Mopar Suspension: Struts & Control Arms Explained

When delving into the world of classic American muscle, particularly the iconic Mopar B-Body range, understanding the intricacies of its suspension system is paramount for both restoration and performance enhancement. At the heart of this system lie critical components like struts and control arms. Mopar struts are designed to significantly improve your vehicle's stability and overall control, offering more precise handling characteristics and a noticeable reduction in body roll during spirited driving. Complementing the struts are control arms, which are essential for maintaining the correct wheel alignment and suspension geometry. When opting for Mopar OEM (Original Equipment Manufacturer) parts for replacement, you ensure that these vital functions are preserved or even enhanced, keeping your classic Mopar performing as intended.

The Mopar Front Suspension Design: A Foundation for Performance

Chrysler’s engineers equipped their muscle cars with an excellent factory-designed double-wishbone front suspension system. When this sophisticated design is considered alongside the asymmetrical rear leaf springs and the front torsion bars, it creates a remarkably capable suspension setup straight from the factory. However, as with many classic vehicles, there are opportunities for simple, yet impactful, modifications that can elevate the original design to offer even more pronounced improvements in handling and driver feedback.

Torsion Bars: The Core of Mopar Front Springing

One of the most straightforward yet dramatically effective modifications available for a Mopar B-Body is the installation of larger-diameter torsion bars. Torsion bars are essentially the primary springs that support the front end of the vehicle. Their advantage lies in their relative ease of replacement compared to the coil-over springs commonly found in many GM and Ford vehicles. The removal process for torsion bars is typically carried out in an unloaded state, often requiring just a few well-placed taps from a hammer on the appropriate torsion bar removal tool. For an experienced mechanic with the correct equipment, this task can be completed in a matter of minutes.

The original factory torsion bars were carefully matched to the specific engine package installed in the vehicle. In production B-Body cars, four distinct torsion bar diameters were utilised, ranging from 0.860 inches up to 0.920 inches. These sizes are commonly, though not always precisely, referred to by terms like Slant-6, small-block, big-block, and Hemi bars. This terminology reflects the common pairings, but it’s important to note that an upgraded suspension package could sometimes be found with smaller-diameter bars on larger engines. Nevertheless, when enthusiasts mention having “Hemi bars” for their B-Body, it’s generally understood they are referring to the robust 0.920-inch diameter torsion bars.

Historically, the Mopar Performance catalogue offered an extensive selection of torsion bars, catering to a wide spectrum of needs, from ultra-lightweight options for drag racing to extremely stiff bars for circle track applications. While Mopar Performance has since reduced its product range, several aftermarket vendors have stepped in to fill the void, providing a good selection of replacement and upgrade parts.

The spring rate of a torsion bar is directly influenced by the cube of its diameter. To be precise, the spring rate is a function of the diameter raised to the fourth power. This means a small increase in diameter results in a significant increase in spring rate; doubling the diameter can increase the spring rate by a factor of sixteen. A common production size for Mopar B-Body torsion bars between 1962 and 1972 is 0.880 inches in diameter. If these 0.880-inch bars are replaced with 1.000-inch diameter bars, the front spring rate experiences an impressive 67 percent increase. Such a substantial change in spring rate is highly noticeable and can significantly alter the vehicle's ride characteristics. It's worth noting that such a stiff setup might be less comfortable for passengers, potentially leading to an overly stiff and somewhat harsh ride. The key takeaway here is that even minor adjustments in torsion bar diameter can have a profound effect, and bigger is not always necessarily better; careful consideration of the desired outcome is crucial.

A useful rule of thumb when selecting torsion bars is to aim for a spring rate that is approximately 10 percent of the vehicle's front-end weight. For instance, if a 3,500-pound B-Body car has a front-end weight of 1,800 pounds, one would select torsion bars providing a wheel rate of around 180 pounds per inch for a high-performance ride. It’s important to understand the relationship between spring rate and ride quality: as the wheel rate increases, the ride generally becomes harsher, with more pronounced high-frequency movements. Conversely, lower spring rates offer a smoother ride but require more suspension travel to generate the same force. A car with low spring rates can feel somewhat disconnected and "boat-like," while one with very high spring rates can feel overly stiff and akin to a go-kart. The ideal scenario for normal street driving often lies in finding a moderate spring rate that balances performance with comfort.

Shock Absorbers: Controlling the Motion

The Mopar front suspension typically employs a fairly conventional tube-type shock absorber. However, the specific bayonet-style top mount can limit the availability of aftermarket parts that are compatible. Less expensive replacement shock absorbers often feature a twin-tube design, which is generally not considered ideal for high-performance applications. Upgrading to a more performance-oriented, gas-charged shock absorber is a sensible choice for vehicles equipped with stiffer springs and lighter suspension components.

A variety of manufacturers offer gas-charged shock absorbers specifically designed for Mopar muscle car applications. While these shocks offer a marked improvement over their lower-priced counterparts, many are non-adjustable, and their valving specifications are often not publicly disclosed. This can sometimes turn the selection process into a trial-and-error exercise to pinpoint a shock that performs optimally with a particular vehicle. Furthermore, shock valving can be a matter of personal preference; what feels excellent on a friend's car might not provide the same satisfaction on yours.

For those seeking greater control over suspension tuning, a few vendors provide adjustable shock absorbers for Mopar muscle cars. Many of these are geared towards drag racing, but with diligent searching, road race-oriented adjustable shocks can also be found. Single-adjustable shock absorbers allow for adjustments to be made in one direction of motion (either compression or rebound), while double-adjustable shocks offer independent control over both compression and rebound valving.

XV Motorsports has invested considerable research into Mopar muscle car suspensions, utilising this expertise to develop a comprehensive line of shock absorbers specifically for B-Body cars. Their XV shocks are available in both twin-tube and mono-tube designs, with each version offered in non-adjustable, single-adjustable, and double-adjustable configurations. These specialised shocks, like those designed by XV, can be considerably more expensive than standard, mass-market shock absorbers.

Race Car Dynamics (RCD) is another company that excels in producing shock absorbers tailored for muscle cars. They offer an extensive range of shocks compatible with most popular muscle cars, including Mopar vehicles from 1962 to 1972. Several reputable vendors, such as Firm Feel and Hotchkis, stock RCD Bilstein shocks for Mopar applications. These Bilstein shocks represent a cost-effective option, typically featuring a large-diameter, gas-charged mono-tube design without adjustability. While non-adjustable, RCD possesses the capability to engineer custom valving for unique or highly specific applications.

Edelbrock has also entered the market with its own line of Mopar shocks. These Edelbrock shocks share the characteristics of large-diameter, mono-tube, gas-charged designs but incorporate an Inertia Activated System (IAS) for enhanced control, particularly over rough road surfaces.

Ride Height Considerations

The selection of shock absorbers for Mopars is generally quite good for vehicles maintained at their stock ride height. However, a significant number of vehicles modified for performance applications are lowered to a stance below the original factory height. Depending on the method used for lowering (as detailed in Chapter 14 of the referenced book), it may be necessary to use shocks that are slightly shorter than the stock units. Crucially, one must prevent the shock absorber from bottoming out before the vehicle's frame makes contact with the bump stops. The shock absorber and its associated mounting brackets are not engineered to bear the full weight of the vehicle. A bottomed-out shock absorber is prone to failure and can potentially lead to damage to other vehicle components.

Modifying shock absorber mounts, especially on the front of a Mopar, can be a complex task. Therefore, meticulous measurement and careful research are essential to find shocks that are the correct fit for your specific car. For heavily modified vehicles, it is often advisable to consult with a specialist vendor, such as Race Car Dynamics. RCD has the expertise to cross-reference numerous applications, increasing the likelihood of finding a suitable shock absorber for your particular vehicle.

Upper Control Arms: Refining Geometry

In recent years, numerous vendors have entered the market offering fabricated upper control arms for a variety of Mopar vehicles. The original stamped sheet-metal control arms found on production vehicles were often a compromise design. While cost-effective to manufacture, the factory design lacked adjustability, and the thin sheet-metal threads could be easily damaged during ball joint replacement. The various aftermarket offerings address these shortcomings by providing additional adjustment for caster and camber, and by utilising thicker-walled tubing where the upper ball joint is threaded into the arm.

A wide array of control arms are available, making it prudent to explore the options thoroughly. Some arms offer adjustability, while others feature fixed mounting points. Several vendors produce upper control arms specifically engineered to subtly alter suspension geometry for enhanced performance applications. It is vital to discuss your intended vehicle use and current ride height with the vendor. Some upper control arms are designed to accommodate tall knuckle conversions and provide increased clearance for larger brake systems and wheels with greater backspacing.

Lower Control Arms: The Load Bearers

The lower control arm plays a pivotal role in the Mopar front suspension, as it is responsible for supporting the weight of the vehicle's front end. While many high-performance vehicles continue to utilise the original lower control arms, several vendors do offer specially designed lower control arms that are either lightened or otherwise modified. Substantial modifications to the lower control arms are often required when replacing the original torsion bar suspension with a coil-over system; consequently, lower control arms designed for such conversions differ significantly from the stock units.

Several vendors, including Firm Feel and Mancini Racing, supply plates that can be welded to the underside of the factory lower control arms to fully box them. The original design features a U-channel shape. While this U-channel design is robust enough to support the vehicle's weight, it was not engineered to withstand the torsional loads generated by a stiff sway bar. Welding a reinforcing plate to the bottom of the lower control arm effectively boxes the structure, strengthening it to transmit the forces from the sway bar to the chassis without twisting or deforming.

Strut Rods: Maintaining Positional Integrity

The front strut rod is a relatively simple component that links the front of the K-member to the lower control arm. Its primary function is to operate in tension, transmitting braking forces from the tyres back to the K-member. The aftermarket offers several types of strut rods, including lightweight versions optimised for drag racing and adjustable units designed for performance tuning. The original OEM design is constructed from solid rod and tends to be quite heavy. Lighter, stiffer, tubular units are available as replacements for the original bars. Another valuable option is to purchase adjustable strut rods, which allow for compensation for other suspension-related issues.

For example, some replacement strut rod bushings are manufactured with greater thickness than the factory originals. This can result in the lower control arm being pushed back slightly, leading to a reduction in caster. An adjustable strut rod can rectify this problem by enabling the lower control arm to be precisely repositioned. It is imperative that the strut rod is of the correct length and that it articulates smoothly through its entire range of motion to effectively control the position of the lower control arm. If the lower control arm is permitted to move fore and aft excessively, front alignment settings will vary dynamically with suspension travel, resulting in sloppy handling characteristics. It is also possible to integrate the sway bar mounting point onto the strut rod, but this requires a strut rod specifically designed to handle such loads.

Sway Bars: Reducing Body Roll

Sway bars were an available option across all generations of the B-Body, although they were a relatively uncommon feature in the earlier years. The design of the sway bar, along with its mounting brackets, underwent several revisions over the production lifespan of the B-Body. Early B-Body cars featured a design where the end links attached directly to the strut bars. These early sway bars are quite rare, and finding replacement parts can be even more challenging.

A second design for the B-Body was implemented from 1966 to 1969. This iteration relocated the sway bar end links towards the outer edges of the lower control arms. The third B-Body design, used from 1970 to 1972, moved the end links inboard, attaching them to the middle section of the lower control arms. This third design offered improved clearance for brake calipers and is generally considered the preferred mounting method for most street-use applications. The E-Body cars shared suspension components with the 1970-1972 B-Body vehicles, meaning that sway bars are interchangeable between these models.

The closer the sway bar end links are positioned to the ends of the control arms, the greater the leverage the sway bar exerts on the suspension. This principle implies that a smaller, lighter sway bar can achieve the same degree of roll stiffness as a thicker bar mounted closer to the centre of the control arms. If you possess the fabrication skills to create custom mounting points for a sway bar, the optimal solution involves utilising rear-hung brake calipers and positioning the end links outboard on the control arms. It is also feasible to construct a system that ties the end links to the strut arms, though this approach necessitates some custom fabrication. This level of custom work is generally only required for highly dedicated road race vehicles; for most situations, the standard mounting systems should suffice.

The original factory-installed sway bars were typically of modest diameter and contributed only a limited amount of roll stiffness to the suspension. For high-performance applications, numerous aftermarket vendors offer upgraded sway bars for B-Body cars. Firm Feel is recognised as one of the original suppliers of comprehensive sway bar systems for Mopar vehicles, providing a full range of bars and mounting kits. Firm Feel currently offers large-diameter sway bars suitable for all generations of B-Body cars and can also produce custom bars if required.

For weight-conscious builders, Hellwig manufactures tubular sway bars designed for various Mopar applications, including most B-Body models. The tubular design offers a weight saving of approximately 6 pounds on the front end of the car while still delivering a high degree of roll resistance. Manufacturing a sway bar from a tube is an effective strategy, as the inner material of a solid torsion spring does not contribute significantly to stiffness. A hollow tube only needs to be slightly larger in overall diameter to match the stiffness of a solid bar, yet it is considerably lighter. While the tubular design incurs higher production costs, the weight savings represent a valuable benefit. Hellwig sway bars are available in diameters up to 1.500 inches and are designed to fit 1962 to 1972 B-Body vehicles.

Frame Isolators for Later B-Body Cars

The 1973 and newer B-Body cars incorporated rubber isolators positioned between the front frame section and the remainder of the unibody structure. These factory isolators can be replaced with precision-machined aluminium spacers to minimise body flex. Firm Feel offers these isolator plates specifically for the later B-Body models.

Coil-Over Front Suspensions: A Modern Approach

The original torsion bar design is well-suited for general handling and road racing, but it does present certain limitations, particularly for drag racing. Drag racing demands high-output engines, which in turn require larger exhaust headers and oversized oil pans. These components often compete for space with the torsion bars and the steering linkage. The original Mopar steering system is a rear-steer arrangement, typically paired with a centre-sump oil pan. While this rear-steer design performs adequately for moderate performance use, a rear-sump oil pan generally offers better clearance and performance in faster vehicles. Years ago, vendors like Milodon began producing rear-sump oil pans with integrated tubes to accommodate the steering linkage. This solution represented a compromise, affecting both the oil pan's capacity and ease of maintenance, but it did serve its purpose.

Eventually, several vendors developed innovative solutions by fabricating replacement front suspension systems that effectively addressed most of the challenges enthusiasts faced with the factory setup. While a number of vendors offer these replacement suspension systems, they generally employ coil-over springs in place of torsion bars and transition to a front-steer arrangement utilising modern rack-and-pinion steering. The adoption of coil-over springs eliminates the torsion bars from beneath the car by relocating the springs to the area beneath the inner fenders. This not only frees up valuable space for the exhaust system but also removes the need for the torsion bar crossmember. As detailed in Chapter 16 of the referenced book, the torsion bar crossmember can be a significant obstacle when attempting to install larger transmissions, such as a Tremec T-56 6-speed. An additional advantage of the coil-over suspension is the ease with which spring rates and pre-load can be adjusted. The coil-over spring design is exceptionally popular, leading to a broad vendor base manufacturing springs and associated shocks. This extensive supply chain contributes to lower prices and readily available parts.

Rack-and-Pinion Steering: Precision and Efficiency

Although the original Mopar steering box is highly regarded for its recirculating ball design, it cannot quite match the precision and directness offered by a rack-and-pinion steering system. The vast majority of modern vehicles are engineered with rack-and-pinion steering due to its excellent performance characteristics and cost-effectiveness. While a rack-and-pinion steering system could theoretically be developed for a rear-steer application, nearly all aftermarket vendors have opted for a front-steer design to move the steering linkage away from the oil pan. In a front-steer configuration, the rack is positioned ahead of the spindle centerline, which, in most vehicles, places it slightly forward and below the crankshaft nose. The combined effect of eliminating torsion bars and adopting a front-steer design results in a significantly more open space for the oil pan and exhaust headers.

In a coil-over suspension design, the original K-member is replaced with a new subframe constructed from either round or rectangular tubing. This replacement frame must incorporate all the necessary mounting points for the steering rack, engine mounts, and front suspension pickup points. While these are not insurmountable tasks for a skilled fabricator, some of the earliest aftermarket products unfortunately suffered from issues related to general fit and finish.

The weight reduction achievable with an aftermarket coil-over suspension system can be quite substantial. Minimising front-end weight on a drag race car can significantly improve 60-foot times. Reduced weight at the front helps the nose to lift, thereby transferring more weight to the rear tyres. If a drag racer is required to carry additional weight to comply with regulations, it is generally more advantageous to distribute that weight towards the rear of the car rather than concentrating it at the front.

One notable drawback of some coil-over suspension systems is their reliance on Mustang II front spindles. While the Mustang II spindle is not inherently a poor component, its use necessitates the discontinuation of the original Mopar brake system installation. The Mustang II spindle enjoys widespread popularity, offering a vast selection of brake components and related items. However, many Mopar owners have already made a considerable investment in Mopar-specific brake components, which may become incompatible with such conversions.

It is evident that the front suspension is a design area of critical importance, involving numerous components competing for space and complex geometric considerations that demand careful resolution. Some early adopters of coil-over conversion kits encountered challenges with turning radius, bump steer, and alignment issues. Initial customer feedback often reported geometry so compromised that it hindered the ability to turn the car while reversing or resulted in an excessively large turning radius, making low-speed manoeuvring difficult. While some of the original vendors have ceased production, new companies are continually emerging in the aftermarket. It is always advisable to exercise caution and thoroughly research product reviews before undertaking such a significant modification to your vehicle.