Safer and Stronger Tyres on way
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THE development of new giant tyres is advancing rapidly, covering a very wide range of new materials, constructions and tread patterns. Manufacturers must continue to concentrate on tyre features to improve road-transport economics, meaning continued developments to improve tread life, remouldability and fuel consumption, whilst higher road speeds and an increasing number of vehicles on the road bring with them the requirements of increased wet-road grip and progressive contributions from tyres to improve vehicle ride. These were the conclusions made by Mr. D. W. Badger, development manager, giant tyres, Dunlop Rubber Co. Ltd., bt a paper presented in London to the Institute of Road Transport Engineers last week.
Of the factors affecting tyre-performance properties, tread life was normally the first consideration, according to Mr. Badger, and new developments were concerned with case construction, tread compound and tread pattern. So far as case construction was concerned, the radialply type of tyre—with either a single steel ply or several textile plies—could give an average advantage of 30 per cent in tread life compared with the cross-bias textile tyre under favourable service conditions, but the advantage was minimal on motorway use at normal loads.
New developments in the polymer field Were leading to tread compounds with improved wear resistance, whilst so far as tread patterns were concerned Mr. Badger pointed out that the modern flatter treads were designed to give even pressure distribution across the tread, and that further improvements could be achieved by packing more rubber into the centre of the tread pattern, where most of the wear occurred. In this connection the new three-rib p.s.v. tyre ga, good results.
Speed Performance Dealing with speed performance on motorways, the speaker pointed out that high speeds and high loads could cause tyre failure because the strength of the tyre compounds and that of the cord-torubber bonds were reduced at high temperature, thus the strains caused rupture. Nylon textile casings give an increase of 10 m.p.h. in the safe operating speeds recommended by the Tyre Manufacturers Conference, whilst radial-ply steel tyres ran between 20°C. and 30°C. cooler than cross-ply textile tyres; thus the steel tyre could give a further increase in the speed safety margin.
Mr. Badger said it was well known that tyre grip on wet roads was controlled by the tread pattern which could bite down through the water film and the rubber polymer that could give the final grip, and that combinations of tread pattern and high-grip rubber compounds could give a total improvement of up to 50 per cent in braking on extremely slippery test surfaces. Tests comparing radial-ply tyres and cross-ply textile tyres had shown that these two constructions gave equal braking grip on wet roads. High-speed coaches and _lorries needed improved grip to give significant performance advantages, but skill was ,required to combine a high-grip compound in a high-speed tyre as these compounds ran hotter: thus the high-grip
and wear-resistant compound had to be confined to the cap of the tread, so that• a cool-running compound could be used in the tread base to restore this temperature balance to a safe limit.
With regard to fuel consumption, Mr. Badger said that laboratory tests showed that between 16 and 17 per cent fuel saving could be achieved by the use of radial-ply tyres compared with a thicktread p.s.v. tyre of conventional construction, whilst accurately controlled road tests gave a remarkably close figure of 13 to 14 per cent. In these days of traffic delays, however, the advantage could be reduced to about 5 per cent.
The author went on to point out the value of correctly balanced tyres, wheels, hubs arid brake drums for motorway use. On the subject of weight reduction, Mr. Badger said that over the past few years progressive developments had given an 11 lb. reduction in the weight of a 9.00-20 cover, which was almost 1 cwt. on a three-axle vehicle. Whereas conventional radial-steel tyres were usually heavier than cross-ply textile tyres, the radial construction" could save weight when used in a tubeless tyre, and mention was made of the new 3B wide-based rim with rubber sealing ring, whilst a further development was the use of a 15° taper drop-centre rim for use with tubeless radial tyres. The author said that the new types of wide single tyres were being developed mainly for specialized services where weight saving was the main requirement, although there were sonic penalties in terms of tread life and vehicle stability.
Actual Developments Regarding actual developments, the author said his company had found that transport engineers using giants required the following tyre properties, and in this order: tread life, structural performance and remouldability, road holding, and fuel consumption. Both cross-ply textile and radial-ply steel tyres would give a high mileage, and the increase in straight running on motorways would give a reduced difference between the tread lives of the two types of construction. The new Dunlop three-rib tyres which had been developed for North America were giving a 30 per cent improvement in tread life. Whilst both types of tyre had a reasonable remould potential on good roads, the introduction of such new cords as Nylon gave the textile tyre a useful advantage on remouldability. Where natural hazards gave a risk of accidental damage the cross-bias textile tyre was more readily, repaired and had a better chance of being suitable for remoulding.
P.s.v. tyres for urban services could contain extra rubber below the pattern with a textile construction because the operating speed was usually below 45 m.p.h., and this usually led to a life advantage compared with a radial-ply tyre, although the latter could give a useful fuel saving. Mr. Badger then said that his company was on the point of introducing a special radial-ply tyre for urban p.s.v. operation that would give a tread life equal to the textile p.s.v. tyre and maintain the low-rolling-resistance feature of the radial tyre.
For high-speed coaches there was a choice between the radial-ply tyre or specially developed high-speed Nylon Lyres. With this type of vehicle it was both desirable and possible to develop new tyrek with improved wet-grip features, and tyre manufacturers would be required to draw upon the valuable experience of tread compound and pattern developments that had been successfully employed with modern car tyres.
Questions Answered
During the course of the discussion following the presentation of the paper, Mr. Badger said that both radialksteel and textile tyres would cope easily with longer high-speed motorway runs than are at present possible in the U.K., and that tests carried out by his company showed that at a cruising speed of 45 m.p.h. tyre temperatures no higher than 70°C would be reached. Regarding stability, particularly of artics, he agreed that it could be assumed that vehicles with steel-cord tyres .,would roll more than those with textile tyres, and Mr. Irvine List, of Michelin, confirmed this from the floor of the meeting, explaining that radial-tyred vehicles could corner more quickly, thus roll obviously would be greater.
Mr. Badger was then questioned on the problems of directional instability which could arise with steel-cord tyres when power steering was fitted, particularly when trying to keep a straight course, and his explanation to this was the "time lag" in such tyres which was difficult to eliminate. When asked about the advantages of demountable rims such as the Swiss Trilex type, Mr. Badger had no comment. In reply to another question. Mr. Badger said that a radial tyre was no more inclined to "climb the kerb" than a textile, but agreed that side-wall cuts were usually more common on radial tyres because the side walls bellied more and were therefore more exposed. Regarding the "grinding" damage to tyre beads caused by rims and flanges, Mr. Badger 'thought that the drop-centre or 313 wheels would give an improved bead performance.