The Dunlop detachable twin-wheel car upgrade.
I first read of this twin-wheel offering from Dunlop in a 1915 trade catalogue that I've owned for some time, its origins somewhat cloudy in my memory now but most likely an eBay find from a few years ago. It wasn't unusual to see competition cars fitted with twin rear wheel setups in the 1920s/1930s, and indeed through to the 1950s - especially those used in hillclimbs (for example this mid-1930s 8C Alfa Romeo) - their drivers keen to eke out as much traction as possible from the tyres that were currently available, but to see a road car fitted with such an arrangement can't have been a common sight in the vintage era.
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I wonder if many conversions sold? This catalogue was published in October 1914, during the early days of the Great War, for the 1915 season. Many manufacturing companies during WW1 switched over to the production of munitions and other equipment much needed by the armed forces.
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Various benefits were attached to the twin wire wheel, as the excerpt further down the page from the original 1915 catalogue recalls. I would imagine that a serious downside would be that the wider wheel/tyre combination may no longer fit beneath a standard car's mudguards, and might well foul the bodywork and/or the chassis, especially at the forward - steering - end of the vehicle (if the intention was to fit them to the front and rear of a car), likewise to the weight of the steering. Still, clearly Dunlop identified a market for this interesting accessory, and were happy to put the wheels on sale alongside their single-width detachable wire wheel, and SMMT-approved steel (artillery-type) wheels. The text accompanying the above illustration reads as follows:
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"The advantages derived from the use of the twin tyres on heavy cars are unmistakably shown by the experience of users of the Dunlop twin wire wheel.
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"The wheel itself possesses the general characteristics of the Dunlop single wire wheel - that is to say, it is superior to any form of detachable wheel with regard to the positive security of the locking device, the rapidity with which the wheel can be removed or replaced, and the simplicity and strength of the whole design.
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"In addition, it should be noted that the Dunlop twin wire wheel is so constructed that each rim is a driven member, the spokes being arranged to take up the weight and driving stresses in a manner which secures the maximum strength and efficiency.
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"The best results can therefore be anticipated when twin tyres are used on the Dunlop twin wheel, and owners of heavy cars are strongly recommended to consider the advisability of replacing single wheels by Dunlop twin wheels, which can be fitted on the inner hub of single detachable wire wheels.
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"The advantages of twin tyres comprise both comfort and economy. It is found that two tyres run as a pair on the same hub, yield a mileage which exceeds the combined mileage of two tyres run singly.
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"This apparent paradox is easily explained. To begin with, the axle load is distributed over just double the tyre area, and under these circumstances, tyres survive longer than when they have to perform the same work not in combination but singly. It is obvious that, when a tyre is supporting singly a great strain, the limit of resistance may be quickly reached; but in the case of twin tyres, the strain imposed is kept well within that limit, and consequently the functions of each unit will remain efficient for a greatly extended period.
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"Another important feature that contributes to the superior economy of twin tyres, is the distribution of the braking strains over a broader surface of contact. The force to be dissipated when the brake is applied is the same whether single or twin tyres are used; but in the latter case, the rolling surface being doubled, the strain is more easily borne, with correspondingly slighter effect on the tyre structure.
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"With regard to the influence of twin tyres on the motion of the car, they impart a remarkable smoothness and secure complete immunity from vibration and shocks. The speed of a car is not very noticeably affected. A point that is sometimes overlooked, is that when twin wheels are fitted, the mudshields must be proportionately wider.
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"The Dunlop Rubber Company will be pleased at any time to give motorists the benefit of their experience on the subject of twin tyres, in order to ensure the selection of the most suitable equipment for particular requirements."
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If anyone has photo(s) of this twin wheel conversion for road cars in action, in the vintage era or later, I'd be interested to see them. Commercial vehicles over a certain size have run with twin rear wheels since the very early days of powered road transport, but the idea never really took off with road cars. In more recent times, increasingly wide single road tyres have adopted the benefits listed for the vintage twin wheel arrangement, with none of the downsides.
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