Our classics are great, but let’s be honest, their suspension systems were designed over half a century ago, and since then technology has come a long way. Today’s handling has spoiled us, and we really notice how deficient the factory handling is when we compare it to our modern daily drivers. This is where bolt-on suspension parts come into play, in this case, an independent rear suspension (IRS) system.
The reason that nearly all modern cars use an IRS is that it’s simply better when compared to a live axle in regard to handling and road manners. In terms of ride comfort and overall driving, it’s superior in dealing with turns and uneven road surfaces. A live axle (sometimes referred to as a solid rear axle or SRA) means that whatever one wheel does affects the other wheel. So, when one wheel hits a pothole or dip in the road, then the other wheel is forced up, resulting in less downward force and hence less traction. Also, with an SRA there’s no way to make suspension adjustments such as changes to the camber (aside from some high-end track-prepped SRA floater kits). The ability of the IRS systems to let the wheels work independently of each other (hence the name) along with the additional suspension tuning that’s possible, makes the IRS a hard system to beat. One downside is that an IRS will have greater unsprung weight, and by necessity will be far more complex, compared to an SRA setup. If done right an IRS system will outperform an SRA in all handling and road driving situations, though an SRA system will win when it comes to drag racing.
The PRO-G IRS kit from Heidts that we’ll be looking at is designed for first-gen F-bodies, but it’s offered for other applications as well. As we said, this is a bolt-in kit that only requires some drilling and a small notch cut into the rear frame rails if you want full articulation on a lowered car. Heidts offers different options to fit your power and performance needs and the upper link of the system produces less than 0.5 degrees of camber curve at ¾-inch of total travel. You’ll also be able to dial in rear camber and enjoy the other benefits of an independent rear suspension. So, follow along as we head over to Outkast Kustoms to bolt an IRS system into a 1968 Camaro using basic hand tools and some common sense.
01 Here’s the driveline end of the PRO-G IRS, which comes with a cast aluminum 9-inch housing and differential along with heavy-duty CV joints and axles capable of handling a good amount of power. If you’re providing your own 9-inch Ford differential, it must be a 31-spline unit to work with the provided axles.
02 The first stage of the PRO-G IRS assembly was building the differential center section, which began with the axle seals. Because these axle seals have garter springs to keep the seal lip in contact with the axle, extra care needs to be taken to keep the garter springs secured in place. We ensured the seal cavity was packed with wheel bearing grease to both lubricate the seal and protect the garter spring. Keep in mind that if the garter spring pops out during installation, the differential will leak.
03 Axle seals were driven into place with a seal driver until they were fully seated. Don’t use a punch, socket extension, or hammer to drive these seals into place. Using punches and the like will distort the seal and result in leaks. If you don’t have a seal driver, rent one or, if you have no other choice, use an appropriately sized socket that’s the same outside diameter as the seal.
04 A mounting plate bolts to the differential as shown. We also made sure to use the longer Grade-8 bolts provided with the differential to secure the plate to the pinion support. This mounting plate joins the differential and lower control arms.
05 Outkast Kustoms suggests using only black high-temp RTV as a gasket on these 9-inch housings and differentials because they tend to be leak-prone. Just a thin bead of sealer as shown here will give us a leak-free bond. Pro tip: Make sure the RTV goes around each stud to prevent lube from eventually seeping through the threads.
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