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.
We bolted on the front plate followed by the control arms and uprights.
The single adjustable coilover shocks and springs are shipped separately, so we needed to marry these two parts before the install. Since we don't have a coilover spring compressor, we needed to remove the knob by taking out the two small button head Allens.
This allowed us to slip the springs on from the bottom. Grant applied a light coat of anti-seize to the body of the coilover to prevent galling the threads.
Grant used a plumb bob to mark the axle centerline in the rear.
Up front, Grant used a couple of squares to find the center. Then we measured the distance between our marks. We got a 115-inch wheelbase, which is stock for this car. We got pretty lucky, because not only did we hit the factory wheelbase, we were also square when measured cross corners.
We slapped on a set of loaner rims we had in the shop, because our original wheel offset doesn't work with the new rear track width. Then we lowered the car on the ground, loading the suspension. We checked our pinion angle, and found we were at 0-degrees. Again, perfect to the instruction's specs.
Knowing we had good wheelbase and pinion angle measurements, we felt confident enough to start drilling our upper brackets. Grant decided to use a roto-broach (or step bit) to drill the required 1/2-inch holes instead of using a drill. He says they leave a truer hole and work a bit better for this. A drill will do the job if you don't have a roto-broach, but make sure to drill a pilot hole first.
With the upper saddle brackets now installed, Grant moved onto assembling the rearend completely. Our nodular iron case features 31 splines, a 3.70:1 gear ratio, and a Tru Trac diff. The 3150 yolk is torqued down to 45 lb-ft.
After a liberal coating of grease, the stub axles slip into the third member. The stub axles are different lengths, and the longer of the two goes on the passenger side. The caliper plates now slip onto the studs, with the threaded inserts in the caliper plates toward the stub axle flanges. They are secured with the supplied Nylok nuts.
Grant assembled the 10-1/2-inch Wilwood rotors to the hats on the bench and torqued the hardware to 180-inch lbs.
Just as a precaution, the hardware on the CV joints was checked. Then Grant greased up the splines.
Now the uprights, control arms, brake rotors, and CV joints were all assembled. The lower arm and upright went first, followed by the brake rotor and CV joints. The outside splined end of the CV is fed into the hub bolted to the upright. There is a large nut that will secure it, but it's left loose for now. Lastly, the upper arm was secured.
Grant installed the calipers so we could evaluate what we needed for the brake lines. We went to our local Earl's store, and had them whip us up two 12-inch brake lines that we coupled with a few elbows and a "T" fitting to another 16-inch line that will eventually hook to our existing hardline.
The Wilwood Dynalite four-piston calipers are mounted to the caliper plates. These can be ordered in numerous colors, but this is the only size available. They are plenty big. Heidts also offers an e-brake set up.
There are two braces that run from the two front bolts on the saddle down to a lower front mounting point on the center section. These triangulate the rear for stability.
This lower strut bar runs from the lower control arm to the front pivot point, and keeps the rear tires planted under hard acceleration.
We are getting towards the end, but still have a few things to button up, like the forward strut rod assembly. Here is an overall shot to give you an idea of how it goes in.
We drilled four more holes to mount the strut rod crossmember.
Getting even closer, boy does it look killer. One thing to note with this rear suspension system is your exhaust. Like ours, most of it will need to be redesigned. You can see our headers point straight at the strut bar crossmember, and out under the axle the tailpipes won't work either. Once we have the exhaust done, we will do a follow up in Super Chevy to show how.
We found our original drive shaft was too short and had the smaller U-joints. Since the rear now has a beefy 1350 U-joint yolk, we decided to have Powertrain Industries make us a longer shaft with the bigger ends. We slipped the new unit in place so we could tighten the large nut on the CV joints.
The lager nut is what keeps the CV shaft mated to the hub assembly in the upright. It is torqued to 100 lb-ft. We still need to fill the rear with good 80/90-gear oil and have it aligned.
We are going to wait on the alignment, as we still need to upgrade the front suspension, and measure for wheels and tires. Until then, the car will have to sit and wait. But what's another month or two when its been sitting for years? We will do a complete story on the front suspension, how to measure for new rims, and a test of the car in future issues of Super Chevy magazine, so stay tuned.