Theme : Suspension – Hydrolastic Rubbery Goodness

A long time ago the Midlands of Britain were at the cutting edge of suspension design.

Hydrolastic suspension: source
Hydrolastic suspension: source

In 1955 Citroen presented their DS which had a suspension system markedly different from the ones with which drivers were familiar. The British Motor Corporation picked up Citroen’s fragrant gauntlet. Their attempt to improve ride and handling went under the name hydrolastic and they offered it first on the period’s equivalent of a bog-standard family car, the 1100-series (born as ADO16).

More hydrolastic: source
More hydrolastic: source

In some obvious ways the Hydrolastic suspension system resembles Citroen’s concept. The springs and dampers of the standard system are replaced by fluid-filled spheres. They are connected from front to back but not from side to side (or all around, as in the case of the DS). Whereas Citroen used gas and fluid filled spheres, BMC’s Alec Moulton used a rubber spring. Fluid displaced by the wheel´s movements is passed through a rubber valve. As the front wheel hits a bump fluid is pushed back to the displacer unit of


the back wheel, lowering it relative to the body. That smooths out the car body’s reaction. Conventional systems require that suspension is uniformly stiff: from side to side (roll) and from front to back (pitch). The front-to-rear interconnection of the Hydrolastic system means that roll stiffness can differ from pitch stiffness. Typically a car that is softly sprung so as to absorb road shocks to all four wheels is also one that suffers body roll during cornering. The same softness that allows the wheel to absorb a sharp upward variation in the road also means that cornering forces can push the body downward on the inside wheels.

This creates the phenomenon so loved by motoring scribes, “the car leaning like a dinghy in a squall”. Hydrolastic can resist body roll while also being smoother-riding. Car bodies have more than one roll axis. The front-to-back compliance of the Hydrolastic suspension means that the car can still squat and dive. If the car accelerates, forces act to compress the rear half of the car. If the car decelerates sharply, it will dive. Attempts to moderate these effects will essentially complicate the system or reduce its effectiveness in handling unevenness at the road surface.

MGF suspension principle: source
MGF suspension principle: source

Hydrolastic was revised by Moulton to become Hydragas and was fitted to the Austin Allegro, the Princess and the Ambassador. Even the Metro received a version of the system. The final car to use the concept was the MGF. When it ceased production in 2002 the hydrolastic/hydragas concept was abandoned. As with Citroen’s system, one gets the impression that the public were not all that interested in advanced suspension technology. The literature on BMC cars doesn’t make quite so much of the suspension as with Citroen. To some extent that indicates its success: it was not problematic enough to warrant the nine metre high warning signs that hover over all oleopneumatic Citroens. By the same token, the public didn’t really care and for accountants that creates a temptation to ask the engineers to desist from their time-wasting experiments.

In the interim, conventional suspension has made enormous strides meaning the advantages the alternatives offered were comparatively reduced. Arguments for lower cost and more reliability defeated the liquid-gas suspension systems. Further, the public were less and less interested in the pleasures of a smooth riding car; somehow a harsh ride was associated with sportiness and performance while ride-quality became the preserve of the non-sporty. In the market place, such an image is poison.

1984 Austin Montego:
1984 Austin Montego:

The remaining question is there is any fundamental alternative to conventional suspension that is reliable, cost effective and performs in a way to square the ride-handling circle.

Author: richard herriott

I like anchovies. I dislike post-war town planning.

5 thoughts on “Theme : Suspension – Hydrolastic Rubbery Goodness”

  1. The ADO16 – I don’t think there’s any car which brings me so much delight and, simultaneously distress. Delight that it was so far ahead of its time, so rational, and so right in its proportions. Distress that BMC produced “The English Golf” twelve years before the far less high-minded German one, then threw it all away with the inexcusable Allegro.

  2. Well at least when the Citroen DS suspension was actually working, the fact that it was engine-power pressurized meant consistent suspension action. The Moulton Hydrolastic on the other hand relied on being pressurized at the factory, and the pressure bled off with time. Our Land Crab, for example, adopted a low-rider stance, more on one side than the other, requiring ministrations at a dealer hundreds of miles away. Hmmm.

    In any case, the idea was predated over a decade before by the wonderful Citroen 2CV. That vehicle used two springs separated by a tight-fitting rubber piston in a cylinder alongside the sills, each spring connected to a swing-arm by a push-pull rod. Small bumps were absorbed by the spring for each wheel, but larger heaves actually moved the piston by overcoming friction and activated the other spring. Same effective front-rear interconnection as Hydrolastic, but nothing much to go wrong. The French win again.

    I used to wonder all those years ago whether a degree of anti-roll could have been achieved by by a cunning cross-connection of the Hydrolastic system fluid pipes side to side front and rear as well as the mere front to rear connection they actually went with. And now, the Tenneco system in the McLaren MP4-12C does exactly that. Pretty logical follow-on.

    More googling will get you a bit more info on the Tenneco system. Cannot help but think that properly commercialized and reduced in cost, this might be as good as you can get without going completely mad. And while they do that, make the fluid magnetorheological for variable damping. GM invented those MR dampers and fluid some years ago and even Ferrari use them these days; no other damper works better.

  3. I had an MGF with interconnected Hydrogas. It arrived overly pumped up with ridiculous ground clearance – even the dealer let it through the PDI like that. Once re-set at the right level, the car’s handling and ride were almost supernatural. Like the Metro that donated the underlying system, one would see many cars collapsed on their bump-stops. So, great concept, crap execution.

  4. I didn’t appreciate what damage the accountants did to BMC when I updated to a series 2 Austin 1800. Amongst other things, the rear swing arm metal bearings were replaced with rubber bushes, with oversteer becoming an option over time.
    Also rust became a given…….in AUSTRALIA FFS ………and all for Pinifarina ‘fins’!.

    1. Hello David:
      The rot set in at BMC in so many ways. Now I come to think of it, I have never driven a British car – especially not from a time when idea meant something.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: