SS701 Rear Trac Bar
Welcome to the third part of our Sprinter 3500 Class C upgrade series! We started out looking at the rear shocks in Part 1, followed by the rear anti-sway bar and how it works with the rear shocks in Part 2. Now, we'll get into the SS701 rear trac bar upgrade for the Sprinter chassis and the difference it made.
You may be wondering, what is a trac bar and how is it different from the anti-sway bar? If you'd like a detailed explanation, we put together a video on this a few months back. To summarize, a trac bar controls side-to-side motion in a solid axle, while a sway bar controls the tendency of a coach to lean.
Any solid axle suspension needs some means of controlling sideways movement in the axle. On a coil spring or air spring suspension, a trac bar (also known as a Panhard rod) is required in order to control that movement. On a leaf spring suspension however, the leaf springs themselves are generally given the task of controlling the sideways movement without help from a trac bar.
The problem with this approach is that the springs have some flex in them, as do the spring bushings and shackles used to connect the springs to the chassis. On a larger vehicle with plenty of weight and overhang behind the rear axle, the rear axle is able to move sideways enough to cause the driver to feel like the back end of the coach is steering the front. We call this tendency "tail-wagging-the-dog", or more simply, tailwag. The side-play in the rear axle can also be felt when passed by semi trucks or hit by gusts of wind, as that pressure on the side of the coach can shove the body sideways relative to the axle. This causes the driver to have to fight harder to keep the coach in the lane.
To test out the effect a rear trac bar would have on our 2021 Sprinter, we wanted to get actual measurements of side movement in the rear axle. We put a tape measure on one othe rear bump stops and attached a pointer to the axle. By simply pushing on the back of the coach at the right frequency, we were able to get over 1/8" of relative side movement between the chassis and the rear axle. After this test, we then installed the SS701 on our coach (here is the install video). The installation is pretty simple as long as the coach's engine exhaust exits ahead of the rear axle and there is adequate clearance between the rear axle and components such as propane tanks or leveling jacks. Sometimes the tailpipe needs to be re-routed to exit in front of the rear axle - fortunately on our Navion, we didn't run into any of these obstacles.
After installing the SS701, we repeated our test. The results of the process can be seen here - we measured about 1/8" before installing the SS701, while afterward we measured about 1/32". While a 3/32" reduction may not sound like much, that's about a 75% drop in side movement compared to stock. When you consider how little of a change in steering it takes to send you into the other lane, that 3/32" reduction could be enough to change passing trucks and cross winds from feeling like a huge push to a slight nudge.
Out on the road with the SS701 installed, we noticed less push when getting passed by semis or when hit by a strong gust of wind. When coupled with other upgrades to improve stability (such as the SS88-1889 Rear Shocks discussed in Part 1 of this series or the SS110 Rear Sway Bar discussed in Part 2), the SS701 made our Sprinter feel much less like a big sail going down the road.
We hope this post helped explain what a rear trac bar does and how it can help the Sprinter 3500 Class C motorhome chassis! Next week, we'll discuss our evaluation of new front struts on our Sprinter.
Sprinter RV Chassis - Better Steering and Handling (Filmed back in 2022, earlier in our testing process)