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ROAD AND WORKSHOP by HANDYMAN
Page 64
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Overheating Through Poorly Matched Tyres
FIRE is known as a good servant but a bad master and the same can be said about its near relation, heat, when this occurs in the wrong places, for example, in tyres, hubs or brakes. The majority of engineers concerned with heavy vehicle maintenance will understand the need for accurate tyre matching on multi-drive vehicles and will be well aware of the mechanical stresses and rapid wear set up when ,operating a vehicle on ill-matched
tyres. Quite often differential units will heat up to danger point, due entirely to tyre changes that have upset revs per mile of tyres on the driving axles, this causing a State of drag and stress that cannot be smoothed out via the differential mechanism. This stress can overload jockey and bevel gears on their plain spindles, these items now being compelled to .accept full engine power on the straight instead of just when cornering.
Quite a lot has been written on this subject and, in theory, surprises should now be few and far between. Nevertheless, there is always something new in transport maintenance and a recent experience of heat in the wrong places is worthy of mention, in that it contained both old and new features.
On the arrival at home base of an eight-wheeled double-drive vehicle and drawbar trailer, the vehicle being of a well-known make that is not equipped with a third differential, the driver reported that he was experiencing poor brakes and a smell of hot oil--he wished the vehicle to he tested before handing over
to the delivery driver. As one approached the vehicle, it was all too clear from smell alone that real overheating was in evidence and that it had built up over a number of hours. Both differential units were too hot to touch or work on, and the oil could be heard crackling and bubbling quite distinctly. There was a trace of oil from each brake backplate vent hole, although no sign of oil from the brake drums themselves; so, this far, the brakes were not flooded out. But the heat had really spread—hubs, axle casings, wheel discs and tyres were still too hot to touch, and tracing back along springs to chassis, it was something of a surprise to find the main frame quite warm up to two feet behind the last spring connection.
This then, was the cause of the brake weakness, which was in fact fade due to the steady build-up of heat in axle, springs, frame and tyres, until a point was reached where further heat released when braking could not escape. The cause was easy enough to find as on the offside first driving wheels there were two new tyres and on the offside rear driving axle a pair of tyres with less than one fourth of life left, and also a different make. Although both pairs were 9-00-20, past experience had made it clear that these two makes could not work together. even when new, as the diameter of one pair when inflated to 90 psi. was f in. greater than the other pair. The actual outside diameter variation of the new and the worn tyres on the vehicle in question
was in., and being on the same side there was no differential action available. Each wheel turned rev. for rev, with the other, and the drag set up had applied its full force to the two axle units. Whilst the cause of the overheating was illmatched tyres, and the brake weakness due to heat fade, the reason for the quite large and unusual heat " spread " into the chassis was not at first clear—until a scraper was applied to the mud on the backplate, springs, axle and chassis, the depth and solidity of this making it almost the perfect insulator.
[Drivers in charge of double-drive. vehicles without a third differential should be instructed to fit new tyres diagonally opposite to the next best pair, if such a case arises; this gives the bogie a chance to dissipate the wind-up in the transmission, through the opposing differentials. This instruction, however, must not be allowed to override the necessity for double-drive bogie tyres to be correctly matched before leaving the depot on the next journey.—ED.1