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Simplification of Giant Tyre Markings

9th March 1951, Page 45
9th March 1951
Page 45
Page 45, 9th March 1951 — Simplification of Giant Tyre Markings
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%URING the past few years the tyre industry, recognizing • the need for the simplification of giant-tyre size mark

3s, has taken certain sleps to achieve this end. Further nplification has been announced by the Society of Motor anufacturers and Traders, which says that by January I, 52, the new markings will be completed for tyres for iginal equipment and replacement purposes.

For many years the tyre industry has had high-pressure d low-pressure giant tyres, the former being marked, say, x 6, and the latter 7.00-20. To simplify the range of :h tyres, high-pressure equipment was made in moulds the same dimensions as those used for low-pressure tyres, Ilist the difference in construction, as related to the numrs of plies of fabric,was maintained.

Dual marking in some sizes was temporarily adopted. A it-marked cover is a high-pressure tyre made to low:ssure dimensions. As an example, a 7,50-20/34 x 7 tyre, a 34 x 7 tyre, so far as price, construction, load capacity I inflation pressure are concerned, but it is made to 0-20 dimensions.

ks a further means for identification, "ply rating" is led. The term is used to identify a given tyre with its ximum recommended load when employed in a specific

e of service. It is an index of tyre strength and does not :essarily represent the number of cord plies in the tyre. kt one time it was possible to determine the maximum d and inflation pressure for a tyre by reference to the nber of plies it contained. A modern tyre, however, may itain more ot fewer plies than is indicated by the ply [hg,' but, for a given size of tyre, the higher the ply rating greater the strength. The simplification of markings has now been completed by the elimination of high-pressure or dual marking. Thus, the 34 x 7 (high-pressure) and the 7.50-20/34 x 7 10-ply rating (dual marked) become identified as 7.50-20 10-ply rating. Thus, tyres will have a particular size marking but different ply and load ratings, such as 7.50-20 8-ply rating for a maximum load of 21 cwt. and an inflation pressure of 60 lb. per sq. in., and 7.50-20 10-ply rating for a maximum load of 27i cwt. at 85 lb. per sq. in.

The following table gives examples of the original, interim (dual marking) and simplified markings for tyres fitting 20-in. rims: Where vehicle clearance limits preclude the fitting of tyres having the simplified marking, tyre manufacturers will continue to supply for a limited period the old high-pressure tyres, such as 32 x 6, 36 x 8, and 40 x 8. The marking of tyres for low-loaders remains unchanged.

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Organisations: Society of Motor

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