Cameron Benty
October 4, 2018

You dump the clutch off the line and the rpm spins the tires sending all that Coyote-bred horsepower rearwards. But just as the independent rear suspension is about to kick your ‘Stang in the butt, the tires start doing that NBA thing, bouncing off the pavement and violently breaking traction. Visions of a quick e.t.’s float away like the wind and you have to back off the throttle until the tires settle before applying power and recalculating for the top end. That was fun, right? Not hardly!

The high tech Independent Rear Suspension (IRS) found in the S550 Mustangs (2015-2019) does a great job planting the tires on your car as you travel over irregularities in the road. Held to the underside of the Mustang with four mounting bolts surrounded by large Polyurethane bushings that flex under hard acceleration or cornering, they are not great for the ultimate in straight-line acceleration. Enter the Steeda line up of high performance suspension components designed to not only overcome that dreaded wheel hop, but get you around the corner like you might never have imagined.

As was the case with the front suspension story we recently posted, Steeda’s Mustang Sport Progressive Springs deliver both comfort when cruising and increased spring rates for fast acceleration or cornering because of their advanced design progressing rate windings. These springs features variable rates 800 to 1200 pounds per inch of travel so that as you call for increased roll stiffness and anti-squat due to acceleration or cornering, the spring compresses to increase spring rate in corners and unwinds when cruising down the highway. In addition, these springs reduce body height by about one inch in the rear, helping with overall vehicle aerodynamics and lowering the roll center of the Mustang for improve handling.

On the heels of our suspension front suspension upgrade using Steeda suspension parts, we moved aft and started to change out the rear suspension parts. Once again we used the talents of Saul Gutierrez at Gear Driven Automotive to help with the change over. It is of critical importance to support the IRS throughout the suspension upgrade operation, as you will be loosening the entire rear suspension during this procedure.

Steeda’s Subframe Alignment and support kit is designed to keep the IRS from walking around and helps keep the tires in full contact with the ground. These impressively designed parts, which can be ordered as part of a kit or individually, stiffen up these mounts and reduce flex around the mounting bolt. In addition, Delrin cylindrical inserts slide around the bolt and fill a void inside the bushing to further reduce flex. When combined with Steeda’s IRS Subframe Support Braces, it dramatically improves rear tire grip without the hop. What’s great about this design is that the original bushings are still intact so that ride quality is not dramatically affected.

One of the key mandates with our Steeda suspension is that it allows plenty of flexibility to adjust the suspension to suit the owner’s driving habits. Steeda’s Rear Camber Adjustment brackets are cool little parts that deliver unheard of camber adjustments of the upper A-arm (we also installed Steeda Upgraded Camber Arm that features additional adjustments as well). To increase roll stiffness and prevent unwanted body lean that can reduce tire contact patch, Steeda’s thin wall 1 1/8-inch sway bar's features press-fitted ends that are made from billet 1018 steel and is a pound lighter than stock. With three holes machined into the end of the bar offering a massive amount of adjustability that is just not present with the factory sway bar.

We were very impressed with the ease of installation we experienced with the Steeda kit and the complete instructions detail every step. The final product is a car that is light years from stock but retains a surprisingly smooth ride on the highway. With the new tires and wheels we had planned for the vehicle along with the upcoming Baer Brakes and oversize calipers, the Steeda Suspension has delivered a very strong platform that will only get stronger with these modifications. Just remember to get the suspension aligned – front and rear – or you won’t get all the benefits this system can deliver and you deserve.

Steeda Part list

555-8210 - 2015-2019 Steeda Mustang Sport Springs - Progressive
555-1016 - 2015-2019 Steeda Mustang Rear Sway Bar
FR3Z-18125L - 2015-2019 Steeda Mustang Rear Shocks
555-4126 – 2015-2019 Rear Camber Adjustment Kit
555-4437 - 2015 Steeda Mustang IRS Subframe Bushing Support System
555-4438 - 2015 Steeda Mustang IRS Subframe Alignment Kit
555-4121 - 2015 Steeda Mustang Billet Aluminum Vertical Links
555-5754 – 2015-2019 Steeda Mustang IRS Subframe Support Braces
555-4120 - 2015 Steeda Mustang Adjustable Rear Toe Links

One of the most sophisticated pieces of S550 (2015-2019 model years) Mustang is the independent rearend. While this type of suspension is far superior at handling cornering challenges in keeping the tire contact patch intact than standard live axle suspensions, under hard straight line acceleration the suspension can experience excessive wheel hop. Steeda has made a science of analyzing the excessive movement of the IRS with components designed to deliver better handling performance without sacrificing ride quality.
Among the key parts that we will be installing to the rear suspension include (from top right and moving clockwise) Adjustable Rear Toe links, Rear Camber Adjustment brackets, Billet Aluminum Vertical links with Delrin bushings, IRS Subframe Support braces and Progressive Rate springs (the two on the left are for the front and the two to the right are for the rear).
The IRS is held into our 2018 Mustang with four main bolts and a series of support brackets. Around each of the four main bolts are heavy-duty bushings that allow the suspension to float and absorb road irregularities. Two bolts hold this support bracket in place.
Here is a close up of the IRS support bushing and the bracket that connects to the IRS. Keep in mind that you will be basically unbolting the full IRS and will need to align the rear suspension again to deliver proper handling performance and tire wear.
Steeda’s patented IRS Subframe Bushing Support System includes four different billet aluminum plates that slip in an around the factory bushings. These plates are different front and rear bushings and not interchangeable. In this case, one slides in from the top and one snaps in place on the bottom.
These support plates are designed to fit within notches that are located in the factory bushing. We found that they fit perfectly within the recesses in the bushing.
The rear IRS bushing mounts (one on each side of the suspension) come apart in the same way by removing the retaining bolt. Make certain to mark the bolts so that they go back in the same place they were removed.
With the upper support plate already installed in the rear mount, the lower plate is actually two pieces that are slipped around the base of the bushing and then screwed together. A little tapping with a dead blow hammer was required to start them on their way and then the supplied screws pull them into place.
Steeda noted from testing that the factory mounted IRS bushing were a large contributor of the excessive IRS movement as shown from their video testing (check out their YouTube site). These little cylindrical Delrin sleeves help keep the IRS from moving excessively.
With our support plates in place, we installed the mounting bolts already outfitted with the Delrin sleeves. This is an indication of how much room is actually available inside the factory bushing. Best of all, the Delrin does not transfer suspension noise to the vehicle interior like a billet steel bushing would do.
We removed the factory shocks by removing the two mounting bolts on top and the two in the lower A-arm from below.
We did not tighten the IRS mounts, leaving the entire IRS loose in the vehicle. This is key to other operations that will be performed, most notably the removal of the rear springs that will not come out if the IRS is bolted up tight.
Since we will be replacing the rear sway bar, we unbolted the sway bar links, first at the lower A-arm mount.
The lower A-arm is unbolted from the spindle (two bolts) to allow you to swing the A-arm down and release the spring. When dealing with springs, it is of key importance to be aware that springs can be dangerous and can release an enormous amount of energy. In our case, the springs could not come free but they will push down on the A-arm so be ready.
As mentioned earlier, the rear springs will not come out of their pocket unless the IRS subframe is loose in the car. Carefully pry out the spring with a crowbar working from the bottom.
As was the case with the front suspension, we reused the factory rubber spring pocket mounts. The new factory Steeda spring fits into the notch in the rubber mount like this.
The factory spring is much softer and taller than the new Steeda spring on the right. Again, notice the progressive rate of the Steeda spring with different rate windings throughout the spring. The wider windings delivers a softer ride and engages the tighter wound section of spring under performance driving conditions. Install them in the same way they were removed but do not install the sway bar end links.
These Rear Camber Adjustment brackets are certainly not the easiest part of the Steeda suspension kit upgrade install but they offer serious performance fans the ability to induce greater amounts of camber for high performance driving. The marks shown on the plate behind the bolt head allows you that ability to denote different camber setting such as one for street driving and one for road course competition.
Because you don’t want your camber adjustments changing when driving the vehicle, Steeda includes nylon-locking nuts to keep it in place. For added security, Saul put a little Loctite Blue on the threads as added insurance
The Camber Adjustment brackets mount on the upper A-arm allows camber input that is not possible with the factory suspension. With the A-arm disconnected, they fit over the mount and the bolt then serves as the mount for the top of the arm.
When the brackets are installed they should look like this. Turning the oblong washer is what moves the A-arm in and out and changes camber settings.
Key to the Steeda suspension kit is the ability to adjust tire attitude like never before. The Rear Adjustable Toe link features adjustment in the shaft that occurs by turning the adjustment nut in the center and locking it down with the two nuts on either side; the factory link (held next to the Steeda parts) does not offer this ability.
To lock down the Adjustable Toe Link and keep this component from turning, Steeda included this locking plate and hardware. The factory mount is unchanged from its factory configuration – these parts just fit that well.
To change out the factory rear shock absorber for the Steeda unit, remove the upper cap and turn out the nut that holds the top bracket to the shock with a deep socket.
Understanding that the Steeda gas-charged shocks will expand slowly once they are released, cut the safety strap that holds the shaft in a compressed state.
In order to get the factory dust cover and bump stop off the factory shock, this collar must be removed. Using an open-end wrench slightly larger than the shaft, we hit the wrench on the shaft with a dead blow hammer to slide it off to allow release of the cover and stop. It will not be reused.
As was the case with the front suspension, we needed to cut the factory bump stops about this much because the new Steeda springs will lower the car when we are finished and cause the shock to bottom out.
After tightening the IRS support bolts to reconnect the IRS (remember this suspension will now require an alignment as soon as possible) to the car, we installed Steeda’s IRS Support braces. These nicely designed braces fit up in the framework without any modifications, bending or cutting.
Here is an example of how nicely these braces are designed to fit the factory chassis. Steeda’s IRS Support braces use existing threaded holes within the chassis and come with their own hardware.
The factory sway bar end links are reused with the addition of this gold plate that is slipped over the factory stud and holds the brake line in place once the bar is located.
The Steeda bar comes with its own straps that mount to the chassis and polyurethane mounts that further increase its effectiveness to add vehicle roll stiffness.
Something not present with the factory suspension, the Steeda rear sway bar features three holes in the end for adjustment of rear sway bar input. This photo shows the exact placement of the factory sway bar link.
This image demonstrates exactly out that nifty gold plate holds the brake line out of harms way. The Steeda rear sway bar adjustments allow the driver to increase or decrease vehicle suspension oversteer (the tendency of the car’s rearend to come around in a corner). If you are a drifter, you are gonna love this but remember drifting is not the fastest way around a road course!