| Author | Message | ||
| Don Evans (Doninwa)
Registered Member Username: Doninwa Post Number: 260 Registered: 1-2007 Posted From: 208.81.157.234  Rating: N/A |
For discussion only, since it is lousy weather today and I just can't get motivated to do something useful. It would seem to me that the pressure in an air bag would be the same to support the weight of the bus whether close to the bottom of the bag range or near the highest level of the bag range. The volume of air would be different, but does the pressure change much? The weight, or needed PSI to support the weight, shouldn't change. What really happens? I know that a car tire with 6 or 8PSI will hold the car up and more pressure will cause it to raise but the contact patch is also deceasing. Does the effective area of an airbag decrease as it fills? Perhaps someone with a manual level system could do a simple check? For instance, see how or if the pressure changes to raise your bus front end 1 inch from normal ride height. I suppose 3 axle buses might have a weight transfer issue, or at least more than a 2 axle rig. So, anyone else bored enough today to get out the measuring tape and give it a go? Don 4107 | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 1125 Registered: 11-2005 Posted From: 173.202.31.47  Rating: N/A |
Don, It is true about the pressure to support the bus. In fact, the bus is supported with no air pressure! However, (there is always a however) the height is the difference. The more air pressure the higher the bus goes. Support is no longer the issue, bus height is. Bags are flexible therefore they expand with higher pressure. The bus mfg figured how much pressure is required to give the height desired and the leveling valves provide that pressure. All this assumes the bus weight remains constant, change the weight and the pressure required changes. You must be really bored!! | ||
| R.C.Bishop (Chuckllb)
Registered Member Username: Chuckllb Post Number: 1164 Registered: 7-2006 Posted From: 75.242.244.32 Rating: N/A |
Great thread (food for the brain), Don. Sure beats Health Care, etc, etc, etc!!!!!!!!  RCB | ||
| marvin pack (Gomer)
Registered Member Username: Gomer Post Number: 835 Registered: 3-2007 Posted From: 71.53.153.91  Rating: N/A |
If me brain is still functioning,I believe it is 31/2 in the rear and 3 in front from the rubber stop to the frame. There is a thread in the archives posted ride height I think.duh gomer | ||
| George M. Todd (George_todd)
Registered Member Username: George_todd Post Number: 957 Registered: 8-2006 Posted From: 99.39.13.50 Rating: N/A |
I gotta side with Don, without going out to measure! If we made an air bag a straight-walled cylinder, the same PRESSURE would hold the body up an inch or six inches. The only difference would be more VOLUME of air in the cylinder to push the piston higher. If we get into rolling-lobe vs. single or double convoluted, its going to take a little more pressure to raise a bus higher, because the top and bottom sides of the bag shrink as it expands upward, just as Don mentions about tires above. There is NO "PRESSURE SETTING" for leveling valves/air bags listed in factory manuals for a reason! The pressure in the bags has to change by weight only. An empty bus, low on fuel, will have air bag pressures many pounds below a loaded bus with a full tank of fuel. Listen to a bus at a stop when several passengers get off, the leveling valves will release air, as the bus rose with less weight inside. A bus being loaded will be low until the leveling valves catch up, which takes a minute or so. G | ||
| Patrick levenson (Zubzub)
Registered Member Username: Zubzub Post Number: 188 Registered: 5-2007 Posted From: 70.52.29.59  Rating: |
So if my car tire has only 10 psi it will ride as high as 30 psi? | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 1020 Registered: 1-2003 Posted From: 67.142.130.35  Rating: N/A |
I don't know about yours, but my ride-height valves work on volume, not pressure. I have rolling-lobe bags, which do not behave exactly like "pistons" but it is close enough for most purposes. If the bus drops below the setting, the valve opens to let more air into the bag. Now, obviously, more air will only go into the bag if the pressure of the supply is greater than that in the bag, but this is virtually a given when the bus is running, with anywhere from 80-120 psi in the reservoirs. Once enough air has been let in to raise that corner, the valve closes. This is all volume, not pressure. The pressure in the bag is not appreciably different after the additional air has been added then before; if it were, indeed, a hard-sided piston, there would be no difference at all. Now if more weight is added to that corner of the bus, the corner will again drop. That alone will increase the pressure in the bag (or piston). Again the valve will open, to admit more volume of air to the bag. Likewise, should weight move off that corner, the air pressure will drop, so the volume must increase, raising that corner (the product of pressure times volume, PV, is constant at a fixed temperature; when pressure drops, volume must increase, and vice-versa). Now the valve will open, allowing a quantity of air out (without a change in pressure) to restore the proper height. So, technically, the height of the coach has nothing to do with air pressure. The weight of the coach, though, has everything to do with it. Now if you do not have a ride-height valve, or the valve is disabled (inlet and outlet closed off), all bets are off. If you had hard-side pistons in this situation, and you connected a constant-pressure air supply, for a given fixed weight, one and only one pressure would hold the coach in position. Any pressure higher than that would result in the piston extending to its maximum height, and lower would result in it bottoming out. In practice, the air supply for the bags is not "constant pressure, infinite volume." Instead it is really "constant volume" within a narrow range of specified pressure. When you use such a system to set coach height, what you are really doing is adjusting the total volume of the system and the amount of air in it. Your gauge may read a steady pressure when you make this adjustment. However, once you close off the valves, you will note the pressure goes up if you add weight, and down if you remove weight. With the valves closed, we are back to PV being constant; additional weight increases pressure, and the volume must decrease to compensate (the piston compresses). Put more simply, the pressure in any air bag is fixed for a given temperature and amount of weight on the bag, no matter the height. At that fixed pressure, the height is adjusted by adding or removing air quantity; the pressure does not change. Tires are a different matter entirely, because, as noted, the area of application is not fixed but changing (unlike a piston, which has a fixed area). Remember the "SI" in PSI is "per square inch" -- change the number of square inches and you change the pressure, or vice-versa. -Sean http://OurOdyssey.BlogSpot.com | ||
| Patrick levenson (Zubzub)
Registered Member Username: Zubzub Post Number: 189 Registered: 5-2007 Posted From: 70.52.29.59 Rating: N/A |
the pressure at a specific weight holds the bus 3" above the axle, more weight is added to the bus , the bus sags the leveler adds quantity of air to compensate, the bus is back to it's original height... has the volume of the bag increased? No (well really a tiny bit of stretch but no where near as much as the volume) Has the pressure increased? Yes? where did the additional air go? It is more compressed ie at a higher pressure. So you add quantity of air into a fixed chamber (it's fixed in that you are using a specific height for the air bag) and the pressure goes up. You have increased the volume of air in the chamber but it is at a higher pressure and so takes up the same amount of space. They are a pair of conjugate variables , you can't really separate them and say it's one or the other. (Message edited by zubzub on March 13, 2010) | ||
| Sean Welsh (Sean)
Registered Member Username: Sean Post Number: 1021 Registered: 1-2003 Posted From: 67.142.130.35 Rating: N/A |
Patrick, To clarify for those following along: don't confuse "volume" with "quantity." The complete formula is PV=nRT, where "n" is the "quantity" or amount of gas (in moles, an obscure measurement of the number of molecules used only by engineers and scientists). R is a constant, and T is temperature, which can be considered constant for the purposes of this discussion. So you are adding a "quantity" of air into a chamber; if the volume of that chamber is fixed, then, yes, the pressure will increase. However, if the chamber can expand, such as a piston or bellows, then the pressure will remain the same. The pressure of gas in a piston is easily calculated. It is the force applied to the piston, which, in the case of an air spring is the weight of the coach on that air spring, divided by the area of the piston. If your bellows has an 8" diameter round plate at both ends, then the area of the piston is pi*4^2 or just over 50 square inches. If you have 5,000 pounds on that corner, the pressure in the bellows will be 5000/50 or 100psi. It doesn't matter how high the bus is sitting, that will be the pressure. The quantity of air in the bellows will determine the height. Now, in reality, a bellows, or a rolling lobe bag, is not really a piston. Everything I just said and said earlier applies, strictly speaking, only to real pistons. Rubber air bags have a balloon aspect to them, wherein the less pressure is in the bag, the more flex in the bag's sides, so in fact, there is a non-linear pressure/volume relationship that has to do with the elasticity of the bag. But, overall, an air bellows functions much more like a piston than a balloon. To clarify further, in a piston the pressure is related only to the force applied to the piston, which is unidirectional. In the case of a suspension component, this is the weight being borne. In a balloon, the pressure comes from the elasticity of the material -- as gas is introduced, the rubber expands, but resists this expansion much like a spring. Rubber air bags have both components; the piston force, related only to weight, and the elasticity of the rubber as it tries to expand. This elasticity provides a certain amount of damping to the suspension; an air spring made from a hard-sided piston would tend to oscillate longer when disturbed. -Sean http://OurOdyssey.BlogSpot.com | ||
| Patrick levenson (Zubzub)
Registered Member Username: Zubzub Post Number: 191 Registered: 5-2007 Posted From: 70.52.29.59 Rating: N/A |
I knew I had a term wrong before. I changed it. | ||
| Don Evans (Doninwa)
Registered Member Username: Doninwa Post Number: 261 Registered: 1-2007 Posted From: 208.81.157.234 Rating: N/A |
It would be interesting to install a pressure gauge on the stock front axle air bags and watch the pressure to see what it does from full fuel and water to low fuel and water with the leveling valve working and adjusted for proper ride height. Does Daytona Pete's system do that? If I install manual leveling I have always planned to keep the automatic system too. Either Pete's system or Sean's. Then there is FF's proposed system of manual cable controlled actuators. I see advantages to all the systems. Pete's is complete with no design work and if air tight does not require more air once set in a parked situation. Sean's is continually adjusting for shifting weight and has less potential new leak points but is harder to procure the parts and to design and requires an air source. It also needs to have the whole auxiliary system tight to keep the compressor from cycling. I have a small compressor planned in anyway for morning air up/emergencies so that is mute. Perhaps a hybrid system with Sean's actuators and inline valves to isolate the leveling system and provide a source from the auxiliary compressor. Then there is FF's system with Telefex cables and a mark for normal ride height. Could not be much simpler but requires a 'tight' system and/or an air supply and isolating valves. Still bored and it snowed last night.  Don 4107 | ||
| ED Hackenbruch (Shadowman)
Registered Member Username: Shadowman Post Number: 264 Registered: 11-2003 Posted From: 75.244.193.222 Rating: N/A |
We had 82 in the foothills today. :>) | ||
| FAST FRED (Fast_fred)
Registered Member Username: Fast_fred Post Number: 1109 Registered: 10-2006 Posted From: 66.82.162.14  Rating: N/A |
Could not be much simpler but requires a 'tight' system and/or an air supply and isolating valves. On most coaches a tight system is done by simply replacing the leveling valves which are cheap , and seem to be the cause of most hassles. FF | ||
| gary throneberry (Garhawk)
Registered Member Username: Garhawk Post Number: 34 Registered: 3-2007 Posted From: 24.158.148.205 Rating: N/A |
Hi Don, Here is what I know by looking directly at four air pressure gauges installed on my bus by Pete Papas. #1 gauge = two front air bags #2 gauge = left rear air bag #3 gauge = right rear air bag #4 gauge = suspension tank pressure This scene is carried out with the RTS's automatic leveling system disabled, the tank pressure at 120 psi and, all three bag gauges registering zero. When beginning to raise a portion of the bus depicted by gauge #1, 2, or 3, the gauge will show an increase in pressure: i.e., a two inch rise indicates more pressure than a one inch uplift. (it appears that approximately 65 psi carries the air bag to its' maximum height) Correspondingly, if air is released from the selected area of the bus, the related gauge will indicate less pressure. What this means and how it relates to the science of compression, I don't know. However, it sure makes it simple for a feeble old mind to level a bus! | ||
| Gus Causbie (Gusc)
Registered Member Username: Gusc Post Number: 1128 Registered: 11-2005 Posted From: 173.202.46.29 Rating: N/A |
Good show, Gary, well said!! | ||
| Tom Christman (Tchristman)
Registered Member Username: Tchristman Post Number: 193 Registered: 1-2006 Posted From: 66.218.33.156 Rating: N/A |
Funny thing about air suspension- the higher you set the bags, the softer the ride since you get more of a pillow effect. On the other side, the lower the bags the less air in the bags, the more reactive the bags are going to be making for a more bouncy ride. If you don't know your ride height, simply deflate the bags all the way down. Measure anywhere on the body-then pump it up all the way till it stops and measure again. Set the height at 5/8 up and that will be just about right. For example-if you measure out 6" of travel, set the ride height 3.75" up from the bottom. Good Luck, TomC |
