The Use and Abuse of Tyres Michelin Executive Describes Relationship
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Between Tyre and Vehicle Design, and-: Advises Operators on the Care .of Their Equipment IN his paper "Tyre Development and the Use and Abuse of Tyres," read last week before the Institute of Road Transport Engineers in London, Mr. W. R. Good, A.C.G.I., M.I.Mech.E., a senior executive of the Michelin Tyre Co., Ltd . said that tyre and vehicle development were closely linked. The pneumatic tyre was the
starting point of self propelled wheeled-vehicle transport as we know it to-day.
In 18)8 the Michelin brothers put what were he believed, the first car tyres on a 4-hp. vehicle which they drove in a 750-mile Paris-Nice race, and a tyre burst every 100 miles.
Designers' Demands
The pneumatic permitted high speeds. These posed tyre problems. and tyres had to be improved to cope with the vehicle characteristics which their use made possible. That competition or collaboration still existed. Designers demanded higher speeds, greater axle loads, better braking, lower centres ot gravity and lower unsprung weight, all calling for modification or improvement of tyres. .
Tyre makers had even anticipated the needs of vehicle makers and permitted desirable developments in vehicle construction. Tyre makers had also to face the problems attendant upon the abuse of tyres, i.e., their use under conditions for which they were not designed. Overloading, underinflation, defects of alignment and speed were all within the control of users. Road and climatic conditions and temperaiures, and types of usage were factors over which users had no control.
Tyre developmtnt was the result of practical experience and empiric approach. The tyre was not a product which could be " worked out mathematically." Too much theory led into false paths, but mathematics had their proper place in design and developmont as a servant of practical approach. Tyres formed a vital part of the suspension and transmission systems of the vehicle.
Gifts of the Pneumatic
A great deal was asked of a tyre. It had to protect the vehicle and the passengers or goods, propel it and stop it—often rapidly—hold the vehicle safely on bends and carry a load, frequently an' overload, long distances at high speeds. It was the pneumatic, v.hich, by reducing shocks and vibration, permitted progressively lighter vehicle construction, low maintenance costs and better comfort.
Protection seemed to follow an inverse square law, and a tyre with 1-in, deflection gave about four times the protection of one with 1-in, dellec-. tion, bu: tyres must be designed for this. It was no use running a highpressure tyre designed for small deflection at a bio one. It caused trouble and vehicle instability. The ratio of rim width to tyre section had risen, but more so in cars.
For giant tyres the technical desirability of wide-base rims was not so pronounced but the 5 per cent. taper seat was a useful development. Giant rims had been reduced to a nearly uniform 20 ins., making a problem for brakedesigners and for 'yre makers, because of the heat transmission. On the Continent, transmission brakes reduced this heating trouble with its inevitable brake fade and tyre dain:ige.
There was slightly, more power absorption. by low-pressure tyres, but this was compensated for by less .vertical lift over . road irregularities. The tangential force on the tread could reach 450 lb. or more per tyre when accelerating on a fast vehicle, and the same when braking at only 50 per cent.
efficiency. There was a, further tangential effect at right angles, from centripetal force and road camber.
Stronger Cords
From the point of view of pooer transmission and vehicle stopping. development was in the direction of cords of higher tensile strength and wider and flatter treads. IReference to the semi-metallic X tyre was made in our issue for last week.—ED.1
Large tyres with many plies generated considerable heat when run as high as 120 degrees C. This heat resulted from the hysteresis loss in the rubber bonding the plies to each other and not from actual contact between cords of adjacent plies. Cotton lost nearly half its tensile strength at the temperature'rnentioned, whilst heat softened the rubber, which caused loosening of plies, tread detachment and burst casings. Rayon retained much of its strength at high temperature, but the most revolutionary step forward was composite cords of high-tensile steel, which required few -plies and retained their strength even at 300 degrees C.
Tubes and flaps had important functions and if incorrect could cause
trouble. Butyl tubes held air indefinitely. Punctures in them did not spread, and even when run flat for some distance the tube did not disintegrate, but this synthetic material had to he imported.
Ply rating was an essential part of giant tyre identification. It was all index of strength, but did not necesSarily represent the number of cord plies. Tyre makers preferred stout rims with no tendency to flange distor tion under heavy loads, and continuous flanges which did not dig into beads.
On the use and abuse of tyres, the author said that modernroad surfaces were usually more rasping and increased the rate of wear, Water was a good lubricant for rubber, and in humid climates tyre wear was less. Atmospheric temperature had a great effect. At 100 degrees F. the wear was nearly five times greater than at 40 degrees.
Excessive toe-in and toe-out could be catastrophic in lyre wear. High speed, rapid acceleration and powerful, braking eat tread rubber, as did taking curves at speed. Under-inflation and overloading might cause premature bead failure, and gave rise to rapid wear. Inflation was often neglected.
Bleeding Tyres Bad
Bleeding tyres when hot was most harmful. To burst a sound high-pressure casing required at least three times
the normal pressure. Twinned tyres never carried equal loads. A solution was flexible hub mountings, as widely adopted on the Continent. It was also helpful To use slightly higher pressure in the outer tyres. Low-pressure tyres were lesssusceptible to tread cuts.
High pressure might mean 100 lb. per sq. in. of ground contact, driving sharp objects into the tread.
In the discussion on the paper the answers were given by Mr. I. List. M.A. (Eng.), Michelin technical adviser. To a query concerning flats on treads, he said that once started a fault remained all the lyre life. In a new vehicle the brakes might at first be out of balance and start the trouble. It was caused also by snatching brakes.
Regrooving Unnecessary
To other questions Mr. List replied that his company sold tyres basically the same on mileage contract' or ordinary work, but, those for public service vehicles obviously differed. He did not think regrooving was necessary if tyre positions were properly changed. There would then never be tyres without pattern until the timecame for retreading. Only about one-fifth of the tread rubber was patternless. On mileage-contract tyres, there was generally better supervision and inspection. There was no reason to treat X tyres any differently from others. There was no possibility of a universal tread to suit all purposes.
Old type tyres, such as the 40 by 8, would be made for 10-12 years, until existing vehicles were scrapped. The main trouble with two-piece wheels was.. distortion, sometimes caused by abase in the matter, of weight and speed, whilst materials were not so good. Legislation had also forced weight saving on wheels. Endless flanges were a wheel-makers' problem. Their condition could be judged by .their flatness and, Ovality. . They were not always handled properly, and would last well if treated gently.