Moulton Automotive Suspension Systems
Almost as soon as he joined the ‘family firm’ - Spencer Moulton - after the Second World War, Alex Moulton set out a case for the adoption of rubber suspension systems on automobiles. He could see that advances in rubber technology, particularly in the field of rubber to metal bonding, allowed new rubber springs loaded in torsion and shear to be developed - and these were particularly suited to automotive use where the wheel travel was much larger than that of railway carriages.
Moulton’s first automotive suspension design was the Flexitor, a roughly cylindrical device that provided location and articulation by a rubber element loaded in torsional shear. Moulton had designed and patented the Flexitor in 1948 and it found many uses (notably in railway carriage gangway springs where it is still used today), but it was not until the mid-1950s that it was used on a car - or rather a 4x4, the Austin Gipsy. Although this was rather a niche vehicle, by this time the seeds had already been sown for the mass-market adoption of Moulton suspension on passenger cars.
Through his friends David and Jeremy Fry (of Fry’s Chocolate fame), Moulton was introduced to Alec (later Sir Alec) Issigonis. Issigonis was already famous as the designer of the 1948 Morris Minor, but initially he was dismissive of rubber as a suspension medium. After Issigonis left Morris for Alvis, Moulton was able to persuade Jack Daniels (Head of the Research Department at Morris) to equip a Minor with an experimental version of Moulton’s rubber suspension. Arduous testing showed that Moulton’s system was demonstrably more durable than coil springs or torsion bar suspensions, and Issigonis was convinced of its superiority.
Alex Moulton designed the ‘Flexitor’ - a torsional shear rubber spring - in 1948. By combining the springing, damping and location in one unit, the Flexitor was a major advance in rubber spring technology. Simple in concept, Moulton struggled for a long time with the design and construction in order to secure a long service life. Extensively used on road trailers - notably by Bramber - the Flexitor also found its way onto the Bond Minicar and, eventually, onto the British Motor Corporation’s Land Rover rival, the Austin Gipsy. Another notable use for the Flexitor - still widely used today - was for gangway springs on railway rolling stock. The Spencer Moulton Company saw a great future for the Flexitor and founded a new company - the Flexitor Company - to promote and exploit it.
Alex Moulton developed the Cone spring in 1951 as an engine mount for Admiralty Minesweepers. These were made by Spencer Moulton in Bradford on Avon. Whilst working with Alec Issigonis on the Alvis TA350 car, Moulton proposed a suspension that used two Cone springs in series ‘nose-to-nose’. Following this came the key discovery that the rubber cone could be used as a fluid displacer as well as a primary spring. Thus the layout was changed to two Cone springs ‘back-to-back’ to form a fluid chamber and enable hydraulic interconnection between the front and rear wheels. Alex Moulton would later recall “I shall never forget the revelation at Coventry, with Alec driving, of experiencing the ‘big car’ ride due to the lowered pitch frequency. The reality of the benefit of fluid interconnection was thus revealed and the seed was sown, not that we realised it then, for a radical new suspension to be made in vast numbers.”
The Alvis TA350 project was cancelled, and the single prototype car was destroyed. For many years no photograph could be traced of this almost mythical car, but some have now surfaced, showing how complete and finished the car looked. Click here for further details. Almost nothing of the car remains other than rubber suspension units here at Bradford on Avon.
When Alec Issigonis was tasked by BMC to design a small car - the Mini - to ‘drive all the bubble cars off the road’, he faced several engineering challenges. HIghly significant amongst these was the use of small wheels to achieve maximum habitability within the smallest vehicle size, and dealing with the great variability in load levels according the number of passengers carried.
Alex Moulton developed his Cone spring into a very compact suspension spring, a levered device based on the Cones used on the Alvis TA350. With a rising-rate characteristic, the Cone spring worked well whether the car was fully laden or with only the driver on board. It also required minimal installation space. - and, last but certainly not least, it provided the Mini with incredible road-holding and driveability. The Mini was launched in 1959 and, after a slow start, became one of the greatest automotive icons of all time.
Moulton and Issigonis had intended for the Mini to have the interconnected Moulton Hydrolastic system right from the start, but it was quickly realised that the low ‘puffing stiffness’ of the rubber cone did not allow for effective interconnection at all frequencies. Thus the ‘dry’ Cone spring was substituted until the Hydrolastic system was fully developed. It should be remembered that the development period of the Mini was incredibly short, especially for a vehicle incorporating so much new technology. Alex Moulton would advise potential innovative engineers that it was wise, when attempting to introduce something new, to have proven technology available as a ‘fall-back position’ if required.
During the Alvis era, Moulton and Issigonis conceived the idea of front and rear rubber springs interconnected hydraulically to give a ‘big car ‘ ride - in effect removing the ‘pitch mode’ of the car and also minimising roll. Moulton’s Hydrolastic interconnected suspension system was not ready in time for the Mini in 1959 so it made its debut on the Morris 1100 in 1962. The 1100 was even more of a 1960s success than the Mini, and for many years was Britain’s most popular car.
In the Hydrolastic system the fluid displacer was a rubber moulded, nylon reinforced diaphragm and the rubber cone spring was hydraulically operated. Thus there was no lag in the hydraulic interconnection, other than that imposed by the integral rubber flap dampers.
The Moulton Hydrolastic system was fitted to the Mini (1964-1969), the 1800, the Austin 3 Litre and the Austin Maxi. The durability and reliabilty of the system, proven by extensive rig testing, was confirmed by a Hydrolastic spring removed from a car that had covered 253,000 miles - tests rated it as within tolerance for a new unit.
As well as providing an exemplary ride and road-holding, the Hydrolastic system was compact and required very little installation space in the vehicle. However, as the units were highly levered (typically 5:1) the suspension loads were very high and on the Mini and the 1100 suspension subframes were required to contain these loads. This added cost and complexity, and - particularly in the case of the Mini - very significant rust traps. A better system was developed by Issigonis and Moulton for the 1800 in 1964 - the front units were placed opposing each other in a structural tubular cross-member and the rears were laid flat within the sill structure.
Hydrolastic units featured integral rubber-flap dampers that were designed - as were the units themselves - to last the life of the car. Even today most surviving Hydrolastic cars, mostly now over fifty years old, are still riding on their factory-fitted original units.
In the early 1970s Moulton Hydragas suspension was introduced, initially on the Austin Allegro. Whilst the principle characteristics remained unchanged, Hydragas used a nitrogen gas spring in place of the rubber ‘cheese’ spring used on Hydrolastic. The interconnection pipe was fitted below the damper valve (‘under-port’ interconnection, previously only seen on the Austin 3-litre). The resulting units were lighter and more responsive than Hydrolastic, and more compliant in ‘bounce’ mode.
Hydragas was fitted to the Leyland Princess, Austin Ambassador, Austin/Rover Metro and Rover 100, and enjoyed a late flowering on the MGF sports car. Later versions of the Austin Maxi were fitted with Hydragas - the Maxi being the only car to feature both Hydrolastic and Hydragas over its production lifespan.