Cobra Kit Car Assembly Guide: Rear Suspension and Differential

Factory Five Racing Mk4 kits can be built with a solid-axle, live rear suspension in a four-link configuration standard, or with an optional three-link setup. The vast majority (95 percent) of FFR Cobra replicas have been built over the years using a solid-axle rear suspension, which is less expensive than an IRS FFR Mk4.

 

---

This Tech Tip is From the Full Book, HOW TO BUILD COBRA KIT CARS + BUYING USED. For a comprehensive guide on this entire subject you can visit this link:

LEARN MORE ABOUT THIS BOOK HERE

 

SHARE THIS ARTICLE: Please feel free to share this article on Facebook, in Forums, or with any Clubs you participate in. You can copy and paste this link to share: https://www.diyford.com/cobra-kit-car-assembly-guide-rear-suspension-differential/

---

 

We decided to source the new IRS from Tasca Ford in Massachusetts, but did order it from FFR. It’s actually the same suspension found in a 1989–1998 Lincoln Mk8.

 

 

Here’s a helpful hint if you wish to use a new IRS in your FFR Mk4. If you contact Ford Racing directly to order an IRS, they may tell you that the company doesn’t make new Lincoln Mk8 IRS systems anymore. That’s what they told us. We then contacted Factory Five Racing and learned that we could obtain a new IRS through the Tasca Ford connection. The alternative is to get a used IRS from an automotive salvage yard.

Our rear disc brakes are Mustang Cobra single-piston disc brakes, purchased new from Factory Five Racing. They are used with the optional IRS and also purchased as an option. Unlike with the IRS, we had an easy time getting a hold of these brakes. These and the ones we’ve already installed in the front stop the lightweight FFR roadster in rapid fashion.

If you’d like even more clamping power, Wilwood recently announced optional brakes for the FFR Mk4 roadster.  

 

Project 1: Independent Rear Suspension Installation

Step 1: Rotate Differential into Place

The correct way to install the differential is to place the unit on your hydraulic floor jack vertically. Point the nose (driveshaft flange) upward and align the halfshaft holes front to back with the chassis. Slowly jack the pumpkin into the IRS cage of the frame, while someone else is holding the differential steady to keep it from falling. Our floor jack did not have a high enough lift, so we improvised with a roll of duct tape. A block of wood works better. With the pumpkin in the cage, correctly position it while it’s resting on the floor jack. Rotate the differential so that the holes for the halfshafts run longitudinally. Tilt it forward to get the center section correctly positioned for installation.

Step 2: Install Rear Bolt to Hold Differential

A bolt holds the differential housing bracket on the subframe. Use a socket and ratchet to torque down the locknut. But first, just fingertighten the bolt so you can align the differential with the other fittings. Use a rear bolt through the rear cover to hold it into place. The center section weighs at least 50 or 60 pounds, so you may need to use a floor jack.

 

Step 3: Install Center Section Bushings and Washers

Install and align all bushings and washers in preparation to install the bolts. Carefully lift the front of the IRS center section and slide in one washer and one bushing for each side of the differential’s front attachment ears. Be sure to have good support for the differential so you don’t pinch your fingers. Use a large Phillips-head screwdriver to line up the bolt holes. Place the bushing and washer on top of each attachment ear, line up the holes, and then remove the screwdriver to drop the bolts in the holes.

Step 4: Install Differential

The center section of the independent rear suspension looks very clean. Torque the bolts on the front that travel through the bushings, and then turn your attention to the rear of the differential and torque that bolt. All four of the differential fasteners, the two through the bushings in the front and the rear fasteners, get torqued to 110 ft-lbs.

 

Step 5: Install Lower Control Arms

Use a 15/16-inch combination wrench and a ratchet and 15/16- inch socket to hand-tighten the rod ends with jam nuts into the lower control arms. Hand-tighten and then back them off four turns. Then install the lower control arms to the chassis with the shock installation mount hanging below the arm. Use three shims on the front side of the front rod end for both the left and right arms. Also install as many of the supplied shims on the other rod ends on either side. We found this extremely difficult and so may you. It’s a bit easier to use a stool. Be as patiently precise as possible. When the car is finished, you need to have an alignment shop professionally install the appropriate number of shims and properly align the chassis. With this accomplished, tighten all four bolts on each side, but don’t torque yet. You are going to remove the two rear bolts, install the right amount of shims, and align later. You can see it’s also a good idea to prop up the lower arm with a stool or something to make your job easier.

Step 6: Inspect Upper Control Arms and Rear CoilOver Shock Assembly

The upper control arms, Koni coil-over shocks, Roadster/ Coupe rear shock kit, snap-ring pliers, 3/4-inch wrench, 3/4-inch socket, ratchet, and ruler are all used to finish the IRS installation.

 

 

 

Step 7: Install Upper Control Arms and Assemble Coil-Over Shocks

Place the lower shock eye in a vice and use a spring compressor to compress the spring. Then use the snap ring pliers to install the snap ring collar on the shocks. Very carefully follow the same correct procedure you used to assemble the front coil-over shocks.

 

 

 

Step 8: Mount Rear Shocks on Suspension

Begin the installation process by mounting the coil-over to the chassis mount. Guide the bolt through the shock-mounting bracket and shock eye. Use two of the smaller-length spacers, one on either side of the shock eye. Align the lower shock eye on the bracket of the lower control arms, guide the mount bolt through the eye, and use the large spacer. As with the front shocks, the shock body goes toward the top. Don’t torque the rear shock bolts yet; wait until the suspension is aligned.

Step 9:Inspect CV Axles, Upper Control Arms, Hub Carriers and Hubs

How cool do the CV axles, upper control arms, hub carriers, and hubs look? Even better than their looks is how well they perform when installed in an FFR roadster IRS chassis.

 

Step 10: Install Passenger-Side CV Axle

After you’ve placed towels on the arms to prevent scratching the CV axles or the lower control arms, carefully push the CV joints into the differential by hand and rest the CV axles on the lower control arms. The arms are in position if you can feel and see when they reach their appropriate position.

Step 11: Install Hub on Spindle

Make sure that the hub is even with the spindle and press it into the spindle opening. Use your workbench vise to make sure that the hub is pushed down into the spindle evenly. Before putting the hubs through the spindle, they may require a small amount of fine and even sanding so they fit in the spindle holes. Because our CV axles were fully assembled, we didn’t have to put them together the way the FFR Mk4 Complete Kit assembly manual details. But you may need to.

 

 

Step 12: Inspect Suspension Components

Installing these components may be the most challenging step in building your IRS. The rear spindles/ hubs, fasteners, upper control arms, and ears for upper control arms all have come together to create one unit. The disc-brake caliper mounting holes are on the back part of the spindles when you install them.

Step 13: Bend Out Control Arm Lower Attachment Ears

If the attachment ears on the lower control arms are too narrow to accommodate the spindle sleeves, you can wrap a shop rag around the attachment ear to protect the black powdercoat. Use a hand vise and a pry bar to pry out the ears before negotiating the spindle sleeves into their new home. Prying against the hand vise enables you to pry out the entire attachment ear in an even fashion. Slide the hub assembly and spindle on the outer CV joint.

 

Step 14: Bend Upper Control Arm Attachment Ear

The attachment ear for the upper control arm to the top spindle sleeve also needs a bit of fine-tuning. Use the workbench and a trusty shop rag for protecting the powdercoat to offer some stationary and stout leverage. You now know why many mechanics have Popeye forearms.

 

 

 

 

Step 15: Tighten Hub Bolt

Tighten the three fasteners for each rear spindle and the 36-mm hub nuts, but don’t torque them yet; the independent rear suspension is aligned later.

 
 
 
 
 

 

 

Project 2: Rear Brake Installation

Step 1: Inspect Brake Components

From this photo of the rear brake components—the rotors, calipers, flex brake lines, and brackets—you’d guess that the rear brake installation is a snap. The tools used to install the rear disc brakes are a 3/4-inch wrench, 3/4-inch socket, ratchet, 3/16-inch hex key, flat-head screwdriver, torque wrench, and some thread locker.

 

Step 2: Separate Rear Caliper from Caliper Bracket

Using Rust-Oleum gloss black, paint the brake caliper adapter brackets and the brake flex line brackets in preparation for installing the brakes (not shown). The first actual build step is disassembly. While held in a workbench vise, remove the caliper bracket from the rear brake caliper.

 

Step 3: Loosely Attach Brake Caliper Bracket and Torque Caliper Mount Plate

Attach the rear brake caliper bracket to the dry adapter plate using just the upper bolt and spacer. The threads on this bolt and all the others that don’t use locknuts get Loctite or other thread locker prior to installation. After locating the proper placement of the driver-side caliper-adapter plate (mounting bracket), torque the fasteners to 32 ft-lbs. Unless you’re an experienced mechanic, you should always consult the manual.  

Step 4:Install Caliper and Put in Brake Pads

Assembling the driver-side caliper and installing the brake pads can be a snap. Be sure to use Loctite thread locker on any bolt threads that don’t also have a locknut, as with the caliper adapter bolts.

 

 

Step 5: Install Passenger-Side Rear Brake

Slide the rotor into place on the hub, swing down the caliper, and be sure to torque the caliper fasteners to the appropriate torque specifications, as indicated in your manufacturer’s kit assembly manual. Your rear disc brakes should look something like this when you’re finished with this part of the assembly, which also completes the installation of the IRS and brakes.

 

Written by D. Brian Smith and Posted with Permission of CarTechBooks

GET A DEAL ON THIS BOOK!

If you liked this article you will LOVE the full book. Click the button below and we will send you an exclusive deal on this book.