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.