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Air over Hydrolic Suspension

Hossbaby50

Hossbaby50

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Ok, need to get some life in this forum and I found something I need talked about and explained.

rear2.jpg


Somebody shed some lift on this in simple terms. I am a visual learner so alot of involved text is not going to help me. From what I can gather they are using 16" travel 2.5" hydrolic tierod rams in a setup similar to airshocks correct? Air on top hydrolic fluid on the bottom. Explain the setup to me if you will please. I understand the 4 link part of the suspesion so that is not and issue, just want to know how the hydrolics work. Thanks

Harley
 
I can't tell from the picture , but do we know if they are tied together with hydraulic lines ?

I don't have school smarts , but could they be connected so when when a tire goes over an obstacle and one side of the suspension goes up , that the other side is pushed down , making it articulate better ?????
 
They are not setup as fully hydrolic. There is air in the system so it acts like a spring instead of a hydrolic ram. The amount of air pressure can be changed to change the spring rate and ride height. Just trying to figure out how it all works.

Harley
 
I was just thinking about it because I remember reading about some suspensions the oems were working on . As to your post , it got me thinking and a quick search got me this :


 HYDRAULIC FLUID AND AIR SUSPENSION

A suspension system that is a combination of hydraulic fluid and air has been developed in which the elastic medium is a sealed-in, fixed mass of air, and no air compressor is required. The hydraulic portion of each spring is a cylinder mounted on the body sill and fitted with a plunger that is pivotally attached to the wheel linkage to form a hydraulic strut. Each spring cylinder has a spherical air chamber attached to its outer end. The sphere is divided into two chambers by a flexible diaphragm, the upper occupied by air and the lower by hydraulic fluid that is in communication with the hydraulic cylinder through a two-way restrictor valve. This valve limits the rate of movement of the plunger in the cylinder, since fluid must be pushed into the sphere when the body descends and returned when it rises. This damping action thus controls the motion of the wheel with respect to the sprung portion of the vehicle supported by the spring. They are also called as pneumatic suspensions.


Which sounds to me ( dummy I am ) like it can support the suspension through all its travel with control . I have ridden in big rigs with TJ before and they don't really bounce around as the suspension does its thing without you really feeling it , like the body is on a cushion of air all the while the pistons are moving up and down . O fcourse these won't use a compressor though . I hope someone who knows posts up , because I love to learn too .
 
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Those rams are probably set-up in a similar way to an aircraft landing-gear strut. A floating piston separates the hydraulic fluid from a pressurized-gas charge (usually nitrogen), and the gas is at the top of the cylinder. The gas pressure determines ride height. The rest of the cylinder functions as a normal hydraulic shock absorber, with a piston attached to the lower rod (attached to the axle). The piston has calibrated openings to pass fluid from one side to the other, to dampen the compression and rebound strokes.
 
So , they are springs to support the body , and shocks in one . Thats what I gathered . Why aren't a lot of guys running something simmilar :thinking:
 
What you describe is provided by Fox and others as an "air shock". Targeted as a lower cost replacement for coil-overs, and using pressurized nitrogen for the gas charge. By varying the oil, valving, and gas charge, you can tune it all sorts of ways. But they have a number of notable shortcomings. Limited weight capacity is one of the more glaring problems, as well as special equipment and some have concerns over longevity of the pressure seal, particularly on higher weight buggies. Some of the performance characteristics have also caused even light weight comp-buggies to revert to coil-overs after trying "air shocks".

However, what was shown in the first photo is not the same thing, though it may be attempting to emulate the same idea in a home-brewed fashion. I have not commented otherwise because I really have no idea what they were after and can't really speculate without seeing more of what went into their setup.
 
One thing I have seen done on a comp machine is to use hyd rams and an open center valve. It had normal suspension and the rams were used to suck the nose or the rear up/down depending. With an open center valve the cyls just float until the driver moves the valve's lever.

JarheadK5's description is nearly a perfect one of how a monotube damper like a Bilstein works. Not a surprise at all that they're similar.
The problem with trying to convert a hyd cyl is that the piston has no holes in it. That means all fluid movement from one side of the piston to the other has to happen outside the cylinder. A "by-pass" damper in other words. Can do that, and it has been done b4. For simply a damper, i.e. no other actions involved, that's the stuffing it's arse with bricks method of killing the cat.

Citroen did the gas over hyd suspension system starting in the 50's. Those oddly shaped black cars with the taillights almost on the roof that the FrenchPolice in movies always drive (DS-19's) have that system.
Each corner has a hyd ram and one of the spherical accumulators. Don't know if there is a dividing membrane in those or not. Anyway, the hyd fluid acts as the 'linkage' btwn the suspension and the gas 'spring' and as the damper. Citroen put in a curve though, the working fluid is also the PS fluid (unique stuff as I recall, not normal PS fluid or ATF). The driver has the ability to add or subtract fluid. This effectively changes the 'length' of the linkage which changes the ride height. Epiphamy: Greg72?

Some things interesting about the Citroen system are that everything power in those cars (windows, door locks, brake booster, etc.) are hyd and not vacuum or electrical. Another is the speed vs ride height sensitivity. The higher the speed the lower the car sits on it's suspension. Bear in mind that they did all of this with mechanical logic, no electronics. The cars normally bleed down overnight, which is why in some movies the cars raise up when first started.

That any of this relates to the pic above, I've not a clue. :D
 
OK, let's see if I can get this into words that paint an easy to see picture.
Oil is not compressible. Putting it under pressure does not change it's volume. Air is compressible and when you put it under pressure you reduce it's volume.

There are two separate methods those links talk about.

The first method is basically using a hyd. ram with air instead of oil. Because air changes volume under pressure, the ram's shaft can move in and out when the system is capped off. In other words, you could put a tire valve in the top (extension) port of the ram, pressurize it with air, and have an air 'spring.' Moving the ram's shaft in will raise the pressure of the air because the volume has been reduced. The pressure vs. volume thing works in both directions. If the temp is held constant (near impossible) the relationship of the volume to the pressure is very simple and easy to predict. The changing temp is the wrench in the works of this method. There are some partial ways around this, but none of them cure the problem.

For the second method I'd best first describe what an accumulator is just in case there is uncertainty. Think of a hyd ram. It has a tight fitting piston on the end of it's shaft. Imagine if that piston didn't have the hole in it for the shaft and was allowed to move back and forth at will. That is the mechanics of an accumulator. How it is used is that one end of the body has something like a tire valve on it and the chamber on that side of the piston is filled with air.
The other side of the body is connected to a hyd ram and is filled with oil. So now we have a hyd ram attached btwn the frame and the axle housing and it's filled with oil. The ram has a hose from it to an accumulator. So far what's happened is that the air volume has been moved to a remote location and the oil is the "linkage" that drives the accumulator's floating piston in order to compress the air so that the ram can collapse and the suspension can compress.
If we add more oil the piston in the ram moves out, which is effectively the same as making the "linkage" longer. If some oil is bled off the reverse happens and the "linkage" is made shorter. If the air pressure is increased then it takes more force to compress the ram. That is an increase in spring rate. Opposite is true for reducing the air pressure.
 
Rather than making one long post I thot I'd break it up into several smaller posts.

The first method is a really simple system. Few parts and no complexity. One problem is that as you compress air it gets hotter. Hot air expands making it have a greater volume. That upsets the volume to pressure ratio which results, or can, in a harsh ride. The best partial cure is to not use air, but to use Nitrogen instead. Nitrogen has a much flatter tempature to volume curve. It changes it's volume much less than air does for the same change in temp.
Another problem is that unless the ram has a huge piston it's hard to generate enough 'spring rate" to carry a heavy vehicle. When the whole point was small and light, a huge piston is not going in the right direction. So maybe an increase in the pressure is the cure? There is a point where the seals just can't go any higher in pressure and contiue to seal.
 
In the second method it is possible to take advantage of hydraulics to carry more weight. By using a small bore accumulator with a larger bore hyd cylinder the accumulator can be light, and still be strong enough to generate enough force to support a heavy vehicle.
This method also lends itself to more easily changing the 'length' of the linkage for ride height adjustments and control when twisted up.

The spherical shaped accumulators used by Citroen and shown in the POoR thread work a little differently. I'm not all that familiar with them, but I think it safe to say they do not have a floating piston in them. One posting said that there was a diaphram in his. This would act in the same manner that the floating piston does. I believe that the Citroen accumulators do not have a diaphram, but due to their mounting the gas does not tend to get mixed into the oil.

As an aside, cylinderical accumulators as described above are known as "reservoirs" when talking about monotube type shocks.
 
Ok, I think everything just clicked in my head of how it works. Pretty simple actually if it works the way I am picturing in my mind. I am going to confer with my suspension guru buddy tomorrow and make sure I am getting this down right.

Why is this suspension not used more? What are the drawbacks? Seems like it would be a pretty decent suspension setup for slow speed applications. It looks like it would take some time to dial the suspension in but that life.

What are the price of accumulators that a suspension like this would need?

Harley
 
The change in temp of the gas (air in particular) is a big problem since it effectively changes the spring rate. The high pressures usually needed mean that the system is prone to leaking and settling. Compressing the gas or air results in a rapidly rising spring rate. Say it takes an additional 50 lbs to compress the first inch. The last inch could take 100 lbs. It's hard to dampen such a 'spring'. For a comp type buggy it might work fine. For anything that moves at a higher speed it's been tried in various ways and has not been a huge success. I think that is the reason you haven't seen it much. People hear that it doesn't work so they don't try it, but they aren't told under what conditions it doesn't work.

I've no clue, other than a Citroen, where to get the spherical accumulators. A cylindrical can be made from a hyd ram or even a KYB damper. A more spendy way to do it would be to use four Canton Accusumps for the accumulators. Accusumps ARE accumulators, engine oil in their intended application.
 
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