Hydrolastic Suspension

If you've got this far, you'll remember that Dr. Alex Moulton originally wanted the Mini to have Hydrolastic suspension - a system where the front and rear suspension systems were connected together in order to better level the car when driving.

The principle is simple. The front and rear suspension units have Hydrolastic displacers, one per side. These are interconnected by a small bore pipe. Each displacer incorporates a rubber spring (as in the Moulton rubber suspension system), and damping of the system is achieved by rubber valves. So when a front wheel is deflected, fluid is displaced to the corresponding suspension unit. That pressurises the interconnecting pipe which in turn stiffens the rear wheel damping and lowers it. The rubber springs are only slightly brought into play and the car is effectively kept level and freed from any tendency to pitch. That's clever enough, but the fact that it can do this without hindering the full range of motion of either suspension unit is even more clever, because it has the effect of producing a soft ride.

But what happens when the front and rear wheels encounter bumps or dips together? One cannot take precedent over the other, so the fluid suspension stiffens in response to the combined upward motion and, while acting as a damper, transfers the load to the rubber springs instead, giving a controlled, vertical, but level motion to the car.
Remember I said the units were connected with a small bore pipe? The restriction of the fluid flow, imposed by this pipe, rises with the speed of the car. This means a steadier ride at high speed, and a softer more comfortable ride at low speed.

Hydrolastic suspension is hermetically sealed and thus shouldn't require much, if any, attention or maintenance during its normal working life. Bear in mind that hydrolastic suspension was introduced in 1965 and you'd be lucky to find a unit today that has had any work done to it.

The image below shows a typical lateral installation for hydrolastic rear suspension. The suspension swingarms are attached to the main subframe. The red cylinders are the displacer units containing the fluid and the rubber spring. The pipes leading from the units can be seen and they would connect to the corresponding units at the front of the vehicle.



Hydrolastic suspension shouldn't be confused with Citroën's hydropneumatic suspension (see below). That system uses a hydraulic pump that raises and lowers the car to different heights. Sure it's a superior system but it's also a lot more costly to manufacture and maintain. That's due in part to the fact that they don't use o-rings as seals; the pistons and bores are machined to incredible tolerances (microns), that it makes seals unnecessary. Downside : if something leaks, you need a whole new cylinder assembly.

Hydrolastic was eventually refined into Hydragas suspension.

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Hydragas Suspension

Hydragas is an evolution of Hydrolastic, and essentially, the design and installation of the system is the same. The difference is in the displacer unit itself. In the older systems, fluid was used in the displacer units with a rubber spring cushion built-in. With Hydragas, the rubber spring is removed completely. The fluid still exists but above the fluid there is now a separating membrane or diaphragm, and above that is a cylinder or sphere which is charged with nitrogen gas. The nitrogen section is what has become the spring and damping unit whilst the fluid is still free to run from the front to the rear units and back.



Hydragas suspension was famously used in the 1986 Porsche 959 Rally car that entered the Paris-Dakar Rally, and today you can find it on the MGF Roadster.