From a hot rodder’s perspective, it’s never an easy decision to completely replace a vintage car’s front suspension with something newer that is of a completely different design. The torsion-bar suspension found on vintage Chrysler cars from the ’60s and ’70s is perhaps the poster child for this, as arguably no manufacturer of the era had a more capable independent front suspension (IFS) design. (Learn the history of the Mopar A-Body here.) The fact that it was so good, ironically, is the problem, and needs explanation before we get into why we’re replacing it in this 1974 Plymouth Duster, a project vehicle owned by HOT ROD tech center mechanic Chris Arriero.
Mopars Are Different, Here’s Why
As an economic recession hit the U.S. in the early 1960s, many of Detroit’s automakers redesigned their car platforms and powertrains for less mass and greater fuel efficiency. Then, just as today, that meant finding new and creative ways to package the mechanical systems. Chrysler’s move to a torsion-bar suspension wasn’t new, but integrating it so thoroughly into the unibody’s new construction was.
The resulting combination of chassis rigidity, superior suspension geometry (by the day’s standards), and mass reduction made Chrysler’s across-the-lineup adoption of the torsion-bar suspension a success story. When combined with an almost limitless number of powertrain choices, Chrysler Corp cars were at the head of the class when it came to chassis and powertrain engineering.
The Issue: An Intrusive Crossmember
Most critical to the success of the Chrysler chassis design was the torsion bar’s supporting crossmember. Unseen and largely forgotten, few parts of any car up to this point had taken on so many important roles. In the absence of a full frame, the torsion-bar crossmember provided the bulwark for a significant amount of the chassis’ stiffness while also supporting nearly all the load of the front half of the car. Its proximity to the boxed c-channels, the cockpit, and cowl area meant very little chassis flex, a superior ride, and better-than-average handling for its day. Comparable coil-spring suspensions from competitors were not only heavier but also moved the mass out nearer to the corners, costing them a measure of handling capability.
Reasons Your Mopar Needs a Coilover Suspension
It seems there’s a lot to recommend the torsion-bar suspension, so why change it? (Love your Mopar’s torsion-bar suspension setup? See a rehab and upgrade here.) The truth is, you may not want to change it—unless you do. Such was the case for Arriero and his 1974 Duster, so what gives? Like other owners of Mopar A-Bodies (19641974 Plymouth Duster/Valiant/Barracuda and Dodge Dart/Demon), Chris discovered he needed more room for the six-speed Tremec T56 transmission he was eyeing, and the Chrysler torsion-bar crossmember just doesn’t allow any room for it.
But there are more benefits to converting a Mopar to coilover suspension than just a bigger transmission. Advancements in aftermarket suspension geometry, the availability of bigger disc brake systems from makers like Wilwood and Baer, rack-and-pinion steering, extra room for long-tube headers, and adjustable coilover technology offered Chris a host of options on top of simply supplying more room for a six-speed, so he made the decision to jump ship.
The torsion bars themselves can also create havoc when you start adding long-tube headers, bigger oil pans, aftermarket starters, and steering mods. Those who modify their Mopar A- and B-Body cars for drag racing or aggressive street driving will literally bump up against interference problems in all these areas relative to competitive cars of the same vintage.
The Reilly MotorSports Coilover Solution
Reilly MotorSports knows this dilemma, as RMS founder Bill Reilly found himself in the same position many years ago when he discovered there was no way to convert to Chrysler’s recirculating-ball, sector-gear steering box to a lighter and more responsive rack-and-pinion steering setup. Due to inadequacies in the Chrysler’s suspension layout, the quest to build a rack conversion for his 1969 Dodge Dart snowballed into a complete five-year suspension redesign. That, in turn, snowballed into building them for other people, and his company—Reilly Motorsports—was born.
RMS calls its front suspension AlterKtion, a play on words that acknowledges the complete replacement of the Chrysler front k-frame with an adjustable coilover system. At roughly $5,000 for the RMS AlterKtion kit (not including a companion disc brake kit), this setup isn’t cheap, but it is super easy to swap into any Mopar with a torsion-bar setup, and that’s what Chris will show us today.