TESTED AT 310 M.P.H.

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TESTED AT 310 M.P.H. How they made Sir Malcolm Campbell’s Tyres

THE tyres on which Sir Malcolm Campbell will attempt to break his own speed record, run at a temperature twenty per cent, cooler than any tyres hitherto made for him at Fort Dunlop. “Very rapid heat generation is one of the main factors tending to destroy tyres like these,” we were informed by a Dunlop official last month. “At 280 miles an hour the Blue Bird’s wheels revolve about 2,500 times a minute, or 42 times a second and continuous hammering of the sand at that rate naturally generates heat very

“The tyres about to be used on Daytona beach have each been tested on a Special machine on which tyres can be run on load at speeds up to 310 miles an hour. Here tyre and wheel, the latter made at Coventry in the Dunlop rim and wheel works, and dynamically balanced at speeds of over 200 miles an hour, are mounted on a loaded arm which allows them to rest on a massive electrically

driven steel flywheel upon which they run. This machine is controlled from a room containing the switch gear, separated from the machine itself by a steel plate shield and the progress of tests is watched through peepholes in this shield. “It was found in this test house that the tyre temperature rose about forty degrees centigrade per minute. That shows the very rapid heating at speed, although during an actual record attempt on sand there are other considerations to be taken into ,account. On the one hand, as sand is a deformable flat surface, the

area of contact is much greater than on our testing machine. On the other hand. Daytona beach sand is usually damp and helps to cool the tyres ; moreover, while on the testing machine a tyre .carries a vortex of air round with it, whereas during a speed bid the air rushing past the tyre at over 250 m.p.h. causes very considerable cooling. ” Among the kuproveinents in the materials and construction of this year’s tyres which make them better than any yet made is the manufacture of the cord twist, made from specially selected Sakellarides Egyptian cotton fibre and specially woven, which now during flexing absorbs less power than before and is therefore cooler running. Relatively hard spots

which would overheat the tyre locally at high speed have also been avoided by having the inner tubes specially made so as to be absolutely uniform in thickness round the circumference. ” In addition to rapid heat generation the other main problem with such tyres is how to cope with the effects of the high centrifugal force created during a speed bid. The tension in the wire coil of each bead due to centrifugal force is approxi mately seven tons and the tension in the

side wall, tending to tear it from the bead wires is approximately 1,250 lbs. per inch width. Special high tensile bead wire, with a strength of 150 tons per square inch, has been used for this year’s tyres, and the tread, which is made from raw rubber taken from a special blend reserved for racing tyres, is very thin, partly to avoid the tendency of high centrifugal force to throw the tread off the casing. A further improvement this

year is the use of a special compound for the outer covering, the better to resist cutting and reduce the rasping action of the sand. “Another effect of centrifugal force is to cause the tyre to increase approxi mately one inch in diameter at 250 miles an hour, even with the very high inflation pressure of 125 lbs. to the square inch. Manufacturing processes for the present covers have been improved in the direc tion of reducing the tolerance allowed on size to produce greater accuracy in shape, and so truer running, and the whole tyre shape has been modified to give a larger

contact area during running and therefore distribute the stresses more evenly in the tyre.” All materials used for the tyres have been put through a laboratory test at

each stage before being passed on to the next stage. Five factory departments have been concerned in making the tyres, and fifty operatives have handled the material for them.