The '73-77 GM rear suspension in our '75 Chevy Laguna project car is similar to its predecessor from the '64-72 A-body. Both use a triangulated four-link geometry; four bars that locate the rear axle laterally, longitudinally, and that provide for a range of vertical motion that is optimal for pinion angle. The key to understanding a triangulated four link is to understanding that each bar--along with the joint at each end of each bar--serves multiple functions. In order to keep the rearend planted in a proper orientation during driving maneuvers, the bars undergo forces of compression, stretching, and their joint ends--rubber bushings--see a good bit of twisting too.
GM used the four-link rear suspension all throughout the muscle car era on its midsized intermediates like the Chevelle, Tempest/LeMans, Cutlass F-85, and Skylark. In fact, all of the rear suspension pieces for these intermediates are interchangeable from 1964 to 1972. For 1973, GM redesigned its intermediates, this time employing computer design for the first time ever. The result was a suspension with a high level of comfort and exceptional geometry; they called it Radial Tuned Suspension, or RTS for short.
The RTS in our '75 Chevy Laguna project car may look a lot like the four-link in the earlier '64-72 A-body, but due to the updated design, none of the rear suspension pieces interchange with the earlier cars. In fact, none of the gazillion aftermarket suspension parts for the '64-72 A-body will swap with the '73-77 A-body (although technically, an adjustable upper rear control arm would work due to its variable length). If you happen to own one of the 7.4 million A-bodies GM made during this time, you have four choices: 1) tough it out with the stock parts and just replace the bushings, 2) doing the same thing, and adding an adjustable upper control arm, 3) gutting the entire stock suspension and going all custom-fabricated, like a race car, or 4) retain the excellent stock suspension geometry and upgrade all the parts with Global West pieces.
This last choice gives us a suspension with maximum utility on the street, and on the racetrack, and it's perfect for the enthusiast with minimal fabrication chops. As we hope to prove at a later date with some track testing, the Global West setup should offer the Laguna over 90 percent of the handling of a custom-fabricated suspension for a fraction of the cost. It's also a breeze to install in a few hours.
The Global West doctrine for designing suspension parts is pretty rigorous, which is fortunate for late A-body fans, since it's their only choice. For one thing, Global West likes to test everything in a race environment, something that proprietor, Doug Norrdin, has been doing for decades. The San Bernardino, California-based company has a well-burnished reputation for no-nonsense suspensions fitting Camaros (all years), GM A-bodies (all years), Mustangs (early and late Fox platform), Tri-Five Chevys, full-size GMs ('58-70), Mopar B-bodies, and Novas (all years).
The excellent geometry of the stock RTS suspension notwithstanding, the actual infrastructure of these 2-ton-plus coupes leaves a bit to be desired. One of the problems inherent in a triangulated four-link is that the even most minimal deflection in any of the joints--rubber donut bushings in the case of our Laguna--allows a relatively large excursion in axle motion. These lateral axle movements detract from the four link's otherwise flat and predictable handling. Moreover, the deflection of these rubber bushings creates an additional problem with roll bind, whereby the vertical motion of the links is impeded by friction in a non-linear fashion. From a driver's perspective, the sum of these traits feels like mild understeer at the beginning of a turn, changing to increased understeer mid-turn, and then transitioning to lots of snap oversteer as the car's mass finally overcomes the binding in the bushings and the axle deflects laterally.
Global West is the only manufacturer...
Global West is the only manufacturer we could find that makes a complete rear suspension system for our '75 Laguna project (a '73-77 GM A-body), and we searched for months before making our decision. That suited us fine, as Global West only makes top-shelf stuff. The Global West rear four-link system consists of adjustable upper control arms, lower control arms, rearend bearing inserts, QA1 double-adjustable shocks, and lowering springs. The tally was about $1,700 for everything.
The way Global West attacks this problem is two-fold. First, they use strong materials for the arms in order to better resist deflection. A CNC-machined 0.250-inch-thick wall mild-steel billet is used for the upper rear arms, and a 0.125-inch-thick DOM mild-steel tubing is used for the lowers. These are powdercoated to resist corrosion. Secondly, all eight articulated joints (two for each link) are designed for zero deflection, while allowing friction-free movement.
The upper double-adjustable arms (part No. TBC-37) use a high-misalignment spherical bearing on the chassis end, and a polyurethane bushing on the axle side. We do, however, recommend upgrading to the spherical bearing kit (SP-47) for the axle side, which makes it a solid link from the axle to the chassis. The spherical bearing kit allows the control arm to twist without binding under heavy body roll--the big plus being that it requires no modification to your axle housing, which for us is an 8.5-inch 10-bolt that we plan on keeping. The Global West uppers are also double-adjustable with a hexagonal section threaded for both left- and right-handed threads. This allows the user to adjust pinion angle without disconnecting the arms (to change their length).