Tiresome matter of metrication
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IN CM August 2 issue, there are two letters about tyres — one from Michelin and one from Dunlop. And in both of them the same error occurs in metric/Imperial conversion.
The figure in question is the weight allowed on a front axle. In both letters the figure of 6 tons appears, with its metric equivalent, in brackets, given as 6610kg.
Now 6610kg equals 6.5 not 6 Imperial tons; my calculations show 6 tons to be 6095kg.
This error in CM's conversion between metric and Imperial is not just academic in this case. It happens to be crucial in the points being made by the tyre companies.
The whole question of maximum allowable weights on steered single-tyred axles of heavies is, it seems to me, wrapped in confusion. According to the wall chart published by CM on permitted vehicle weights, up to 7 tons (7110kg) can be imposed on a steered axle. Can someone tell us what minimum tyre size this implies?
Would that size of tyre bring clearance problems under wing valances on existing heavies? If fitted at the rear as well (in twin formation) in the interests of spare wheel interchangeability would such big tyres make the vehicle over width: beyond 2.5m? Indeed, are dished wheels available to suit such tyres without "cheeking" occurring on twin rears?
If 16-ton rig ids with 7-ton front axles were obtainable the present problems of axle overloading caused by a forward or rearward biased centre of gravity (through a non uniformly distributed load) would be greatly. eased. And chassis designers could aim for say a 6.5-9.5 ton (Imperial) front-rear axle loading when the load was distributed uniformly.
ALAN BUNTING Harrow, Middlesex