Evan J. Smith
July 17, 2014

Mustang owners love aftermarket hardware, especially when the word billet is used. Parts machined from ingots of aluminum can be downright sexy, and the pot is sweetened when they serve an actual performance purpose.

With that, one of the most popular exchanges for late-model Stangs owners is to install aftermarket lower control arms (also called trailing arms) in place of the stock stamped-steel units. Truth be told, the stock stamped-steel arms are pretty strong, especially on the current S197 Mustangs. Ford wouldn't employ the design on the 662hp Shelby GT500 if they were weak.

But while the stock arms are suitable for the daily commute or occasional track attack, the soft rubber bushings can deflect under load and hurt feel and performance. The good news is older arms can be upgraded to a set like these from Bob's Auto Sports. In fact, most aftermarket billet-aluminum and some tubular-steel arms feature polyurethane bushings in place of rubber (race-type arms often feature spherical bearings).

Aftermarket control arms with polyurethane bushings are an affordable way to add performance to your Mustang. These billet-aluminum arms from Bob’s Auto Sports can also shed about six pounds.

No matter the use (street or track), your suspension must provide a safe, controllable, predictable, and comfortable ride. So any time you modify your suspension, it's important to select parts designed to work with each other. This relates to spring rates, shock valving, suspension alignment, suspension link arms, and tires.

The stock arms on all non-IRS '79-present Mustangs are of the stamped-steel type, and the S197 models have weights attached on the inside to dampen vibration and enhance ride. While inherently strong, the rubber bushings can deflect more than performance enthusiasts would like, and they degrade over time, leading to poor ride quality and performance. In contrast, virtually all aftermarket arms come with either polyurethane bushings, delrin bushings, or spherical bearings at the mounting points.

Polyurethane is a great high- performance bushing material—it transfers loads quicker than rubber since there is less deflection. This results in better vehicle feel and performance, yet provides some flexibility, thereby maintaining tolerable NVH.

To kick up performance of this '09 GT, we installed billet arms from Bob's Auto Sports ($219.95 shipped in the U.S.). They feature a sturdy, yet lightweight design, with polyurethane bushings and Zerk fittings fore and aft. Powdercoating is available for $20, and you can select Grabber Blue; Got To Have It Green; gloss black, white, or red; or matte black.

Model: Brooke Weisbender

S197 Mustangs use a three-link design (one upper and two lower arms) with a Panhard bar to locate the rear (to prevent side-to-side movement). From a functionality standpoint, the lower arms connect the rear axle housing to the chassis, but allow it to move up and down through its dynamic range of motion.

The housing is designed to “float” up and down in order to absorb imperfections in the road (such as bumps and depressions); this allows the tires to maintain compliance with the road surface. The spring and rear shocks work in unison to control ride height and dampen the ride. If the rear was locked solid under the car, it would skip across bumps much like a skateboard—the tires would lose contact, and controlling the vehicle would be difficult if not impossible.

Control arms also transfer loads (force) generated by engine torque and vehicle motion. In fact, when you see a Mustang yank the front tires, you can thank the rear control arms as it's those arms that transfer the torque to the body to create lift.

Every action has an opposite and equal reaction, so when power is applied to the drive tires, there is an opposite force on the rear housing that causes rotational force in the opposite direction. Under acceleration, this force pulls rearward on the upper link (and also on the body of the car at the attachment point).

Simultaneously, torque is transferred through the lower control arms that are pushing forward and upward (depending on the angle of the bars) on the body of the vehicle. This causes pitch rotation (or more simply, the nose lifts), which is desirable to drag racers. The forces, however, are reversed under braking and cause the nose to dive. As you can imagine, side loading also occurs during cornering, so the control arms are subject to a multitude of forces, from braking and cornering to accelerating and cornering at the same time.

In short, installing aftermarket control arms is an easy and affordable way to drop e.t., improve feel, and/or quicken your lap times. They also look pretty cool, and can be installed by just about anyone. With that, take a look and follow along with the install on Adi Jordan's '09 GT. She loves to bang the gears so hopefully Bob's Auto Sport control arms will help her improve track times in the quarter-mile.

01. Rear lower control arms (also called trailing arms) attach the rear housing to the chassis (unibody) of the car. Installation of aftermaket arms is relatively easy on S197 Mustangs (’05-present). You can accomplish this in your driveway or garage with basic handtools.

02. It’s best to swap one arm at a time. We used a lift and then took the load off one side of the rear housing using a pole jack. You can use jackstands and a floor jack to accomplish the same thing. With the car elevated and wheels removed, we disconnected the parking-brake-cable retaining clip and unhooked the cable. This is necessary because they are routed through the stock control arms.

03. Next, Shop Manager Darrell Kunda removed the rear control-arm bolt using an 18mm socket.

04. He then unbolted the front bolt and removed the control arm.

05. One of the shortcomings on the stock arms is the rubber bushings. While they provide a quiet comfortable ride, they can deflect quite a bit under hard acceleration, hard cornering, or hard braking. Additionally, they will deteriorate over time.

06. Before installing the new arms, it’s important to lightly grease the external faces of the new bushings. Also, make sure the Zerk fittings (grease nipples) are facing down. The polyurethane bushing material is stiffer than rubber, so it will transfer torque loads quicker than stock. The stiffer material can also transfer more NVH through the vehicle, but it’s a worthwhile tradeoff.

07. With grease applied, we slipped the front of the arms into the torque box and installed the bolt (we didn’t tighten it at this time).

08. The rear of the arm was lifted into the bracket on the rearend housing. It may be necessary to tap the arm into place. Once engaged in the bracket, we used a pointed prybar to align the bolt hole.

09. Once both bolts and nuts were started, we cranked them down.

10. Kunda then greased the bushings using a hand-operated gun and the Zerk fittings.

11. Here is a comparison of the stock arms and Bob’s Auto Sports control arms. The stock steel arm comes in at 7½ pounds, where the aluminum ones were 3 pounds lighter per side.