Fox Mustang Independent Rear Suspension Install

The companion flange on '03-'04 Cobras is a larger diameter than other Mustang 8.8s, including those on stick axles and earlier IRS assemblies. This flange mates with an extra-beefy driveshaft in deference to the big power of the supercharged 4.6. Brian's '92 was already equipped with FRPP's popular aluminum driveshaft, which he wanted to keep. Here, he removes the '04 flange in preparation for swapping to a standard 8.8 flange. Prior to removal of the pinion nut, Brian measured the preload in the ring-and-pinion free-play.

After installing the smaller companion flange, Brian applied Loctite to the pinion nut before tightening it to match the preload measured earlier (10 in-lb). This is an important step. Under-tightening may lead to the pinion nut backing off, while over-tightening over-compresses the crush sleeve-either leading to failure.

Kenny Brown's rear shock-tower brace helps beef up the rear portion of the Mustang chassis whether running a stick axle or IRS, but it's a particularly nice piece when going the coilover route as you'll see shortly. Advertised to fit '94 and later Mustangs, a quick check in a stripped '90 showed it could easily be modified to work in a pre-'94-particularly if interior trim panels were being tossed aside.

The IRS coilover assembly used here is made up of Koni shocks, H&R springs, and Kenny Brown upper shock mounts. The Konis use a threaded aluminum body with spherical bearings at each end and are custom built to Kenny Brown specifications for this application. The H&Rs are 10 inches tall, and feature a 400-pound rate and 2.5-inch inside diameter.

Brian temporarily installs the upper shock mount via the top stud in preparation for drilling the sheetmetal for the shock mount's two horizontal bolts. Once the location is marked, the mount will be removed and the holes drilled with a 51/416-inch bit.

With the shock-tower holes drilled, the entire coilover assembly and upper shock mount were assembled on the bench, then secured through the normal shock access hole in the interior trim panel. Despite its integrated design, the rear shock-tower brace isn't required as a part of the coilover installation and will be modified to fit this Fox application later.

Several brake-line revisions are required to mate the Fox body to the IRS. Here, Brian has already cut and flared the factory hardline (note the rearmost line nut), but he's holding the original piece in place to illustrate where his cut was made.