Affordable Independent Rear Suspension Upgrade For Your Classic Trucks

Flat Out Engineering’s Corvette-Based IRS For Early F100s

By Ron Ceridono   –   Photography By The Author

When it comes to suspension design there’s a reason many high-end luxury and performance cars have independent rear suspension: it’s the simple fact that an IRS rides and handles far better than a solid axle. Like many classic truck enthusiasts, Paul Willis wanted to improve the ride and handling of his ’55 Ford F100 with an IRS update, but the heart-stopping cost of the aftermarket designs put such a swap out of reach. That was before he learned of Don McNeil’s Flat Out Engineering installation kits that use C4 Corvette suspension components.

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02 This particular C4 Corvette IRS assembly has a Dana 44 center section
There were two generations of C4 rear suspensions. This is the ’88-96 version. This particular IRS assembly has a Dana 44 center section with 3.43:1 gears and a Positraction differential. As usual Scooter the shop dog finds a way to get in on the action.

McNeil has a long and storied career as a hot rodder and entrepreneur. He’s been involved in drag racing, Bonneville competition, built a long list of modified cars and trucks, as well as developed a variety of successful businesses. Today McNeil’s focus is Flat Out Engineering that specializes in affordable, easy-to-install kits to adapt ’84-96 C4 Corvette suspension components to a variety of cars and trucks—that includes a kit for the rear of our ’55 F100.

03 We bought this assembly without a spring to save money as we would be converting to Aldan coilovers
In stock form the C4 IRS used a transverse spring. We bought this assembly without a spring to save money as we would be converting to Aldan coilovers.

Read More: How To Install Wilwood’s Forged Narrow Superlite Big Brake Kit

As McNeil explains it: “The C4 Corvette suspension was a design milestone in sports car suspensions. GM engineers worked for years on this to have a ‘World Class Sports Car’ that would outperform the finest high-priced European sports cars on the market. The forged aluminum spindle uprights, A-arms, half shafts, and control arms are not only light weight but incredibly strong. The lightweight aluminum components give the suspension system a sprung to unsprung weight ratio that is unsurpassed. This provides excellent ride quality with cornering and handling that outperforms any other production vehicle.” McNeil points out another big advantage to the C4 Corvette suspension is the availability of parts. “Ball joints, brake parts, tie-rod ends, and any other parts that might wear with frequent use are standard GM parts and easily available at parts stores anywhere.”

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04 With the stock F100 rear axle and springs out of the way all four spring hangers were removed
With the stock F100 rear axle and springs out of the way all four spring hangers were removed. We’ve found the best way to do that is to drill the heads off the rivets and knock them out with a punch.

To install a C4 IRS under a ’53-64 Ford pickup there are two kits available. One is for use with ’84-87 Corvette rear ends, the other is for ’88-96 style. Both kits maintain the correct geometry and handling characteristics C4 Corvettes are known for. Each kit includes bolt-on style rearend mounting brackets, pinion mounting crossmember and pinion snubber bracket with urethane bushing, upper and lower coilover shock mounts, rear boxing plates, forward control arm mounting brackets, all necessary hardware, and detailed installation instructions.

05 To begin the installation of the Flat Out Engineering kit Willis put the frame on jack stands
To begin the installation of the Flat Out Engineering kit Willis put the frame on jack stands at ride height with a slight nose-down rake (2 degrees) and leveled it side to side.

During the production run there were two versions of the C4 IRS. The ’84-87 version is 61 1/2 inches wide wheel flange to wheel flange and uses parking brakes inside the rotor hats. The ’88-96 is 1 inch wider; 62 1/2 inches and the parking brakes are built into the caliper. Both series of rear ends use the 5-on-4.75 wheel bolt pattern, the same as Chevrolet passenger cars, so there are many styles of wheel to choose from. From the factory, ’84-87 C4s used 16 inch wheels; in 1988 16-inch wheels were standard with 17-inch on special models; ’89-96s all used 17-inch wheels.

06 Classic truck frames often led a rough life and as a result may be out of square
Classic truck frames often led a rough life and as a result may be out of square. The best way to check them for accuracy is by cross measuring at several points. This frame was on the money, but if and when there is frame damage it must be corrected before proceeding.

Read More: New Parts For Your Classic Truck October 2022

Another difference in C4 rear ends is the center section. Both generations came with either a Dana 36 or Dana 44. Factory gear ratios for the Dana 36 include 2.59:1, 2.73:1, 3.07:1, and the rare, hard-to-find 3.31:1. Dana 44s can be found with 3.07:1, 3.33:1, or 3.45:1. Of course there is a wide range of gear ratios available for both the Dana 36 and Dana 44 from the aftermarket.

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07 the IRS brackets would be taken off the ends of the frame rails we checked to make sure both sides were identical
As many of the necessary measurements to position the IRS brackets would be taken off the ends of the frame rails we checked to make sure both sides were identical—they were.

Installing Flat Out Engineering’s C4 IRS kit is surprisingly simple. The instructions are easy to understand and follow, with all the necessary measurements included. All the brackets bolt into place with no welding. While the kit is easy to install it’s wise to clamp all the brackets in place and double check all the measurements before drilling any holes.

08 This is the complete Flat Out Engineering IRS kit
This is the complete Flat Out Engineering IRS kit. Not only are all the components well thought out and beautifully built, they fit like they should, simplifying the installation process.

Along with the basic installation kit we opted for two Flat Out Engineering options. We added the

“Free Spirit” Adjustable Rear Control Arm Kit. It replaces the four stock Corvette strut rods and eliminates the “bind” produced by the rubber or urethane bushings in those rods by incorporating  premium spherical rod ends. They provide increased suspension travel for a smoother ride and complete adjustability—plus they look great.

09 Replacing the stock Corvette rear spring will be a pair of single adjustable Aldan shocks
Replacing the stock Corvette rear spring will be a pair of single-adjustable Aldan shocks. Aldan coilover springs are available in black powdercoat or a polished hard chrome finish.

Read More: Making A Canyon Carving C10: Big Brake Kit & Suspension Upgrades

Another option we chose was the Tubular Rear Toe Bar Kit that replaces the stock Corvette assembly (it provides toe adjustment for the rear wheels and maintains it during suspension travel). The replacement kit lowers the bars by mounting to the bottom of the steering arms, thereby providing additional frame clearance in lowered ride height applications. This kit provides increased adjustability and with the billet mounting bracket and the tapered aluminum adjusters it enhances the rearend’s appearance as well—wait until it’s all polished and painted.

10 The knob at the top of the Aldan shock body adjusts rebound
The knob at the top of the Aldan shock body adjusts rebound. Full counterclockwise is the lowest setting. Turning the dial clockwise increases rebound stiffness.

With an easy-to-install Flat Out Engineering kit and a readily available C4 IRS (ours came from Dino’s Corvette Salvage) you can give your classic truck the ride and handling qualities of a contemporary sports car for a price that won’t break the bank.

11 Aldan offers a thrust bearing kit that fits between the adjuster and coil spring
To make suspension tuning easier, Aldan offers a thrust bearing kit that fits between the adjuster and coil spring. The bearing reduces friction when adjusting the preload on the spring.
12 A spanner wrench is used to adjust the Alan springs preload
A spanner wrench is used to adjust the Alan spring’s preload. A good starting point is to compress the spring 1 inch before installation.
13 Once the final preload adjustment is made the threaded adjustment collar is secured with a set screw
Once the final preload adjustment is made the threaded adjustment collar is secured with a set screw.
14 The stock Corvette control arm bracket
The stock Corvette control arm bracket (arrow) is removed—the arms will attach to the new frame brackets that are part of the installation kit.
15 Before the IRS assembly can be slid into place under the frame rails the square damper brackets on the Corvette crossmember
Before the IRS assembly can be slid into place under the frame rails the square damper brackets on the Corvette crossmember, or batwing, that mounts the differential case must be removed.
16 We clamped everything in place and triple checked all the measurements before drilling any holes
The front frame brackets are positioned 47 1/8 inches from the end of the frame rails to the center of the rear upper mounting hole (10 5/8 inches from the axle centerline). We clamped everything in place and triple checked all the measurements before drilling any holes.
17 the batwing brackets were positioned 29 7 8 inches to the center from the end of the frame rails
With the center section supported on jackstands, the batwing brackets were positioned 29 7/8 inches to the center from the end of the frame rails and clamped in place.
18 the stock F100 shock absorber crossmember interfered just enough to keep the batwing brackets from fitting properly
In our case rivets securing the stock F100 shock absorber crossmember interfered just enough to keep the batwing brackets from fitting properly against the bottom of the frame rail.
19 We elected to remove the four rivets and eliminate the crossmember
In addition with the stock Ford shock crossmember in place there wouldn’t have been room for a nut on the top right bolt for the batwing bracket. We could have ground off the rivet and notched the crossmember to provide clearance for the nut. We elected to remove the four rivets and eliminate the crossmember.
20 The Corvette batwing uses metric fasteners
The Corvette batwing uses metric fasteners. To use the hardware supplied in the kit the inserts in the bushings must be drilled to remove projections in the sleeves that accommodate the metric bolts.
21 We used a tri square an angle finder and the measurements in the instructions to duplicate the mounting position
Satisfied that the radius rod and batwing brackets were properly located all the necessary holes were drilled. We used a tri-square, an angle finder, and the measurements in the instructions to duplicate the mounting position of the radius bracket on the other side of the frame.
22 With the pinion angle set at 2 degrees up the crossmember was clamped in place
Next to be installed was the crossmember that secures the center section. With the pinion angle set at 2 degrees up, the crossmember was clamped in place.
23 With the pinion at the desire angle the holes in the frame bracket are marked and drilled
The pinion crossmember has three parts, two frame brackets that bolt to the frame rails and the crossmember that bolts to the brackets. With the pinion at the desire angle the holes in the frame bracket are marked and drilled.
24 With the crossmember bolted in place the supplied bracket is attached to the differential housing
With the crossmember bolted in place the supplied bracket is attached to the differential housing and bolted to the bushed bracket on the crossmember. A pair of struts (one can be seen, arrow) were used to hold the suspension at ride height.
25 There are two factory holes in the frame at 3 1 8 and 6 3 8 inches forward of the rear axle centerline
There are two factory holes in the frame at 3 1/8 and 6 3/8 inches forward of the rear axle centerline. They are drilled to 3/8 inch and are used to attach the coilover brackets. Two additional holes must also be drilled.
26 One last round of measurements verified all the brackets were precisely where they were supposed to be
Here all the frame brackets have been bolted in place. One last round of measurements verified all the brackets were precisely where they were supposed to be.
27 To attach the Aldan coilovers to the hub bracket arrow bolt to each hub
To attach the Aldan coilovers to the hub bracket (arrow) bolt to each hub. The brackets are attached by the radius rod bolts and the factory lower shock absorber mounting hole (shock stud has to be removed).
28 we opted to replace the stock control arms with Flat Out Engineerings new aluminum adjustable links
To dress up our installation, and to provide some adjustability, we opted to replace the stock control arms with Flat Out Engineering’s new aluminum adjustable links.
29 Before installation the length of the control arms was matched to the originals by aligning them with long bolts
Before installation the length of the control arms was matched to the originals by aligning them with long bolts.
30 Here the Aldan coilovers and adjustable control arms are in place
Here the Aldan coilovers and adjustable control arms are in place. The links’ rod ends are less restrictive to movement than the GM factory bushings.
31 The final piece to be installed was the Flat Out Engineering tubular rear toe bar kit to replace the stock Corvette assembly
The final piece to be installed was the Flat Out Engineering tubular rear toe bar kit to replace the stock Corvette assembly.
32 All finished Its a credit to Flat Out Engineering that something this trick would be so easy to install
All finished. It’s a credit to Flat Out Engineering that something this trick would be so easy to install.

Sources
Aldan American
(310) 834-7478
aldanamerican.com

Flat Out Engineering
(714) 639-2623
flatout-engineering.com

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