Guide to Properly Lowering your S197 Mustang

Guys, I found this thread on The Mustang Source, posted by "Galaxie" (Steve) and it is so thorough and informative, that I thought it would be worthwhile to copy it over here and link it as well:

EDIT: Forgot to add the link: http://forums.bradbarnett.net/showthread.php?t=60937

Factory Mustang suspensions are usually designed as an attempt to be all things to all people. If you’re just using it as a highway cruiser, then the stock system will be more than adequate for you. If you are a member of the other side of the coin, who enjoys making passes at the dragstrip, hitting open track and autocross events, modifying the factory suspension is one of the best ways to get your power to ground. By optimizing the suspension geometry, the potential to take turns at higher speeds and knocking down your 60’ times at the dragstrip increases drastically. Usually, an added benefit is a lowered stance, which helps fill up the wheel gaps and adds a more appealing look.

When picking out your suspension, the first step should be to look at what your end goals are: more competent street driving, drag racing, lapping days, full racing. No suspension setup will be ideal for all of those conditions. At the same time, many of us are looking for a happy medium, picking suspension bits that will make the car more fun to drive on a daily basis, take the car to the odd open track and test and tune night at the dragstrip. This setup probably won’t get you the best possible results for those situations, but will make your car more enjoyable on the weekends and the drive to work every day.

I will describe some of the factors that have to be considered when you are lowering a car for improved looks and performance. There will be simplified explanations of some issues and can be researched in more detail if desired. I have consulted with Gus from Steeda about some of these issues. He was very helpful in putting together this package, in addition to being a sponsor of the site.

Front Suspension

There are two critical points with the geometry of a front suspension, the center of gravity and the roll center.

The center of gravity as defined in highschool physics: is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated

The roll center is an imaginary point created by the intersection of the extension of the lines that follow the control arms. On a car with McPherson struts, it is found by drawing a line through the lower ball joint and lower control arm pivot (bushing). You follow that by drawing a line starting from the upper strut bearing, perpendicular to the line of travel of the strut.

The roll center is where these two lines intersect. Keep in mind, the roll center changes with body roll and suspension travel as it is a dynamic point

The ideal suspension design keeps the roll center and center of gravity as close together as possible. The farther they are apart, the more pronounced body roll will be. Essentially, the distance between these two points results act as moment arm when cornering forces are applied, the larger the distance between the two points, the greater the body roll.

By lowering a car, you change the relationship between these two points, moving the two points farther apart and actually increasing body roll. This can be counteracted by using a fatter swaybar. To truly correct the geometry though, requires tweaking the locations of pieces in the front suspension.

Steeda sells two different kits to raise the instant center. The X5 Ball Joint and the Control Arm Relocator Kit. The X5 ball joint kit raises IC slightly less than the relocator kit. The difference is minute enough that only a seasoned track driver would notice the difference. While the ball joint kit is slightly more expensive and does not change the IC as much, there is much less labor to install the joint, as no welding is required. If the car is being lowered substantially (ex. Eibach Sportlines), then both kits can be paired up to correct the geometry.

These two kits are compatible with any quality suspension system on the market.

Note: The X5 balljoint kit requires the use of an 18" wheel because the lower control arm mounts lower at the front spindle and will interfere with 17" wheels.

Bump Steer

When changing the front geometry, you can also negatively affect a property called bump steer. Technically, bump steer is the change in toe as the suspension travels. What happens is because the tie rod is traveling at a different arc than the control arms, it will tend to change the toe when it hits a bump. What that means to you is when your suspension is bottoming out or hitting a hard bump, the tire will move as if the car was steered slightly causing the car to jump in one direction or the other. Obviously, this is not ideal, especially when running at the limit on a road course.

The fix for this is the bumpsteer kit. It corrects the angle of the tie rod ends in relation to the spindle to minimize the effect. Bump steer is most pronounced the more a car is lowered (1.5” and more) and if the suspension geometry is changed.

Camber Adjustment

The factory struts and spindles do not have enough camber adjustment to get proper factory alignments (and even tire wear) when the car is lowered. If you do a search on the forum, you will see that people using mild drops (Steeda, Roush, Saleen), can sometimes get their cars aligned to spec. The majority of users having steeper drops (Eibach, FRPP) almost always need to have some kind of camber correction kit.

Options:
Camber Bolts. (Fox style shown)

These bolts mount off-center with an eccentric. This allows the technician to get the camber within spec. This is the cheapest solution; you do however run the risk of getting the camber out of spec if you hit a large pothole for example. These kits are more than acceptable for street use.

Camber Plates:

These plates allow a large range of motion and allow for custom camber settings for track days. They are more expensive than camber bolts but allow the greatest adjustment and least potential for camber change.

Upper Strut Mounts:



This is potentially another point of adjustment in addition to curing an additional problem. The upper strut mounts are made from plastic on the new cars. The service manual even suggests replacing them in the event the strut assembly is disassembled. Many members have reported the strut popping returning after using lowering springs on the car. Steeda offers a new upper strut bearing which is CNC machined and will cure any strut popping problems down the road. One major benefit is that there is +/- 1 degree of camber adjustment. This means that you can additional front suspension adjustability without the use of camber bolts of caster/camber plates. For custom alignment settings on road courses, caster camber plates should be used in tandem with this setup.

Springs

The purpose of lowering springs is to bring down the center of gravity of the car. Because the effective height of the spring is decreased, the spring rates (stiffness) are usually increased to keep the suspension from bottoming out. The obvious tradeoff to this is a harsher ride.

Breakdown of Steeda Springs: (all the springs lower the car 1-1.25”)

Ultra-lite: This spring is ideal for someone looking for ride quality. The spring is about 3% softer than the sport spring. It also uses a hollow-core wire for less total weight and inertia.

Sport Spring: This spring is ideal for someone who would attend some kind of track event every once in a while, in addition to aggressive street driving. This spring has a similar design to the Ultra-Lite, but uses a solid core wire giving it the additional stiffness.

Competition: Ideally suited for track use but still streetable. These springs are used by Steeda’s racers such as Robin Burnett, American Iron Champion for his division in 2005. They provide superior handling, brake dive reduction.

Shocks

If you’re lowering a car strictly for looks, or not very low, you can probably get away with using the factory shocks knowing that the ride will not be ideal. A shock is designed to dampen the oscillations of a spring with a specific height and spring rate. By not matching the springs and shocks the ride of the lowered car will not be ideal. By using lower, stiffer springs the car’s ride will be harder than a stock ride, using properly designed aftermarket struts and shocks will help dampen the spring oscillation so that the ride will be comfortable and safe on the track and on the street.
There are many quality aftermarket struts available on the market. Most are compatible with the lowering springs on the market. The new Steeda shocks are made by Tokico to Steeda specifications and can be used on lowering springs up to 1.75” drop. On an even lower drop they will not be as effective. The Tokico D-Spec shocks and struts are one of the premium models available and can be fine tuned for the street or track.

Part 2: Rear Suspension

Rear Suspension Geometry

Looking at the side profile of your Mustang, there is also a center of gravity point. On front-engine, RWD cars, it is typically found slightly forward of the middle of the car, about 2 feet from the ground. A typical 3-link suspension applies power or lift at its instant center, which is not necessarily the center of gravity. The instant center can be found by extending lines through the mounting holes of the rear suspension all the way until they intersect (similar to the front suspension). This position changes as the car is loaded and the control arms change their angles. By changing the location (and subsequently the angle) of the upper and lower control arms, you can move the instant center location either above, directly on, or below that 100 percent antisquat line. When the instant center is positioned below that 100 percent antisquat line, the rear of the car will squat on acceleration and "hit" the tires relatively softly. When the IC is positioned above the 100 percent antisquat line, the rear of the car will tend to rise on acceleration and "hit" the tires harder. Obviously, if the IC is placed directly on the 100 percent line, the rear will remain neutral. There is a reciprocal effect to this in braking nosedive. If the instant centre is above the 100% antisquat line, it will tend to exhibit harder nosedive on braking than when neutral or below the antisquat line.

The antisquat line is found from drawing a line from the contact patch of the tire to the instant center. The antisquat line should run across the vertical line through the front tire. Typically, antisquat characteristics are found if the point crossing the centerline of the front tires is within 50 percent to 80 percent of the center of gravity height. If it is desired that the rear suspension lifts more, then that point should be above 80% of the center of gravity height.

Many vendors sell a rear control arm relocation kit. Most of the kits have different mounting points to change the geometry of the rear control arms. For example, when taking your car to the track, you can change the rear control arm angle to allow the tires to plant harder. The tradeoff is that the car will have a much more pronounced nosedive when braking. This is not usually an issue at a drag strip, but the suspension setup can easily be changed to have more balanced handling while driving during the week.

Pinion Angle

The pinion angle is the angle between the pinion and the driveshaft. When a vehicle is lowered, the angle changes due to the change in ride height. An incorrect pinion angle can cause many different negative results. An incorrect angle can result in premature wear in the universal joints. Additionally, under hard acceleration, as the rear differential torques “up” the differential housing may contact the floorpan (this is in extremely lowered situations).

The way to correct the pinion angle on a lowered 3-link suspension is by using either adjustable upper or lower control arms. Steeda’s fixed upper control arm uses a shorter length than factory allowing the pinion angle to reach stock specifications. Because it is fixed, the adjustablility is not there to fine tune the angles for hardcore racers.

Upper Control Arms

The rear upper control arm also takes part of the load from the rear axle during hard launches on the street and dragstip. The factory piece has a soft rubber bushing in addition to being a stamped steel piece. A good idea for anyone lowering their car (or even keeping stock ride height) is to upgrade this filmsy factory piece with a more stout aftermarket unit. The harder rubber bushing and stiffer design will eliminate deflection and reduce wheel hop under hard acceleration. There may be some small increase in NVH from going to stiffer bushings though.

Lower Control Arms

Because the S197’s relocated the spring mounts from the lower control arms to the top of the rear axle, the rear control arms are much smaller than previous iterations of the car. They are still built using steel stampings with large soft bushings. To stiffen up the rear suspension even more, quality aftermarket control arms should be considered. There are 3 basic options to consider for your car when looking to upgrade your control arms; fixed length street, adjustable street and adjustable race. The fixed length street control arms typically are tubular or boxed steel design with urethane bushings at either end. These control arms are the most road friendly of the available control arms. Adjustable street control arms have a threaded portion which can be used to correct pinion angle. Additionally, some adjustable “street” control arms can come with spherical rod ends on one side. According to Steeda, control arms like this are liveable on the steet and will have a slight increase in NVH. The full race rear control arms are similar to the adjustable street control arms, but they usually have spherical rod ends at both ends of arm. The benefit to this is a very stiff design with no bushing deflection. The negative aspect of these control arms is a significant increase in NVH which would likely be unbearable on the street. One thing to note which has been shown on the forums quite a bit is that following the lubrication instructions for the bushings is critical. Using the incorrect lubricant or too much of it will result in premature failing of the bushings.

Panhard Rod

Because the rear suspension uses a panhard rod to locate the rear axle in the lateral direction, as the suspension is lowered this can cause the rear suspension to shift to one side. This effect will become more pronounced as the car is lowered. An adjustable panhard rod will correct this shift in the axle and allow the rear wheels. Aftermarket panhard rods are also built sturdier and with stiffer bushings than factory pieces. Just like the rear lower control arms, some panhard rods feature spherical rod ends and some urethane bushings. The same NVH and deflection issues as with the lower control arms should be taken into consideration with your purchase.