The interconnected fluid suspension developed by Alex Moulton for BMC is one of the most distinctive engineering features of the cars that use it, and it relies on a specialist fluid that is unlike any other liquid found on a classic car. The Hydrolastic system, fitted to a range of BMC front-wheel-drive saloons of the 1960s, replaces conventional springs and dampers with rubber-and-fluid displacer units interconnected front-to-rear by hydraulic pipes, a small fluid volume transferring between the front and rear units to maintain ride height and damp out road inputs.
The later Hydragas system, fitted to the MGF, develops the same principle using gas-and-fluid displacers interconnected by the fluid, giving the MGF its distinctive ride characteristics. The fluid stocked by MGOC serves both systems, manufactured to British Leyland standards with the spec sheet matching the original specifications the cars were filled with at the factory.
Fluid Specification
The suspension fluid contains selected corrosion inhibitors in a solution with methanol, giving the low coefficient of expansion that the closed-circuit system requires across its operating temperature range, a water-alcohol-antifreeze mixture formulated to give the right viscosity and freezing-point behaviour. It is supplied in containers sized for a single-car top-up and for a complete system refill, sufficient for a complete system service on the cars that use it. The fluid has specific viscosity, thermal, and chemical properties matched to the rubber seals and metal components within the system, and is specific to the suspension, not interchangeable with brake fluid, power-steering fluid, hydraulic oil, or any conventional fluid found elsewhere on the car. Pumping the wrong fluid into the system will cause progressive damage to the rubber displacer seals and the internal valves, a difficult mistake to recover from once the fluid has circulated, so confirming the correct specification before any topping-up or refilling work is essential.
How the Fluid Works in the System
In the Hydrolastic system the fluid is the spring and damping medium itself, transferring between the front and rear displacer units through the interconnecting pipes as the car rides over bumps, while in the Hydragas system the fluid transmits force between the front and rear displacers and provides the interconnection that gives the system its ride character, the gas in the displacer providing the spring action and the fluid providing the interconnection and damping. In both systems the fluid level and condition directly affect the ride, contaminated or degraded fluid affecting ride quality and the speed of the interconnection response, and a system that has lost fluid sits low, the first symptom typically being the car sitting low on its suspension, particularly noticeably on one corner where a displacer has lost more than its share of the fluid. On the MGF the interconnection pipes run beneath the car similar to the coolant pipes and are subject to the same corrosion risks, though fluid loss typically manifests as a change in ride behaviour rather than a visible leak.
Top-Up, Re-Gassing & Recommissioning
A system that has not been touched for decades will typically have lost some of its original fluid through micro-leaks at hose unions and seal-to-displacer interfaces, and the correct repair is to identify and fix any leaks, pressure-test the system, and refill to the correct specification using a dedicated pump, a hand-operated displacement pump that builds the system pressure to the required level, which on the MGF Hydragas system is part of the re-gassing procedure that restores the correct ride height and spring rate. The fluid may need replenishing during system servicing, re-gassing, or if a leak develops in the interconnection pipes or at a displacer connection, and the fluid level and condition should be checked whenever the system is serviced.
For owners of the BMC Hydrolastic saloons, conversion to dry-suspension components fitting conventional springs and dampers is an alternative for those who prefer to leave the system behind entirely, but for cars where originality is the priority, refilling and recommissioning the original system is the right approach, and on the MGF the Hydragas system is integral to the car's design and character so maintaining it correctly is the standard approach. The technical team is available to advise on the right approach for a specific car, including the pressure pumps and adaptors needed to do the work properly.