The success of the Citroen hydropneumatic suspension no doubt inspired the Moulton Co. to tum to gas as the suspension medium; also to use a tapered pedestal or piston to give a variable effective diaphragm area, the technique originally used by Firestone.
The Hydragas principal and the detailed design of a Hydragas spring unit. It will be seen that the effective piston area acting on the diaphragm increases with the increase of wheel movement in bump and decreases under rebound. Nitrogen gas under pressure and compressed by a flexible diaphragm acts as the spring.
Even in the position of the damper valve, the Moulton gas spring is almost identical to the one in the Citroen system. In the Hydrolastic suspension system, the interconnection between the front and rear was made from the liquid compartment above the damper valves.
The main resistance to pitch is given by the change in diaphragm area by the piston taper. Since one diaphragm increases in effective area as the other decreases, the system behaves in pitch like a conventionally sprung car, but with no conventional damping. Some damping is given by the long interconnecting pipes.
On a typical Hydragas suspension such as the Austin Allegro, the pitch rate would be 13 kN/m, the front bounce and roll rate, 20 kN/m and the rear bounce and roll rate, 18 kN/m. On the 1750 HL Allegro, with a fully laden sprung mass of about 1050 kg and a front/rear weight distribution of 63/37, the front period in bounce is approximately 1.2 Hz and the rear 1.5 Hz.
The pitch frequency is about 1.0 Hz, a very low value and well removed from the bounce frequencies. I myself have used an Allegro for personal transport for several months and found the ride to be as good as that given by many larger cars. The low pitch frequency is no doubt responsible for the favorable comments on the good ride given in the back seat.
Before wasting time and money on making a product, we should always establish that a need really does exist. The Ford Co. no doubt still remembers the Edsel. There was nothing much wrong with the Edsel, but the American public was not in the market for yet another big car when they introduced it!
Very little market research seems to have been carried out to discover how much the typical car owner will pay for a car with a better ride, or a reduced tendency to roll when cornering. Even the men who design the cars do not seem to be of one mind when the question of the roll is considered.
French cars, for example, usually roll more than British cars. What, indeed, is the ultimate suspension behavior when cornering? Should there be a limited degree of roll, no roll, or even what is usually called ‘banking’, i.e.
Against this failure, we can see a successful application in British Rail’s high-speed train, which has a mechanism to make the carriages bank up to an angle of 9° when negotiating curves. This is simply to reduce the side forces on the passengers. A similar mechanism could also be used on an automobile.
In 1961, I drove a Chevrolet Impala sedan that had been converted by Mr. J. Kolbe of Menomonee Falls, Wisconsin, the USA to the bank when cornering. Measurements taken from a photograph show the stock Impala with a body roll of about 6 ° on a particular turn.