Modern Mustangs come standard from the factory with independent rear suspension (IRS). While it is basic stuff these days, it wasn’t always the case with Ford’s Pony car. For decades the Mustang chassis rocked an old-school solid rear axle, often referred to as a “truck axle.” This design was loved by drag racers, but criticized by car magazine writers who argued the design hindered the Mustang’s ability to handle corners well. Eventually, Ford got the message, and when Ivan Korda’s 2001 Ford Cobra originally rolled off the assembly line in Dearborn, Michigan, it was one of the first Mustang models to rock an independent rear suspension (IRS) system. Our goal is to make this IRS system even better through the use of Maximum Motorsports products.
But, with an IRS system comes more parts, more articulation points, and more complications. With those additional parts and complications, when age starts to affect bushings and alignment settings, things can really start to feel interesting coming out of a corner. To be blunt, when the rear of the car starts to steer for you unexpectedly coming out of a fast corner, your butt starts to pucker. Let’s be clear, this is a bad thing.
The front wheels are connected to the steering wheel and are for turning the car. With Ivan’s 2001 Cobra hitting the ripe age where it doesn’t need a fake ID to purchase whiskey, all of those bushings keep things aligned in the rear of the Cobra (and there are a lot of them) have started failing to do their job. With Project Apex being built specifically to attack the track, it was time to overhaul the rearend of the snake to keep things tidy.
After running the car through some track day events, Ivan upgraded to a Maximum Motorsports Road and Track Box suspension kit, which made massive improvements to the car’s handling. To really dial the car in correctly, and get rid of the nervous rear-wheel steering consistent with an aging IRS, Ivan decided to install the Maximum Motorsports IRS Rear Grip Super Street and Competition Package.
With the multitude of bushings that needed to be replaced, Ivan disconnected the driveshaft, removed the rear portion of the exhaust, and then dropped the entire rear axle assembly and suspension out of the car. This made upgrading the IRS system much easier. Maximum Motorsports included the tools required to press out and press in the bushings for the IRS. This proved to be invaluable during the project, so big props to the guys at Maximum Motorsports for the help.
The softer the bushings are, the nicer the ride will be while cruising down a country road with pot holes. However, that soft bushing will deflect quite a bit under load, and any deflection changes the alignment of the car. In the case of Project Apex’s rear suspension, bushing deflection can change the rear toe angle which causes that nervous rear-steering feeling. Replacing the stock rubber bushings with polyurethane or Delrin (a firm plastic) bushings keeps the vehicle’s alignment settings relatively unaltered while under the stress of hard cornering with sticky racing tires.
Chuck Schwynoch, owner of Maximum Motorsports, detailed how their own in-house testing showed huge results when they installed IRS systems into their car and how they upgraded those systems with Delrin bushings. “There was no question, as soon as we put the IRS system in and our driver went around Buttonwillow Raceway, we were faster,” Chuck says. “The key to faster times was driver confidence, and that confidence came from keeping the rear steer to a minimum with new, stiffer Delrin bushings.”
Since the rearend was out, it was the perfect time to upgrade the differential itself. Another key to making a race car quick around the track is a good limited-slip differential. The Cobra came with a limited-slip diff from the factory, however, it was a clutch-type, which can wear out over time, especially with increased horsepower and sticky tires.
You know right away when the differential isn’t working properly anymore, because the inside rear tire will spin on corner exit, harming the acceleration abilities of the car and thus lap times. You can install new clutches and shim them tighter for an improved OEM differential but it is a quick fix that often doesn’t last. To properly remedy this issue, Ivan chose to upgrade to a Torsen T2R differential, which is a fantastic choice for any road racing Mustang.
Using data from previous track days at Barber Motorsports Park, Ivan recognized that he probably needed a better final drive gear ratio to maximize the powerband between corners on the track. While the IRS project was being upgraded, he sourced a set of 3.73 gears from Ford Performance to replace the stock 3.55 gears. This will be an obvious improvement in acceleration. Pro teams change final drive ratios between race tracks constantly.
Ivan installed the rear suspension without the rear springs in the car on purpose. It may sound crazy, but the reason this was done was to use a bump steer gauge from Maximum Motorsports and measure how much the rear toe changed when the suspension articulated up and down. If the spring was in place, jacking up the suspension would simply jack up the car.
With the springs out of the way and the new bushings installed, Ivan is able to use a jack to move the suspension up and down and measure readings. In order to correct bumpsteer (the changing of toe-angle when suspension is compressed or unloaded) Ivan just needed to install shims on the Maximum Motorsports tie rods.
The process of correcting bumpsteer is not a difficult one, especially with the tools and shims from Maximum Motorsports. However, deleting bumpsteer correctly does take a bit of time to set up. Patience is required to accurately measure the changes in toe and add the install the shims to correct it.
Once the rearend was back in the car, bumpsteer adjusted, and springs back in, it was time to install the adjustable sway bar end links. The ability to adjust the length of an end link allows you to install the sway bar in the car without preloading it. You want sway bars to not limit a suspension’s movement until it is the appropriate time to keep the car from leaning. Adjustable end links help dial in that point.
With the IRS system installed and everything shimmed, adjusted, and aligned, plus a new limited-slip differential and gears in place, it was time to head to the track.
Track Time And Drivers Feedback
We were able to secure a two-day track session at Barber Motorsports again to test out our new setup with Ivan Korda driving.
“Having spent two days track testing and putting 50 laps on the car with the IRS Grip box, I am very impressed with the performance,” Ivan says. “The Cobra felt more stable on corner entry, mid-corner and in corner exit. These were all areas where I felt like the car was unpredictable before the installation. I can throw the car into the corner and am confident that it will go where I am asking it. The oversteer is gone unless I overdrive the corner.”
Unfortunately, not everything went according to plan and we experienced brake issues throughout the weekend. Originally, our brake setup had supplied ample braking abilities, but the performance wasn’t able to keep up with the much faster pace and hotter track temperatures compared to our last track outing.
“I boiled the brake fluid two times this weekend. Saturday, after two sessions, the pedal was soft and I had to abort. The fluid was cooked and my track pads had to go into the trash, after just three total track days on them,” Ivan explains. “Even with the Kenny Brown Brake Cooling Duct kit, I knew I would find the limit of the factory braking system, but I didn’t expect it to be so soon.”
We tried to replace the cooked brake fluid and toasted brake pads with new fluid and a set of our original track pads replacements, but after two sessions the brake pedal went away again and it was time to retire for the weekend. While not trying to inflate Ivan’s ego anymore than it already is, the pads just couldn’t handle how hard he was working the brakes and the pad material was disappearing quickly.
When we tested at Barber in November the temperature was 37 degrees. That day, things were cooler and the brakes were fantastic. This weekend the temperature was 66 degrees and I struggled with the brakes a lot. They did not have the same performance as they did in November at our last test.
Even though the times only show a .6 second improvement, we think there was potentially 1-2 seconds left on the table, if the brakes were performing as they should. Unfortunately, preparing for brake fade had Ivan slowing the car down pre-corner entry much sooner than we would like and the results are visible in our lap times. However, that’s why we test. We look forward to making the necessary revisions and heading back to the track!