Tyre wear on tri-axle antics
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• The Goodyear Tyre Co Ltd has published findings of a detailed survey into tyre life on three-axle semi-trailers. Because the wheelspread of a three-axle bogie is around 8ft 6in, the scrub effect and resultant tyre wear on corners is appreciable. Tyres on the front and rear axles of the bogie are dragged sideways causing excessive strain on the side-walls and a high rate of tread wear.
On closer inspection, Goodyear found that, in a number of cases weight distribution over the three axles of the running gear was far from even due to the balance beams on each side of the suspension being unable to cope with the deflection angles brought about by extreme angles of slope on semi-trailer frames. For the self-compensating linkage to be effective, each equalizer beam must have an approximately equal amount of clockwise and anti-clockwise travel available when the trailer is on level ground.
In one case an operator, Lloyd's Animal Feeds Ltd, had put a trailer based on a fifth-wheel coupling height of 5Iin., into service, coupled to a Volvo F86 tractor, whose fifth-wheel was mounted at a height or only 49in. by interposing packing pieces between the springs and axles. As a result the lin.-thick packing between the first axle and its springs had the effect of forcing the front pair of equalizer beams beyond their normal range of travel so that load compensation between axles 1 and 2 was not effective. Test weighings showed that the weight distribution on front, centre and rear axles, instead of being equal-was in the ratio 47, 31 and 22 per cent respectively. The leading axle tyres suffered bursting failures after only 3000 miles.
The problem was solved by doing away with the spring axle packing, by raising the fifth-wheel height the required 2in. and by substituting Goodyear hi-Miler cross-ply tyres in place of the radials originally fitted. In this way tyre life improved to approximately 24,000 miles. Any operators whose three-axle trailers are not giving the hoped-for tyre life could well call on the Goodyear specialists for advice.