When we first heard that Heidts Hot Rod & Muscle Car Parts would be making its killer Pro-G IRS system fit under a Tri-Five, we were excited to say the least. The company has used Pro-G IRS equipped vehicles in our Super Chevy Suspension & Handling Challenge for two years straight, first with a second-gen Camaro, then with a '66 Nova. Both of them worked exceptionally well, so you can tell why we got excited. We happened to have a clean '56 in our Super Chevy stable that was in need of suspension upgrades for more than one reason.
The owner of the car decided to install NASCAR-style trailing arm suspension he lifted from a '67 C10 pick up. While the components fit, the supporting airbags and shocks just didn't ride very well, and, in fact,the car rode so poorly the owner just parked it and only brought it out for local cruise nights. In total, the car has only been driven 500 miles since completion almost 10 years ago. Having a car this nice, yet not being able to drive it is just wrong. Heidts' IRS is not just a fix, but a huge upgrade.
There are a few key benefits to converting to an independent rear set up. Most notably is the left and right sides are now independent. When a solid axle car goes into a corner, the outside squats, which in turn lifts the inside. With an IRS, the outside can squat without affecting the other side. This allows for a larger area of the tires' contact patch to stay in contact with the ground. In addition, when the car rides over uneven ground, the control arms move independently, so the ride is smoother and more controlled.
The Pro-G IRS system is based on a 9-inch center section, and uses heavy-duty CV joints to transfer the power out to the wheels. Hooked to the center section is a set of control arms made from DOM tubing that run out to heavy-duty uprights. The upper arm has Heim joints for smooth articulation and adjustability. Holding the weight and providing control of the bounce is a set of billet single adjustable coilover shocks with chrome springs.
Braking comes via a set of inboard-mounted Wilwood Dynalite four-piston calipers and 10.5-inch rotors. Heidts also offers drilled and slotted rotors and an e-brake set up for an additional charge. Because the brakes are mounted inboard, the car sees a reduction in unsprung weight.
The upper and lower arms are fully adjustable in both camber angle and toe, which greatly improves handling capabilities. Also, since the center section is solid-mounted, there will be no pinion angle change under acceleration.
The IRS is available with a number of gear ratio and spring combinations. The set up is true bolt-in installation with only a few holes needing to be drilled. It comes complete, but disassembled, unfinished, and retails for $7,995.
Here is the underside of the '56 we are upgrading. The home-built truck trailing arm suspension, while a neat idea, just didn't work correctly. The bags sitting directly over the axle provided a harsh ride and even with the shocks extended up into the trunk floor still didn't help. Grant Peterson, our west coast Snap-on Tech Center guru, removed all of this from the car in preparation for the IRS install. We will put the 9-inch into something else, so we'll keep that, but the trailing arms will be sold at the swap meet. One other notable thing on this car is the wheelwells; they have been widened by a couple inches, basically over to the framerail enough to fit the 295/50/16 BFG's. No mods to the framerails were made other than adding a crossmember for the bags.
Here is the Heidts Pro-G IRS unboxed. It comes with just about everything you need minus brake lines. Heidts offers a brake line kit that is sold separately, or you can build your own. We decided to build our own. The control arms are made from DOM tubing, while the rest of the stuff is thick steel. The center section is a beefy cast aluminum unit that will bolt to a new crossmember, which you can order in as cast or polished. Most everything else will then hang from the center section. The CV joints are heavy-duty units that will feed the power out from the nodular iron case third member with a 3.70 gear ratio, True Trac and 1350 yolk.
Before we assembled anything, we took the time to clean all the components. While they are shipped pretty clean, it's a good idea to give everything a once over to make sure nothing got in during shipping.
There are two seals that go into the ends of the center section. They will go all the way down and bottom out on a machined step. Its best to use an actual seal installation tool to drive the seal in squarely without damaging it. If you don't have one then look for a large socket or drift. But whatever you use, make sure the seal is not deformed during installation. If it is, than you can easily get replacement seals directly from Heidts.
There are three areas that will get studs: the sides and the face where the third member mounts. The two sets on the sides will need to be left 7/8-inch while the third member ones stick out 1-3/8-inch. Thread locking compound is used to keep them in place.
The main top crossmember bolts to the top of the center section with 1/2-inch hardware torqued to 75 lb-ft for a solid connection.
Now we need to do things a little differently than the instructions. You see, Heidts based its measurements off of the rear leaf spring mount. Unfortunately, this was not on our car anymore, so we needed to partially assemble the rearend to get our axle centerline so we can position the upper brackets via wheel base measurements. If you still have your leaf mount, then your task will be a bit less involved.
The outer bearing assemblies bolt to the uprights. The uprights are constructed from mild steel and are fully boxed and beautifully welded. By now you can probably tell that all the steel items come in a raw finish. It will be up to us to have them powdercoated, chromed, or painted. Heidts recommends doing a pre install to make sure it all goes well, just in case you need to modify something. We didn't need to modify anything, so we will tear it all back off and send it out to get powdercoated, after we are done with the future front upgrades.