The S550 platform represents the most technologically advanced Mustang in history. Despite advancements that make the S550 a better car in nearly every aspect than its predecessors, there are always places to improve its performance. One of those places exists in the suspension system. For that we turned to the experts at BMR Suspension, who have been busily developing parts for the latest Mustang since last fall.

Rather than take the route of many project cars of the past with a street/strip build, Project 5-liter Eater, our 2015 Mustang EcoBoost, is serving triple-duty. The car is primarily a daily driver and weekend autocross racer, but it will also see some dragstrip testing as well. The latest improvements we’re installing to the rear of our project car from BMR will improve both the handling characteristics and its performance on the dragstrip.

Deflection

“The biggest part of the S550 IRS that needs improvement is by far the bushings. Ford uses soft rubber bushings with big voids for the cradle and differential. These do a few things, some are good, and some are bad. They do a great job of absorbing noise, vibration, and harshness (NVH), giving you a smooth comfortable ride. But, you’d cringe if you saw what was actually happening under the car,” says BMR’s Pete Epple.

Deflection occurs as the suspension reacts to acceleration or cornering forces. The energy of that motion is initially absorbed by the bushings, however, the bushings then must do something with that energy. It is released back through the bushing and causes the suspension to move again. This is the most common cause of wheel-hop, and if the condition is violent enough, or occurs often, the result is typically damaged parts.

“This deflection in the cradle or differential is so extreme that it allows the entire cradle to shift, deflect, and oscillate when power is applied. These waves of deflection cause the tires to rapidly grip then lose traction, which is extremely hard on the rear suspension and driveline components,” Epple says.

Securing The Differential

On S550 Mustangs the rear differential is held in place by four mounting points, each with a cradle bushing. These absorb NVH but, they contain voids in the material, and are a rather low durometer, which equals soft performance.

However, they do a great job of keeping gear, differential, and road noise out of the passenger compartment. But, these bushings have so much deflection, that after removal of the stock parts we found just dropping them from a height of around four feet onto the shop floor caused the OEM bushing to bounce and roll across the shop on its own.

According to Epple, when power or load is applied to the driveline these bushings are not only soaking up unwanted noise, but also power that should be making its way to the tires.

“When power is applied and transfers to the differential, the differential bushings deflect. This causes the differential to rotate. The pinion rises and the rear cover drops. This rotation absorbs power, which causes the car to react slower to power application. It also changes the pinion angle. Changes in pinion angle can affect NVH, but with the IRS will not be as critical for power application,” he says.

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The entire rear cradle must be removed to install the differential bushings. To do this, the exhaust and driveshaft must first be removed. Next, the rear brakes calipers, shocks, and parking brake cables are disconnected. Finally, the four bolts that secure the rear cradle to the chassis are removed. We used a rolling cart to lower our rear cradle onto and out from under the car. This allowed us to easily work on all of the rear suspension components, and made installation of the entire assembly easier once we had completed installation of all the new BMR parts.

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We used a drill (top) to cut out the voids in the rear bushings. This was followed by a sawz-all to cut the thicker portions of the bushings. Finally a sawz-all must be used to cut slits in the metal sleeve, and a large hammer is used to knock them out from the inside of the cradle.

BMR supplied us with their polyurethane rear differential bushing kit (PN BK049). These bushings have a 95 durometer rating and are a direct replacement for the OEM rubber bushings. The rear subframe/cradle must be removed to gain access for this installation, however the increased performance is well worth the effort. There is a trade-off with increased NVH, however, as we’ll discuss later, it’s small, and the performance improvements are worth a little extra noise on the road.

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(Left) With the differential bushings in place, slide in the new sleeves. (Right) The finished installation of the differential bushings should look like this.

Vertical Links

The S550 Mustang IRS features a unique vertical link which connects the rear spindle to the rear control arm. This design is a vast improvement over the old ’99-’04 Cobra IRS system, which had a tendency to rotate under hard acceleration loads.

Epple says, “Because of the design, the vertical links see tremendous amounts of force applied to them every time you hit the gas. We needed to design a link that was strong enough to take the abuse without adding a lot of weight, and billet aluminum was the material of choice. The factory vertical links have rubber bushings, which have similar deflection characteristics to the cradle and differential bushings. This deflection causes the rear spindle to rotate back and forth slightly as the bushing deflects. This absorbs power and contributes to wheel-hop.”

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BMR's upgraded billet aluminum vertical links are substantially stronger than the OEM stamped steel pieces. The BMR parts feature spherical bushings versus the OEM rubber bushings.

Replacing the vertical links does not require removal of any suspension components other than the rear wheels. Constructed from heavy duty billet aluminum, versus the stamped steel design of the factory parts, the upgraded BMR links (PN TCA049) use spherical bearings, instead of the factory rubber ends. Epple says these links can take the abuse and won’t cause much, if any additional NVH.

Keeping Your Toe

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BMR's toe rods aren't just substantially stronger than the OEM parts. The new toe rods also offer more rear alignment stability than than the OEM parts, and according to BMR do not alter the car's NVH characteristics.

According to Epple, the toe rods play a more important role than most enthusiast realize. He says that under extreme cornering loads, or under extreme acceleration loads, like what is seen in a drag strip or road course environment, the factory rear eccentrics can actually move and cause alignment changes.

“If there is any deflection in the factory toe rod or toe rod bushings, that deflection is directly transferred to the alignment of the rear wheels. We wanted to eliminate as much deflection as possible. This is why we use heavy-duty rod ends and eccentric lockout plates. The eccentric bolts make alignments fairly simple, but if a car has more than factory power levels and sticky tires, the eccentrics don’t do a good job of keeping the alignment consistent.

Imagine starting a lap with your rear toe setting in one location and ending in another. The way the car handles is going to change with each lap. The eccentrics can move under aggressive straight-line acceleration also, so drag racers aren’t safe from toe changes. By eliminating the eccentrics and using the arms for the adjustments, you add a large amount of consistency to the rear alignment,” Epple says.

Having the car aligned properly, and ensuring those settings stay consistent throughout an event, are keys to solid performance. The BMR toe rods (PN TR005) are another component that is easily installed, and in spite of the use of rod ends with spherical bushings, they do not contribute to additional NVH when driving the car.

Locked Cradle

It may sound odd, but the entire rear cradle that the 2015 Mustang rear suspension is built around attaches to the car with only four bolts. There are also two shear plates that help locate the cradle. Unfortunately, like much of the rest of the rear suspension, the bushings for the rear cradle contain a series of voids to reduce NVH. The bushing design also uses a bolt that is significantly smaller than the inside diameter of the bushing for the bolt to pass through. While this allows for adjustment in extreme alignment situations, it also creates inconsistencies in the way the rear suspension performs, and may allow alignment to shift under hard cornering loads.

Replacing the bushings with high-durometer material could contribute significantly to NVH. BMR has come up with a different solution, which positively locates the rear cradle in the form of its rear cradle lockout kit (PN CB005).

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The cradle lockout kit can be installed without completely removing the rear cradle assembly. Once in place it solidly secures the rear cradle in position and braces it better than the factory sheer plates.

“We knew the soft cradle bushings with voids were going to be a massive contributor to wheel-hop. We also knew most people don’t want to change bushings or have the added NVH of polyurethane or solid bushings for a street car. We started looking at the cradle bushings and how it attaches to the car and realized there really wasn’t much locating the cradle and this causes much of the deflection,” says Epple.

“We designed the cradle lockout kit to capture the top and bottom of the outer bushing cups on the cradle, as well as the inner sleeve of the cradle bushing. The result is greatly reduced fore, aft, and lateral cradle bushing deflection. There is a stainless plate that mounts on the bottom side of the rear bushing, which prevents vertical cradle movement. The factory sheer plates are replaced with BMR’s fabricated chassis braces. These braces have a large lug that inserts into the cradle bushing sleeves, positively locating the cradle. These braces tie the entire system together.”

Supporting Role

BMR also sent us a set of their new adjustable sway bar end links (PN ELK012) to compliment our previously-installed BMR sway bars. While most of us might not think much about replacing the end links, Epple pointed out that the stock end links have a tendency to flex under extreme load, especially when using them with a sway bar upgrade. This flexing then reduces the amount of work the sway bar is able to do, hindering the performance of the upgraded bars.

“The amount of force exerted on the end links is unbelievable. They keep a 3,500-plus pound car from twisting as you push it around a turn as hard as possible. Any deflection in the sway bar end links is a reduction in sway bar rate applied to the car, and that means more body roll. The adjustable ends are designed to do two things: be stronger than stock for the reasons listed above, and allow you to keep the sway bars setup properly on lowered Mustangs. Sway bars are designed to operate at a certain working angle (for most applications this is parallel to the ground). When the car is lowered, the working angle of the bar changes and can impact the effectiveness of the bar. Having an adjustable end link allows you to change the working angles of the sway bar for optimal performance,” Epple says.

Install

Installation of most of these parts can be performed with the wheels off the car and on the ground. We took our car to London Chassis Dyno, in London, Kentucky, where owner Chad Epperson and the main shop man Willie “Red” Taylor helped us with every step. We’ve highlighted the install throughout this article. Getting the differential bushings swapped out does require removing the entire rear cradle from the car, and removing the differential/center section. We recommend the help of a professional or at the very least using a lift to safely get the car high enough for the work to be performed.

The entire installation takes about a day to perform on a lift. If you’re doing all of this on the garage floor, plan on it possibly going over into a two-day affair.

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Side-by-side our OEM suspension vs the installed BMR upgrades.

Road and Competition

On the street, we instantly noticed a difference in how the car handles with the new BMR products installed. While there is some additional gear and differential noise from the rear, it’s nothing that is intolerable. The noise transfer reminds us of older Mustangs with a solid rear axle. There’s no perceivable difference in the actual ride quality of the car, the suspension changes aren’t harsh, and the ride quality is not compromised.

Just two days after performing this installation, we headed back to the autocross with Project 5-Liter-Eater. This is where the car was really able to shine. At NCM Motorsports Park for the Kentucky SCCA Region’s Points Event 5, we were able to push our EcoBoost Mustang harder than ever before. The results are more impressive when we consider we were running on the OEM Performance Pack Pirelli PZero tires, which are not made for autocross, and ours have more than 80 course laps on them.

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“It’s amazing how different the car can feel when the rear suspension is solidly connected to the chassis. The parts we sell are designed and engineered to change the IRS as a ‘system’ and not just individual components. By reducing or eliminated bushing and cradle deflections, vertical link deflection, and making the toe setting consistent, the IRS can work and not be limited by its shortcomings when the rear suspension is solidly connected to the chassis. The parts we sell are designed and engineered to change the IRS as a “system” and not just individual components. By reducing or eliminated bushing and cradle deflections, vertical link deflection, and making the toe setting consistent, the IRS can work and not be limited by its shortcomings,” says Epple.

We had previously dealt with some understeer issues with our car. The BMR parts reduced understeer so much that we struggled on our first two runs on the autocross to find the car’s limit in high-speed sweeper turns. The car was rotating so well, that we actually missed an exit gate for one of the sweepers on an early run, driving inside of a gate where we would have normally been far to the outside.

We were expecting the car to take a wider line, however it rotated so quickly that we found ourselves inside the gate with no time to correct. The result was bad for that run, but better for the car, and every run after. If the car was fun to drive before, it is outstanding with these parts in place. Our autocross outing was so successful we were able to take home third place in the CAM class that day, Editor Don Creason’s first ever autocross trophy.

With the suspension now able to put the car in a better position on the track, we just have to find ourselves a good set of track wheels that will be able to keep up with these new BMR parts next season.