A common thread on the RV class A form deals with handling, sway bars, steering stabilizers and other add on devices to keep the RV going straight down the road. Some chassis/body combinations appear to be more suseptible to bad handling than others. It appears that the length of the overhang, the distance from the rear wheels to the back bumper, as compaired to the overall length of the rig, has a lot to do with stability. Probably the worst, hands down, handling RV that I have ever owned was a 33 foot class A on a Chevrolet P30 chassis. It’s overhang was better than 10 feet, or over one third of the total length of the rig.
The best is the 38 foot DP that we currently have. Be advised here that the DP is not new as it is a 1990 model year, and the Oskosh chassis under it is listed as an 89. Now the P30 chassis had all the add on steering stuff that I could find to help handling while the DP is the way that it was delivered from the factory. The difference? The ratio between the total length of the rig and the over hang, that is how much of the coach hung out beyond the rear wheels.
However overhang is only a part of the reason why the tail wags on some rigs and not on others that have the same overhang. Overhang can probably be the root of the problem of instability because weight hanging out behind the rear wheels will react just like the laws of inertia says they will. The P-30 had the water tank under the left side of the rig right against the back wall. On the right side in the very rear compartment was the generator. Right about a thousand pounds of weight acting like a pedulum as the rear of the vehicle began to wag.
The rear wheels then act as the pivot or the fulcrum of the seesaw transfering motion to the rest of the rig to swing the other way. At this point the tires come into play. First the stiffness of the side wall and then the grip of the tire tread on the pavement. And believe it or not the rear leaf springs, as heavy as they look, will twist sideways and allow for more wag in the rear. Now with the front end moving over in the opposite direction the driver adds input trying to correct the path of the rig down the road. The longer the overhang the more the reaction of the pendulum effect. The more weight on the end of the overhang, the more the pendulum effect, simple junior high school physics.
Now add wind, passing 18 wheelers, road surface, low tire pressure, and something hanging off the trailer hitch and things can get down right nasty. Stay tuned as next week we look at some of the add ons that are supposed to make it all better.
Brad