Comparison of Michelin Pilote tyres with American wide base rim development, focusing on rim widths, load capacities, and construction.
| Identifier | ExFiles\Box 156\3\ scan0214 | |
| Date | 25th November 1940 guessed | |
| -2- a tabulated statement of the characteristics of Michelin Pilote tyres with a copy of their drawing U.558, detailing the Pilote rims available in this country early in 1939. The following points emerge from all this information. (1) The Americans are examining the possibility of adapting existing tyres and therefore existing tyre moulds to the wide base rim development. We doubt whether this will be successful without substantial changes in bead and carcase construction. (2) The original American programme as tabulated in the Seiberling letter of August 16th. is based on a ratio of rim width to inflated section width of 80% whereas according to the modified programme they will test ratios of 75% and 80%. The corresponding ratios for recommended practice and permissible practice with existing 16" rims are respectively 65% and 70% whereas the Michelin Pilote value for this ratio is approximately 73%. In other words, the Americans are using relatively wider rims than Michelin. (3) The load carrying capacities allocated to wide base tyres are less than those of present tyres relative to sectional widths. For example, an axle load of 20 cwts. on present standards requires 6.00/16 (4 ply) tyres of sectional width 6.25"; to carry the same load, Michelin would offer their Pilote size 185 x 400 of sectional width 7.0"., whereas the corresponding fitment according to the T. & R.A. wide-base testing programme would be 7.1/2-16 of sectional width 7.95". Similarly your present equipment of 6.50/16 (4 ply) tyres would have to be replaced by Michelin 205 x 400 Pilote tyres of sectional width 8.25" and by 8-16 T. & R.A. wide base tyres of sectional width 8.55"; at the same time rim widths would increase from 4.50" to 5.90" if Pilotes were fitted and to 7.00" with T. & R.A. wide base tyres. In considering all these comparisons it must however, be remembered that the load carrying capacity of a tyre is a somewhat intangible quantity, readily susceptible to a variety of factors, e.g. speed, atmospheric temperature, sectional shape, construction, fatigue characteristics of the cord etc. Present rated loads have been arrived at by trial and error studies of many constructions all similar however, in sectional shape. For a new sectional shape therefore, the rated loads must of necessity be arbitrary and more properly to be described as intelligent guesses rather than accurately calculated values. (4) The Seiberling statement that for a tyre of given inflated width, a wide base tyre would cost less than a conventional is strictly correct, but it is equally clear from the examples cited above that for a required carrying capacity the converse will be true - the wide base tyre will be larger, more costly and probably heavier. | ||
From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).