Many owners of 1999-2004 Mustang Cobras have a love/hate relationship with their Mustang’s independent rear suspension (IRS). They appreciate the handling confidence and smooth ride that an IRS provides, but don’t like the side effects of the compromises Ford engineers made when designing the IRS. After all, it was meant for a car that was never supposed to have an IRS, and had to fit without any modifications to the chassis!
The objective of this 2003 Cobra is to create be a well-rounded, capable street car with modifications that maximize driving enjoyment and minimize noise, vibration, and harshness side effects. Modifications are focused on the chassis and suspension, the most significant being a Maximum Motorsports coil-over conversion using 400 lb-in front springs and 650 lb-in rear springs. Power modifications are minor: just JBA mid-length headers, home-built X-pipe, and a Flowmaster cat-back. Actual horsepower is unknown…probably 450 around at the flywheel.
Firmer bushings were desired to help reduce driveline shudder and wheel hop, and also help the car track better through corners. The IRS bushings helped on both counts, without any major side effects. Perfectly in line with the purpose of this car.
Which Is Better, IRS or Solid Axle?
Over the years, the aftermarket has taken two approaches with the IRS: scrap it for a solid rear axle, or refine it. Maximum Motorsports (MM) has chosen the latter path, and developed a series of components to turn the IRS into a confident dancing partner for corner carvers.
The biggest shortcoming of the Mustang IRS is the deflection of all the rubber bushings. -Chuck Schwynoch, Maximum Motorsports
“Whether or not to swap from the IRS to a solid axle depends on your goals for your car, and if the IRS in your Mustang is causing a problem that a solid axle can solve. Especially in the early days of the Mustang IRS most people in the Mustang world were unfamiliar with the IRS, and did not know how to solve the issues of the OEM Mustang IRS,” says Schwynoch.
Twelve years later, Maximum Motorsports know what works. “The biggest shortcoming of the Mustang IRS is the deflection of all the rubber bushings,” claims Schwynoch. “Deflection of the rubber allows the alignment to change when driving, and also contributes to wheel hop/driveline shudder.”
Compared to a solid rear axle, Schwynoch says an IRS, when fortified with Maximum Motorsports’s products, “…will provide better ride quality when street driving, and better cornering ability. The IRS absorbs bumps much better than does a solid axle, and on the road course that prevents bumps from upsetting the car as much as with a solid axle. A solid axle equipped with a torque-arm suspension will not corner quite as well as an IRS, but will do better at putting the power down at corner exit, and at the drag strip. The IRS requires more care with the alignment settings and bumpsteer correction than does the Mustang front suspension, but that is usually a one-time setup.”
IRS Product Development
Maximum Motorsports knew that in theory, the IRS should be faster. But the stop watch never lies, so to be sure, They went to the race track for testing. Schwynoch recalls, “We swapped out the solid axle with its well-sorted torque-arm suspension in our American Iron race car for an IRS fitted with all of our newly developed parts. The race car was a known quantity on the track; it had won races and set track records. To ensure a good test procedure, we had been racing the car with the rear track widened to match the IRS. When the IRS went in we also swapped the rear brakes from the solid axle over to the IRS; that included the rotors, calipers, and pads. The car weight increased by 40 pounds.”
Deflection of the rubber allows the alignment to change when driving, and also contributes to wheel hop/driveline shudder.” -Chuck Schwynoch, Maximum Motorsports.
According to Schwynoch, “The IRS is relatively sensitive to imperfect alignment and excessive bumpsteer. The actual IRS geometry is decent enough, even when the car is lowered.” We already had IRS Adjustable Complete Tie Rod Kit installed, which addressed the rear bumpsteer curve concerns, however we needed to adjust the static wheel alignment for the stiffer Maximum Motorsports IRS bushings.
With the OEM bushings, we had our test car’s rear toe set to nearly zero, because the soft OEM rubber bushings allowed the outside rear tire to “toe in” during cornering, which helped stability. However with the Maximum Motorsports Delrin® bushings, we needed to add a little bit of “static toe in” back to the rear, since the rear wheels will no longer “gain” toe from cornering forces.
Our test car already had some of Maximum Motorsports’s IRS products, and we completed its fortification with MM’s polyurethane IRS subframe bushings, polyurethane sway bar bushings with adjustable end links, and Delrin upper and lower control arm bushings. These bushings minimize the compliance in the rear suspension pivots, keeping the wheels pointed in the right direction for improved handling and traction.
Delrin versus Polyurethane
Delrin bushings are a lot more expensive than the ubiquitous polyurethane bushings. Schwynoch says that, “Delrin is a thermoplastic made by DuPont with properties that make it an excellent choice for some automotive suspension bushings. According to Schwynoch, Delrin is high in strength, stiffness, and hardness. It is thermally stable, has a low coefficient of friction, low moisture absorption, and is self-lubricating. It can be machined to shape, in a manner similar to machining aluminum. Since Delrin has less stiffness than most metals, it transmits less noise into the chassis when used as a bushing material.
“Polyurethane (also called simply “urethane”) is a polymer plastic that requires lubrication if movement is required. Without some sort of grease, urethane causes friction in pivoting applications, and will make a squeaking noise. Unlike Delrin, urethane can cold flow, changing shape in unwanted ways,” says Schwynoch.
With the wheels back on and the car on the ground, we were ready for our test drive. As expected, the car felt more nimble and tracked more accurately through corners. Whenever installing stiffer suspension components, there’s a tradeoff. Less compliance usually results in increased noise, vibration, and harshness (NVH). Happily, we didn’t notice any increase in vibration or harshness after installing the Maximum Motorsports components, and only a very slight increase in gear noise from the differential—but just at 50 MPH. Given how much stiffer the MM bushings were compared to the OEM rubber pieces, we don’t mind a little extra gear noise!
The most noticeable benefit of the Maximum Motorsports bushings, when combined with a set of BFGoodrich G-Force KD tires in place of our worn Yokohama ES100 tires, was that the driveline shudder that plagued our IRS was virtually eliminated! It was finally a pleasure to spin the rear tires from a roll without fear of shaking the car apart.
Overall, it was a great success. The Maximum Motorsports IRS bushings were straightforward to install, and we realized some major benefits with only a minor increase in rear differential noise. For owners that remain faithful to the Cobra’s independent rear suspension, we think the Maximum Motorsports IRS bushings are definitely worthwhile!