THE modern automobile with all its multitudinous accessories and components has now reached so high a state of dependability that the average motorist tends to take everything very much for granted, accepts things without question, and seldom pauses to tbink how and why such reliability is obtained.

Tyres, for example. Of the thousands of car owners whose motoring is carried out from one year’s end to another with complete immunity from trouble, few, very few know of the precise, intricate processes which are embraced in the manufacture of tyres and tubes, and a visit to the Birmingham works of the Dunlop Rubber Co., would, without doubt bring a bewildering revelation. Within this vast, very modern single-storey factory, a huge personnel work in the production of tyres for every purpose. All is high systematized and highly scientific.

los Piloted by a guide the visitor first enters the raw rubber store where the material is kept in the form of “smoked sheet.”

Raw compressed rubber is almost as hard as wood, and to make it workable it needs to be softened. This is accomplished by passing it between rollers mining at different speeds which tear it apart, and masticate it ready for the next process. It is then ready for cleaning, which is effected by forcing it through a fine metal mesh of 2,500 holes to the square inch. A great deal of heat is generated, and the machine has to be cooled by water. The rubber when it emerges looks like brown cords slightly warm, sticky and easily pulled apart. In order to get the tough elastic substance required for inner tubes, raw rubber has to be mixed with sulphur and various other substances ; colouring matter, heat resisting material and the like are also put in. The raw rubber and the various powders are passed between enormous steel water-cooled rollers and gradually amalgamated. When the process is complete, the rubber is cooled, and sent to the tube-making department.

After its journey, the tube stock is softened once more by passing it through rollers. Then follows one of the most important factors in making a good tube, the repeated passing of the rubber between close-set rollers to crush out any lumps, of powder which may have been left. An inflated tube is subjected to great stresses, and if a fragment of hard material is left in it, a weak spot will occur, with a consequent failure at an early stage. The ” straining ” of the raw rubber and the repeated passage of the mixed compound effectively removes foreign bodies.

The pinkish, mixed rubber is then conveyed to the extruder. This device has been likened to a sausage machine, and by means of a screw forces the rubber through dies in the form of a tube, the tube in fact The extruder is watercooled like the raw rubber straining machine, and by means of a chalk” carburettor,” chalk is blown into the interior of the tube, which prevents it sticking together. The extruded tube then passes along a belt conveyor to a series of water jets, which cool it after its passage through the die, air jets dry it, printers mark the place for the valve patch, and a cutting machine chops it off to length, all without being

touched by hand. The lengths are then taken off the conveyor to be weighed, but the control is so accurate that few pieces come outside the narrow limits set down. The next stage is to convert the tough length of tubing into the elastic substance

which we know as rubber. This is done by vulcanising. In order to make the finished tube fit into the cover without creasing, it is forced over a curved duralumin mandrel of circular section and clamped into place with a special clip which at the same time bevels the end of the tube ready for the joint. The slipping over of the tube requires considerable skill and strength, for the unvulcanised rubber has very little stretch, but by applying a blast of compressed air between tube and. mandrel the expert operator slides it on like a glove. The loaded mandrels are next hung in long boilerlike tanks, and subjected to heat for about 30 minutes. When the tubes come out they have become the springy substance with which we are all familiar, and after removal, the lengths are hung on conveyor hooks, and carried round to the next stage of manufacture.

First the valve hole is drilled, the ends of the tube solutioned and the valve thrown into the interior and the ends put together, all without removing the tube from its conveyor. It is then whipped off, and put on a joint-vulcanising machine.

The tube afterwards goes on its travels once more, and comes to a girl who finds the valve, pushes it through the hole, and with the aid of a power spanner, quickly screws on the outer parts. Ultimately the tube is tested in a water tank.

The tubes are then deflated (the last trace of air being removed by a vacuum pump), examined, numbered and finally packed.

The outer cover consists roughly of two parts— the tread and the carcass. The manufacture of the former resembles the beginning of the inner tube process. Blocks of “smoked sheet” provide the raw material, but for tread purposes it is much more heavily ” compounded ” in order to provide the necessary physical qualities. There are the vulcanising agents, the accelerators of vulcanisation, the reinforcing and toughening agents, the dispersing agents, the antioxidants, and so forth. And it is in the technologists skill in this compounding that much of the success of the tyre is obtained. The mixture is weighed out on platform scales fitted with magnifiers, so that quantities are closely checked ; the tread stock is then carted away to be mixed, an operation performed either by the ordinary rollers, as in the case of tube rubber, or in the Banbury mixer. This latter device is able to deal with 800 lbs. of tread mix ture at a time, and consists of rotors revolving in a steel casing. The sulphur has

to be added afterwards, owing to the great heat generated, and the mixture when it comes out of the machine looks like some sort of black road mending material. This lumpy condition is soon overcome by passing it through rollers, where the sul phur is put in, and it emerges as smooth black sheets. These sheets are passed through further rollers called profile calenders, shaped so as to form the tread with side walls attached. The tread strips are quite smooth, the pattern being

imprinted at a later stage of manufacture when the tyre is in the mould for the purpose of vulcanisation.

Making the Cord.

The cord for Dunlop tyres is made in. their mills at Rochdale. It comes down in hanks and is wound into cone-shaped balls and taken to the impregnating machine. The cones, 1,700 in all, are arranged in racks, and the threads are brought parallel to one another and fed in between two rollers. Raw mixed rubber is fed from the top and below, and the cords form a web about 54 feet wide, coated and held together with rubber. The web is wound off between rolls of cloth to prevent sticking and the complete roll is taken off and brought to the cutting machine. The ” Spadone ” cutter stands about 12 feet high, and when set, feeds a pre-determined length of the cord web over a roller at the top. Across the web, as it hangs down, runs a slotted steel guide which can be set at any required angle, a steel knife works in the guide across the web, and severs the sheet. Each strip cut off is called a ply, and two plies assembled together with cord threads running in opposite directions is formed into a “pocket.” Most passenger-car tyres are made on cylindrical wooden formers. A pocket is put on, its ends overlapped and wiped with solvent and stuck together. The cords run diagonally across the ply, and two are so arranged that the bias runs in the opposite direction. Two or three pockets are put on according to the type of tyre. Filler strips which protect the bead are then fixed about quarter way from each rim, and over them go the bead wires. Two more plies come on the top of this, then the parts of the lower plies projecting beyond the beads are folded over, locking the wires immovably. Rubber cushion strips go round between the

beads, and on top of the casing in certain sizes of tyre are put fabric breaker strips. These strips distribute road shocks transmitted through the tread, and form a second line of defence to objects which have penetrated it. The outer breaker is usually what one sees when a tyre is “worn down to the canvas.” A strip of the tread and sidewall material is now taken, and wrapped round and stuck down by naptha ; the construction of the tyre is now nearly complete.

The built-up tyre comes off in the shape of a flat, band, and the rounded crosssection is given by stretching on an airshaper, which in appearance resembles two umbrell as with aluminium spokes arranged points outwards. On the ends of the spokes are fixed aluminium sectors, and the tyre is put over these. Pressure on a pedal makes the tops of the” umbrellas” come together, forcing out the sectors and stretching the tyre into the finished shape.

The finished tyre gets its shape from being expanded against the side of a mould, and in order to do this an air-bag is put inside.

The moulds consist of enormous pairs of discs of steel, five or six feet across with half the pattern of the tread cut into each. The tyre is dropped into one hall mould, while its fellow half descends on an overhead track and is dropped into place over the other. The assembly is then given a push along a roller conveyor into a pit, where the ” curing ” takes place. The pit or autoclave, to give it its correct name, contains a hydraulic ram. This is at the top when the first mould goes in and is lowered for each following one. As it goes in, each air-bag has to be connected to an air-supply to keep it blown out when curing is taking place. When the pit is full the lid is lowered, air is turned on, the ram raised and steam is admitted. The vulcanising takes about two hours, and when the tyre comes out of the mould, it is almost finished. The tread pattern, size, and other marking has been embossed, the joint where the tread was joined has disappeared, and the rubber and the layers of cord all united

together in the form in which they appear when one buys the finished product. Giant fingers expand the cover and allow the air bag to be withdrawn, a man trims off the mould ” spue ” and the tyre is then cleaned and packed.

Testing Methods.

The days of trial and error in tyrebuilding are past, and the laboratories at

Fort Dunlop form one of the most important departments. Every batch of rubber and mixture is tested at frequent intervals for quality and uniformity. Laboratory tests cannot altogether replace actual trial on the road, so a fleet of vehicles ranging from the heavy com

mercial type to pedal-cycles is in constant use. Most of the running is done on ordinary main roads, moving the tyres from wheel to wheel every 500 miles, but the testers also use certain very rough sections of road to imitate overseas conditions.

For intensive testing on the premises an interesting batch of machines is installed. One type consists of a circular concrete pit in which a beam pivoted in the centre carries an axle with wheel and tyre. An electric motor drives the axle, which is fixed to the beam by car-type springs and shock-absorbers. One of the pits has a normal surface, while the other has patches of tarmac, loose stones, or any other material on which the designers may wish to test the tyre.

The apparatus most interesting to readers of MOTOR SPORT is the high-speed tester. In lay-out it follows the lines of the testers just mentioned and has the same pivoted beam. The tyre however, is forced into contact with a drum. Treads are liable to fly off at high speeds, so the wheel is surrounded with a steel shield, and the apparatus is operated by means of remote controls from a room outside the test-house ; the tyre is watched through peep-holes in the door.

This apparatus will run up to 300 m.p.h. and is used for testing tyres like those used on Sir Malcolm Campbell’s “Blue Bird.” Incidentally Sir Malcolm uses straightside rims on the front, and well-base rims with fillers on the back. A special cord casing is used, and the tyres have no tread at all in the usual sense of the word, the cords showing plainly through their rubber coating.

The world record tyres stand at the opposite end of the scale to the ordinary and Fort tyres, wearing qualities being unnecessary compared with heat-resistance and structural strength. The tread of the Fort tyres will withstand many thousands of miles on the road, and is safe up to speeds as great as 120 m.p.h. Beyond that, the racing type has to be used, as centrifugal force acting on the tread would cause it to disintegrate. Racing tyres, therefore, have stronger carcasses and lighter treads, wear being a secondary consideration.

There is a restless spirit about Fort Dunlop, a feeling that no one is content to rest on achievements already attained and while such institutions are with us, Great Britain’s supremacy in the realms of motoring sport will not fail.