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).
Comparative table of different servo lining materials and their performance characteristics.
| Identifier | ExFiles\Box 87\5\ scan0034 | |
| Date | 26th October 1934 | |
| -4- He/PJH.7/KW.26.10.34. Type of Servo Lining. | Average /u when cold | Decrease in /u when hot per cent | Remarks. --- | --- | --- | --- Cast Iron | - | - | "Picks up" immediately. Mintex Rubber | 0.40 | 25 | Objectionable smell. Ferodo SS{S. Smith}042 | - | - | Plastic when warm. Judders and breaks up. Ferodo VM.30 | 0.45 | 22 | Long life. Silent. Ferodo Special E.Z. | 0.43 | 34 | Long life. Silent. Duron | 0.52 | 37 | Silent. Ferodo SS{S. Smith}060 | 0.44 | 30 | Silent. He/P.J.Hardy. | ||