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).
Article from 'The Motor' magazine detailing the Bentley Mark V chassis and its features, including its new frame and modified power unit.
| Identifier | ExFiles\Box 160\5\ scan0227 | |
| Date | 29th January 1941 | |
| The Motor 564 January 29, 1941. The Bentley Mark V and A Modified Power Unit With New Overdrive Gearbox Fitted to an Entirely New X-Braced Frame. Independent Front Springing Used for First Time on a Bentley Chassis IN The Motor dated December 4, 1940, there appeared the story of a road run made on a new Bentley chassis, due to have made its public appearance at the 1939 Motor Exhibition, but, of course, since put out of production by the war. This car is not available to the public to-day, and will not necessarily be listed exactly in its present form even as a post-war model, but we believe that it has sufficient technical merit to justify a comprehensive description in our pages. It will be remembered that the run evoked great enthusiasm from the experienced drivers who handled the car, who were agreed it was exceptional in combining first-class performance with silence, comfort with stability, and high acceleration on top gear with good maximum speed. Although a direct development of previous Bentley designs, the design is an entirely new conception and represents the essence of nearly a decade's work on one type of motorcar. Although basically conventional, many novel details are embodied and it is this combination that results in the first-class practical results that are evidenced on the road. There were to have been two chassis available—the “Mark V,” designed to carry capacious coachwork capable of seating five or even taking limousine type bodies, and the “Corniche,” which, amongst other modifications, had a higher final drive ratio. This was designed to take aerodynamic bodywork and to achieve speeds in excess of 110 m.p.h. Two Models: Speeds of 97 and 120 m.p.h. The Mark V car is lower geared, the overdrive giving 3.6 to 1 which means that the car will do approximately 90 m.p.h. at the peak of its power curve, although capable of a good deal more than this under favourable conditions without exceeding the 4,500 r.p.m. limit for the engine. To be specific the Mark V has been timed over a measured stretch of road to achieve 97 m.p.h., whilst the Corniche has lapped Brooklands at 109 m.p.h., which indicates an all-out maximum approaching 120 m.p.h. The power unit shows little external change from previous models, but there have been a number of internal modifications which are of value. The number of timing wheels has been reduced from 5 to 3 and a modified form of tappet introduced, whilst the exhaust valves, as before, are faced with exceedingly hard material known as stellite and will run for approximately 100,000 miles without regrinding. The main and big-end bearings are also designed to achieve this mileage without attention. The load-carrying capacity of the light-alloy big-end material (developed at Derby and called AC9) has made it possible to reduce the width of the big ends and increase the width of the main bearings. For technical reasons the latter are of white metal and test cars have been driven great mileages in the most arduous conditions without a sign of wear. The crankshaft is, of course, hardened, and lubrication is by a gear-type pump with a General filter on the by-pass line. Another point of interest in the engine design is the use of pistons with a specially raised crown which fits into a rectangular combustion chamber. That this shape permits an unusually high compression ratio having regard to the cylinder diameter is shown by freedom from pinking and detonation, even when using comparatively low octane “pool” petrol. The drive is taken through a single-plate clutch, an interesting point being that the flywheel is flexibly connected to the crankshaft through the medium of a thin disc of saw steel, a feature that has been found of considerable value in obtaining smooth running. The gearbox has a new internal design giving synchromesh between the overdrive, direct and second speeds. On the Mark V, the ratios are 3.6, 4.3, and 6.15 respectively and on the Corniche, CRESSWELL A WAR CASUALTY—This drawing shows the Mark V Bentley chassis, the production of which has been prevented by the war. It will be seen that there are big changes compared with any previous Bentley model, notably independent front springing, open coil springs and a X-braced frame of very deep section. The wheelbase is two inches shorter than on previous models, but there is greater body space, the engine having been moved forward substantially in the frame. The slightly modified engine, new synchromesh gearbox with overdrive, divided propeller shaft, variable-rate rear springs, improved braking system and other points are all described and illustrated in detail in this article. | ||