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).
Technical description of the Bentley Mark V chassis, covering the exhaust, engine, clutch, gearbox, brakes, and frame.
| Identifier | ExFiles\Box 160\5\ scan0331 | |
| Date | 1st May 1941 | |
| 146 AUTOMOBILE ENGINEER THE BENTLEY MARK V CHASSIS (Continued) the carburettor outlets hot-water jackets are arranged. Exhaust system As shown in Fig. 4, a three-branch exhaust manifold is bolted to a vertical pipe leading through a right-angle bend to the first expansion chamber which is fitted between the cruciform member and the frame side-member (Fig. 7). A further pipe leads the gases to the second expansion chamber just in front of the rear axle (Fig. 10). From here a swan-neck pipe passes over the axle and there is a Z-bend, so that the pipe extends outside the frame side-member beyond the petrol tank (Fig. 6). The piping passes twice through holes in the cruciform member where it is supported by flexible mountings in addition to similar supports, one on the rear cross-member and a fourth at the rear extremity of the frame. Ignition Battery ignition is employed with automatic timing control as standard, though it was proposed to fit certain cars with hand control in addition. As shown in an illustration (Fig. 2) two coils are fitted on the right-hand side of the engine, one wired up and the other as a spare. Fig. 6. Rear end of frame, with petrol tank and support for tail of exhaust pipe. Cylinder cooling Water - cooling spaces are on the generous side, but owing to the cylinder construction the jacketing is not full length and does not cover that portion of the cylinder traversed by the piston skirt at the bottom of its stroke. Circulation is by water pump thermostatically controlled. A bellows-type thermostat is fitted in the water outlet pipe to the radiator, so that the flow of water through the radiator is automatically restricted until the water in the cylinder jackets attains a temperature of about 75 to 80 deg. C. A thermometer is provided on the instrument board to indicate that the thermostat is operating properly and there is no shortage of water. Driven by a V-belt, a five-bladed fan is mounted on a sleeve formed integral with the pulley. The unit runs on two small ball internal bearings on a spindle the flanged end of which is attached to the water outlet on the cylinder head. The fan blades are of thin sheet metal and unevenly spaced for quietness. Lubrication Engine lubrication is by pressure from a gear-type pump to the crankshaft bearings, big ends and gudgeon pins. A dual relief valve provides a positive low pressure supply to the valve rocker shaft and thence to the rockers, push rods, tappets, etc. A renewable General by-pass filter made by British Filters, Ltd., is fitted on the delivery side. Adjacent to the water pump on the near side of the power unit is the crankcase oil filler, an oil level gauge being fitted close to the filler on the crankcase where it can readily be seen when filling. On previous models it was lower down and rather difficult to see. A breather pipe is formed in the funnel casting, the opening being just beneath the filler cap, but partially covered by a lip so that it is not possible for oil to enter when replenishing the crankcase. A vent pipe leads the fumes down below the undershield beneath the front portion of the engine. Electrical equipment A twelve-volt system is incorporated with an automatic regulation of the dynamo output by vibrator control. A separate starter motor is used with pinion providing gentle engagement. It is housed in a recess cast in the clutch casing. The battery is of approximately 55 ampere-hour capacity. Engine suspension Beneath the thermostatic water control is the front engine support. This, it will be observed, is quite high up and offset from the centre line of the engine. A rubber bush formed in two halves is employed. The unit is bolted to a shallow channel-section arched cross-member in the form of an inverted V.{VIENNA} The rear attachment is beneath the gear box where the unit rests on a substantial rubber pad carried on a member attached to the frame side-members. In addition, some lateral movement is permitted and controlled by a banjo-shaped member bolted to the rear end of the gear box as seen in Figs. 7 and 8. On the extremity of each arm rubber pads are sandwiched between the arm and a plate attached by a central bolt to upstanding arms bolted to the cross-member. In positioning the attachment points of the suspension scheme, care has been taken to place the front point in line with the centre of gravity of the engine, thus reducing the disturbance of the chassis when the engine rocks. At certain engine speeds, vibration will coincide with the natural frequency of the engine mass on its mountings. This usually occurs at low revolutions. If the front suspension point is low down and out of line with the centre of gravity there is a force sideways in addition to a torsional one. By raising the mounting point and bringing it in line with the centre of gravity, all sideways force is eliminated, leaving only the torsion. MAY, 1941 Clutch A dry single plate clutch (Fig. 9) is fitted of a specially modified Borg and Beck type. It is actuated direct, i.e., without servo. Several features are of particular interest, the first of which is the method of mounting the flywheel. Bolted to the flange formed on the extreme rear end of the crankshaft is a flexible disc, and to this both the flywheel rim and the front friction member of the clutch are bolted. As is well known, the impulses from the rear cylinder can be imparted to the crankshaft extension, setting up undesirable vibration. With this flexible mounting, the natural frequency of the flywheel is so low that it cannot coincide with the cylinder impulses. The central driven disc of the clutch is a Long type with special linings. It is of exceedingly light construction and has the usual cushion centre consisting of a ring of eight coil springs with the centre lines in the plane of the clutch faces which are a standard Borg and Beck feature. Another interesting point is the insertion of heat insulating washers between the presser plate and the coil springs which normally hold it in contact with the driven member. Withdrawal is through the usual pivoted levers, which in this case are fitted with centrifugal weights which make the clutch particularly easy in operation and give increased pressures, so ensuring absence of slip as the engine speed rises, despite the light springs employed for the presser plate. A large ball bearing is employed for the clutch withdrawal thrust member, provision being made for its adequate lubrication from the central system. A light sheet metal cover encloses the lower portion of the clutch housing. Gear box It was proposed to fit two alternative four-speed gear boxes, one with a direct top speed and the other with an overdrive. Gear ratios for the overdrive box are first 4·24:1, second 1·43:1, third 1:1, and fourth 0·830:1. The ratios for the box having a direct top drive are first 2·98:1, second 2·01:1, third 1·38:1, and fourth 1:1. The construction of this box is shown in an accompanying line drawing, Fig. 9, and the accompanying notes are confined to this model. The only difference between the two gear boxes lies in the diameters of gears and a rearrangement of the operating mechanism to give the correct gate arrangement, namely, to have the same position of the lever for the top gear in both cases. An unusually long first motion shaft is employed, the front end of which has a spigot mounting in a ball journal bearing, the flanged housing of which is bolted to the flange terminating the crankshaft. The eighteen-tooth constant mesh pinion is formed integral with this shaft, and adjacent to it is a special Ransome and Marles duplex double-purpose ball bearing in which the inner race is made in two. MAY, 1941 AUTOMOBILE ENGINEER 151 Figs. 16 and 17. Independent front wheel suspension with lower wishbone lever taking brake torque reaction. angle as the suspension moves from the normal to full bump position. An advantage of this is increased directional stability when braking. Front brakes Interest attaches to the method adopted for actuating the front brakes so that their operation is unaffected by the up-and-down movement of the wheels. This is usually somewhat of a problem with independent front-wheel suspension when mechanical rather than hydraulic means are employed. The commonest solution is a flexible cable. In this case the usual Girling type vertical pivot member is mounted on the front cross-member, the upper arm being connected by a short link to the centre of a short vertical lever, as shown in Fig. 15. At its lower end this lever is pivoted to a frame bracket, while the upper end is pivoted to the pull-and-push link which actuates the brake. As the anchorage pivot of the vertical lever is adjacent to the fulcrum of the upper wishbone lever, both swing through the same arc. Fifty-seven per cent. of the total braking effort is taken by the front brakes. Between the servo and the front brake equaliser there is a small hydraulic damper to give quiet application of the brakes in all states of adjustment. This is located on the off side of the engine just below the ignition coils. When high-pressure washing hoses were first introduced at garages, considerable trouble was experienced owing to water entering the front brakes. Introduced on the previous model, the Mark V retains the particularly effective water excluders fitted adjacent to the drums. They are capable of keeping out water directed under pressure at the drums. Fig. 15. Detail of front brake actuation. Frame Particular interest attaches to the frame construction (see Fig. 18), which has obviously received the closest attention. Such unusually flexible suspension, combined with the generous isolation by rubber of the power unit and transmission, permits of a frame of exceptional rigidity both in bending and in torsion. Deep channel-section side-members are employed, nicely tapered from end to end. Of box-formation, the first cross-member is positioned beneath the radiator. It is built up of a deep-flanged member, the open base being closed by a flat sheet. The second cross-member is of shallow channel section, the open side uppermost. It is just below the front end of the gear box, and carries the rear support of the power unit which is beneath the gear box. Marles cam and double-toothed roller type of steering is fitted. It operates the front wheels through a divided track rod, as is usual with independent front-wheel suspension. Sufficient flexibility is introduced into the system connecting the steering wheel to the road wheels to give freedom from road reaction on the steering wheel. Fig. 18. Plan view of chassis, showing frame construction. | ||