James Shaw III

Jim Shaw and the Rover P8

Full Power Hydraulic Braking at last?

three-quarter sketch of motor car
Rover P8 - The car that got away

Why use Full Power Hydraulic Braking?

As Rover's Brake Project Engineer at the time, my father, Jim Shaw, hoped that the Rover P8, would be the first Rover car designed from the start to be fitted with Full Power Hydraulic Braking (FPHB). This was a system that had been under consideration since at least the mid-1950s, having been trialled on P4 and P6 models and that he had championed throughout his career.

Ever since hydraulics had first been used to operate brakes on vehicles in 1917 there has been a concern that there was a reliance on there being an incompressible fluid to transfer the motion of the driver's brake pedal to the brake operating cylinders. This system fails if it contains (compressible) gas or vapour or indeed insufficient fluid as a result of leaks. Unfortunately it isn't possible to tell if such a system is functional when it isn't in use, which is the normal driving state.

Although the reserve of brake fluid can be monitored there is no way of checking that the hydraulic transmission system (brake pipes) are intact and charged with (only) fluid as pressure in the brake pipes is nominally zero when not in use. To mitigate this problem various 'split' braking systems have been employed, some legally mandated in various parts of the world. Effectively either components of the brake system have been duplicated or isolated from each other or both, so that multiple-failures are needed before the brake system will fail totally.

FPHB gets around some of these problems by storing hydraulic fluid under pressure, making it possible to continuously check that there is a source of hydraulic power available to power the brakes. There still remains an un-pressurised part of the system, the final connection to the brake operating cylinders, but because of the reserve of high-pressure fluid it is possible to achieve the required pressure at these cylinders even if there is a degree of leakage. (The drop in hydraulic pressure in the high-pressure accumulator(s) would trip a brake failure alarm, but by then the vehicle will have been safely brought to a stop).

Design Proposals

Although FPHB could be implemented on its own on a vehicle it makes sense to use the hydraulic system for other purpose too, such as suspension or ride-height adjustment or power steering. For P8 it was proposed that there should be a hydraulic power system for adjusting the ride-height and the braking. The braking system ultimately would incorporate some sort of anti-skid control.

Lockheed had drawn up a 'Power Braking & Levelling System For Rover P8' by January 1969. It incorporated a single cylinder pump, two accumulators, a dual brake control valve, two-cylinder front calipers, singe-cylinder rear calipers modulated by a brake pressure regulating valve controlled by the suspension system, consisting of a levelling valve and a suspension strut operating against a pneumatic/hydraulic spring.

In July 1969 my father visited Rolls-Royce at Crewe for a conversation about the system that they were discussing with Lockheed. Visits were also made to Lockheed and they offered to demonstrate their skid-control system. These discussions seem to have been at an 'engineering level' they didn't want the disruption of a 'top-brass' involvement.

Around the same time my father was visiting Girling to see their anti-skid rig with Peter Wilks. They anticipated demonstrating it on a vehicle at the Honiley test track during August/September 1969. My father says that Peter Wilks was fairly definite in wanting to P8 to be the first production vehicle to use the system, (but not from initial product launch).

He quotes Peter Wilks as saying that P8 was to be:-

“in the forefront of technology. We are leaving the antique motorcars to Jaguar”.

Evaluation

sketch of hydraulic systemr
System Diagram - From the December 1969 report

By 30 December 1969 both Lockheed and Girling had offered-up designs and my father complied a report comparing the two systems.

The Lockheed and Girling systems each consist of a fluid reservoir feeding a single cylinder plunger pump and charging a pair of gas spring accumulators through an unloader valve. Each non-return-valve-protected accumulator feeds “half” the brake system through the pedal operated control valve. The front calipers are 4 cylinder opposed piston units and a corresponding pair of cylinders from each caliper forms “half” the system. The other “half” comprises the remaining cylinder pairs of the front linked to the rear calipers through a ratio-control valve modulated by rear levelling pressure. Pressure for rear levelling is tapped off at the brake accumulator feed point and regulated to the rear levelling unit by a control valve sensing suspension height mechanically.

US Federal regulations required manufacturers to provide 'consumer information' relating to stopping distances in the event of a partial systems failure. From a speed of 60mph [97 km/h] these comparison were made:

  • US Fed. 105 Proposal - stopping distance 387' [118 m]
  • Rover 2000 TC & Automatic - stopping distance 359' [109 m]
  • Rover 3500S - stopping distance 323' [98 m]
  • P8 (Girling) - stopping distance 253' [77 m]
  • P8 (Lockheed) - stopping distance 436'[133 m]

The report concluded:

The performance of the two systems is very similar. In three particulars a considerable difference appears which may simplify the technical choice.

  1. Without benefit of differentially bored front calipers the Lockheed Residual Performance will be inferior if not inadequate.
  2. The policy of Girling to use standard brake fluid avoids the legal/technical arguments that seem inevitable with an attempt to deviate from the use of standard brake fluid.
  3. Lockheed have long experience of the manufacture of spool type control valves but Girling have avoided the necessity for this high precision continuous vigilance, manufacture by invoking a simpler technique of which they have long experience, the tipping valve, used in all their conventional tandem master cylinders. The simplicity ought to be reflected in the cost of the control valve and in lower expectation of production and service trouble.

Despite this apparent technical preference for the Girling proposal it was the Lockheed scheme that went on to be developed for the production car.

Post-Mortem

By 1971 Rover was no longer in control of its own destiny, it had merged with Leyland Motors, a truck maker, in 1967 and by 1968 that group had merged with British Motor Holdings, the parent company of volume car maker BMC. At the beginning of 1971 it became evident that further work would be required to achieve the required crash-worthiness. This was an expense too far for the British Leyland board and the P8 project was cancelled in March 1971.

In a post-mortem report from March 1971 it is stated that eight cars had been completed with the brake and levelling system. Lockheed had proposed combining parts of the system to simplify it and to keep the costs down. They had intended to:

a. Combine the preference valve with the pedal control valve

b. Combine the suspension levelling valve and the ratio valve with the levelling strut forming one unit.

A new axle incorporating an internal drum handbrake rather than an external band hand brake was waiting to be fitted.

The boot on the suspension ram had blow off several times and some leakage had been encountered, particularly during a test in Finland. There was a proposal to provide the ram with its own return pipe, instead of the common one currently fitted. Ferodo 2430 brake pads and DTD585 mineral oil were being used and subject to trial would be used for production. A cheaper mineral oil was being sought.

Also on 9 March 1971 my father wrote to Dick Oxley [Chief Engineer]:

The Dual-line system fitted to 3500 [P6] models suffers from what threatens to be a perennial problem and the twin-servo of the P6, although remarkably free of complaint, is hardly an elegant solution.

The cancellation of P8 and the resultant spare apacity for the production of full power actuation offers an alternative specification to the above models. If we do not utilise the system on one of our models someone else will and this will further degrade our market-place viability. If failed servo-assistance performance requirements get much more severe, any system which can be interpreted as servo assistance will be in difficulty at our weights.

The increased cost, relative to present P6, P6B split specifications is:

  •   +£11.75 for P6
  •   +£12.50 for P6B

possible utilisation options:-

  • 1) P5B, if this can be made “crash worthy”, etc.
  • 2) XJ6 used for an “RJ1” [Rover-Jaguar 1]
  • 3) Range Rover
  • 4) BMC 3l [litre] or 1800 shell for a “RAM 1” [Rover-Austin-Morris]

Having written off a great deal of capital on 1 March [1971] any “new” model which can pick up the fruits for free is set fair to hit the market place with a high return on a falsely low capital investment.

J. Shaw

[signed]