Tom Wilson
August 17, 2011

Horse Sense: Maximum Motorsports says setting up an IRS suspension follows the same philosophy as it uses on live-axle Mustangs-set up the front axle, then tune the rear spring, shock, and bar rates to match.

For a number of reasons, the independent rear suspension in the Mustang Cobra has proven the most controversial bit of Mustang engineering since the '73 gas crisis spawned the Mustang II. Constrained to fit where the normal live axle goes, and prompted at least as much by a marketing need to offer an IRS than any zeal to make the Cobra faster around a track, the IRS has its detractors. If you're serious about performance, trash that IRS and move back to the proven stick axle, is the battle cry.

On the other hand, the IRS does ride better on the street, it offers the promise of adding static negative camber to the rear tires of road racers, and-if nothing else-it's already under Mustang Cobras, so why not work with it? That was the beginning of IRS parts development at Maximum Motorsports, where it was decided development-not replacement-was the better answer.

So we asked Chuck Schwynoch, the big link at Maximum, to detail the pros and cons of Ford's Mustang IRS. He began with the obvious-it's already under the powerful, desirable Mustang Cobras, and upgrading is faster and less expensive than replacement. In addition, the ride is better-on the racetrack the IRS action means an upset on one side of the car should not translate into reduced traction on the other side. And there could be some advantages to IRS, such as camber and toe tuneability. Furthermore, Chuck noted that limited, anecdotal evidence from road racers around the country indicates there is no gain in reverting to a live axle, even when the live axle is optimized with the latest technology.

Downsides to the IRS make a longer list than the upsides, but many have workable solutions. For starters, the IRS was not a clean sheet of paper design, as it had to conform to the chassis as optimized for a live axle. This has no doubt led to geometry and fitment issues, although Chuck admits Maximum hasn't spent the considerable time to fully reverse-engineer the geometry. A quick look at it shows the geometry isn't that bad-such as the hopeless, binding four-link mess in live-axle Mustangs. Maximum decided to work with the stock geometry, thus it hasn't really needed to plot out the geometry to the last second of angle. With the exception of optimizing the IRS' bumpsteer characteristics, which Maximum has already done, geometry upgrades may come from Maximum. If so, they'll be well into the future.

The Mustang IRS is also heavy-Maximum's shipping scale says 66 pounds heavier than a live axle.

Incredibly, many IRS Mustangs have been delivered with 12mm bolts in the front subframe-to-chassis attach points where 14mm bolts are supposed to go. The subframe thus shifts with changing loads (see Clunk! sidebar).

Typical of anything developed around Michigan's cratered roadways, there is rubber everywhere in the IRS. And all these bushings are naturally on the soft side, the better to damp out the last nuance of road noise or driveline vibration.

But all that rubber-coated compliance means everything in the IRS is doing the funky chicken. The subframe moves relative to the chassis, the differential moves relative to the subframe, and the control arms move relative to the subframe. Put some power and cornering loads through it, and no doubt the IRS can look like the scarecrow in the Wizard of Oz giving directions. Hammer it with some real power and it can wheel-hop like a freshly landed bass.

Given these issues, Maximum Motorsports' philosophy is to work with the stock geometry, and preserve as much ride quality as possible, yet reduce compliance, improve the bumpsteer, get the correct subframe bolts installed, and offer more sophisticated, better acting shock and spring options.

None of this can be a bad thing for Mustang Cobra owners on or off the track. While dedicated road-racing machines will likely provide bench-racing fodder on IRS pros and cons for years hence, for the legions of Mustang Cobra owners looking to optimize their suspensions for street use, the Maximum Motorsports offerings seem a definite help.

Stock Arrangements

For those not familiar with how the Mustang IRS is constructed, the basics are as follows. An 8.8-inch differential is mounted into a tubular steel subframe. The subframe is bolted to the chassis using the same pickup points as the live axle suspension. The rear suspension, which uses upper and lower control arms, is also attached to the subframe. Drive axles with universal joints run from the differential to the wheel flanges. The outer end of the suspension uses what would be called a spindle if it were on the front of the car, to provide a bearing mount for the axle and attachment point for the control arms.

Toe control is provided by links from the subframe to the "spindles." These look just like the outer tie rod ends on the front suspension.

Nearly all attachment points in the IRS are relatively soft rubber bushings with steel crush sleeves to isolate noise and vibration. There are four subframe-to-chassis attachment points-two front and two rear. The differential uses three attach points-two at the front of the diff, on either side of the pinion shaft, and one at the center rear, by the differential cover. The control arms have bushings at both their inner and outer ends, of course.

Clunk!

Does you're '99-or-later Cobra make noise when shifting from Forward to Reverse or vice versa? Do you get a shifting sensation, as when the thrust line of the powertrain shifts while getting hard on the gas? If so, chances are large you have 12mm bolts in the front lower subframe-to-chassis attach points. The cure is to install the proper 14mm bolts (Maximum has them, see the photos).

How did the 12mm bolts get where 14mm bolts are supposed to go? Ford mistakenly put them there on the assembly line on a huge number of '99-and-later Cobras, and might still be doing so for all we know. You can easily tell which bolts you have because a portion of the threads of these bolts extends past the nuts. Simply try fitting a 12mm or a 14mm wrench over the protruding, threaded section of bolt. If the 12mm wrench goes on, your bolts are too small and should be changed to the proper 14mm variety.

The rear of the subframe attaches where the quad shocks mount on live-axle cars. These have always used 12mm bolts, and there are no issues with them, just the fronts, which mount where the front of the lower control arms meet the chassis (the torque box) on live-axle cars.

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