Chrysler enthusiasts have lived with prehistoric leaf-spring rear suspensions for so long, it's easy to see why settling for way less than the rest is the status quo. To their credit, Mopar fans have done an excellent job of managing their expectations, as making do with a suspension design straight out of the 17th century is no mean feat. Take heart: Since 2006, Reilly Motorsports (RMS) has been offering its Street-Lynx four-link suspension to classic Plymouth and Dodge owners (1962-76 A-Body, 1966-72 B-Body, 1970-74 E-Body). The Street-Lynx is a godsend to Mopars with true handling intentions; its lightweight dual-adjustable Viking coilover shocks and tunable link adjustments are just what the doctor ordered.
But simply having the ability to adjust coilover ride height, or to set your instant center by moving the lower control arm pickup point doesn't begin to tell the whole story. The RMS does do all this, but the immediate and dramatic improvement in ride quality and road-holding ability—due mostly to the drastic reduction in unsprung weight and the correction of geometry—will be appreciated the most.
To understand why the big improvement, let's look at the stock leaf spring. First, the leaf spring takes on the roles of both supporting the weight of the rear axle assembly, and locating the axle laterally. In order to do all that, it must be simple and massive. As the rear axle encounters bumps, all that iron must react. It's a slow, ungainly, thunderous dance that puts tire grip and ride comfort as a last priority. In smaller cars like Mustangs and Camaros, it's almost livable, but in larger cars like Mopar B- and E-Bodies, it'll rattle your fillings out.
Another problem on the performance side is rear roll steer. As a by-product of the leaf spring's arch design, when a leaf-spring car corners hard, the inboard spring shortens slightly as it arches more, while the outboard spring lengthens (as it flattens out). This actually rotates the axle and causes oversteer—it's unpredictable, it's nonlinear, and it's scary. If all this is happening while the tires are going over an uneven surface, the rear roll steer combines with the unsprung mass trying to react to the road, and you've got some real monkey motion going on. It should go without saying that you won't be able to add power in this situation.
For drag-only and street/strip cars, a leaf-spring rear creates problems with axlewrap on the launch pad. The more torque the engine puts out, the worse the problem gets. In this situation, the torque of leaving hard with a sticky tire causes the leaf spring to de-arch, or deflect into an "S" shape. As the leaf bends, it stores increasing energy, until that energy overcomes the grip and spins the tire. When it happens repeatedly in sympathetic resonance, it turns into wheelhop. Outside of the negative effect on launch traction, the pinion angle is distorted to the point where axle and U-joint breakage is common. The "fix" is often to put ever thicker, heavier leaves on, and/or add links to the already massive leaf spring's unsprung weight.
The RMS Street-Lynx rear suspension solves all of this and adds some cool extras, like being able to easily set your ride height for the most bitchin appearance. With the Street-Lynx, the coilovers and the four-links divide the job of the leaf spring into two tasks—the links locate the rear and correct the arc of the axle travel, they maintain pinion angle, and they offer adjustment for instant center. The coilovers, by contrast, support the full weight of the axle, while doing it at a fraction of the weight penalty of leaves. Here, RMS relies on a pair of U.S.-made billet aluminum Viking Warrior double-adjustable shocks, which have independent control of compression and rebound damping—all in a lightweight package that looks like jewelry. Relating to the overall weight issue, we obtained four-corner weights of our Valiant project car prior to any modifications. Once the project is complete, we'll weigh the car again to determine the overall balance and weight. We think our goal is reasonable: replace the Slant Six engine and stock driveline with an all-aluminum 657hp Indy-built Wedge, upgrade the 904 automatic with a built TCI 727, fortify our suspension with the RMS AlterKtion and Street-Lynx bits, and beef up the rear with a Strange S60. We expect we'll gain minimal weight, or even zero weight in the process, and the Street-Lynx will be a contributing factor.
Other cool Street Lynx fact: converting from leaves to a four-link provides an extra ¾ inch of tire clearance on the inside. With B- and E-Bodies, the lower bars are flush with the wheelhouse, allowing tire sizes as large as 28x12.5 without any other modification. There is even a pair of easy trailer tie-down holes in the axle mounting plates (which match the AlterKtion's front suspension tie-down loops in the front).
Here, we're following up last month's RMS AlterKtion front coilover suspension with the company's Street-Lynx four-link rear suspension. Work on our '68 Plymouth was once again performed by the knowledgeable Ron Aschtgen of Outlaw Motorsports in Riverside, California. Outlaw does nothing but muscle cars and race cars, and their skills run the range from engine building and chassis work, to sheetmetal, paint, and wiring. They are a complete one-stop shop with everything done in-house, so if you're considering having your work farmed out instead of doing it yourself, we recommend them highly. Outlaw got the Street-Lynx done in under four hours, not including stripping the old parts off, cleaning, and repainting the A-Body's chassis with Eastwood Extreme Chassis Black. The Street-Lynx job was significantly easier because there's no cutting involved and only a few easy welds need to be made.
Soon, Outlaw will help us tackle the installation of our front Wilwood brakes. We'll take a close look at those, and show you a cool piece from RMS that makes installing Wilwood's GM-based master cylinder into a Mopar a cinch!
To check out a video of the RMS Street-Lynx suspension installation at Outlaw Motorsports, go to the video page at PopularHotRodding.com and search for "Project Valiant: RMS Street-Lynx Rear Suspension Installation."