Here is my theory. In a stock truck, the weight in the rear (the truck itself + any load) applies downward force. The leaf springs resist this force. They compress under that force and apply it to the axles. The axles apply the force to the wheels and the wheels apply it to the tires and the tires to the ground. At that point, every component of the suspension is under load. The only component that gives in to all this force is the leaf springs (and the tires a little). The compression of the leaf springs makes the rear of the truck sag and we don't like that so we install airbags. The airbags take some of the weight of the truck + load off the leaf springs so they don't compress as much. The load on the airbags is applied directly to the axle, bypassing the leaf springs. We decide how much of the load they take by applying more or less air pressure to the bags. The force applied to the axle/wheels/tires is exactly the same.

In many trucks, the weakest link of the rear suspension is the leaf springs. Hypothetical and theoretically speaking in regards to a fictitious situation my friend asked me about, installing airbags would (in a mechanical sense not a sticker sense) effectively increase the payload capacity of the rear suspension by transferring force from a weaker component and applying it to one that is stouter.

Is this correct? What I mean, is the concept correct? I'm sure I chose some incorrect terms. If I am not understanding the concept and you want to explain it to me, please use small words. Thanks.

 

Usually, the lowest denominator of the Load capacity is the tires/wheels. Most of our trucks run "E" rated 10 ply tires rated at 3750 lbs ( 275/65r20) and wheels that rated in the 3500-3600lbs range.
The F350's usually have 7,000 lbs rear axles which pairs well with the 3500 lbs wheel rating ( 2 wheels at 3500 = 1 axle at 7,000)

So adding the airbags, Levels the truck, But doesn't really change the load rating of the axle, wheels or tires.

 

Correct - to a point.
The GVWR is determined by the lowest rated component of the load carrying system. The assumption that the springs are the weak link in the system is just that, an assumption. The springs could very well be rated for higher loads than the axle in which case air bags (in your example) would be shifting weight from a higher capacity component to a lower capacity component.

Ultimately, the manufacturer specifies that nothing you can do will increase the GVWR from what is specified by the factory.

 

I understand that airbags don't change the rating of anything. We all get that there is no way to increase the load rating on any component. I also understand that they are designed to level a truck. I am trying to get down to the next level. My assumption is that the lessen the load on the leaf springs. That is made apparent by the fact that the leaf springs don't compress as much. Less force applied to the leaf springs = less compression.

Using my truck as an example, the lowest rated component is the leaf springs.

 

There is no doubt that they will share the load on your springs. Thus reducing what the springs carry.

 

No, not really. You're right you've transferred some load from the springs via the airbags, but that really has nothing to do with 'Payload'. 'Payload' in and of itself is an artificial limitation. It's an estimator based on the sum of all the components. What you need to do, is make sure you don't exceed the individual ratings of the individual components. Wheels, tires, axles, suspension, frame, etc all have their stress points. What I find interesting is the theory out there that 1 lb under a rating is way safer than 1 lb over a rating. Either way, as you apply load to a component, it's under stress. The more load, the more stress. The more stress applied, the quicker it will fail. Period.

 

I was afraid this would happen. The GVWR has nothing to do with any component of your truck, it only has to do with how the manufacturer wants a particular truck classified. That is not to say that there aren't potentially differences between classes. Unless you special order a truck and choose to have it derated, an F250 will have a GVWR of 10,000 lbs. and a SRW F350 has a GVWR of 11,500 lbs. Within class, those GVWRs have absolutely nothing to do with how the truck is equipped. It doesn't matter if an F250 is an XL, XLT, or Platinum, has a Sterling axle or a Dana axle, 17" wheels or 20" wheels, all-season or all-terrain tires...none of it matters. The GVWR will be 10,000 pounds. The same is true for 350s and 450s. The GVWR is an assigned number and how you choose to equip your truck has no impact on it.

Now back to how airbags work.

 

If I was going to tow close to my weight limit I probably would be installing airbags. Not to go over the limit but to help ease the burden on the leaf springs which spread the load from two points of contact with the frame to three points of contact (on each side).

 

What about airbags vs adding a leaf? Literally, the only difference between a 350 and my high capacity 250 is an additional leaf in the main pack. I'm guessing that adding a leaf could actually result in a significantly higher weight carrying capability. Once again, I am inquiring about actual mechanical capability, not GVWR or payload capacity which I understand are impossible to increase. Any real advantage going the additional leaf route? I'm sure it would stiffen up the ride some but I don't know how noticeable it would be.

 

The way I see airbags they are simply a leveling tool. They just help you keep the rig more level when you are "using the payload/weight" within the limits the manufacturer has rated the truck.

 

This. Otherwise you should've bought a "bigger" truck.

 

airbags are an additional vertical adjustable spring. I used them on my 2012 dually to level and try to prevent engagement of the overload spring, which caused a
very harsh ride. thus they moved some spring from the leafs to the bag. they did not add any capability to the truck, only moved it around.

 

Some of you are really overthinking this.

Airbags do what they were designed for. The help the suspension handle the weight, and in most cases make the truck handle better while weighted down.

It doesn't increase payload capacity from a legal standpoint, but it certainly gives you the ability to add more weight than you could on a factory setup alone.

 

Correct. Contrary to the desired outcome of less sag, airbags actually decrease your truck's payload capacity by the amount equal to the weight of the air bag system.

 

Eating a big lunch and not taking a ***** would also do the same thing. Guess we are spitting hairs at this point over what 50lbs? You are 100% correct tho.
Bags just add a tune able spring rate, and keeps the truck off the overloads of what ever it has.