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).
Detailed overview of the Bentley Mark V chassis, covering the suspension, brakes, gearbox, and axle components.
| Identifier | ExFiles\Box 160\5\ scan0333 | |
| Date | 1st May 1941 | |
| 150 THE BENTLEY MARK V CHASSIS (Continued.) AUTOMOBILE ENGINEER MAY, 1941 AUTOMOBILE ENGINEER 147 (Left Page Text - Page 150) Fig. 12. Detail of rear brake actuation. arms actuate the brake cross rods. As the longitudinal brake rod and the radius rod swing through the same arc, which is approximately that of the rear propeller-shaft, up and down motion of the rear axle does not affect the brake operation, while the flexible connection between radius rod and axle shaft takes care of any axle twisting movement. Brake judder arising either from rough roads or wheel lock is in this way entirely eliminated. The hand brake lever operates on the rear wheels only. The balance lever mounted on the extension of the radius rod provides compensation between the two rear brakes, while a similar lever at the front compensates between the front brakes. In addition, compensation is effected between front and rear at the servo. Should any connection break, stops come into effect which retain the braking effort on the three remaining wheels, no matter what the state of adjustment of the brakes. Rear suspension For the rear suspension, two long semi-elliptic springs are employed 14in. apart. They are anchored beneath the rear axle and are hydraulically damped by Rolls-Royce shock absorbers. These are controlled by a governor actuated by the speed of the car, which has the effect of progressively stiffening the springs as they depart from their normal amplitude. There is also a control lever mounted on the steering wheel. The action of this control is to vary the tension of the relief spring in a gear pump on the gear box, which raises the pressure in the control pipe lines. For high speeds or heavy loads improved riding is obtained by increasing the damper loadings. The gear pump utilises the oil in the gear box, but does not actually displace any oil into the dampers. These are provided with a special arrangement of flexible metal diaphragms for their relief valve abutments, the diaphragm being deflected by increase of oil pressure, thereby increasing the damper relief valve loadings. Both torque and brake reactions are taken by the road springs. Enclosed in leather gaiters, the springs are lubri- ...cated from the centralised chassis system. On the "Corniche" model, the springs are of the Hadio Woodhead grooved type described in the issue of The Automobile Engineer for March, 1940. Tension shackles connect the springs to the frame, that is, they are fitted below the spring eye and not, as is more usual, above it. With the shackle in compression the rate of the spring is reduced in either extreme position, whereas it is increased with tension shackles. In this way a lower rate of spring may be used. Fig. 13. Brake servo mechanism. Front axle and suspension The chief novelty on the chassis is the use for the first time on a Bentley car of independently sprung front wheels. The scheme employed differs considerably from that used on the Rolls-Royce cars. Wishbone levers are mounted above and below the supporting member carrying the stub axle, as shown in Figs. 10 and 17. The upper wishbone is comparatively short and is pivoted on the central member of a piston-type shock absorber. These front shock absorbers are a special adaptation of the Gordon Armstrong principle so developed to ensure freedom from hydraulic and hearing noises. They have a fixed adjustment and are not controlled with those at the rear. Each wishbone is built up of two separate members bolted together through a distance piece to which is attached a semi-spherical rubber member which acts as a limiting stop. Considerably longer, the lower lever is of somewhat complex construction. The front arm is pivoted below the centre of the cross-member beneath the radiator. This arm is formed with a boss which carries a single robust open coil spring. The rear arm, formed separately, is of deep I-section tapering towards the end, which is pivoted beneath the frame side-member. The position and angularity of this lever is such that it functions as an effective brake torque member. The two arms are fastened together by a bracing member so that the structure is of A formation. The web of the I-section member is pierced with lightening holes, through the largest of which the divided steering track rod passes. Rubber bushes and buffers are freely used throughout the system. As the coil springs have a very low rate, a softer front suspension results compared with the previous orthodox suspension scheme. As the normal position of the wishbone levers viewed from the front is inclining upward from the wheel pivot, the roll centre is above ground level. The wheels are slightly out of parallel, but not sufficiently so to affect tyre wear. A slight caster action is provided unaffected by wheel movement except that at maximum travel the amount is slightly increased as is advantageous. Each pivot pin has two roller bearings for all radial loads, thrust being taken on a plain bronze washer. A substantial anti-roll bar joins the front wheels, giving increased stability when cornering. Attached to the front arm of the lower wishbone is a short vertical rod fitted with two rubber washers, seen in Fig. 16. This forms the attachment to an arm of the stabiliser, which is mounted below the front extensions of the frame, side-members where rubber bushes are fitted. Its function, of course, is to add stiffness when rolling, but not when bouncing; in other words, only when one moves independently of the other. The stabiliser is placed at the front end to give the desired relationship between the roll stiffness of the two ends of the car. This relationship is important from the viewpoint of over or under steering on corners. There is an interesting difference between the Bentley and the Rolls-Royce arrangement of the wishbone levers. On the Rolls-Royce the levers are positioned so that the wheels trail from lever anchorages, while on the Bentley the levers are directed forwards. With the Bentley layout, it is easier to use rubber joints because of the greater distance between the two frame anchorages of the lower wishbone. It also has the effect of increasing the caster Fig. 14. Method of mounting steering head on frame side-member. (Right Page Text - Page 147) ...halves. It is particularly adapted for taking thrust in both directions with less endwise deflection than with a standard deep grooved ball bearing. For lightening purposes, the rear end of the shaft is drilled out, and is machined to take a constant roller bearing acting as a spigot for the third motion shaft. Behind the constant mesh pinion, the shaft is bored out and machined with a ring of internal teeth, forming with a mating set of teeth on the adjacent slider a dog clutch for the direct drive. Engagement is facilitated by a synchro-mesh device of the tapering wedge type. Moving the same sliding member in the opposite direction engages an exactly similar dog clutch in the third speed gear again controlled by a similar synchromesh device. With the usual spring-loaded type of synchromesh, teeth engagement can take place before synchronisation takes place. This is not possible in the Bentley design. First movement of the sliding dogs carries the synchromesh cone into engagement with its mating cone, the end load exerted on the sliding dogs on the cone being transmitted to inclined cam surfaces. The frictional torque resulting from engagement of the cone surfaces, combined with the end thrust exerted by the dogs, produces loads normal to the surface of these inclined cam surfaces. While the frictional torque exists, the sliding dogs cannot proceed any further to engage their mating dogs. When synchronisation occurs, however, there being no friction torque on the cone surfaces, the inclined cam surfaces slide over each other, allowing the dogs to engage. Particular interest attaches to the anti-rattle device adopted for the sliding member (see detail views in Fig. 9). Two holes are drilled through the mainshaft, and in them are inserted small coil springs, both having a ball at each end. The springs hold these balls into contact with grooves on the sliding member and thus prevent rattle. Similar devices are employed for the second speed synchromesh device and also for the sliding first speed gear wheel. It is possible that these anti-rattle devices may be found unnecessary and discarded. For engaging first and second speed, a gear is arranged to slide on the third motion shaft so that when moved towards the front of the car the teeth engage directly with a pinion on the layshaft, while when moved in the opposite direction a dog clutch engages with the second speed wheel assisted by a synchromesh device. Bottom gear would only normally be used when starting on a severe gradient. The mainshaft is unusually long but is well supported. A similar Ransome and Marles bearing is used at the rear end to that employed at the front, while a third bearing is housed in a central web and supports the centre of the shaft. This is a standard Hoffmann ball bearing. For the layshaft a stationary plain solid shaft is fitted in the aluminium gear box casing and locked with a pin at the rear end. It carries two Timken taper roller bearings, and it is particularly interesting to note that no separate outer races are employed. In other words, the rollers are in direct contact with the layshaft proper, which is a long sleeve formed in one with its five gears. By this means there is considerable saving in outside diameter, and the scheme seems quite sound as the outer race has a considerably greater periphery than the cone and, of course, they are both revolving. As a result, any tendency to wear is considerably reduced compared with that of the cones. On a smaller scale the reverse gear shaft is similar to the layshaft. Here, again, a solid spindle is inserted in the gear box casing and locked at one end by a long setscrew with a plain end. Through most of its length a flat is formed on the shaft for lubrication purposes. On this spindle is mounted a long sleeve formed integral with two gears which are slid into engagement to obtain the reverse. Between the second-speed gear and the rear ball bearing on the main shaft is a sleeve formed with a two-start worm. This drives the brake servo, the relay pump for the rear shock absorber, hand control and the speedometer. It is interesting to note that engine oil (S.A.E. 30 viscosity) is recommended for gear box lubrication. The capacity of the box is five pints. An interesting detail is the drilling of four holes in the sleeve surrounding the first motion shaft. The holes are about midway between the clutch thrust bearing and the first bearing in the box. In the experimental models, it was found that owing to the centrifugal action of the rapidly revolving clutch parts, a depression was set up in the inner clutch casing, causing oil to be sucked into the clutch from the gear box, the liquid finding its way between the shaft and this sleeve. A complete cure followed the drilling of these holes, which relieved the depression by connecting the enclosed space with the atmosphere. Contrary to usual practice, the gear lever as well as the brake lever are on the right-hand side of the driver. They are set rather farther back than on the previous model to facilitate entry and exit on the driver’s side. The gate is mounted on the side-member of the frame, and the gears are operated through a short shaft having a universal joint at each end. The flexibility is particularly desirable in view of the suspension scheme adopted for the power unit. Rear axle For the final drive, hypoid spiral bevel gears are employed, and an open divided propeller-shaft is used. Three Hardy Spicer grease retaining needle-type universal joints are incorporated, the first one having the forward half bolted to a flanged member mounted by splines on an extension of the third motion shaft. This short propeller-shaft passes through a tunnel in the centre of the cruciform cross-member and is supported at the rear end by a sealed ball bearing mounted at the apex of the cross-member. To provide the necessary amount of flexibility required with a drive involving three universal joints, a rubber sleeve is used between shaft and bearing. The second universal joint is immediately behind this bearing, while the third has its final member bolted to a flanged sleeve mounted by taper, key and nut on the bevel pinion shaft. A sliding sleeve coupling is, of course, provided in the second shaft. One advantage of a divided propeller-shaft is that with a shorter effective length combined with a hypoid final drive, the floor of the rear compartment may lie flat. Objectionable feet wells or tunnel are eliminated, thus enabling three people to occupy the rear seat with comfort and ample leg room. It will be noticed that in the chassis general arrangement drawing (Fig. 15), a single propeller-shaft is shown, this being an earlier scheme which became obsolete. Fig. 7. Cruciform frame construction, dual exhaust expansion chambers and brake linkage. | ||