel, with width the radius of the wheel, was folded into sharp corrugations. This accordion pleat was then fanned into a circle, the inner folds being closer than the outer. This disk was then cast into an iron hub and folded tabs on the outside of the sheet riveted to the pulley rim. These pulleys were produced by J & E Hall of Dartford. Plans were also made to develop this into a railway wheel, with the corrugations pressed into the shape of a half-torus.[4]

The first use for a road wheel was by Roy Fedden for his Cosmos car of 1919.[5] This pressed steel wheel used a disk with six spokes on alternating sides and three retaining bolts.[6]

Steam locomotives

Steam locomotives use spoked driving wheels of cast iron or cast steel. A few makers have favoured variants of this, seeking a more weight-efficient design by using various forms of hollow or girder spoke, rather than solid oval-section spokes. One of the most successful of these designs was the Bulleid Firth Brown or BFB wheel, often incorrectly confused with the Boxpok wheel. This had some features of the wobbly web, being developed from a flat disk wheel with short, wide "spokes" formed in it by displacing teardrop-shaped sections of the disk into another plane. Except for a few small lightening holes, this disk wheel is consistently one wall thickness thick, all around the disk.

Like the Lotus wheel, the BFB wheel also displays wide shallow pleats at its outside diameter, with narrower steeper pleats towards the centre (see image).

Although superficially visually similar to the BFB wheel, the true Boxpok design is, as indicated by its name, derived from "box spoke". This is a spoked wheel, with square box-section hollow spokes. The wheel disk in section is either two skin thicknesses at the spokes, or zero (i.e. open space) between them.

Aircraft

Aircraft with cast light alloy wheels appeared in the 1930s. A common form in British military practice used a single-thickness disk, displaced sideways in sharp-edged segments and joined by radial webs. Such wheels were fitted to, amongst others, the Spitfire. This basic design served with little change into the 1960s and the jet age.

The first distinctly wobbled web appeared on the Convair CV-240 of 1948.[7] This used a wheel cast with nine narrow radial spokes formed from a curved disk.



Lotus racing cars

The best-known example of the wobbly-web wheel was that used by Lotus for their racing cars of the late 1950s and early 1960s. These wheels first appeared on the Formula 2 Lotus Twelve of 1957.[1][8] A similar wheel was used by Vanwall from 1958.[1]

The wheel's design was, in typical Lotus fashion, intended to achieve the lightest possible weight.[3] This is particularly important for wheels, as they are unsprung weight and the cars intended to carry them were extremely lightweight. Their designer was Lotus' Gilbert 'Mac' McIntosh although, again in typically Lotus fashion, they are often credited to Colin Chapman.[3]

Unlike earlier 'waved' wheels, the Lotus design and its distinctive 'wobbles' was a deliberate feature, even down to the way that the wobbles are deeply indented near the hub and soften outwards to the rim.[3][2] For ease of manufacture by casting, the wheels were designed with a constant wall thickness. Their stiffness (force/area) was required to be highest around the hub, so here the wheels were deeply and steeply folded. Towards the rim, distribution of this same force over a greater circumference and metal cross-section thus required less folding and their profile became a gentle wave.[3]

As for many other Lotus components, the wheels were cast from Elektron magnesium-aluminium alloy.[3] They were cast by Stone Foundries[9] of Charlton.[3] Cooper were already using magnesium wheels and apart from their technical advantages, Charles Cooper had found it highly profitable to sell drivers the many spare wheels required for racing, rather than having an external supplier take the business, something that Chapman was keen to emulate.[3] McIntosh and Chapman also obtained foundry scrap, failed wheels from Coopers, and studied these failures to understand the failure modes of a magnesium racing wheel.[3]

Lotus' team racing colours at this time were green and yellow, often small patches of each with the bodywork substantially of bare polished aluminium sheet. With the advent of the wobbly web wheel, rather than the previous wire wheels, the wheels were painted yellow and the car bodies green. This glossy bright yellow colour could be applied easily over the greenish-yellow zinc chromate primer used on the wheels to prevent corrosion.

4 stud wheel controversy at Le Mans

Lotus' main design principle at this time was Chapman's famous maxim, 'Simplicate and add more lightness'.[note 1] The Formula Junior Lotus 20 and Lotus 22 extended this to their front wheels, which used a 4-stud fastening, compared to the previous 6-stud that was still used on their rear, driven, wheels.[11]

At the 1963 Le Mans, Lotus entered the new, and somewhat unfinished, Lotus 23, a widened version of the Lotus 22 two-seater. It was disqualified from competition before the race and did not take part. This was on the basis of its wheels, although not specifically due to the wobbly-web design.[12] The car, as designed, used Lotus' typical 6-stud wheel at the rear but a 4-stud mounting at the front. The wheel rims were also wider at the rear, 6" vs. 5", with wider tyres of 5.50×13 rather than 4.50×13.[13] As the rules for Le Man were still framed as a "sports car" endurance race, they required the carrying of a spare wheel. The 23 had such a spare, but obviously it could not be fitted to both ends of the car. The scrutineers rejected this and barred the car from competition. Matching 4-stud rear hubs were flown hurriedly from England, avoiding the incompatibility problem, but the scrutineers now objected that if 6 studs had been required before, 4 must be inadequately strong and still refused to allow the cars to compete. After this, Chapman replied "We shall never again race at Le Mans.", a promise that Lotus kept until 1997, long after Chapman's death in 1982.[12]

The following year's Can Am sports car, the Lotus 30, switched from wheel studs to centre-lock wheels. This was mostly due to the far greater torque of its large V8 engine, but it also avoided this situation re-occurring, as the 30 also carried a single spare wheel.[14]

Spoked wheels

A drawback to the use of a disk wheel, particularly for front wheels, is the lack of airflow through the wheels, which is important for cooling the brake disks. The rear wheels of single-seat racing cars are driven by driveshafts from the transaxle and the brakes are usually mounted inboard in any case, away from the wheels.[15] Later Lotus designs, from the 26 onwards, used spoked wheels, although this was more about the shrinking diameter of racing tyres, especially fronts, rather than brake cooling. With an open-wheel racing car, any shrouding from the wheels was never a serious limitation.[15]

Alloy problems

As for many articles made from Elektron alloy, corrosion of the magnesium was a problem for long-term service. Even though Lotus only used these wheels on its racing cars, not its road cars, they had a limited service lifetime. Lotus cars of the 1960s are still popular for historic racing, but these wheels are now no longer acceptable for competition scrutineering, no matter what their apparent condition. As the wheels are such a distinctive part of the car's appearance though, there are modern reproductions available.

See also

References

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