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).
Comparison of dimensions and specifications for the 7.00-21' Dunlop Silent Tread 'Fort' Reinforced Tyre against the standard 33 x 6.75' tyre.
| Identifier | ExFiles\Box 45\2\ Scan169 | |
| Date | 14th September 1929 | |
| X4050. Hs{Lord Ernest Hives - Chair}/AJL.1/MJ.14.9.29. x4050. x7820. x990 7.00 - 21" DUNLOP SILENT TREAD 'FORT' REINFORCED TYRES. This size of tyre has been produced as an alternative to the 33 x 6.75" silent tread tyre at present standard on 40/50 cars. The advantage claimed for them is longer life due to having a greater thickness of 'non skid' tread. They have the following dimensions - | | 33 x 6.75" Std. Silent Tread Tyre. | 7.00-21" Silent Tread 'Fort' Reinforced Tyre. | | :--- | :--- | :--- | | Diam: of tyre inflated to 35lbs/sq.in. but with no load. | 34.62" | 35.2" | | Width of tyre at largest section. | 6.750" | 6.9375" | | Effective running diam: at 35lbs/sq.in. on rear of 48 cwt. car. | 33.13" | 33.83" | | Revs./mile under above conditions. | 608.7" | 596.4" | | Weight of outer cover. | 38.5 lbs. | 44 lbs. | | Engine revs./min. at 60 m.p.h. in top gear (11/41 axle ratio.) | 2269. | 2223. | | Alteration to speedometer reading due to adoption of 7.00-21" tyres. | | 2% slow. | | Is present side spare wheel carrier O.K. | Yes. | Yes. | | Is present rear spare wheel carrier O.K. | Yes. | No. Wheel requires raising, fouls side members. | | Steering locks. | L.H.-36.5° R.H.-32° | L.H.-35.5° R.H.-31° | | Depth of non skid tread. | .28125" | .4375" | | Length of arc of tread. | 4.625" | 5.125" | ROAD TESTS. The 7.00-21" tyre is found to give very poor results | ||