Let's talk air bags...
My 1999 7.3 dually has airbags installed by the previous owner. I've generally not bothered to inflate them unless towing a trailer with my Bigfoot camper (4000lbs loaded). Today while driving on Cali...
Forum Discussion
ajriding wrote:
Guys that say bag can only stiffen the ride are dead wrong. There is much more to suspension dynamics than just a knee-jerk reaction can offer.
The way they are usually installed in a pickup truck, they stiffen the ride. There is no mystery to how air suspension works, they are installed on 80% + of the over-the-road heavy trucks and well understood. Air helper springs as installed on pickups are not the same thing: they are convoluted bag instead of a rolling lobe bag, and the installed height of the bag is very low. To support a given load, the ONLY dimension that can change spring rate is installed height of the bag. At a given installed height, the spring rate will depend on the load it is supporting. In real air suspension, the installed height is typically about twice what it is on an aftermarket air helper spring. Since the travel of the convoluted bag is usually less than the truck leaf springs you more less have to inflate them to retain a ride height near the unloaded height. This will almost always increase spring rate.
Typical numbers for a pickup: rear spring rate 1000 lbs/in, doesn't change much until the overloads hit their perches. Load a 4000 lb camper and you are down 4" in the back. Typical helper air bag has 25 sq in active area each side, so to remove the 4" sag takes 80 psi. A pair of 5" high bags has a spring rate of 1000 lbs/in, added to the leaf springs (which are still there and active) gets you 2000 lbs/in total, twice the stock springs. Only if the bags were installed so that the leaf springs were inactive (very unusual!) would the spring rate be lowered.
If the air bags are used to keep the overloads from engaging it is more complicated, they may be softer than the overloads or not.
Air springs are also progressive, and short ones like those in pickups are highly progressive. The leaf spring is close to linear. So the above air bags have a spring rate of 1000 lbs/in over the first inch of compression, but 1670 lbs/in over the second inch of compression. The pressure goes from 80 psi at 5", to 96 psi at 4", to 133 psi at 3" height. This may be why they feel "bouncy" to some. The stock leaf springs would be nearly the same 1000 lbs/in over the same range, vs. 2670 lbs /in with the air bags at 80 psi nominal.
