Tyre Economy

Northern Advocate, 28 May 1938, Page 15

Tyre Economy

I While the primary object of a tyre is that of cushioning, the tyre tread is the point at which power is translated into progress. If. therefore, full benefit is to be obtained from tyres and costs kept as low as possible, some attention —although not very much —is demanded.

Regular maintenance of correct inflation pressures is vital. On this depends the degree of cushioning, the rate of wear, and the incidence of fatigue. Tubes are not perfectly airtight. Air is lost by a physical process known as "‘diffusion,” not by porosity. In the course of a week the pressure falls by about 31b per square inch. Consequently,' tyre' manufacturers have always insisted that pressures should bo checked every week and any loss restored, and that only an inflation pressure gauge that fits directly to the valve be used. It is just as illogical to dispense with the use of a tyre pressure gauge as to do without an oil gauge. The spare tyre needs weekly inflation, so that it is always fit for use when required.

Failure to maintain proper pressures affects adversely both the tyre casing and the tread. Excessive bending of the tyre fatigues the cotton and eventually results in an extensive fracturing of the casing. Again, if the tyre receives a sharp localised blow when in an. under-inflated state, the’ casing is sharply bent to an excessive degree, causing a local fracture of the cotton structure.

Because tread slip produces wear, every effort is made to reduce distortion of the rubber to a minimum; this is achieved partly by the compounds used and partly by so designing the contour of the tread that the load is evenly distributed across the whole width. If inflation pressures are low, the load is concentrated on the sides of the tread, where there is then a high intensity of pressure, bringing about excessive slip and wear. Over-inflation is almost as harmful as under-inflation, for then the load is concentrated in the centre of the tread, which wears rapidly, and blows may fracture the casing by sheer concussion.

It is “unnecessary to reduce tyre pressures in hot weather: the changes in temperature in this country do not involve any appreciable rise in inflation pressures. There is a normal rise that takes place whatever the weather and this is naturally allowed for in the manufacturer’s recommendations.

Blows received by the tyres, as when stinking a kerb, may disturb the alignment setting of the wheels, in which case the tyre runs diagonally upon the. road, a grinding action is set up, and very rapid wear results. The usual setting for cars having axle assemblies of the orthodox type is; rear wheels, parallel; front wheels, parallel or a toe-in not exceeding rath inch.

Driving is another important factor governing tyre wear, for fierce acceleration and sudden deceleration by drastic use of the accelerator, the clutch, and the brakes distort the rubber abnormally. The smoother the start, the gentler the halt, and the more easily roundabouts are taken, the longer will tyres last. That high speeds produce higher tyre temxxeratures and increase slip is well known. Tyre wear at 50 m.p.h, is nearly double that at 30 m.p.h. Oil, grease and paraffin are injurious to rubber, paraffin being almost instantaneous in its effect. Any oil. that may have tieen thrown on to tyres should be removed without delay by a rag moistened sparingly with petrol. When the car is in for overhaul, an opportunity occurs to remove the

tyres for inspection, repair any damage that has not yet caused a failure, and to clean the rims. The latter maybe treated with graphite or a coat of special tyre and rim paint. The golden rule concerning cover'repairs is that, while any damage to the rubber may be prevented from extending by a simple vulcanised repair, any injui’y to the cotton material can be successfully dealt with only by a repairer having specialised knowledge and the proper plant for a fully vulcanised “restoration.”

Oversizing, i.e., the fitting of tyres of larger sectional width than the standard size for the car, is a recognised practice, offering better comfort, increased protection, against damage, more resistance to skidding, and greater mileage. It is necessary, before fitting the larger tyre, to make certain that it is suitable for the existing rim. Old tubes should never be used in new covers, for there is a danger of nipping and creasing. It is false economy to risk the destruction of a cover costing pounds in what will probably be an unsuccessful attempt to save a few shillings. In fitting and removing tyres it is essential that due advantage should be taken of the rim well, to avoid unnecessary strains and damage by tyre levers. The red spot on the bead of the cover should always be located at the valve position—to promote balance. It is a sound practice to change tyres round every 2000 miles —

spare to off rear, off rear to off front, and so on round the car. This ensures even, wear and prevents the spare being out of use too long,—“The Times.’’