5.0 Mustang & Super FordsHow To Chassis Suspension
Mustang's Independent Rear Suspension Kit - Independence Stay
There's No Need To Trade The Factory Cobra Irs For A Solid Axle-Maximum Motorsports Is Improving It For The Handling Crowd
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.