# How to Create Pressure Differential?

• posted
Problem:
2-Way Air Suspension on a vehicle (using Air Cylinders) leans to one side
since the vehicle is heavier on that side.
Suggested Solution:
Create a constant pressure differential between the two cylinders so that
one is always a fixed PSI higher than the other, eliminating the lean.
Implementation ideas:
1) Use a differential pressure regulator (I'm having GREAT difficulties
trying to locate one, anyone have ideas where to get one?)
2) Use check valves to create a pressure differential. i.e. Have the
cylinder inflow port connected to a 5psi cracking pressure check valve and
the outflow port to a 0psi check valve. Problems may occur when the other
side hits bumps and transfers air into the supposedly lower psi cylinder.
3) any more ideas???
• posted
Measure the "tilt". Divert air pressure to the low side *and* bleed a little from the high side. Short busts of very small 5-way, center-ports-blocked valve should do the trick, assuming you have a (relatively) constant supply of air pressure.
David A. Smith
• posted
1) If there are any linkages in the force paths to the cylinders, make differential adjustments to their geometries so the cylinder on the "heavy" side supports a greater load than the cylinder on the "light" side, with equal air pressures.
2) Use different cylinder diameters on each side.
3) Add a "helper" spring to the heavy side.
Art W.
• posted
Hi David,
I just finished measuring the tilt. I require a differential of 12psi to make the car sit evenly.
I'm not sure how your "bleeding" idea would work?
Here is a diagram of the current setup to give a better picture:
I basically want to have one cylinder sit at a 12psi differential to the other, regardless of the input pressure (it will be changing).
Thanks!
difficulties
• posted
Thanks for the tips Art. I don't think I can implement any of them though. This is why I am just trying to achieve a pressure differential (simplest way...)
1) The cylinder is attached to an automotive suspension so I really can't modify the leveraging. 2) Different cylinder diameters would work, but it is costly and involves me tearing out the current setup. 3) There is no room to add a helper spring with the current setup.
Thanks!
difficulties
• posted
Use another valve. Drive one cylinder with the existing valve, and the other cylinder with the new valve.
David A. Smith
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How about using two differently rated air shocks, with the same pressure? If the diameter of one is larger, it should support more weight at the same pressure.
Michael
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It seem the obvious question that nobody has asked yet is..
Why is the vehicle heavier on one side - can the weight be re-distributed?
can a balance weight be added to the other side??
SS
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Cars don't lean when they are new. Fix the illness, don't treat the symptoms. Fix the springs or load it evenly. Ghost
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I think you are missing the point. Your objective is a level vehicle, not a differential pressure. You need two separate, identical control systems, or perhaps even four, one for each suspension element. Each one admits air when it is low and vents air when it is high. Thus each wheel will independently adjusts its pressure to suit its height requirement regardless of load.
Walter.
• posted
Check the Citroen DS ("Goddess") Pneumatic self-leveling suspension - from 30 ? years ago. Beautiful body: rose when the engine started.
Brian Whatcott
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Lets see, who to answer first... :-)
Michael: Two different air cylinders is an option, but would be prohibitively expensive and require removing the entire system.
SS: This is a good idea which will have to be explored, but might involve too much work.
Ghost: Ummm, cars are not necessarily a 50/50 side to side split when new. The original shocks/springs compensate for this, but when you change to air cylinder suspension you are on your own. i.e. there is no illness :-)
Walter: Yes, splitting to a 4 way system would solve the problem, however, this too is expensive and more difficult to control. For example, you will have to adjust BOTH front cylinders when all you really want to do is adjust one and have the other move in proportion (but with differential) to it.
Brian: Yes, my setup is the same idea. The Citroen used a four way system.
Anyone else have ideas? All I'm looking to do is create a 12psi differential between two air cylinders!!!
Thanks!
• posted
Fit a conventional pressure reducing valve ( pressure regulator) on the line going to the side that sits high . Adjust the valve until the vehicle sits level. The PRV will reduce the pressure to the high side,and let it sit lower. To Miller
• posted
Yes, but it will not change as the overall system pressure changes. i.e. it will not create a differential, rather just a fixed output pressure.
• posted
...
Norgren (and others) will most likely have a remote pilot pressure regulator. The remote line opposes the spring, and the output line aids the spring (or is it vice versa). You can adjust it to provide whatever differenital you wish. It will be pretty gross, and you will need the relieving function.
You'd be better off with a servo system, assuming you cannot shift the weight mechanically. If the mechainical shift cannot be performed, you really need a stiffer spring on that side. Otherwise if will handle poorly
David A. Smith
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How about a differential pressure regulator? I looked that up on google, and found a few manufacturers listed in the Thomas register. Spence Engineering in Walden, NY has a type EN regulator which might fit your purposes.
Michael
difficulties
• posted
You are assuming a constant differential will solve your problem. Will it? Surely the weight distribution is not constant. It will vary from load to load. Furthermore, when going around curves, a given mass produces different forces. Assuming there are only two points to control, you have a choice:
1 - Use one control system to control the combined pressure and a second to control the difference between them (Do not assume either of these is constant.)
2 - Use one control system for one wheel and a second for the other.
If you have two degrees of freedom and you need two controllers.
Walter.
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I would check GlobalSpec