Air Bag question

[Ziegelsteinfaust]

Something I never understood about air bags is how do you set the effective "spring" rate?

http://www.theslamstore.com/PhotoGal...&PhotoNumber=3

Lets say I use a RE7 bag that has a minimum height of 3", and a max height of 11 inches. So if I go for a 8" bag height I have a force of almost 2500 at 80 pounds. What does that mean?


On the web site they have a force chart, but how does that compare to the normal in/lb they normally sell springs in.

I so badly want an air ride suspension system one day although not likely anytime soon, but I am skeptical because I know others have done it that would meet my ideas. I just don't know where to start with a build set up.

[lolife99]

I would ask Rob from NoLimitEngineering about spring rates.
He's the guru,...

[andrewmp6]

With air bags the more air in it the stiffer the spring rate will get.Think of it as a spring that has a adjustable spring rate.

[earl84]

I must confess, I am a little in the dark on this as well. I understand the more air, the stiffer the spring, and that is great if you are hauling something. How do you set the bags up to be at the proper ride height is what I don't get.
For example, I want to lower the rear by 4 inches. So how do I install a bag that gives me 4 inches of lowering? There must be a designed drop distance in the bag itself. Cuz if I have to adjust height by adding more air, will the ride not be either harsh or the back end raised up to a height I don't want it to be at?

[Sammy]

One way is with presets memory on the newer controllers. Otherwise setup ur truck with what you want, then you know 30 psi front 40 rear is your ride height for example. The variation in air pressure will have very slight differences in your ride because you won't be driving at fully inflated or deflated bags. Best if you take a ride in one or at least drive one.

[broey]

Originally Posted by earl84

I must confess, I am a little in the dark on this as well. I understand the more air, the stiffer the spring, and that is great if you are hauling something. How do you set the bags up to be at the proper ride height is what I don't get.
For example, I want to lower the rear by 4 inches. So how do I install a bag that gives me 4 inches of lowering? There must be a designed drop distance in the bag itself. Cuz if I have to adjust height by adding more air, will the ride not be either harsh or the back end raised up to a height I don't want it to be at?

that is something that you will have to mess with once you get the bags installed to figure out. most people that install bags use them to lower the truck as much as humanly possible. not just 4 inches. so we usually start by laying the truck out as much as we can go. and then from there you just lift the truck to the desired hight or smoothness you like and go. my last truck that was bagged could drive 1/8th inch off the ground all the way to 6 inches off the ground before it got too stiff.

[ryanroo]

The spring rate depends on the amount of air in the bag at a given height. it also changes constantly as the bag is cycling. the height for a given pressure is dependent on the size of the bag as well. if you run a smaller 5" diameter, it will take more pressure to achieve a certain height than a bag with a 6" diameter bag and so on. so the smaller bag will have the higher rate. and it will also ramp up faster than the bigger bag so it will ride stiffer. the smaller bag will reach its capacity with less weight for that reason as well.

so, to set the rate you have to know how much weight is being suspended vs how big the bag is. i dont know how that translates to a normal inches per lb rate. also, as you let pressure out to lower the ride height the rate will decrease and visa versa as it increases. unless you are changing the weight being supported. when a controller has different height settings they are actually pressure settings that achieve the height you are looking for. so a lower height will have a lower rate. if you set a system up to maintain 1 height no matter the load, it will have a more constant rate since it is maintaining a static height for a given load.

this is all my understanding of how it works, but i may be wrong

[andrewmp6]

What year truck are you bagging and is the rear a link set up or is it the trailing arms the 60s trucks used?Do you have drop spindles or not and what size wheels you going to use?

[Ziegelsteinfaust]

Originally Posted by andrewmp6

What year truck are you bagging and is the rear a link set up or is it the trailing arms the 60s trucks used?Do you have drop spindles or not and what size wheels you going to use?

I would only bag a truck after I got all the other big items done first, and my truck needs a lot of work first since it is my dd.

None the less I have been wanting to bag one ever since I saw my first one driving around a few years ago, but for me to be happy it needs to handle well. I am not into cruising as of yet, and its not looking good for next year either.

While I flip threw these forums reading various build threads. I see guys bagging trucks, but I always wonder how they know how to set it up for the ride they desire.

[ulakovic22]

It's all personal preference and experience. The bag has a certain recommended height for the best ride so you can use that as a rule of thumb. Also many people post questions asking what everyone runs for pressures with what bags. Most everything I have read is about 45-50 psi in the front and about 30 in the rear. When you install them you can air them up to those pressures and see if you like the ride and the height.

If you want it to ride lower from there then you have to find ways to adjust the suspension without effecting your bags. For instance you can use lowering blocks on the rear to move the axle closer to the frame. You can cut the front cups to allow the bag to sit further in the pocket or cup your LCA. You can get drop spindles as well that will lower the vehicle without effecting the bags or shocks. There are a lot of things that you can do, but experience and personal preference dictate how you achieve that.
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[AndrewSz]

I comprehend the more air, the tougher the springtime, and that is excellent if you are transporting something. How do you set the purses up to be at the appropriate drive size is what I don't get . . .

[made2drag]

like broey said....after you decide how low you want to go with the truck, put the bags in and air it up enough to drive it....then add more or less air in by increments of 5 psi until you like the ride the best. when you find the best ride... look at the air gauge....air it up to that psi every time you drive it.

on my 86, i have an inch and a half of clearance in the front aired out....it takes 100 to lift to ride hieght....i usually ride at 110 and over bumps and speed bumps i lift to 120. i would say my front bags are 50% full at 100 and 60% full at 110 to 120. i could get massive lift with 150 psi but its as hard as a brick. the rear takes 30 to lift and i cruise at 30 or 40.

i have 2500's so i would expect if they were 2600's i could subract 20 or 30 psi to these numbers for the front.

put the biggest bag in you can (dont let it rub) and use 1/2 valves and adjust your speeds for up and down with reducers if you want it slower....i have 1/2 inch up and 1/4 down and it works great.

since you didnt say what truck you have start with a 3 inch spindle if its available and buy the bolt in plates and cups. you'll be fine.

[AndrewSz]

Originally Posted by AndrewSz

I comprehend the more air, the tougher the springtime, and that is excellent if you are transporting something. How do you set the purses up to be at the appropriate drive size is what I don't get . . .

Edited:
No spam allowed.

[wraprail]

Originally Posted by made2drag

like broey said....after you decide how low you want to go with the truck, put the bags in and air it up enough to drive it....then add more or less air in by increments of 5 psi until you like the ride the best. when you find the best ride... look at the air gauge....air it up to that psi every time you drive it.

on my 86, i have an inch and a half of clearance in the front aired out....it takes 100 to lift to ride hieght....i usually ride at 110 and over bumps and speed bumps i lift to 120. i would say my front bags are 50% full at 100 and 60% full at 110 to 120. i could get massive lift with 150 psi but its as hard as a brick. the rear takes 30 to lift and i cruise at 30 or 40.

i have 2500's so i would expect if they were 2600's i could subract 20 or 30 psi to these numbers for the front.

put the biggest bag in you can (dont let it rub) and use 1/2 valves and adjust your speeds for up and down with reducers if you want it slower....i have 1/2 inch up and 1/4 down and it works great.

since you didnt say what truck you have start with a 3 inch spindle if its available and buy the bolt in plates and cups. you'll be fine.

Dam 100 psi. to get to ride hight? How does it ride? Guess the 2600 bags were the right choice for me after all. On my 85 I have never aired any of my bags to 100psi. Normal cruise is around 50-60 psi and ride is great. 25-30psi and I'm on the bump stops

[DJDAudio]

One thing I do not believe has been mentioned is Valve location and line size.

The spring rate will be shifted stiffer if the valve is mounted closer to the air spring.
The spring rate will be shifted softer if the valve is mounted at the Air Tank.

Also if the valves are mounted at the tank, the line size will be a factor. For it effectively changes the 'volume' of the air spring.

In laymen terms it gives the air a larger volume to compress into.

The Force to pressure ratio is fixed, for example a 7" diameter bag has a surface area of 38.47 Square in. So if you need it to lift 1,000 lbs that would take 26 pounds of force per in. or 26PSI this is a fixed number based on air spring size.

One more example lets say same system but with a 5" diameter air spring. You now have only 19.62 Sq. In. of area now pushing the same 1,000lbs this will now take 50.26 pounds per inch. or 50.26PSI to do the same lift ( Almost double that of the 7"). But now at this higher pressure it will be a stiffer spring rate.

The key is to match the air spring size to the weight it is lifting to give the spring rate you are after.

Here is where it gets even more confusing. if you increase line size/length or even install small accumulation tanks, this will increase the volume of the system but not change the pressure requirement. This just means it will take the same pressure to lift but will allow a greater stroke on the spring rate curve. ( Softer ride )

I hope that made sense haha

[wraprail]

Originally Posted by DJDAudio

One thing I do not believe has been mentioned is Valve location and line size.

The spring rate will be shifted stiffer if the valve is mounted closer to the air spring.
The spring rate will be shifted softer if the valve is mounted at the Air Tank.

Also if the valves are mounted at the tank, the line size will be a factor. For it effectively changes the 'volume' of the air spring.

In laymen terms it gives the air a larger volume to compress into.

The Force to pressure ratio is fixed, for example a 7" diameter bag has a surface area of 38.47 Square in. So if you need it to lift 1,000 lbs that would take 26 pounds of force per in. or 26PSI this is a fixed number based on air spring size.

One more example lets say same system but with a 5" diameter air spring. You now have only 19.62 Sq. In. of area now pushing the same 1,000lbs this will now take 50.26 pounds per inch. or 50.26PSI to do the same lift ( Almost double that of the 7"). But now at this higher pressure it will be a stiffer spring rate.

The key is to match the air spring size to the weight it is lifting to give the spring rate you are after.

Here is where it gets even more confusing. if you increase line size/length or even install small accumulation tanks, this will increase the volume of the system but not change the pressure requirement. This just means it will take the same pressure to lift but will allow a greater stroke on the spring rate curve. ( Softer ride )

I hope that made sense haha

...Line size and valve location? Come on dude....

[DJDAudio]

Originally Posted by wraprail

...Line size and valve location? Come on dude....

To each his own

Only way I can think to explain it,

Lets say if you have a 7" diameter air spring (bag) at 5" height that has ~ 0.83 Gal of air inside at 100PSI.

With the valve mounted at the bag, you compress it from 5" to 1" the pressure inside will now go up. for you now have an internal volume of only 0.16 Gal. The pressure would not be at 518PSI. ( This is crude but what happens when you hit a large bump )

With the Valve mounted at the tank, lets say you have 1/2"ID line that is 10' in total length from bag to valve. The line will hold ~0.10 Gal of air.

So with the same above analogy from 100PSI at 5" compressed to 1" the volume change now goes from 0.93Gal to 0.26. This would make the pressure only go up to 357.69PSI.

that is a difference of 160PSI. So latter option would have a softer spring rate.

Again I exaggerated slightly on the compression range, but I wanted to show that it does make a difference, if only slightly. Now the longer the line and the larger the line the more volume the air can compress into.

This is why some early OEM air strut application used accumulators to soften the spring rate, for they did not have the room to fit a larger spring.


So you do not think of this as the ramblings of a mad man here is an example,



I pulled that from the suspension design book of the Audi A8

http://www.volkspage.net/technik/ssp/ssp/SSP_292.pdf

[thekid54]

DJDAudio, while your theory and math are correct, no one is going to feel the difference between a valve at the tank or a valve closer to the bag. Your example uses a 1/2" ID line. I don't know too many people running that big of an air line. Most folks opt for 3/8" OD, which has an approximate ID of 1/4". That would bring your air line volume down to about 0.03 gallons, or 5.88 cubic inches. That's pretty negligible considering a 6" bag at a height of 7" has a volume of about 200 cubic inches.

[DJDAudio]

Originally Posted by thekid54

DJDAudio, while your theory and math are correct, no one is going to feel the difference between a valve at the tank or a valve closer to the bag. Your example uses a 1/2" ID line. I don't know too many people running that big of an air line. Most folks opt for 3/8" OD, which has an approximate ID of 1/4". That would bring your air line volume down to about 0.03 gallons, or 5.88 cubic inches. That's pretty negligible considering a 6" bag at a height of 7" has a volume of about 200 cubic inches.

I agree

I forget where I am some days
I come from the world of dare I say it 'minitrucks' They love to use crazy line , bag, and valve sizes some as large as 5/8" line.

With the smaller line, I think you are correct and the 'seat of your pants' feeling would be negligible.

My Sofa ( yes Sofa ) has 1/4" line with 3/8" valves, I did not worry about valve location, but then again, I do not drive it on the road,