Dual air bags

boogie_4wheel wrote:
The larger volume will contribute to this.

The air compress in linear way.
Double volume divided by double surface will produce the same characteristic.
What would change characteristic would be adding additional reservoir to existing bags.
Mercedes does it on its Airmatic systems.
I would think about easy project on truck >>>> get couple of empty cylinders after refrigerant, or helium used for parties, make T-connector to air bag and here you go.
Connector should be max dia possible to lower chocking effect.

I can do fab work, I'm a hot rod guy who has built cars & trucks from the ground up, and I'm a truck & heavy equipment guy for a living. I'm not afraid of steel and a welder. But this really doesn't even appear to need any custom work at all. The bags I have clamp on top of the leafs and the top brackets bolt to the outside of the frame rails. There's plenty of room to use two per side.
I just like the idea of spreading the stress out over more area and more hardware.
I almost can't believe nobody's already done it.

Rate verses force explains it - Thanks.

Bedlam wrote:
Please explain this. I thought that volume of the bag had more to do with ride quality and weight distribution than the shape or height. I can understand having less suspension travel with a shorter bag, but that is different parameter than what we are discussing.

I said it wasn't inuitive :).

First let's define and assume: Spring Rate is the rate of change of spring force, not spring force itself. Ride quality is primarily controlled by suspension frequency, which in turn it proportional to spring rate.

To support a given load you need a matching spring force. Spring Rate though, can be nearly anything at a given spring force, depending on the design of the spring. In an air bag, the force is simply pressure x effective area. To get the required spring force, you can have high pressure and a small area, or low pressure and a large area. Intuitively, it seems like the low pressure version would have a softer ride, i.e. a lower Spring Rate. This is not the case however.

The Spring Rate of an airbag is due only to the incremental increase in pressure when the volume is reduced by compression of the bag. The gas law says pressure x volume = constant. If you compress the airbag to half of its height, the volume will be cut in half and the pressure will double. The Spring Rate then is equal to the original load divided by 1/2 the ride height (the increase in force divided by the distance moved). This is true regardless of the size of the bag or the pressure it is running at initially. To support the same load, a bag of twice the diameter will run at 1/4 the pressure (4 times the area) - but the Spring Rate will be the same supporting the same load. Make the bag twice as tall, you will need to compress the bag twice the distance to get to half volume (and twice the force), so the spring rate is 1/2.

Another way of thinking about it is that bag effective area appears in the numerator of the force equation, but in the rate equation it appears in both the numerator and denominator and so it disappears. Yet another is that by increasing area, the proportional change in volume (and pressure) with a certain amount of travel does not change, so the rate is the same. If you instead increase height, the proportional change in volume (pressure) is less for the same amount of travel, so the rate is less.

Rolling lobe bags (most air bag assists are convoluted bags) can be designed with odd shaped pistons to affect the spring rate curves a bit by changing the effective area with motion, but the same basic rules apply. If you somehow increase the volume of air being compressed while keeping the bag area fixed, then you reduce the spring rate. This is what 'ping tanks' do - add an additional volume of air connected to the bag by a hose, resulting in a change in pressure not directly proportional to a change in height, reducing Spring Rate.

Not sure I explained that well enough. Both Firestone and Goodyear have engineering manuals available online which explain all this stuff in some detail. It is all minutia I learned when attempting to design my own air suspension.

Please explain this. I thought that volume of the bag had more to do with ride quality and weight distribution than the shape or height. I can understand having less suspension travel with a shorter bag, but that is different parameter than what we are discussing.

OK, now it's clearer what you have and are trying to do. Yes, two bags per side would cut the load on each of them and their mounts by 1/2. It will distribute the load into the frame over a slightly wider area, depending on how you mount them.

It will not improve the ride any. Though it will seem unintuitive, at a given load the spring rate of air bags is proportional only to the installed height of the bag, and independent of bag size, number, and pressure.

Bedlam wrote:
Firestone sells two different bag mounting systems that may accommodate what you want to do:

1. The first type is bolted to outboard side of the frame rail and sits over the leaf spring's U-bolts.

2. The second type is bolted under the frame rail (replacing your bump stop) and sits on the bump plate inboard of the leaf springs.

I assume you would plumb the two left bags to each other and the two right bags to each other to equalize pressure. You could plumb each bag individually for redundancy in case of failure, but it would require more work to manually equalize pressure and not have one bag doing most of the work for the two.

This is what I was thinking. ^ It may all fit without any custom work.
My opinion though is that's a lot of air to be sitting on. My truck's still kind of a bouncy castle with 50-60psi, sitting on the overload leaf with a big wig sway bar.
Before adding a second set of bags right where the greatest bending moment is between the spring mounts, I'd likely opt for adding a leaf or 2 in the stack or in the overload spring. Or even some bolt on add a leafs.
You already have the Kelderman system in place to tame the buckboard empty ride.
Or, is the eye to eye distance the same for springs out of a 45/5500 Ram or F4/550? May be possible to get some heavy junk yard springs out of a cab n chassi truck?

Out of curiosity, and I know this will be open season for the weight police, how much weight are you carrying and what do you need to run the bags at currently?

Firestone sells two different bag mounting systems that may accommodate what you want to do:

1. The first type is bolted to outboard side of the frame rail and sits over the leaf spring's U-bolts.

2. The second type is bolted under the frame rail (replacing your bump stop) and sits on the bump plate inboard of the leaf springs.

I assume you would plumb the two left bags to each other and the two right bags to each other to equalize pressure. You could plumb each bag individually for redundancy in case of failure, but it would require more work to manually equalize pressure and not have one bag doing most of the work for the two.

The problem with adding the second set is, nobody's done it yet and there is nothing designed for this application.

You're completely on your own designing the mounting and ensuring that it isn't going to put undue strain anywhere and make things worse than if you left well enough alone. I mean, it's not rocket science and you'll probably come out okay, but there is always that chance you miss something important.

The 2nd set of bags are going to place the 'same' amount of force on the truck frame as the single-pair that you have now.
The bag brackets are designed for their maximum capacity, don't be afraid to pump them up.
You might get a slightly better ride by running 2 pair of bags. The larger volume will contribute to this.