Description of the Braking System
On both Bentley S1, Bentley S2 and Rolls Royce Silver Cloud I / II cars, the braking system incorporates a gearbox-driven friction type servo motor which is engaged by the foot brake pedal. The output from the servo is transmitted through hydraulic cylinders which operate Girling Hydrastatic brakes incorporating special Rolls-Royce features.
In addition to operating the servo, the foot brake pedal is also connected to the rear brakes through a mechanical linkage of rods in tension, as also is the handbrake, and supplies 40 per cent (Bentley SI cars) or 30 per cent (Silver Clou 2 cars) of the effort applied to the rear brakes, the additional 60 per cent (S1 cars) or 70 per cent (S2 cars) being provided hydraulically.
The handbrake also is connected to the rear brakes through a mechanical linkage.
Two Shoe Brake System — Front Brakes
Front brakes are of the ‚two trailing shoe‘ type employing two brake operating cylinders for each wheel. The lower ’shoe factor‘ due to absence of self-wrapping effect with trailing shoes renders them less prone to pulling and grabbing troubles, sometimes associated with leading shoe type brakes. The drum diameter has, of necessity, been reduced to I l in. (28.575 cm.) and this, combined with the absence of self-wrapping effect, necessitates the employment of higher shoe tip operating pressures to obtain the same total braking. This higher shoe tip force is derived from the servo motor and increased hydraulic leverage, made possible by the use of self-adjusting shoes.
The importance of self-adjusting shoes in this system lies in the fact that the displacement of fluid when the brakes are applied is much less than in a normal system, Since there is virtually no initial shoe clearance to be taken up. With less fluid movement, it has been possible to employ master cylinders of smaller bore diameter than that of the wheel cylinders, thus providing an increased hydraulic leverage without introducing excessive linear travel of the master cylinder pistons.
On all except Rolls-Royce Phantom V rear brakes, self-adjustment of the shoes is achieved by a friction device known as the ’shake-back stop‘. This device retains the front shoes in the expanded position when the hydraulic pressure is released, thus achieving near zero shoe clearance. The ’shake-back stop‘ is located radially by a steady post attached to the brake carrier plate. Sufficient clearance is provided in this location to allow slight shoe to drum clearance with the brakes off. The tension of the brake shoe return springs (or more correctly ‚bias springs‘) is such that the shoes are in equilibrium with the springs in the wheel cylinders. These exert a slight pressure on the back of the operating rubber cup and piston in the expanders. In consequence, there must be sufficient friction between the shake-back stop and the brake shoe web to prevent the shoe being shaken back by the vertical forces imposed on it when traversing rough roads.
As the front brakes are applied entirely by the servo there would normally be no increase in pedal travel as the linings became worn. A strong pull-off spring is therefore fitted to the shoes which takes effect only when the linings are near the end of their life, and, by the increase in pedal pressure above that which would normally be required, gives warning that the linings require renewal.
Four Shoe Brake System — Front Brakes
On the Bentley Continental S2 the front brakes are of the ‚four-shoe‘ type.
The four shoe brake consists basically of four neutral shoes mounted in pairs on two trailing shoe carriers. The shoe carriers are operated by two wheel cylinders mounted on a torque plate. A water excluder is fitted to the back of each front brake drum and is positioned between the carrier plate and the stub axle.
With four shoes a greater lining area is obtained, while the arc length of each shoe remains low.
Two bosses with fine limit bores are welded to each shoe carrier and the shoes pivot on hardened pins which are retained in the bosses by spring clips. As the pivots are set well back from the shoe lining surfaces, there is a tendency for the shoes to tip forward due to the rotation of the drum. The two shoes overlap in order to make the tendencies of the shoes to tilt, mutually self-cancelling.
`Shake-back stops‘ are fitted to the brakes which prevent the shoes from shaking away from the drum when travelling over rough surfaces.
On the four shoe brake, one shake-back stosiis fitted to each of the two trailing shoe carriers.
Rear Brakes for Bentley S-Serie- Rolls-Royce Silver Clouds I / II
The rear brakes comprise a leading and trailing shoe expanded by a double acting wheel cylinder in conjunction with a mechanical flat wedge type expander, the latter being operated by direct foot pedal linkage and also by the handbrake. The shoes are interconnected by a special equal-wear linkage which converts the floating shoes to a ‚fixed-cam‘ equivalent, thus reducing the self-wrapping effect of the leading shoes and allowing the use of a Single shake-back stop on the trailing shoe which simultaneously adjusts both shoes to near zero clearance; it should be noted that shake-back stops are not fitted to the rear brakes on Phantom V cars.
As lining wear takes place with consequent automatic adjustment, the shoes move away from the tappets of the mechanical expander in the ‚Off‘ position; an adjuster is therefore provided to enable the shoes to be repositioned thus bringing the heels into contact with the tappets again.
A light rubbing when the brakes are ‚Off‘ is quite normal with this type of automatic shoe adjustment.
Initial movement of the foot brake pedal first applies the rear brakes through the mechanical linkage and supplies 40 per cent (S1 cars) or 30 per cent (S2 cars) of the effort applied to the rear brakes. Further pressure on the foot brake pedal engages the servo motor which operates the hydraulic master cylinder(s) and supplies the additional 60 per cent (Bentley S1 and RR Silver Cloud I cars) or 70 per cent (Silver Cloud II and S2 cars) of the effort applied to the rear brakes; the front brakes are operated by the hydraulic system only. On releasing the foot brake pedal, the hydraulic master cylinder piston(s) return by means of an internal spring; the servo operating levers return to their normal positions by means of a torsion spring fitted between the two levers.
The handbrake is also connected to the rear brakes through the mechanical Linkage.
Figure abowe shows the brake linkage for a right-hand drive car with the arrangement fitted to left-hand drive cars shown inset. On Phantom V cars an extra relay lever is fitted between the intermediate linkage and the rear axle. The lever is fitted to a bracket welded to the right-hand rear member of the propeller shaft tunnel. The Single rod (see 2, on the Figure above) fitted on standard cars, is divided into two rods on the Rolls-Royce Phantom V, the ends of which are pivoted on the relay lever mentioned above.
A servo motor is mounted on the right-hand Side of the gearbox and is driven at approximately one fifth of the propeller shaft speed. A pull-rod (see 7, on the Figure above) is operated by the brake pedal and is coupled to a lever on the servo motor shaft. The lever has inclined cams formed on the face of its boss which engage, through the medium of steel balls, with Similar cams formed on the boss of a second lever (5). From the latter lever, rod (4) actuates the rear brakes through an intermediate lever which is pivoted on a bracket bolted to the crossmember of the frame and through rod (2) and the rear equaliser linkage, mounted on a bracket suspended from the rear axle.
Initial movement of the servo lever cams engages the servo motor, and its output is taken to the master cylinder operating lever assembly by one of the two output rods, depending upon whether the motion of the car is forward or reverse.
The handbrake is mounted under the facia, and is connected by an enclosed cable to the handbrake lever carried on the master cylinder support bracket. This lever is linked to a second lever which is connected to the intermediate lever (I l) by means of a pin. The remaining linkage is the same as for the foot brake. The intermediate lever is permitted to slide along the slotted link on rod (4) so that application of the handbrake does not disturb the foot pedal.
Hydraulic Systems of the Bentley S-Serie and Rolls-Royce Silver Clouds
On early Rolls-Royce Silver Cloud, Bentley S1 and Bentley Continental S1 cars, a Síngle master cylinder operates all the brakes. In the event of failure of the hydraulic system, the rear brakes remain effective through the mechanical linkage; Similarly, provision is made to ensure that the hydraulic system remains effective should the mechanical application fail.
Later Bentley S1 cars and all Bentley S2 cars are fitted with two reservoirs and two master cylinders; the upper cylinder (0.980 in. (2.489 cm.) piston dia.) operates the upper shoes only in the front brakes and all shoes in the rear brakes, while the lower cylinder (0.747 in. (1.897 cm.) piston dia.) operates only the lower shoes in the front brakes.
Two brake fluid resovire for each master cylinder, are mounted on the wing valance and incorporate fine mesh gauze filters. The pipes leading to the master cylinders are arranged to run in such a manner that no air trapping takes place; any air present in the brake fluid cylinders or pipes is expelled from the reservoirs.
Note: All brake fluid is hygroscopic, i.e. the fluid will absorb and chemically combine with water from the atmosphere.
To overcome this problem, the fluid should be exposed to the atmosphere only for the minimum time. It should always be stored in and used directly from small sealed containers.
Check valves were introduced on early Bentley S1 cars to obviate the possibility of ingress of air at the wheel cylinders, but it was later established that the spreaders were fully capable of maintaining an adequate interference between the lips of the cup seals and the bores, thus preventing air from being introduced into the system. No retrospective action is necessary to remove the check valves.
The two sub-divisions of the hydraulic system are not interconnected hydraulically and balance between the two cylinders, and therefore between front and rear brakes, is obtained mechanically by means of a balance lever.
In the event of failure of one system, braking is still retained on all four wheels by the mechanical application of the rear brakes and the operation of the second cylinder.