Dunlop's radial steel cord tyre
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• Four years of development and road tests covering over 14m tyre miles have gone into the Dunlop SP111 radial-steel-cord tyre which has been introduced this week. It is claimed that the new tyre which has a thicker, flatter tread and utilizes an improved tread compound which gives high resistance to wear and cutting, can give up to 10 per cent more mileage than any other radial truck tyre. A new manufacturing technique developed for the SP111 gives better uniformity and other features claimed for the tyre include cooler running, better wet grip throughout the life of the tyre, reduced stone trapping and minimal side-wall damage through the use of a thicker kerbing rib.
Dunlop uses the new international standard for marking the tyre in which a letter indicates the profile width—A=7.50, B =8.25 and so on—with a small R following it to indicate that it is of radial-steel construction. When this is used, the tyre is a "high-load" design the equivalent to the maximum ply rating for the particular size) and the tyres are dual marked with conventional figures following but with the profile dimension followed by an "R". Where the code-letter marking is not shown, the tyre has a ply rating below the maximum thus the DR-20 /10.00R-20 with 16-ply rating while ''10.00R-20" is the same size but with 14-ply rating. The SP111
The new Dunlop SP111 has four circumferential grooves with an uneven zig-zag pattern and a thicker tread raised at the shoulders to give a fatter tread profile with more even contact pressure. These features help to prolong tread life,
is made in four tubed sizes-8.25, 9.00, 10.00 and 11.00-20 "high load" and 10.00 arid 11.00 20, 14 -ply rating. There are also 10.00 and 11.00-22.5 tubeless with 16-ply rating and an 11.00-22.5 14-ply rating.
The SP111 replaces the RB6 radial tyre made by Dunlop in the sizes quoted hut the latter tyre will be continued in other cases. The tread pattern of the new design differs from that of the RB6 with the use of an uneven-zig-zag pattern for the four circumferential grooves. Tread depth is increased effectively by 2mm and there are additional 1rn recesses at intervals the full way round the two middle grooves. The grooves are V-shaped with variable width around the tyre circumference, this reducing the danger of stone trapping. The feature has been patented by Dunlop and it provides for stones to be forced along the tread groove as the tyre rotates and to be squeezed out of the wider part of the groove.
A second feature that helps give the tyre ,-better adhesion qualities than other similar tyres is that the tread has grip slots which extend the full pattern depth and help the wet grip capabilities by "mopping up" surface water on the road after the main part of the tread has cleared away the bulk.
Tests carried out by Dunlop are said to have shown that the SP111 provides up to 10 per cent higher peak braking performance on wet roads than any other similar tyre. It is also reported that some sets on trucks in the Dunlop test fleet are approaching 90,000 miles and that many have covered 60,000 miles and still have sufficient tread left to complete at least 15,000 miles more.