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Double-weighing
Page 52
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settled back into the previous rolling condition when the brakes were released. He thinks end-to-end weight shift is caused more by changes in mean wheelbase than in changes in the centre of gravity of the load.
He believes that measurements of axle weights, whether by weighbridge or built-in device, when a vehicle is rolling are unreliable because they record the weight "as it has settled at that time"—and he is convinced that, particularly with suspensions having slipper-ended spring layouts, friction in the suspension and the effects of braking can cause quite wide variations in axle spacing—or mean wheelbase—and hence axle weights.
Mr Brock has carried out some more extreme tests on these lines, to show the effect on the axle weights of the tractive unit standing on the weighbridge while one of the
three axles of the trailer is lifted clear of tin approach road. Lifting the trailer's first axlt by only lin. transferred some of its weigh rearwards, while lifting the rearmost traile, axle increased the load on the kingpin. An when an axle was reweighed after it ha( been lowered, the weight recorded wa different from the original -figure.
The whole series of tests has served ti confirm an earlier decision to weig] three-axle semi-trailers separately—that ii uncoupled and alone on the bridge—for th purpose of obtaining the weights of load chargeable by the ton. Mr Brock maintain that the suspension is exceptionally sensitiv to slight deflections caused by surfac irregularities and braking forces. The wid variations obtained make the results c double-weighing very unreliable; in teF. three, for example, the change in th recorded weight of axles 2, 3, 4 and when the brakes were released indicated shift of 0.275 tons from the tractive un front axle. The wheelbase of the unit wa 10ft lfin. (308.41cm) and the kingpi load was imposed 18in. (45.72cm) forwar of the rear axle.