It’s a well-known fact that cars with unibody construction—aka unit-body, subframe, or stub frame design—are designed to satisfy an automaker’s need for both strength and low manufacturing cost. Quite literally, the body panels are what provide the chassis with stiffness. Yet in spite of some very clever engineering, both of these goals are ultimately in contradiction with each another. As you lower the cost of production, the stiffness—and thus the handling and ride quality—goes down. In the 1960s, Chrysler was arguably the most successful of the Big Three at walking the cost/performance tightrope, but as the decades rolled on, the compromise of a unibody chassis comes increasingly to light. As we'll see, chassis subframe connectors can cure this problem.
The compromise of using only the body panels for chassis stiffness manifests itself as shaking, twisting, and flexing. If unchecked, this shows up as cracks in paint, wrinkles in sheetmetal, misaligned body panels, and cracks in glass. The cause is that the front suspension cradle is not linked solidly to the rear suspension structure—except through the thin body panels of the floor and greenhouse.
...over time, the weld points and stressed areas of the body shell begin to yield. Even the best-kept cars fall victim.
In the ’60s and ’70s, the Mopar A-, B-, and E-body chassis offered some of the best performance available at that price, but over time, the weld points and stressed areas of the body shell begin to yield. Even the best-kept cars fall victim. Since rebuilding the Mopar’s unit body from scratch is impractical, the best bet it to install a set of weld-in subframe connectors from US Car Tool, sold through Reilly Motorsports (RMS) for $199 a pair.
Trial fitting the subframe connectors is an iterative process, and here Ron is grinding aw
When the front suspension moves one way and the rear moves another, the old body panels pay the price. But that’s not all. The suspension is unable to do what it’s designed for and the car can handle in unpredictable, non-linear, and unsafe ways. And we haven’t even begun to talk about adding more power, bigger brakes, better suspension, or stickier tires.
These subframe connectors are beefy form-fitting c-channels that connect the front and rear frames of the uni-body Mopar, and tie to the body at the same time. Moreover, these are also available for Mopar B-body, and E-body. Note that while we are installing them on an A-body, they are even more effective on a Mopar B-body, since these cars are longer and heavier. The installation of the B-body version of these subframe connectors is nearly identical, so fans of intermediate Mopars take note.
Since our 1968 Plymouth Valiant (a ’64-76 Chrysler A-body) is already equipped with the RMS AlterKtion front coilover suspension and the RMS Street Lynx rear four-link coilover conversion, we would be committing structural suicide by not installing a set of subframe connectors. It was a no-brainer to order the subframe connectors, and it only took a half day to install them using our Millermatic 211 MIG machine.
Ron Aschtgen of Outlaw Motorsports (Riverside, CA) is helping us assemble Project Valiant, and he handled the subframe connector installation for us. As Ron discovered, the US Cartool units fit our A-body very closely, their contoured sides needing very little trimming and grinding to create a gap small enough to make effective MIG welds with the Millermatic 211.
Some minor fine-tuning with the fit is to be expected on something like this, and along those lines, we also had to relocate the service brake, parking brake, and fuel conduits. Nevertheless, the end product of vastly improved chassis stiffness was worth that small amount of trouble.
Now we’ve got the best of both worlds, with plenty of stiffness to handle the torque...
As a result of this half-day’s work, our Valiant is for all intents a full-frame chassis—in fact it’s even better than that. We have given up absolutely none of the existing strength in the Mopar’s gusseted and boxed unibody—a strong point that traditional full body-on-frame cars—such as GM’s A-, B-, and G-bodies—never had.
Now we’ve got the best of both worlds, with plenty of stiffness to handle the torque from our 657hp Indy low-deck wedge, as well as the higher cornering and braking loads from our RMS suspension and Wilwood disc brakes. This ol’ Mopar chassis now has another half century left in her!