I'm quite ready for the bags-are-for-shopping crowd on the one hand, and for those, on the other, who hold that "doing it right" means doing it the way it is always done. This notwithstanding I can never leave an idea alone once it's got hold of me, and some of you might know how often I come back to suspension hydraulics, especially hydraulics used not as a mere means of altering ride height but as one to get suspension movements to a place where one can play them against one another in all kinds of cunning ways. In the course of that I've been looking at both Citroën and Moulton/BMC hydraulics. The former includes a lot of brilliant lateral thinking and the parts are available and surprisingly cheap, but the system is engine-powered, which means that the car gently settles down on its bump-stops when the engine is turned off. The latter is completely passive and avoids piston-sealing issues entirely, but it comprises nasty sealed assemblies which are moreover not very pretty, and which are becoming quite scarce. Neither is ideal, but it would be so cool to be able to combine the Citroën system's diy-friendliness and creative-configurability with the Moulton system's trouble-free, plastic-bag-with-water-in-it simplicity. It was therefore probably inevitable that the ubiquitous mental light-bulb should sooner or later illuminate the thought of taking an air-bag spring and putting an incompressible medium like hydraulic fluid in it. The thought following immediately is as inevitably that there must be a million reasons why it won't work. Imagine my delight, then, upon reading in a Firestone manual for industrial air springs that their use with a water/glycol mixture as medium is perfectly accepted practice! And before the point is made, there is no essential difference between industrial bags and automotive ones: in some cases they are exactly the same thing. It must be pointed out, to pre-empt yet another objection, that doing this causes the bag to stop being a spring and to become an hydraulic cylinder. The spring action in a bag doesn't come from tensile elasticity of the envelope but from the compressibility of the column of air inside it. Replace the air with the abovementioned mixture and the bag becomes a rigid strut. One would obviously put back the springiness by incorporating accumulators elsewhere in the system: the very point is that there are all kinds of ways to do this, depending on what else one wants to achieve. Now, bags are also not paragons of beauty; but I'd expect that one could safely use smaller bags in any given situation, because ride quality no longer depends on bag capacity. One just has to be safely within the structural capability of the bag. I'd got to this scratching my head around the Morris Minor's suspension. It was going in the direction of hydraulics somewhere in the system, but I was a bit wary of seals that start to leak in spite of all that expensive precision machining. Bags would be trouble-free and a lot cheaper, and used thus as cylinders the very smallest rollover-sleeve bags should be well up to the job. Thoughts?