TYRE PRESSURE AND LOAD

Evening Star, Issue 23492, 5 February 1940, Page 11

TYRE PRESSURE AND LOAD

Not all motor owners are aware 'why tyro pressure is all-iraportaht arid is so essential to the wellbeing of the . tyre and to ensure low running costs. If all tilings be normal, mainly, that is, if inflation pressure and load per tyre be normal, the tyro will assume its normal working shape. This shape is important, for on it depends to a large extent the service and longevity of the tyre. Tyre covers are so designed that the correct shape is automatically assumed when the pressure in the tyro is appropriate for the load. A distorted condition, due to under-inflation or overloading, is responsible for most tyre failures. Excessive flexing of the walls of cover is the most common form of tyro distortion. Few motorists appreciate that there is a definite relationship between- air pressure, the load, and, the area of ■ground contact. The laden weight of the vehicle falls on the part of the tyre which touches the road. The rest of the tyre is doing no work whatsoever until the revolution of the wheel calls it into use. For example, with a fourwheeled motor vehicle of a laden weight of, say, two tons, each tyre carries approximately 10cwt, thus each tyre or the part of it touching the ground is supporting 1,1201 b. This is the downward pressure. If the area of road contact be 20 square inches, then a downward pressure of 1,1201 b is carried on this area, equivalent to 561 b per square inch. As this pressure is exerted pn the tyre from above, it is obvious that, unless the cover has equivalent air pressure inside, exerting an Upward, lift, then the cover cannot assume its normal shape : in other words, it will be forced to yield to the pressure of the vehicle. If a tyre be required to carry a certain weight it is designed by its maker to present a definite area of tread to the road, which, in relation to the air pressure, is capable of counterbalancing the weight of the vehicle. If the air •pressure in the tyre be reduced, increased deflection takes place, and the tyro is not supported in its proper shape bv air pressure; in fact, it is practically sandwiched between the vehicle and road surface, a condition which is bad for the tvre and tends to shorten its service. To ensure satisfactory tyre mileage it is essential that the air must be always sufficient .to counterbalance the load the tyre has to carry, and thus enable it to retain its normal working shape.