FUNCTIONS OF TYRES Cushioning.—Road impacts are of two kinds, shock and drop. When a tyre strikes an obstacle protruding above the road surface the road and the tyre both experience shocks, designated respec tively as shock impact to the road and shock reaction to the ve hicle. Studies by the U.S. Bureau of Public Roads indicate that shock reactions increase progressively with the speed of the ve hicle up to 40 m.p.h. and assume constant value at further in creased speeds. If travel over the obstacle raises the axle of the vehicle the tyre, after passing it, drops back into contact with the road, producing what are termed drop impact and reaction. These reach maximum value between 20 and 4o miles per hour. Above these speeds the tyre envelops obstacles more completely and drop reactions decrease in magnitude. The fact that pneu matic tyres, especially the low pressure type, are far superior to others in reducing shocks to the road has prompted legislation favouring the use of pneumatic tyres.
Depending on the material and smoothness of road surfaces and on conditions of the weather, the coefficient of friction between tyres and road may range widely, —from o•02 to 0.70. Dry surfaces offer high resistance to slip page ; whereas wet surfaces, especially immediately after rain be gins, are extremely slippery. Non-skid designs are formed on treads during the vulcanization process when the rubber bags in side the tyres expand them against an engraved mould presenting the desired design in reverse. The designs most effective in pre venting slipping and skidding on wet, hard surfaces are those which remove water from the highway under the tyre through channels in the tread, permitting the rubber to grip a nearly dry surface. On dirt roads traction and steadiness are enhanced by deformation of the road material as it sinks into depressions in the tyre tread. This condition is encountered with farm tractors which are used over ploughed ground. Tyres for such equipment are consequently made with sturdy, thick crossbars of rubber. To improve traction still further, farm vehicle tyres are weighted either with iron weights applied to the wheels or by almost filling the tyres with a non-freezing water solution of a suitable salt.
Even over smooth pavements some non-skid designs produce objectionable noise and vibration espe cially if, after some thousands of miles of travel, the tyres are unevenly worn. Intensity of sound caused by tyres has been meas ured by microphone and radio-receiving instruments and great improvement has been made in eliminating noise from.this source. Elimination of isolated knobs or buttons, and the use of con tinuous ribs or of unevenly spaced rectangular blocks to avoid regular vibrations, has been effective in accomplishing this change.
Proper design of the cross-sectional con tour of tyres, the manner in which the cords are arranged, and the width of rims all affect steering of the vehicle. Properly de signed and properly inflated tyres run true without weaving. Dynamic balance of tyre and tube assembly must be maintained within certain limits specified by car manufacturers. This require ment is designed to prevent uneven tread wear and "shimmy" or "tramp," which were frequently encountered when balloon tyres first came into use.
Aside from the expense of replacement, long uninterrupted service is demanded of tyres to-day for other rea sons, particularly when the vehicle is used for commercial pur poses. Failure to meet appointments or to maintain a bus sched ule, and delay in transportation of goods, must be avoided. The quality of modern tyres is remarkably high and failure of tyres properly used is extremely rare.
The following figures indicate the economic importance of the rubber tyre industry in the United States.
42 Number of wage earners in 1935 . . . . . . . 57,128 Wages in 1935 ....... . . . $ 78,253,000 Value of tyres and tubes produced in 1935 . • • $446092000 At one time the useful life of tyres was ordinarily terminated by failure of the fabric in the carcass. To-day, however, car casses of tyres almost invariably outwear the tread, even though tread life has been improved greatly during the past decade. Consequently the practice of retreading or of recapping treads is now economical and many tyres will last until a second tread has been worn through. The development of retreading compositions which vulcanize around 26o° F. has made possible application of new treads on worn tyres without deterioration of the carcass, which would occur at somewhat higher temperatures.
Wheels for many purposes are now fitted with rubber tyres. Not only bicycles and motor road vehicles but railroad cars, in dustrial trucks and tractors, farm tractors and farm machinery, wheelbarrows, lawn mowers, juvenile vehicles (notably coasters), roller skates, bullock carts, and rickshas are now equipped with rubber tyres. Aeroplanes, also, are now normally equipped with low pressure tyres. The cost of pneumatic tyres is low to-day compared with 20 years ago and the quality vastly higher. Con sequently, the average number of tyres required per car each year has decreased from about five to less than one and a half. Lower costs of tyres and increased safety and freedom from delays resulting from tyre failure have been important factors in promoting the extensive use of motor cars for pleasure and busi ness purposes. (W. H. P. ; J. W. Sc.)