The Morris brochure stated that 'there are no moving parts, no glands to leak or wear, and no requirement for maintenance'. It is doubtful whether this was actually true when the car was new and it's certainly not the case 35 years later!
To achieve this, a displacer unit consisting of a rubber spring and a fluid chamber was placed at each wheel. The fluid chambers were then connected longitudinally by a hydraulic pipe. If a front wheel encountered a bump in the road surface then the upward motion of the wheel would operate a piston at the base of the displacer unit. This would deflect the flexible liner of the fluid chamber thus increasing the pressure in the chamber.
To a certain extent, the rubber spring at the top of the fluid chamber would act as a cushion, but more importantly some of the hydraulic fluid in the chamber would be forced through the interconnecting pipe to the fluid chamber in the rear displacer unit. This would result in an increase of pressure in that chamber forcing the piston downwards, so raising the rear of the car to the same level as the front. When the rear wheel hit the same bump in the road the opposite would happen. The body of the car would therefore remain level at all times. In practice the system took a little getting used to and progress of an 1100 over an uneven surface was somewhat 'bouncy'.
Another problem was the 1100's tendency to adopt a nose up attitude when heavy loads were carried in the rear. This was caused by the weight displacing fluid from the rear units to the front. Inertia induced weight transfer to the rear under hard acceleration would also cause the front of the car to lift.
Many driving instructors found that the 1100 was an excellent tool for teaching hill starts. With the handbrake hard on and the car in first gear, it was possible to slowly let out the clutch and, as it bit, the front of the car would rise up as the front wheel drive tried to pull away against the handbrake. As the learner saw the bonnet lift, he knew it was now safe to release the handbrake.
Overall the system was hailed a success and BMC were to use fluid suspension in many more of their models over the coming years. In September 1965 even Mini saloons were so equipped, although only until October 1969, except for the 1275GT. Mini Coopers with fluid suspension were commonly referred to as 'wet' models.
A further development of the Hydrolastic system arrived in 1973 when the Austin Allegro was born. This was the advent of the Hydrogas displacer unit. The main difference of the Hydrogas displacer unit was the replacement of the rubber cushion with a nitrogen filled chamber aimed at further improving ride quality. This type of displacer was sometimes used independently as well as in tandem and was used right up to the days of the Rover Metro/100 and MGF. Hydrogas and Hydrolastic units are not interchangeable.
MAINTENANCE
By far the most common fault with Hydrolastic or Hydrogas suspension is the its inability to maintain the vehicle at the correct ride height due to fluid loss or low pressure within the system.
Fluid loss can be due to a faulty Schraeder valve (through which the system is pressurised), a leak at the valve seating, a perished or split interconnecting pipe, or a damaged fluid chamber membrane within the unit. Low pressure without fluid loss can be caused by extremes of temperature affecting the fluid, which is a mixture of water and alcohol together with an anti-corrosion agent.
Should the system need pressurising or depressurising in order to carry out other work on the vehicle then a special maintenance unit is required. Most established garages will have the necessary equipment and will, for example, pressurise a deflated system for around £40 - £50 + VAT. DIY maintenance units are also available but will cost around £200.
Should a system need to be pressurised then it is safe to drive the car to and from the garage, but a maximum speed of 20mph is recommended as the car will be riding on its bump stops. Unfortunately displacer units are sealed and cannot be repaired if damaged.
UPRATING
As far as everyday road use is concerned, Hydrolastic suspension provides good handling and ride quality. If you want to push your car a little harder, there are certain modifications that can be made, though not surprisingly, most of the hardware available for such mods is aimed at the 'wet' Minis rather than the other Hydrolastic equipped models.
Early 1100s were fitted with an auxiliary spring and roll bar set up at the rear to introduce an element of variable rate to the suspension. It is therefore possible to alter the spring setting to raise or lower the ride height of the car. Later models were fitted with progressive bump stops and these can be substituted with uprated items.
In the 1960s 'wet' Minis were often lowered by carefully machining the displacer pistons though this is specialised work and needs to be done professionally. Rear anti-roll bar kits are also available for the Mini as are progressive bump stops.
It is possible to fit the higher rate displacer unit from a post-1966 Cooper S to other 'wet' Minis, but these units will not fit other models with fluid suspension. Auxiliary shock absorber kits are available; the Hydrolastic suspension of the late sixties/early seventies Works Austin 1800 rally cars were successfully uprated with the addition of Armstrong adjustable shock absorbers.