The independent rear suspension helped distinguish the “New Edge” Mustang Cobra from its predecessors promised to address some of the solid rear axle’s shortcomings on the road course and the street.
When released in 1999, the new Mustang Cobra piqued the interest of track rats, but the jury was still mostly out.
Maximum Motorsports of San Luis Obispo, Calif., had picked up a 2004 Cobra to begin understanding the new tech. They were soon fielding calls from all sorts of curious enthusiasts who wanted to know if the juice was worth the squeeze. “The IRS was an unknown quantity then,” said CEO Chuck Schwynoch.
The main appeal of the independent rear is the way in which it allows each corner of the car to articulate independently of each other, though the increased compliance and reduction in unsprung mass helps the wheel move more deftly over surface changes is intruiging. There are other perks, too.
While converting to a rear coil-over spring on the shock does not eliminate friction as with a coil-over conversion on the front, a coil-over conversion puts the spring closer to the spindle. That means the spring will provide a higher wheel rate for the same spring rate, which causes less degradation to ride quality.
A coil-over conversion also allows easy ride height changes and the ability to corner-weight the car. There are also more springs available for this arrangement. This is all quite handy, since the IRS is more particular when it comes to finding the right settings.
Installation
To delineate the impact this suspension design made on the track, the team took an IRS from a Cobra Mustang and implanted it into their American Iron-winning Fox body-based race car which had already done great things with a torque-arm-equipped solid axle.
Swapping an IRS into a Fox body or an SN95 Mustang is not overly complex. The SN95 chassis (1994–2004) can accept the IRS with minor drilling and exhaust/brake line modifications. Fox body Mustangs (1979–1993) also accept the IRS, but require additional fabrication like nut plates and support brackets.
The solid axle’s lower control arm mounting point on the chassis is where the independent rear suspension cradle mounts. The holes in the chassis where the control arm mounts to the body use a 12mm bolt in the 1979-98 cars, and a 14mm bolt in the 1999-04 cars. Maximum Motorsports has cradle bushings for both.
The IRS cradle mounts to the rear of a 1999-04 frame rail with a bracket that bolts right into existing captive nuts. For swaps into earlier cars, one of the two captive nuts on the side of the frame rail for the quad shock mounting bracket aligns with the IRS bracket. A second quad shock captive nut can be used if a new hole is added to the bracket. The two bracket bolts on the bottom of the frame rail will need either captive nuts added to the bottom of the frame rail, or a crush sleeve installed to prevent crushing the frame when the vertical bolts are tightened.
Whether the differential pinion flange needs to be changed to match a Fox body driveshaft depends on the year of the IRS. “Our 1999 IRS pinion bolted right up to our Fox body driveshaft, as did a 1999 IRS we put in a 1995 Mustang. I think it would be easier to install a rear flange on the drive shaft to match the IRS pinion than to change the pinion flange,” Schwynoch said.
When installing an IRS in a Fox body car, there are a couple of interference points for the chassis to cradle that are easy to overlook, and then they will cause mysterious fitment issues. Just watch everywhere when bolting the cradle to the chassis. The rear IRS cradle brackets often hit the lowest sheetmetal of the inner wheel housing, below the frame rail. Some chassis have extra sheet metal on the passenger side near the fuel tank that can interfere.
“We started by using the 2003 Cobra Mustang as a test vehicle to develop performance parts for the 1999-04 IRS. The IRS unit swapped into our race car had Delrin and urethane bushings, a coil-over conversion, and a bump-steer kit right from the beginning,” Schwynoch began.
Replacing the rubber bushings in the IRS system with harder bushings, such as Delrin, urethane, or aluminum was incredibly important in helping it work by minimizing deflection.
“Which material to use depends on the location. Delrin is best for the control arm bushings. The exception to this point is the differential mounts; aluminum transmits gear noise, so is acceptable for a track car, but not a street-driven car. Urethane is best for the IRS subframe cradle bushings, which should retain some compliance to reduce NVH. Polyurethane is often the ideal compromise here, but ours were custom-made and nearly as stiff as Delrin, which were benign on the street. They’re also malleable and fit better in the oblong sleeve caused by the welding of the cradle,” Schwynoch explained.
Testing at Buttonwillow
The car chosen to carry both suspension types was their successful American Iron Fox body. Its 321-cubic-inch engine produced 350 horsepower and 330 pound-feet of torque, which is respectable power in a car that crosses the scales at 3,150 pounds. Custom-valved Bilstein coilovers, Stoptech four-piston fronts, and serious G-Stream aero rounded out the proven package that had taken several American Iron wins in Mike Croutcher’s hands.
Following the car’s final race with its torque arm rear, the team returned to the Buttonwillow Raceway Park for an evaluation with the new suspension and nearly twice the spring rate with track-spec springs suited to the new IRS suspension. They used the same tires, wheels, and Cobra rear brakes as used the previous race weekend, but they didn’t stop there.
“We had previously widened the track of the solid axle to match what it would be with the forthcoming IRS swap, and had been racing it that way for a year. The swap from a solid axle with a Torque-arm and Panhard bar to the IRS increased the car weight by 80 pounds. We later swapped the stock rear hatch and glass for carbon fiber and Lexan, which dropped 80 pounds,” Schwynoch added. That means a Fox body is the lightest Mustang you can get and then modify to accommodate an independent rear suspension. There are advantages to be had in American Iron with such a recipe.
Dave Royce stepped in to perform the testing. At the time, Royce was the head engineer at Eibach, an experienced road racer, and a capable development driver as well.
“I was driving Mustangs in NASA long before the Maximum Motorsports guys reached out to me. I assume they picked me because I was developing chassis parts and give good feedback to the product development team,” he said. “They had an interesting testing protocol and tried to get the most honest feedback possible: they wouldn’t always tell me what changes they had made before asking my impressions.
“When I first drove the car, it was equipped with a torque-arm setup, which was then the best launching device for the Foxbody. It had so much bite off the corner, you could understeer off the track if you were too greedy with the throttle” he said.
“At first, the car felt spooky,” Royce recalled. Immediately, the team set to work learning what the IRS wanted. “We found out that we couldn’t run any rear static toe-out whatsoever – even a little made it handle like a shopping cart. We also had to ensure the bump-steer curves were symmetrical side-to-side,” he added.
After three spring changes, three bump-steer changes, and four different alignments, the team found the right setup. “The car is much more sensitive to anything not well done in the rear alignment than it is to a poor front alignment,” Schwynoch noted.
“Both designs were superior to the factory setup. You could always throttle-steer the car without too much difficulty. It never snapped with either setup, but when you went over the limit, the IRS would re-center more quickly and more smoothly,” Royce elaborated.
“When the IRS car was loaded in a fast steady-state corner like Riverside, you could lay the car on the outside tires and apply more throttle since it wouldn’t understeer as much. You could also brake a little bit later and trail-brake into the corner a little longer without upsetting the car. It always comes down to having more tire on the ground at any time,” Royce elaborated.
Both setups could generate a peak of 1.75 G on Toyo DOT tires, but the IRS seemed to hold that peak grip longer and more consistently. “At Riverside, we’d sometimes pull four car lengths on the others in our class!” Croutcher reminisced.
By noon, Royce had run a lap time right at the American Iron lap record. During the final session of the day, he put down a lap a full 2 seconds under the AI lap record. “We’re not going back now,” he declared.
The camber gain in mid-corner, the stability over bigger bumps, safer direction changes, and the change in balance were the most notable benefits. Through complex sections like Bus Stop and Phil Hill, the IRS setup gained significant time. “With the solid axle, you have to miss the berms in the Esses after the Sweeper. With the IRS, you could almost flat-foot it and make those Esses a straightaway toward Sunset,” Croutcher added.
One of the biggest differences Royce remarked on is what happens when putting the power down at corner exit. “With a solid axle and a torque-arm, the driver usually has to deal with understeer, but with the IRS, the driver is usually managing some oversteer at corner exit.”
Later Findings
Successive tests revealed the impact the swap had on certain ancillary parts. Without two big steel tubes to act as heat sinks, the IRS’ differential would get hot enough to require a fluid cooler.
However, the brakes would run much cooler. “When our car was equipped with the solid axle and torque-arm suspension we had to run cooling ducts to the rear brakes. Even then, front and rear rotor temperatures exceeded 1,200 degrees Fahrenheit,” Schwynock explained.
“After testing and development of a new rear brake kit for the IRS, we learned we were able to adjust in more rear brake bias. Even better, the front and rear rotor temperatures dropped to under 900 degrees Fahrenheit, even after we removed the cooling ducts to the rear,” he added.
The products developed around the Cobra’s rear suspension have done well with road racers and even a handful of drag racers trying something different.
The IRS swap transforms the Mustang into a more refined, better-handling machine on street and track. However, that comes at a cost: the IRS is more complicated than the torque-arm version of the solid axle, requires an IRS to be sourced, is more demanding in terms of setup, and more labor-intensive as well. However, it’s clear that 2 seconds on the same track, with the same experimental controls in place, is a step forward in anyone’s eyes.
