Why the uncertainty in this story’s subhead? A task is either doable or not, right? It’s easy or hard, no?

Well, no.

Depending on who you talk to, mounting an independent rear suspension (IRS) in a street rod is either a fairly straightforward process that a handy guy with good common sense and welding skills can accomplish, or it’s so difficult it should be left to chassis professionals only.

See? Not exactly a consensus.

Here’s what we can tell you with absolute certainty: Several companies manufacture kits and/or chassis subframes that allow you to replace your street rod’s stock rear suspension with the IRS out of a Corvette. Using a Vette’s rearend, as opposed to the more traditional Jag IRS, has its benefits. After that, the water gets pretty murky. So we’ll give you the facts first, then more opinions, and let you decide if you want to tackle a project like this.

Why IRS?

There’s no denying the appeal of an independent rearend. All those moving parts simply look way cool. And if you’re serious about improving your rod’s handling, what better to put between your tires than a sports car’s rearend? A properly installed IRS system will plant your rear meats like no solid axle can, since it allows each tire to follow the road surface individually. Both tires maintain a solid, consistent footing, even if one dips into a pothole, for example. You don’t get the action/reaction seesaw of a solid axle. Ride height and ride quality can improve too, especially if you choose a system that adds adjustable coilover shocks to the equation. Regarding late Corvette suspension, most agree that the real benefits come from adding them front and rear.

Vette Specs

If you’re retrofitting an original-equipment IRS (as opposed to a fabricated aftermarket assembly) to a solid-axle car, the Corvette rearend is far more popular than the once-beloved Jag. The assemblies themselves are more plentiful than Jaguar rearends, and replacement parts—like bearings and seals—are much easier to find, as the Vette uses a Dana differential. The later-model all-aluminum Corvette rears are lighter than the steel Jags (200 to 250 pounds for the Chevy versus 350 to 400 for the Jag). They’re cheaper, too. Depending on where you are in the country, a C4 (’84 to ’96) Corvette rearend can be bought from a wrecking yard for as little as $300 to $500. Some rod builders may want to buy an entire Corvette to salvage not only the rearend but the front suspension, engine, trans, engine computer, and so on.

A Vette IRS does have its limitations. If your car puts out more power than, say, an LS1 or LT-4 small-block (in the * neighborhood of 350 hp and 350 lb-ft of torque), you may be better off with a stouter rearend, like a 9-inch. You will need to fabricate a new driveshaft to mate the rear to whatever tranny you’re using. You’re also limited in wheel choices due to the size of the brake rotors. The 12- and 13-inch rotors used on C4 rearends don’t clear anything smaller than 16-inch wheels, and even some 16s don’t fit. If you have to run 15s and still want Corvette IRS, you can opt for the older C3-style rearend, which has smaller brakes. But not all kits adapt to this narrower assembly.

Which brings up the width issue. An ’84 to ’89 C4 rear axle is 62.5 inches from tire-mount surface to tire-mount surface. The ’90 to ’96 axles are ¾-inch wider. Many fat-fender and shoebox cars can easily accommodate this girth with no resizing of the framerails or the rearend itself. But try to put that same rearend under a ’32 or similar-sized hot rod and your rear tires may stick out like jug ears.

Options? There’s the narrower C3 rearend, or you can have the late-model half-shafts narrowed to fit your application. A couple of the Corvette IRS kit manufacturers offer crossmembers and/or bracketry designed especially for the narrower frames. One company, Progressive Automotive, manufactures a complete subframe assembly specifically for these narrower cars.

But another chassis specialist flatly refuses to make a Corvette IRS kit for anything but fat-fendered or newer cars, saying that the narrowing process negatively affects the rearend’s suspension geometry. “If the swing-arm lengths get too short you can develop camber problems,” warns Steve Brownfield of Art Morrison Enterprises. “The tires won’t want to stay flat on the ground.”

Then too, GM lengthened the half-shafts from 13 inches on the C3 models to 18 inches on the C4 models to combat vibration, says Mike Jonas of Stainless Steel Brakes. His company specializes in the calipers, rotors, and other components necessary to make a Vette rearend work. “If you shorten the half-shafts you could throw them out of balance. There goes your smooth ride,” Mike says.

And so begins the disagreements between retrofit IRS mount suppliers.

There are two ways to mate a Corvette IRS with street rod bracket kits that attach to the existing framerails, and weld-on clips that replace the back of the frame. Of the three companies we spoke to, one fell solidly into the bracket camp, another solidly into the weld-on camp, and a third used both.

Where To Get ’Em

Southern California’s Flat Out Engineering made bracket-mount kits that were pretty much limited to classic Chevy and Ford trucks until it recently prototyped mount systems for pre- and post-War Ford passenger cars. (One of the prototypes for a ’48 Ford Tudor sedan is shown in this story.) Other kits are on the drawing boards, including a version for ’60-’72 Chevy trucks. Flat Out’s kits include all the crossmembers and brackets needed to install a C4 axle, and you can choose to keep the stock transverse-leaf suspension or replace it with coilovers. Flat Out kits either bolt or weld onto the existing chassis, depending on the application. Retail prices for Flat Out’s kits range from $590 for a kit using the stock leaf spring to just over $700 for a coilover kit (exclusive of shocks). Flat Out can supply a C4 IRS setup with either a Dana 36 (used in automatic transmission Vettes) or Dana 44 (for manual trans cars—a better choice), for anywhere from $1,000 to $1,200, “but you’d do better [price wise] finding your own,” says Flat Out’s Don McNeil, “And that cost doesn’t including shipping.” A Vette IRS may be relatively light in the suspension world, but not on your local UPS truck.

Art Morrison Enterprises in Washington state manufactures a weld-on rear clip that replaces the back of a fat-fender car’s frame with components that are “friendly with the component parts of an ’84-’87 Corvette axle,” says Brownfield. The clip is made from mandrel-bent 2x4 tubular framerails that eliminate the transverse-leaf mounts and instead can adapt to the builder’s choice of coilovers. Retail price of the weld-on clip is $1,025, exclusive of shocks. The Morrison customer needs to supply his own IRS assembly.

Progressive Automotive, based in Ohio, uses both mounting types for its Sweet Ryde rear IRS kits. Sweet Ryde kits adapt to a wide variety of cars and trucks, ranging from pre-War and shoebox Fords and Chevys to some ’30s Plymouths, Willys, and even Dodge trucks. The company’s kits make use of the full range of Corvette IRS assemblies, from the first independents back in 1963 through the C4s. Aldan coilovers replace the stock transverse leaf spring. All Progressive Automotive IRS kits require welding, since “we don’t trust nuts and bolts,” says President Bob Shetrone. Kits range in cost from $1,250 for a simple bracket kit that welds to the framerails to $2,750 for a subframe assembly for narrower cars. While the company can supply Dana 36 versions of the C4 assemblies as well, it charges from $1,250 to $1,300 for the IRS. For your money, though, you get a rearend that’s guaranteed to be complete, straight, and working—not a guarantee you find at most wrecking yards.

DIY vs. DIFM

So what about the do-it-yourself versus do-it-for-me question? All our * experts agreed that an IRS installation is not your typical weekend bolt-on. Keep in mind that you are messing with your vehicle’s suspension. If you’ll excuse the pun, a lot is riding on your work—from your car’s precious sheetmetal to your own personal safety. You should be supremely confident, not to mention meticulous, in your ability to cut, fit, and weld pieces properly. “If you’re a ‘measure five or six times and cut once’ kind of guy, you should be able to handle this,” Shetrone says of his kits.

Art Morrison’s Brownfield disagrees. His company sells the entire rear clip with crossmembers and brackets already professionally attached to reduce the possibility of error. And most of his customers are chassis professionals who are doing the work for their clients. “This job is not as easy as it may seem,” he tells us. “It’s unrealistic to say that a moderately skilled person can do it.”

The photos of Flat Out’s prototype post-War car system will give you an idea of the work involved with installing a bracket kit. Look them over and measure the tasks against your own ability and experience. You can see the steps required to mount a Progressive Automotive IRS kit on a ’38 Ford sedan on the company’s Web site: www.progressiveautomotive.com.

Consider this, too: What we’ve covered here is the simple act of attaching the IRS assembly to the frame. We haven’t yet talked about any of the related tasks in getting a rearend set up correctly, from adjusting ride height to making sure the wheel and tire are centered in the wheelwell. And what about your car? Will it need floor modifications? A new gas tank? Tubbing? As one of our sources put it, “A better title for your story is probably ‘Opening a Can of Worms.’”

If you have any doubts about the installation or the related procedures, find a professional to do the work. Any of the companies mentioned here can steer you to a local pro. Progressive Automotive even has a list of nationwide dealer/installers on its Web site, so you can buy the kit and have it mounted at the same place.

Installation costs

We asked one of Progressive’s dealers, Don Lemke of Sweet Ryde Unlimited in Mesa, Arizona, to give us a ballpark idea of what professional installation would cost. We could hear him taking a deep breath over the phone. “That depends on so many things,” he began. “What condition is the car in? Have we done this kind of car before, or are we doing it for the first time? Do we know where the wheel centerlines are? Are you using one of the narrow, older axles or a modern, wider one?” And the list of questions went on from there.

Lemke’s bottom line was this: Under ideal conditions—meaning he’s done a car like yours before, he can easily remove the old stuff, and there’s little other work required to get the proper fit—the job will take about eight hours. His shop charges $40 per hour. If there’s more work to be done, “it could take up to a week,” he admitted. You do the math.

“Remember, building your street rod’s chassis is like building the foundation to your home,” Lemke said. “It has to be done right for everything else to work and for you to be happy with it.” Good advice to consider as you ponder your IRS installation choices.

SOURCE
Stainless Steel Brakes
Clarence
NY
8-00/-448-7722
ssbrakes.com
Art Morrison Enterprises
www.artmorrison.com
Flat Out Engineering
Rancho Cucamonga
CA
Progressive Automotive
Baltimore
OH
Sweet Rydes Unlimited
Mesa
AZ  85210