There are several ways to make an early Mustang handle better than stock, from basic spring and shock upgrades, to more involved parts installation, up to completely redesigning pickup points and suspension geometry.
When Ford designed the Fox chassis for 1979 and later, they did away with the previous Mustang rear suspension design of leaf springs to suspend and locate the rearend and converted to a four-link with coil spring setup. This made for a marked improvement in handling, but the four-link design wasn’t very effective at keeping the rearend centered under the car. During hard cornering, the rearend housing was allowed too much sideways movement, which causes the car to point in a different direction than where the driver is steering. This effective “rear steer” makes the car unpredictable.
There are two common fixes to keep the rear centered with either suspension—a Panhard bar or a Watts link. You probably know what a Panhard bar is; it’s a solid bar that connects to the chassis on one side of the car and runs parallel to the rearend housing, attaching to the housing on the other side. A Panhard bar is effective at centering the rearend, but it still allows some side-to-side movement due to the basic geometry in the design, which puts a small arc in the bar’s movement. A better, but more complicated solution is a Watts link, which attaches to the rear housing with a pivot point, with arms that bolt to the car’s body. As the body moves up and down, the pivot point will move up and down, traveling in a line. Since the pivot point is traveling in a line, it keeps the axle located directly under the car.
This is Mustangs to Fear’s Watts Link setup with their standard semi-floating 9-inch rearend housing—using an existing housing requires welding on the shock support brackets, swing arm mounts, and pivot point, and that can be tricky to avoid housing warpage and get everything located properly. Inspect and lay out all the parts before starting with the installation. Use red Loctite on all the bolts during final assembly.
Traditionally, Watts linkages have been bulky, with brackets and bars hanging off the housing, but Mustangs to Fear (MTF) just came out with a design that is unobtrusive, a bolt-in installation (if you get the 9-inch rear housing from them), and works just as well as any of the more complicated designs. The kit is available three different ways: with a full-floater 9-inch rearend; a standard semi-floating 9-inch; and a “knock down” kit that requires you to weld on the Watts link shock supports, swing arm mounts, and pivot mount to your existing 9-inch housing. If you add MTF’s front sub-frame kit, the Watts link kit essentially turns the Mustang into a full-frame design without cutting out the entire floorpan of the car.
What are the differences and pros/cons between a full-floater versus a semi-floater rearend? With a semi-floating rear (which the vast majority of automotive rearends are, including the 9-inch), the wheel bolts directly to the axle. This is a simple, inexpensive, and effective design, but if an axle breaks there’s nothing keeping the wheel and tire from flying off the car. Furthermore, hard cornering imparts deflection and a lot of load on the axle and wheel bearing, creating all sorts of potential problems.
By contrast, with a full-floater the wheel mounts to a two-bearing hub and not the axle; therefore, there’s no axle deflection since the hub carries the weight of the car, not the axle and bearing. If the axle should break, the wheel remains on the car and won’t exit the car. The axle only carries rotational power, no vertical load. Which type of rear do you want? If you’re using the car for mostly cruising and not planning on heavy-duty handling such as an open track event, the semi-floating design is more affordable and simple to deal with.
Here, we show what goes into installing the MTF Watts Link under a 1966 Mustang hardtop. Kits are in stock at MTF now and retail for $2,495 (knock down kit), $6,300 (with 9-inch semi-floating assembly), and $7,600 (full-floater).
Install the end links on all of the swing bars and Watts links if not assembled. Note that they are threaded as left handed and right handed on each of the arms. Make sure you leave about 3/4 inch of thread showing for adjustment later.
Make sure the vehicle is well supported using jackstands or a lift, and then make sure the framerails are clean, straight, and have no interference issues that may prevent a good fit.
Insert the bolt sleeve that is needed for the framerail installation. It is installed as pictured.
Slip the frame brackets over the existing frame. Begin by aligning the front spring mount hole to the frame brackets supplied as pictured above. Fit and insert a 1/2 x 6 1/2-inch spring bolt.
Using a jack or clamps, fit the rails as tight as possible to the existing frame. It’s a normal fit if there is a slight gap at the rear between the existing frame and the Watts bracket. Once raised and fitted, drill 1/2-inch holes through the frame supports’ existing holes and through the existing frame. Insert 1/2 x 3-inch bolts with washers. Tighten all 1/2-inch bolts to approximately 45 lb-ft of torque, being careful to not crush the inner frame.
Next, install the upper shock support. A drift pin is helpful at aligning the support holes. Once fitted, insert 1/2 x 2-inch bolts with washers and tighten them to 70 lb-ft.
Install the upper and lower swing arms next. Align one end of the lower swing arm to the lower swing arm frame bracket mounting hole. Again, a drift pin may be required. Once fitted, insert a 5/8 x 3-inch bolt with washer. Install 1 1/4 x 7/8 x 5/8-hole spacer toward the outside of the lower swing arm frame bracket mount. Install the upper swing arm to frame bracket mounting hole—no washer is used on the upper bolt head side and the bolt will be inserted from outside of the frame inward. Use a 5/8 x 4-inch bolt and tighten to 100 lb-ft. (There are no spacers for upper swing arms.)
Using a vise press the bushing into the Watts link pivot arm.
If using the “knock down” kit with an existing rearend housing the pivot stud must be welded to the top of the housing, along with, of course, the shock brackets. Install the Watts pivot arm to the upper rear axlehousing with the 90-degree bends facing up. Install 5/8-18 washer and nut and tighten it until the locknut washer makes light contact with the top of the pivot arm. Insert the grease fitting into the upper rear axlehousing. Apply grease until it starts to come out from the sides of the pivot arm. Install one end of the Watts arm into the pivot arm and tighten to 35 lb-ft, then install the jam nut and also tighten to 35 lb-ft.
Install one end of the Watts link arm into the pivot and tighten to 35 lb-ft of torque. Raise the axle assembly to the lower and upper swing arms. Using a drift pin, align the holes of the upper swing arms first and insert 5/8 x 3-inch bolts with washers. Install the lower swing arms into the differential bracket’s mounting hole and tighten all swing arm bolts to 100 lb-ft. Insert the other end of the Watts link arm to the Watts link mounting holes on the frame bracket rail. Once fitted, insert 1/2 x 2-inch bolt and washers and tighten all bolts to 70 lb-ft.
Attach the other end of the Watts link arm to the frame bracket mounting holes and adjust length as needed. Then use 1/2 x 2-inch bolts to mount and tighten to 70 lb-ft. This is how the Watts links should look.
Raise and lower the differential and adjust the Watts link arms for smooth operation by using a twisting motion on the arms. Through the travel up and down, there should be no binding. After the adjustment has been made and the jam nuts are snug, raise and lower the differential and check for any binding again.
All of Mustangs to Fear’s kits come with QA1 adjustable shocks. Using a vise press the insert 5/8 inch into the shock. Assemble shocks and springs using the manufacturer assembly instructions that are with the shocks.
Install the shocks to the upper shock support mount holes using a drift pin. Once fitted, insert 5/8 x 3-inch bolts with washers. Insert a 5/8 x 6-inch bolt with washer on the shock bushing side, then into the shock and add a 1 3/4-inch spacer between the shock and the mount. We recommend installing the lower shock to the middle hole on the lower shock mount. Tighten all 5/8-inch bolts to 100 lb-ft of torque.
Adjust the axle to be centered in the wheelwell using a ruler to find center, then make even adjustment of the swing arms to center the axle and set the pinion angle at the desired setting. It is recommended a professional four-wheel alignment be performed along with suspension tuning for even weight distribution and correct ride-height settings.
Voilà, this Mustang now has a race-bred Watts link rear suspension to securely locate the rearend under the car and maximize handling potential.