I promised an update once the root of the problem had been discovered. The answer lay with both the ride height setting and the shock mounting location of the early 4 bag system.
First for those not familiar with how an air suspension works a quick review. The air bags are the springs. Their "rate", the stiffness of the spring, is controlled by how much air pressure is maintained in the bag. The more pressure, the higher the "spring" rate.
Ride height is also controlled by how much pressure is held in the bag, with more pressure the more the bag extends and raises the ride height. So ride height and the "spring" rate of the bag is directly related to the amount of pressure in the bag. And the amount of weight of the coach being supported by the suspension will determine how much pressure is needed to achieve the desired ride height, more weight more pressure.
However the "compression" rate for air in the bag is not "linear" but progressive. The amount of force to compress the air in the bag goes up in an exponential manner, meaning that each additional inch of compression will require a higher force than the previous until the air in effect becomes solid. A crude example would be while 100 lbs of force might compress the bag 1 inch, 200 lbs of force would only compress the bag 1 3/4 inch, not 2 inches. And each additional 100 lbs compressing smaller and smaller movements.
The Damage :
The amount of force that I was encountering actually tore the vertical part of the "I" beam to which the front axle bolts. The crack was at the location where the "I" beam is welded to the "U" channel that runs from side to side with the air bags at each end. In short the bags were in essence going "solid", as if there was no suspension, allowing the upward force on the axle to try and rip the "I" beam off the "U" channel.
Bag Air Pressure Issue:
The issue to be determined was if the bags were completely compressed to the point the "inner movement stop" was hitting the upper bag mounting plate OR if the "spring" rate of the bag had gone up so high that it was in effect "solid", not able to compress the air any further.
In this case when I first got the coach it needed to have the ride height and level checked and correctly set. I search the web and this forum for any figures and the best I could find was setting the bags at 10 3/4 to 11 inches at ride height which was done prior to actually using the coach.
After the recent trip and the discovery of the problem I took the coach to Leale's RV in San Jose, CA at the the advice of the folks at Henderson's, from whom I had purchased the Koni shocks recommended for the 1996 4 bag suspension as found on the Beaver and Safari coaches of the mid 90's.
Only after several conversations with the folks at Leale's and the engineer at Henderson's did we discover that in fact the bag height was too high. Fortunately there was a long term employee at Henderson's who remembered the correct height was 10 inches. Once the bag heights were set correctly the ride improved dramatically. The correct bag height was especially important in light of the fact that my coach runs well under the rated weight capacity at both ends thereby needing less pressure to achieve the desired ride height and ride rate. In essence the air in the bags were going "solid" and transferring the forces directly to the suspension structure instead of "absorbing" the movement of the axle. The "springs" were way too stiff.
As to Shocks
Unfortunately the original design of the bag system was deficient in using just a pair of shocks mounted to the axle rather than as on the later systems putting a shock at each air bag location. The amount of the "unsprung" weight of the axle, "I" beams and "U" channels, along with the leverage due to their location is too great for a pair of shocks with the size pistons used.
One of the advantages to being in the racing business is our access to shock engineers. I had one of the engineers from the Heavy Duty Division at Koni check out the suspension. He confirmed there was inadequate shock for the amount of forces needed to be controlled, the size of the internal pistons in the shocks being too small. In the absence of the ability to place a shock at each bag location a much more "robust" size shock is needed to correctly control the forces. In his estimation the common complaint of excessive "porpoising" in class A coaches has a lot to do with with chronically under shocked designs.
We are in the process of doing a rough calculation of the "unsprung" weight of the suspension and then will configure a shock similar to those used on the big off road race support trucks used in the long distance desert races. These will be a re-buildable and adjustable twin tube design, a one time purchase good for life .
I'll follow up once we have completed this upgrade.
