Hydrolastic suspension

Hydrolastic suspension /hy-dro-las-tik suh-spen-shuhn/ noun (uncountable)

Hydrolastic suspension is a type of interconnected vehicle suspension system, designed by Alex Moulton and used by the British Motor Corporation (BMC), that uses a combination of fluid displacement and rubber springs. This was a particularly British piece of engineering, an uncommonly clever attempt to give small, lightweight cars the smooth ride of a luxury saloon. The system used a network of rubber cones and water-filled bags, all plumbed together so that when one wheel hit a bump, it would help lift the wheel at the other end. It was a complex solution that provided a very level, floaty ride, at least until it started to leak. For a generation of mechanics, the sight of a sagging Austin 1100 was a familiar sign of trouble.

The Full Story of Hydrolastic Suspension

The system was the brainchild of Dr. Alex Moulton, the same engineer who had designed the rubber cone suspension for the Mini. Not content with that success, Moulton wanted to solve the problem of "pitch"- the choppy, nodding motion that short-wheelbase cars suffer from. His solution was to connect the front and rear suspension on each side of the car.

Instead of a conventional spring and shock absorber at each corner, a Hydrolastic car has a "displacer" unit. This contains a rubber spring and is connected via a pipe to its partner at the other end of the car. The whole system is filled with a mixture of water and alcohol. When a front wheel hits a bump, it compresses its displacer, forcing fluid through the pipe to the rear unit. This pressure at the rear pushes the back of the car up slightly, counteracting the pitch and keeping the car remarkably level. It gave a small car the ride quality of a large one.

The first car to be fitted with the system was the 1962 Austin/Morris 1100. Its smooth ride became its defining feature and a major selling point, making rivals feel crude by comparison. The system was subsequently fitted to the Mini, the Austin 1800 "Landcrab", and the big Austin 3-Litre.

In the 1970s, the system was evolved into "Hydragas". This refined version replaced the rubber spring with a sealed sphere containing nitrogen gas, similar in principle to Citroën's famous suspension. This gave an even better ride and was fitted to a generation of British Leyland cars, including the Austin Allegro, Princess, and Rover Metro.

For all its ingenuity, the system was not without its flaws. It was more expensive to produce than conventional suspension, and while it was impressive when it worked, it could be a nightmare when it failed. Leaks were the common problem, causing the fluid pressure to drop and the car to sag dramatically on one corner. Repairing it required a special pump to re-pressurise the system, a tool many garages did not have. As conventional suspension design improved, the complexity and cost of Hydragas meant its days were numbered, and the last car to use it, the Rover 100, ceased production in 1998.

For The Record

Who was Dr. Alex Moulton?

He was a British engineer and inventor, famous for his work on vehicle suspension systems and for creating the small-wheeled Moulton bicycle. He designed the rubber cone suspension for the Mini and then developed the Hydrolastic and Hydragas systems for BMC.

Was the fluid just water?

Almost. It was a specific mixture of 49% alcohol, 49% water, 2% corrosion inhibitor and a green dye. It was often referred to simply as "hydrofluid," and using plain water would cause the system's components to corrode and in winter, freeze solid. 

Was it the same as Citroën's suspension?

No, though they are often confused. Citroën's famous hydropneumatic system used a high-pressure, engine-driven pump to power the suspension, brakes, and steering. Hydrolastic was a much simpler, sealed, un-pumped system. The later Hydragas system, which used nitrogen spheres, was conceptually closer to the Citroën but still lacked the central pump.

Why did the cars sag when it failed?

The fluid in the system was under pressure and helped support the car's weight. If a pipe or a seal failed and the fluid leaked out, the pressure was lost, and the corner of the car would sink dramatically towards the ground as the displacer unit depressurised.

Can you still get a Hydrolastic car repaired?

Yes, but it requires a specialist or a well-equipped enthusiast. The fluid, seals, and displacer units are available through classic car parts suppliers, but the key is finding someone who still has the special Hydrolastic/Hydragas pump required to depressurise and re-pressurise the system correctly.