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.

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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.

Here’s all the bits we needed to upgrade our test car’s IRS. Maximum Motorsports’s polyurethane IRS subfrane bushings and removal tool, polyurethane sway bar bushings and adjustable end links, and Delrin® upper and lower control arm bushings

 

The biggest shortcoming of the Mustang IRS is the deflection of all the rubber bushings. -Chuck Schwynoch, Maximum Motorsports

Sure, some owners just swap the IRS for a solid axle, but Maximum Motorsports rarely recommends this approach. Chuck Schwynoch, C.E.O of Maximum Motorsports wants customers to keep in mind that, “A stock Mustang solid axle does not provide good performance from the factory. The solid axle needs a lot of upgrades to improve its performance, and so it should be no surprise that the IRS also needs upgrades.”

“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.

We opted for Maximum Motorsports’s Low Profile IRS Rear Sub-Frame Bolts. The OEM bolts are the first to contact wide tires, so the low-profile bolts gain a bit of valuable clearance. And, if the tire does rub the bolt during hard cornering, the rounded surface of the head is less likely to damage the tire.

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.”

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Left: Dropping the IRS is a lot easier than it sounds. We started by getting the car on four jack stands with the rear higher than the front to so we had additional clearance when dragging the IRS out from under the car. We checked that we had at least 19” between the floor and the rocker panel pinch weld. Center Left: The wheels and the cat-back exhaust came off first. Next, we unbolted the driveshaft from the pinion flange, and hung it from the vehicle floor with zip ties. Center Right: The brake calipers stay attached to the car, we removed them from the rear spindles by disconnecting the parking brake cables. The brake hoses route below the upper control arm, so we needed to move the hoses before dropping the IRS subframe. We scribed marks indicating the upper spindle camber bolts’ eccentric position, removed the bolts, and raised the upper control arms enough so we could reposition the brake hoses above the control arm. We then loosely reinstalled the camber bolts. Right: We then removed the lower control arms and their bushings using the tools Maximum Motorsports included.

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.”

With the calipers, brake rotors, shock bolts, and ABS sensors removed, we supported the IRS assembly with a 2×6 and a floor jack before removing the four IRS subframe mounting bolts. A helper stabilized the IRS assembly as we lowered it to the floor. Voila! We rolled the IRS assembly to an open area in the floor and prepared for disassembly.

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.”

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Left: With the spindles and axles removed from the IRS assembly, we unbolted the upper control arms from the IRS subframe, and set them up in a vice with some soft jaws. Center: Maximum Motorsports includes a tool to remove the upper control arm bushings. The bushing needs to be cut to make room for one of the legs of the removal tool. To avoid cutting into the control arm, we stopped before cutting completely through the shell, and worked the last portion loose with a pliers. Right: With the removal tool straddling the bushing, we pulled the bushings out.

Deflection of the rubber allows the alignment to change when driving, and also contributes to wheel hop/driveline shudder.” -Chuck Schwynoch, Maximum Motorsports.

Continuing Schwynoch says, “We tested at our home track, Buttonwillow Raceway—a challenging, very technical road course. After a day spent swapping in rear springs with different rates, adjusting the rear alignment, and experimenting with different amounts of rear bumpsteer, test driver Dave Royce put down a best lap time three seconds under the American Iron track record. Needless to say, we have not returned to the solid axle. That testing helped us gain the knowledge to assist our customers with improving their IRS. A very important thing we learned while track testing was that the IRS is very sensitive to alignment and bumpsteer settings. To ensure optimum handling, both the static alignment and bumpsteer must be adjusted to the proper settings. Our track testing also helps us provide good comparison information to people who are deciding between a solid axle or an IRS for their Mustang.”

Subframe Bushings: Left: The lower control arm’s rear tab has a washer tack welded to one of the tabs. To remove it, we cut into the spot welds with a Dremel and a cut-off wheel. Then, we used a chisel to pry off the washers. Center: We then drilled a series of 5/16-inch holes around the bushing near the shell to weaken the bond between the bushing and the shell. Right: To make room for the Maximum Motorsports polyurethane bushings, we removed all of the remaining rubber from the shell with a wire wheel and a 2-inch sanding disc.

Alignment Considerations

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.

The Maximum Motorsports Bushing Solution

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.

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Left: To make room for the Maximum Motorsports polyurethane bushings, we removed all of the remaining rubber from the shell with a wire wheel and a 2-inch sanding disc. Center: To straighten the lower control arm tabs, we used our adjustable wrench and a big C-clamp to straighten the tabs. We then used Maximum Motorsports’s included tool to flatten the tabs. We finished by dressing the surfaces with a sanding disk and checking for uniform clearance with the Delrin bushing’s crush tube. Right: We pushed the control arms all the way forward on the bushings, and chose the largest shim stack that that fit into the gap between the bushing shoulder and the rear edge of the upper control arm. Then, we removed the control arm and split the spacer stack in half, placing half of the spacer stack on the rear bushing, and the remaining spacers on the front bushing. We then reinstalled the control arms, adding a thrust shim on each side of the bushings, and rechecked for bind. The control arms moved easily by hand, so we turned our attention to the lower control arms.

Test Drive

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!

Left: After we ensured that all the bushings didn’t bind, and the arms articulated smoothly, we were ready to lubricate the bushings. Maximum Motorsports included several packets of Teflon grease to lubricate all the sliding surfaces. Center: Before installing the upper and lower control arms, we installed the IRS subframe mount bushings. We lubricated the bushing shells and the outside diameter of the bushings. The bushings fit perfectly, and slid into place with firm hand pressure. Right: We wrapped up assembly of the IRS by installing the Maximum Motorsports Adjustable Sway Bar End Links. With the spacer stacks specified in the instructions, we inserted the end link mount bolts into the control arm and sway bar.

Balanced on a wide plank and our floor jack, we rolled our newly-fortified IRS assembly back under the car, and jacked it back up into place. We had a friend help stabilize the IRS while we jacked it up and inserted the mounting bolts. The rest of the assembly process is pretty much the reverse of the teardown.

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!

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