How to Install the Pro-G IRS on a 1956 Chevrolet Bel Air

Tri-Five Independence, The Install - We convert the back of a '56 to an exotic independent rear suspension with Heidts bolt-in Pro-G system.

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We bolted on the front plate followed by the control arms and uprights.

Tri Five 1956 Chevrolet Bel Air Bolt Front 2/31

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.

Tri Five 1956 Chevrolet Bel Air Single Adjustable Coilover Shocks And 3/31

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.

Tri Five 1956 Chevrolet Bel Air Install Springs From 4/31

Grant used a plumb bob to mark the axle centerline in the rear.

Tri Five 1956 Chevrolet Bel Air Plump Bob 5/31

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.

Tri Five 1956 Chevrolet Bel Air Measure Center 6/31

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.

Tri Five 1956 Chevrolet Bel Air Pinion 7/31

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.

Tri Five 1956 Chevrolet Bel Air Drilling Upper 8/31

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.

Tri Five 1956 Chevrolet Bel Air Assemble 9/31

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.

Tri Five 1956 Chevrolet Bel Air Stub 10/31

Grant assembled the 10-1/2-inch Wilwood rotors to the hats on the bench and torqued the hardware to 180-inch lbs.

Tri Five 1956 Chevrolet Bel Air Assemble 11/31

Just as a precaution, the hardware on the CV joints was checked. Then Grant greased up the splines.

Tri Five 1956 Chevrolet Bel Air Grease 12/31

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.

Tri Five 1956 Chevrolet Bel Air Uprights Control Arms Brake Rotors Cv Joints 13/31

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.

Tri Five 1956 Chevrolet Bel Air Install 14/31

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.

Tri Five 1956 Chevrolet Bel Air Mount Caliper To Caliper 15/31

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.

Tri Five 1956 Chevrolet Bel Air Braces Lower Front Mounting Center 16/31

This lower strut bar runs from the lower control arm to the front pivot point, and keeps the rear tires planted under hard acceleration.

Tri Five 1956 Chevrolet Bel Air Lower Strut 17/31

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.

Tri Five 1956 Chevrolet Bel Air Forward Strut Rod 18/31

We drilled four more holes to mount the strut rod crossmember.

Tri Five 1956 Chevrolet Bel Air Drill Holes Mount Strut Rod And 19/31

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.

Tri Five 1956 Chevrolet Bel Air Rear Suspension Exhaust 20/31

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.

Tri Five 1956 Chevrolet Bel Air Powertrain Industries Longer Drive 21/31

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.

Tri Five 1956 Chevrolet Bel Air Cv 22/31

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.

Tri Five 1956 Chevrolet Bel Air Final 23/31


Heidts Automotive
Lake Zurich, IL 60047
Powertrain Industries
Garden Grove, CA 92841


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