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Independent Rear Suspension Kit - Independence Play
Installing Maximum Motorsports' New Bilstein-Specific Coilover Kit Makes The Cobra Irs More Fun
Both '99 and '01 Cobras featured an IRS sprung with 470-lb/in coils, while the 2000 Cobra R wore 800-lb/in springs. The '03 Cobra coupe features 600-lb/in springs. Meanwhile, the '03 Cobra convertible sports familiar 470-lb/in springs in the rear.
There was much rejoicing among Ford enthusiasts when an independent rear suspension (IRS) was finally grafted onto the back of the Mustang Cobra. But, as with everything on Ford's famous ponycar, there was also immediate enthusiasm for modifying that long-awaited IRS.
Maximum Motorsports has made a new contribution to the cause with the release of its Bilstein-exclusive IRS coilover spring and shock kit. The IRS kit follows hard on the heels of Maximum Motorsports' coilover kit for the solid-axle Mustangs. The two kits are similar, but the IRS pieces offer some distinct advantages of their own as well as a slightly different installation procedure.
We followed along with the installation of the new parts on David Simmons' '97 Cobra. David, of Austin, Texas, is the type of dedicated enthusiast who can't get out of bed in the morning without planning some new strategy for improving his Mustang. Last year he swapped his car's stock live axle for an IRS setup from a '99 Cobra, complete with Cobra R springs and a Torsen T-2 differential. He installed a Vortech supercharger as well, but recently removed it after deciding to nudge the Cobra more in an open-track/ road-racing direction. His car is well thought out and the modifications nicely executed, and he approaches his hot rodding with an engineer's appreciation for both theory and hardware.
David was working with Maximum Motorsports to test-install a preproduction version of the coilover kit, but the pieces shown will not vary greatly from the production parts, which will be available by the time you read this. David performed the work at a service bay at Motion Dynamics in Leander, Texas, using a floor jack and jackstands to raise the car, the better to approximate the average enthusiast's experience.
The coilovers offer two big advantages over the stock rear springs, and a couple small ones. "You are moving the spring more toward the centerline of the A-arm, or more within the 'A,'" Ehren Van Schmus, Maximum Motorsports' engineer and designer of the kit, says. "Because the way they [Ford] do that IRS, that one spring is hanging way out in front. That's pretty odd. You have a lot of bending and twisting in the control arm due to that, which adds a lot of friction in all the pivots.
"But what I think contributes even more to the benefits of the coilover kit is the fact you're moving that spring closer to the wheel. And when you move the springs closer to the wheel, you can run a softer spring to give you the same wheel rate. You end up with a lighter, smaller spring and with less friction in the pivots because the more inboard that spring is, the more force the inboard pivots are seeing. This means they don't like to move as easily as if you get that spring way out toward the wheel. Then the inboard pivots are kind of just along for the ride. The whole package ends up absorbing bumps a lot nicer."
The coilovers also offer an adjustable ride height and some modest weight savings, although at press time Maximum Motorsports had not yet calculated how much weight was being shaved.
Though details were still being finalized, Ehren estimates the IRS coilover kit will cost approximately $450, plus the cost of the Bilsteins. He figures installation time to be about one hour for a shop or well-equipped do-it-yourselfer, perhaps two or three hours for some-one with less experience and a smaller toolbox. Most of the time will be spent assembling the coilovers and removing the factory springs using a spring compressor. Beyond that, the operation is largely just a matter of swapping shocks.
Additional experience with the coilovers will yield more solid feedback, but so far David's subjective impressions confirm Maximum Motorsports' calculations. "I think it is smoother from a transition standpoint," he said after the first test drive. "It feels more linear in a turn." The Cobra's independent rear suspension has transformed the Mustang's personality, and the coilovers play to that strength. Says David, "I think the good thing is, on the street you lose no creature comfort. You maintain all the positive sides of the IRS."