Muscle Mustangs & Fast FordsHow To Chassis Suspension
1989 Mustang LX Rear Suspension Upgrade - Bringing Back The Bite
Overhaulin' An Outdated Rear Suspension To Handle Today's Horsepower.
Adding a good suspension system is useless without the inclusion of sticky rear tires. The suspension can work perfectly, but if the tires don't grip the ground, then performance will suffer greatly. Adding a set of sticky meats can and will exploit the weakest link inside the rear. We have a pair of 275/60-15 Mickey Thompson drag radials--a.k.a. rear breakers--under the car right now. Our inventory also includes a pair of M/T ET Street 28x12.50 (measuring 10.5 inches wide) DOT tires mounted on another set of wheels for track use. Just bolting on the radial tires made us think twice about the differential and axles. Our old combo used a set of M/T ET Drag 26x10 slicks--far smaller than our new tire line up. The coupe always spun a little out of the gate, sparing the puny 28-spline axles and traction-lock differential.
Our subject LX has survived some 10-second runs and countless 11-second performances over several years with its stock axles. The differential was replaced once, but with a regular Traction-Lok, nothing special. We attribute its lifespan to the fact that the car is equipped with an AOD transmission, sans transbrake. The very nature of the numerically low First gear of the transmission (2.40:1), coupled with a mild 3,200-rpm stall speed, meant this coupe left lightly compared to its stick-shift brethren. That was about to change due to the forthcoming powerplant upgrade.
Unlike the late-model GM F-body cars, the Mustang was built with a stout rearend housing. A few simple mods are all that is required to get the Ford 8.8-inch rear strengthened. Larger axles and a tough differential go a long way in the Mustang world. We tapped Latemodel Restoration for its five-lug conversion kit. Since we wanted to add 31-spline axles, it was a good time to increase our lug count from four to five. Adding five lugs opens up the choice of wheels on the market for both track and street use. Plus, the lug strength is increased due to distributing the load to five lugs, as opposed to four. We have seen the lugs sheer off on quite a few 10-second cars, so we wanted to avoid that unpleasant experience.
A quick phone call to Summit Racing netted us an Eaton Detroit Locker differential to fit the 31-spline axles. The Detroit Locker diff is a nasty unit that is more than capable of handling 9- and 10-second runs, but it still has limited-slip capabilities for subtle street manners. The same 3.73 gears were re-installed in the rear, but only after a thorough inspection. Miraculously, after 15 years of use, the gears looked great without any hint of wear. We saved money by reusing the gears, but still added new shims and bearings that we bought from the local NAPA store.
The modifications took about a day with two DMC techs working on our coupe. The process required a little cutting and welding in order to install the UPR adjustable spring perches on the body. Your author found an old set of five-lug Weld Draglite wheels in the basement for the car. The wheels added that classic and timeless 5.0L look to our coupe.
The new rear-suspension setup complements our recent front-suspension overhaul that included a tubular K-member and A-arm setup, along with coilover struts. The front-end mods removed 67 pounds, including the skinnies. The reduced weight will help weight transfer and let the rear suspension work even better in planting the variety of DOT tires we have for this car. Thanks to UPR, Strange Engineering, Latemodel Restoration, Royal Purple, Mickey Thompson, and Summit Racing, we have brought back the bite to go along with the supercharged bark under the hood.