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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