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that greatly reduces injury from under-inflation and overloading. Development in the two-ply " cable " cord construction and the " multi-ply " construction paralleled each other. In 1920 the merits of the multi-ply construction had prevailed to such an extent that the " cable " cord was no longer made. After the World War European tire manufacturers began to duplicate American multi-ply cord construction in their millimeter beaded edge sizes. The cord tires of 1920 averaged 7,000 to 8,000 m. of service. Durability was also materially improved by increasing the cross-section size of the tires. In 1920 practically all American straight-side cord tires were made 10% over the nominal size. Also the tire manufactures gradually succeeded by persistent educational work, in getting car manufacturers to fit tires ade- quate for the loads to be carried. During this decade a new use was developed for pneumatic tires, namely their application to motor trucks up to 35 tons capacity. The movement made very little progress from 1910 to 1915. "Dual" (or twin), square- woven fabric tires in passenger car sizes were first tried. Then square- woven fabric tires of 8-in. and g-in. sections were employed with results encouraging enough to justify putting a limited number on the market. By 1916 automobile cord tire construc- tion had been mastered sufficiently for trial in the truck sizes. Success with 6-in., y-in. and 8-in. sections immediately demon- strated the superiority of the cord construction and was followed by regular demand from the public. The g-in. and lo-in. sections were used to some extent but their future was in 1921 uncertain. The typical solid tire of 1910 was the British " pressed-on " band tire then in use in Europe and soon to be duplicated in America. These tires were fitted to the wheels by simply forcing or pressing on with a special press, but in the absence of con- veniently located tire presses, some American manufacturers adapted the metal base idea to a " bolted-on " design (called " Demountable "), having bevels on the inside edges of the steel band so arranged that hoop-shaped " wedges " could be fitted to mount the tire, the whole assembly being bolted in place with " side flanges." Early metal base tires in America failed pre- maturely from fracture of the exposed hard rubber at the edge of the base band due to rough streets. To remedy this the band was made in " channel " form and the hard rubber protected by the side of the channel. In 1913 as an experiment, a "channel base " tire made to press directly on the S.A.E. (Society of Automotive Engineers) standard wheel without traction plate or staples, previously considered necessary, was tried. The experiment was successful, and this new type was so much simpler and less expensive that it rapidly superseded all other types in America. In 1915 wide single solid tires were intro- duced in America (8 in., 10 in., 12 in. and 14 in. wide) on the rear of heavy trucks in place of dual or twin tires. In 1920 wide singles and duals were almost equal in popularity.

The growth of pneumatic tire production in the United States is shown by the following figures, those for 1913, 1914, 1915, 1918 and 1919 being estimates: 1913 6,588,000 1917 . . 25,845,656

21,000,000 35,000,000 32,400,000

1914 . . . 8,983,000 1918

1915 . . . 12,840,000 1919

1916 . . . 18,564,957 1920

Structure, Materials and Manufacturing. Solid tire structure is clearly shown in figs. 4 and 5. The tire maker's problem is to attach the tread rubber, which must be of highest quality, to the wheel. This necessitates a steel foundation band, a thin layer of hard rubber specially compounded to adhere to the steel, to which the tread rub- ber will also adhere. No practical way of making the tread rubber adhere to the steel is known. The component parts of the straight- side pneumatic tire are indicated in figs. 2 and 6. The bead portion has imbedded in it a circular inextensible wire core, usually of many strands in the form of braid, cables, or coils (to give a certain amount of flexibility). This wire anchors the tire to the rim, prevents it from blowing off, and gives rigidity enough so as to prevent the tire from creeping on the rim when inflated. The body or " --" " f * t -

1 carcass " of the

pneumatic tire consists of bias " plies " of cotton fabric impregnated with adhesive rubber " friction," insulated from each other by a thin " skim coat " of the rubber, and having the edg^es of the plies folded or " tied in " alternately over and round the wire bead core. (See fig. 7 showing detail of a typical bead " tie-in.")

Since the function of the carcass is to serve as a strong yet flexible container for the inner tube with its charge of compressed air,

the specifications covering the fabric call for great strength, uni- formity of weight, freedom from grit, and particular grades of long staple cotton (Arizona, Sea Island, Egyptian, Sacalarides, Pealer, etc.). The two general classifications are: first, " square-woven " fabric, weighing 17$ oz. per sq. yd., woven from warp and filling, twisted of 1 1 strands of No. 23' yarn and having a tensile strength

TP

SSR

FIG. 7.

of 425 Ib. per in. of width (both warp and filling) for the best fabric. The number of plies used are 3^-in. 4-ply, 4-in. 5-ply, 4j-in. 6- ply, 5-in. 7-ply. Second, "Cord" ply-fabric, which is primarily a warp composed of parallel cords of combed Arizona or Sea I. cotton resembling fish line and weighs 14 oz. per sq. yard. The parallel cords would get snarled up in the tire building processes, so for handling purposes it is necessary to weave a single light filling thread into the cord, 2j picks per inch. Each cord, (((23)s)3) cabled yarn, has a tensile strength of 20 Ib. In cord tires the cords of each ply must cross those adjacent, consequently the direc- tion of the bias is reversed in the successive plies, which number as follows: 3j-in. 4 plies, 4-in. 6 plies, 4-in. 6 or 8 plies, 5-in. 8 plies, 6-in. 8 or 10 plies, 7-in. 10 plies, 8-in. 12 plies, g-in. 14 plies, and lo-in. 16 plies. As mentioned above, the tire plies are " tied in" round the wire bead core. To make a bead proof against rim cutting, etc., the most improved designs include narrow reinforcing strips of frictioned fabric (see figs. I and 7). The outermost of these is named the " chafing" strip. The outside of the carcass is entirely covered with rubber; the sides with a " sidewall " layer, i^-in. thick, and at the tread portion with " cushion stock," " breaker fabric," " undertread," and " tread." The tread is the thick, tough, firm, wear-resisting face of the tire which is in contact with the road sur- face. The forces and stresses of vehicle operation are so severe in their tendency to tear the tread from the carcass that tire makers have found it impracticable to attach the tread directly to the car- cass, and have had to resort to the interposition of the soft elastic adhesive " cushion " and open mesh " breaker fabric " to taper off the severity of the shearing stresses that would loosen the tread. Another very important function of the cushion and breaker is to prevent fabric rupture of the carcass by softening and spreading the intensity of impact of rough roads. The design and quality of the tread rubber must be worked out to wear at least as long as any other part of a balanced tire. The simplest smooth tread is a thin crescent in cross section ; and in the case of non-skid designs they are generally crescent cross section with geometrical depressions or protuberances. The physical properties most desired are toughness, to resist cutting and chipping, and attrition resistance to provide against abrasion from road surface friction.

There are no particular differences of design for the inner tubes; nearly all makes resemble each other very closely. Highest quality rubber with little or no compounding except sulphur is used for grey tubes. The best red tubes are compounded with antimony sulphide. To be satisfactory the tube must hold air; not crack nor check in storage; not stretch out of shape; not stick to the casing; not split nor tear easily; not be affected by heating; and must be repaired easily. " Flaps," made of inexpensive rubberized fabric, are used in straight-side tires to prevent the tube from being pinched or nipped under the edge of the bead, and to keep water and rim rust away from the tube.

Only general ideas of manufacturing can be mentioned. First, there is " stock preparation" ; the rubber and " compounds " are mixed, the fabrics " frictioned " and " skimcoated " with rubber, gauged to a very exact thickness, the frictioned fabrics cut to proper widths on a machine called the " bias cutter," the cushion, under- tread, and sidewall stocks are sheeted out and cut to width on the calender, the tread rubber is prepared either on the calender or the