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Suspension Basics 7 - Hydrolastic & Hydragas Springs

  "It floats on fluid" was one of the marketing taglines - because, basically, it does.

Hydrolastic & Hydragas?

Supporting a vehicle on fluid with a cushion inbetween is the basic principle of both systems   First things first: There is usually quite a bit of confusion as to the difference between hydrolastic and hydragas systems, but it's fairly simple. Hydrolastic suspension has a rubber spring built into it, while hydragas uses a compressed-gas spring. Both are suspension systems rather than just springs, in that they include a damping system to slow the action of the suspension. The overall operation of both systems is the same.

  The principle element is a sealed container of fluid for each suspension unit, one for each wheel. These are then linked fore and aft; the left front linked to the left rear, along with the right front linked to the right rear. In actual fact, there are two containers for each unit, stacked on top of each other. it's the top ones that are linked together, and the bottom ones that are linked to the suspension assembly and carrying the majority of the fluid. In order to flow to another unit, the fluid has to flow through a valve assembly between the two containers, and then through a pipe to the other unit.

Transfer of fluid between units to keep the vehicle level - click for animated version   The idea is that if the front wheel hits a bump, the compression of the suspension pushes the fluid to the rear unit. This causes the rear suspension to extend by the same amount that the front compresses, keeping the car level despite the bump, eliminating the fore/aft pitching that would occur if the bump affected the front suspension alone - as would happen with a "normal" suspension system. The entire process is reversed when the rear wheel reaches the bump, again keeping the car level, and everything works in the same fashion for potholes etc. Click on the right-hand image for a simple animation of the system in action.

  It is the valve between the two containers that limits the rate at which any one suspension unit can compress or extend, and the diameter of the pipe that limits how quickly the transfer between either side's front and rear units can occur, providing the damping within the system.

Hydrolastic suspension unit   Now, the question is; what happens if both front and rear units are compressed at the same time, because they'll cancel each other out? Well, those top containers of each unit have a spring inside them - rubber on the hydrolastic system, just like in the previous section. With the lower containers being squeezed by the suspension, and no way out of the upper containers, the fluid's only choice is to compress the spring, giving pretty much the same suspension action as if the spring itself was being compressed directly.

  Why doesn't the fluid itself compress? Bluntly, because we don't want it to, so it's made up to be incompressible (mostly water, with some anti-freeze and anti-corrosion agents added. Oh, and colouring, presumably to help show up leaks or contaminated fluid easily).

  The only difference between hydrolastic and hydragas is that the hydragas version uses pressurized nitrogen instead of a rubber spring. Nitrogen is used because it is an "inert" gas - it doesn't get upset by things like pressure, temperature, or foul language, so gives a good, stable performance. A compressed gas spring gives a rising-rate performance, too, just like a rubber spring (see previous section), as we shall examine more closely in the following section.

Hydragas suspension unit   Both systems are designed to be fit-and-forget parts, and history has shown that they have a working life in the region of 15 years plus before things like leakage become an issue. They provide an excellent quality of ride without compromising handling, and, providing the damping is set up correctly, don't "wallow" under cornering, braking and accelerating. The continued development of hydragas eventually led to fully cross-linked systems joining both the left and right sides together to make the most of the benefits.

  While there are more modern systems that can outperform them, it should be remembered that hydrolastic and hydragas suspensions achieved their performances over four decades ago, with relatively basic, sealed, mechanical systems that lacked any kind of "active" or feedback/adjust control on them. Also, many Monster Truck builders use the same basic principle (displacing fluid to compress nitrogen gas), minus the linking between units, to provide the suspension springing for their behemoth racers. The only real difference is the use of a sliding piston, rather than a flexible "bag", to do the compressing.

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