Pete Epple Technical Editor
January 28, 2010

It was the intent of Ford Motor Company to take the handling characteristics of the Mustang to the next level with the independent rear suspension (IRS) in the '99 Cobra. The IRS separated the Cobra from the other Stangs in the barn, and gave Snake owners something "upscale" to brag about. With 32 valves and an IRS, the Cobra was on step closer being an M3 killer, or so some thought. The IRS certainly changed the handling characteristics, some say it was better, but that is arguable.

With the introduction of the Terminator Cobra in 2003, Ford added gobs of power and it beefed up the IRS to prevent breakage. As the aftermarket embraced the Terminator, huge power was easier then ever to achieve, and the IRS quickly became the weak link.

As the Mustang Cobra evolved, so has its power potential, resulting in the chance for parts to fail, namely in the rear. Many have traded the IRS in their Cobras for a live axle, yet many more have stuck with the original setup but want a solid fix for IRS breakage.

The fact is, it's not hard to break an IRS. Just be overly aggressive, dump the clutch with sticky tires, and/or surpass 500 rwhp, and try to launch using all the power on a regular basis. In addition to half-shaft failures from excessive power, excessive wheelhop can also be a destroyer of everything IRS.

"Wheelhop occurs when the IRS differential 'flops' or deflects on the rubber bushings," explains Chuck Schwynoch of Maximum Motorsports. This "flopping" is caused by the rear tires rapidly gaining and losing traction, causing the pinion angle to change violently as the differential winds up and unwinds. Many say that the soft rubber bushings that cushion the differential are to blame for what ultimately leads to broken halfshafts and/or a leaky or totally blown out differential housing or cover.

Although there are many opinions on how to remedy the wheelhop issue, we decided to show you how to improve two areas of the IRS-the mounts and differential cover.

When the center section moves under load, it causes a high level stress on the rear mount located on the differential cover. In high horsepower applications, the cover can crack or the mount will completely break away from the rest of the center section. To prevent this, Fore Precision Works and Maximum Motorsports stepped up, manufacturing parts that help eliminate the "flopping" that causes wheelhop.

Maximum's solid rear mount kit replaces the stock mount, eliminating the soft rubber bushings. The new rear mount is a solid billet piece, which connects the cover to the rear of the car. The front mounts are comprised of spherical aluminum bushings, which not only eliminate pinion movement, but also allow for adjustment of pinion angle to help reduce vibrations in the driveline.

"Solid mounts secure the differential and the cover, preventing the cover from breaking," Schwynoch adds. "The adjustability of the pinion angle helps with driveshaft vibrations. The Maximum mounts allow you to properly set pinion angle."

Fore Precision Works' new billet IRS differential cover is a huge improvement over the stock piece. This fully CNC-machined cover starts as a 55-pound block of 6061 T-6 billet aluminum before it is machined down to just over 11 pounds. The finished product boasts many benefits over the stock unit. Axle bearing pre-load bolts, feed and return ports for an external differential cooler, and an O-ring seal to prevents leaks with out the use of silicone sealant are just a few of the features.

"The Fore Precision Works differential cover is designed so something else will break before the cover," explains Justin Fore. "Without the Maximum Motorsports solid mounts, the stock mounts would fail before the cover."

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To install both pieces, we took a trip to Bohemia, New York, and visited Realspeed Automotive where Dan Carlson had a Cobra needing a fix for the dreaded wheelhop. Bill Loper's silver '04 Cobra arrived sporting a Whipple twin-screw and Snow alcohol injection. It laid down a street-friendly 568 rwhp on Realspeed's Mustang dynamometer. When Loper heads to the track, his Snake is treated to a little race gas and added boost, bringing the rwhp number to 640. With a Spec Stage III clutch, aftermarket halfshafts, and Nitto drag radials, wheelhop is still an issue and it was time to put an end to it.

The first step in the process was removing all of the parts that would be in the way. To change the rear mount and differential cover, we needed to remove the center section from the car. Once the exhaust and axles were removed, Rob DeMartinis made quick work of getting the differential out.

At the time of our installation, neither the Fore or Maximum piece were designed to work with each other, but we had a fix. In addition, the Fore cover we had was a prototype unit. In order accommodate both pieces and maximize the strength potential of the IRS, Carlson sent the Maximum Motorsports rear mount to Fore Precision Works where Fore machined the needed changes to make it compatible.

Although the Maximum mount was modified, we still needed to remove some material from the new cover to accommodate the mount. With the cover swapped and the billet mount secured in place, the process of test fitting the differential began. As soon as the center section was in place, it was clear some trimming was in order. With the center section bolted in, DeMartinis marked both the cover and the bracket for the driver's side tie rod, which needed to be modified for clearance. Production pieces are designed to bolt right into place. After Carlson removed a small amount of aluminum from the differential cover and trimmed the tie-rod bracket, the center section went right in.

With a pole jack supporting the pinion, DeMartinis test fit the spherical washers for the front mount. These washers help locate the front of the center section and put no bending force on the front mounting ears of the differential. They also allow you to raise or lower the pinion angle, which helps eliminate vibrations in the driveline. Once the washers were loosely in place, DeMartinis used an angle finder to set the pinion angle at 31/2 degrees positive (remember the pinion doesn't move). After the rest of the rear end was reassembled and the exhaust was reinstalled, Carlson took the Cobra for a spirited test drive.

The solid mounds add a slight more noise than the stock rubber bushings, but the rear felt solid under the car and wheelhop was reduced dramatically. Now with our differential solidly mounted in the car, we're ready to pour more power to the pavement without worrying about wheelhop.

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