Hydrolastic suspension was the system that kept many 1960s BMC cars riding level, in place of the separate steel springs almost everyone else used. It was the work of Alex Moulton, the engineer who had given the Mini its rubber cone springs, and it took one clever idea further: link the suspension units front to rear with fluid, and let the car settle itself.
It is one of the defining pieces of British engineering of the period, of a piece with the sideways engines and front-wheel drive that BMC was pioneering at the same time, and it explains the distinctive soft, level ride of a whole family of classics.
What Hydrolastic suspension is
In place of a coil or leaf spring at each wheel, Hydrolastic uses a sealed unit containing a rubber spring and a fluid. The rubber does the springing. The fluid does two jobs: it damps the movement, doing away with separate shock absorbers, and it carries movement from one end of the car to the other.
That second job is the heart of the design. The front and rear units on each side of the car are joined by a pipe. Press the front unit and its fluid has somewhere to go, which is what makes the whole system work as one connected unit rather than four separate corners.
How the interconnection works
The point of joining front to rear is to control pitch, the nose-up, nose-down rocking that upsets a short car on a bumpy road.
When a front wheel rides up over a bump, it displaces fluid down the pipe to the rear unit on the same side. That fluid pushes the rear up too, so the car rises more or less evenly at both ends instead of pitching forward. The effect is a flat, slightly floaty ride that feels unusually settled for a small, light car, and it is the signature of the breed from behind the wheel.
Which cars used it
Hydrolastic spread across the BMC range through the 1960s:
- The Morris and Austin 1100 and 1300 (ADO16) introduced it in 1962 and became the cars most associated with it.
- The Austin 1800, the big front-drive saloon, used it next.
- The Austin Maxi carried it on into 1969.
- The Mini ran Hydrolastic for a few years from 1964 before going back to its original, cheaper rubber cone springs.
All of these were front-wheel-drive cars with a transverse engine, the other half of the BMC engineering recipe of the period.
Hydragas: the gas-sprung successor
The idea did not stop with rubber. In 1973 the Austin Allegro introduced Hydragas, which kept the interconnected-fluid principle but swapped the rubber spring for a sealed chamber of pressurised nitrogen gas. Gas makes a naturally progressive spring, and the system was simpler to make consistent.
Hydragas proved long-lived. It rode under the Austin Princess and its Ambassador successor, returned in interconnected form on the 1990 Metro, and gave the mid-engined MGF of 1995 its level, comfortable ride. The basic Moulton idea, fluid linking the ends of the car, was still earning its keep nearly forty years after the 1100 first showed it off.
Living with it today
For an owner the system is reliable when sound but particular when not. The fluid can leak and the units can sag with age, leaving the car sitting low at one corner. Re-pressurising or replacing a displacer unit is a specialist job with specialist equipment, not a roadside fix, so a car that sits level on all four corners is worth seeking out, and one that does not is worth pricing accordingly.
Related
- The 1960s page sets the first of these cars in their decade.
- For the broader scope question, see which cars count as British classics.