We have been working on new suspension products for pick-ups for quite some time now, and last week we had the opportunity to work on a Dodge heavy-duty truck that would allow us to use much of the knowledge we have gained working on motorhome chassis. This truck hauls a camper, and although it was already equipped with airbags, the handling was still far from ideal.
After conducting a Road Performance Assessment, (RPA), we learned that this truck was suffering from what we call “tail wagging the dog”. Common in motorhomes with large rear overhangs, the problem is characterized by a “wagging” sensation that comes from the rear after steering input. The overhang creates leverage on the rear axle that causes it to move from side to side whenever the steering wheel is moved back and forth. This truck was fitted with a short box camper that also had significant rear overhang, and it was causing the same type of instability.
This was the perfect opportunity to test our new SS725 Trac Bar for 2500-3500 Dodge trucks. Like the Trac Bars that we use for motorhome applications, the SS725 bolts on to the frame on one side, and clamps to the axle on the other, which positively locates the axle and prevents lateral movement. Like our other Trac Bars, the SS725 bolts on with no drilling or welding required, and although this bar was one of our first working prototypes, it installed in under an hour. Here is a photo with the Trac Bar in place:
During the course of our install, we noticed was that the air bags did not look like they were installed properly. The customer reported that he had addressed this with the installer and manufacturer, but they said “that’s the way they’re supposed to look”. The bags were lopsided, and appeared to be pinching on one side. Further, the bottom plate was bending because half of the bag was supported only by the plate (see photo below). As a point of reference, air bags should always be mounted straight up and down so they can manage the load more evenly and do not get pinched or stretched on one side more than the other. Upon closer inspection, we could see that the plate could be relocated and the air bag problem corrected with a few adjustments.
With the bar installed and the air bags corrected, we took the truck for another drive. There was now a noticeable difference in the steering response, and the the back of the truck no longer continued to move after rapid steering inputs.
The steering was now much more predictable, but there was still a lot of bounce generated by the air bags when driving into our out of a driveway, especially at an angle. The truck would bounce from side-to-side several times before coming to rest.
We have a product called a Motion Control Unit (MCU) that we use on air bag suspension motorhomes, and it calms this sort of motion down significantly, so we tried it on the truck as well. MCU’s are available in varying sizes, so we fitted these comparatively small air bags with our #3 MCU’s, which are about the diameter of a cigarette and are about three inches long. We simply cut the air line and spliced the MCU’s in place. Here’s a photo (the MCU is the light blue part right above the air bag):
The rocking motion was calmed considerably, and combined with the Trac Bar and air bag relocation, made the truck handle much better with the camper in place. A rear sway bar and a set of Bilstein shocks would make it even better, but that’s a job for another day.