Cobb's Tyres

Without Dunlops, British motor-racing would be impossible. Moreover, from the time when special, as distinct from normal racing, tyres became essential for attacks on the Land Speed Record, Dunlop alone have supplied such tyres for British contenders.

For Cobb’s recent successful record runs fifty 7.00 by 31 special treadless Dunlop tyres were made. The tread thickness was 1/50th of an inch, to withstand a centrifugal force of 150 tons at 400 m.p.h. In spite of such thin treads, the tyres have to withstand abrasion due to wheelspin. The tyre was held to the rim by a 180 tons/sq. in. high-tensile steel wire. A special wire-reinforced rubber ring, or “bead-spacer,” also helped to hold the tyre to the rim. Inner tubes and valves were more or less standard design, but with stronger valves made to finer limits. Each valve cap was locked to the stem as centrifugal force opens a valve long before 400 m.p.h. is reached, and so only the cap keeps air from escaping. The rim nuts were strengthened to obviate stripped threads. Special rubber compound was used to limit temperature rise to 210 deg.F.

Cobb’s Dunlops were 30 per cent. stronger and lighter than the tyres Eyston used in 1937. A special plant was used for testing, the tyre being revolved by a 20-h.p. electric motor and a 150-h.p. motor revolving a steel drum that was brought to bear on the tyre for one minute at a load of 18 cwt. The tyres were also checked for centrifugal force effects at 560 m.p.h. on a special turbine assembly at the G.E.C. works, the loading reaching 270 tons.

Cobb’s special Dunlop wheels were of divided type, each weighing about 105 lb. without its tyre. The rim members were machined from a solid nickel-steel ring. The rims had to resist a total internal air pressure of over 40,000 lb. at 400 m.p.h. Thirty-six h.t. held the detachable portion of the rim and each had a tensile strength of approx. 55 tons/sq.in.

At a luncheon at Fort Dunlop on October 29th, Cobb thanked the workers who made these tyres. In replying, Mr. King, of Dunlop’s, said: “I would like to add my congratulations to Mr. John Cobb. He is very modest about it all — very modest about all his difficulties, about the weather and, I expect, about himself.

“It is, I think, a really outstanding achievement — outstanding in regard to the pluck and perseverance and courage that Mr. Cobb has shown in this affair. We have contributed to some extent, but the major part is his. I saw the record of what was happening each day and I was perfectly amazed at the way in which this job was being persevered with in the characteristic English manner and all the difficulties that had to be overcome — to the extent of flying camshafts from England and other difficulties. And as a just reward for all the toil and trouble the record was broken with a really outstanding addition of 30 miles an hour which, on top of the previous record, is quite an amazing achievement. We were used, before the war, to seeing a few m.p.h. added each time, but when Mr. Cobb comes on to the scene he seems to have put a slice on every time and this one is a very big slice! All credit to Mr. Fletcher, who designed the tyres, Rochdale who made the special tyre cord, and the Schrader people who made the valves. This is a striking example of the way things are done in the modern world, not just by one person but by a whole team who work together.

“The wheels were made at the Dunlop works at Coventry and were the ones that were used last time. They were, of course, reconditioned, but it is a triumph that the wheels were good enough to do another 80 miles per hour. I was asking Mr. Fletcher and Mr. Hardeman this morning how much more the tyres could have done and we came to the conclusion from the data collected and the experiments carried out that they would have done 420 without any undesirable effects — that is to say, had the course been normal — but as Mr. Cobb just told you, it was not very normal, the tyres had a good bouncing, and they were not really made for bouncing. [Even so, Cobb’s highest speed was 415 m.p.h. — Ed.] The temperature of the tyres was quite normal, in fact, it was rather low. The dangerous temperature is 100° C., the actual temperature reached was probably about 50° C. When testing the tyres, we started measuring the temperature at about 87° C. — we actually had temperature measuring equipment inside the tyre — and went up to 120° C., but as in actual performance the temperature did not even reach 87° C. it was very much better than we expected. The strength and speed required were also well within the capabilities of the tyres. One tyre did four runs, and two tyres did three runs. That is all I have to say, except to congratulate Mr. Cobb and his team on a magnificent achievement.”