Griggs Racing has provided a very useful bit of history on the background and history of the Mustang platform, its suspension "weaknesses", how to "fix" them and some concise definitions of commonly used suspension terms (e.g., "bump steer", "roll center", etc.).
The entirety of this information can be found at Grigg's webpage (
Griggs Racing Products and more specifically the technical analysis articles on the S197 platform: e.g.,
Griggs Racing Products - Articles - Chassis - S197 GR40 System Reasoning of Design) but an edited version of the basis commentary is set forth below for those who are interested:
(from Griggs Racing's website, with editing as noted)
It is important to understand the heritage of the current Mustang suspension if you want to fully appreciate the significance of it's design shortcomings, and why and how we address them.
Believe it or not, the Mustang has never been a purpose built car (like a Corvette for example); it has always been built from an
existing platform. The first Mustang evolved from the Falcon. The
Mustang II was little more than a reskinned Pinto. The Fox & SN95 Mustangs are direct descendants of the Fairmont, and use the same basic chassis to this day.
The Fairmont was introduced in late in 1977 as a '78 model. At the time this car was designed (work probably started sometime in '74) we were in the middle of an energy crisis. The days of high horsepower cars were gone, most people believed never to return. Few people had discovered the delight of driving a good handling car, and the Big 3 were content to let them buy funny little cars from Germany with the engine in the wrong end.
Detroit's main concern was competing with the inexpensive and economical cars coming from Japan. The Fairmont had to be light and cheap to build. It didn't have to deal with a lot of horsepower, big tires, or threshold braking. Certainly none of the designers ever imagined anyone would ever race them!
Some helpful [suspension] definitions:
Before going into too much detail, let's take a minute to define some of the terms we commonly use that you might not be familiar with.
Roll Center - Both front and rear suspensions have a Roll Center. This is an imaginary point around which the body of the car will rotate in a turn. The attachment points of the
suspension components determine the Roll Center.
Roll Axis - A line between the front & rear Roll Centers.
CG (Center of Gravity) - The center point of the vehicle's mass.
Understeer - When the front tires lose traction first.
Oversteer - When the rear tires lose traction first.
Neutral Steer - The ideal balance when the front & rear tires gradually give up traction at an equal rate.
Spring Rate - Expressed in pounds per inch, it is the force
necessary to compress the spring, i.e. a 200 lb spring requires 200 lbs to compress it 1", 400 lbs to compress it 2", etc.
Motion Ratio - Specifically we usually refer to the relationship between the motion of the wheel and the motion of the spring; i.e. If the spring is half the distance from the control arm pivot as the wheel is, the motion ratio relative to the wheel is .5 to 1.
Wheel Rate - The combined effect of spring rate, motion ratio, friction and/or binding of other suspension components measured at the wheel.
Roll Bind - Any binding of suspension components that occurs as the body of the car leans over in a turn
Roll Steer - Generally refers to a steering effect on the rear
axle as the car leans over in a corner. Caused by the rear control arms pivoting around their forward mounting point, drawing the axle forward as the arm moves up or down.
Bumpsteer - Toe change as the suspension moves up & down
Ackerman - AKA "Toe out in turns". When turning the inside tire must turn more than the outer tire because it is turning on a smaller radius
Camber - Expressed in degrees, it is how much the tire leans in or out
Caster - The forward inclination of the spindle or strut - like the forks on a bicycle
Toe - The difference in the distance between the leading and trailing edge of the tires
Specifically the Mustang's shortcomings are:
Too much flex through the floor of the unibody.
Rear upper control arms whose function is to locate the axle housing laterally as well as control it's rotation (they don't do either very well). They give the car a very high rear roll center,
and bind as the car leans into a corner. The binding causes a sudden increase in the wheel rate that results in the Mustang's characteristic "snap" oversteer.
a. The current front suspension geometry yields only about 2 degrees of caster, which was fine for the skinny whitewall tires that were used on the '78 Fairmont, but is inadequate for today's low profile high performance tires.
b. The front suspension also has a very low roll center (it can actually be below the ground if the car is lowered too much), which combined with the high rear roll center gives you a very steeply inclined roll axis.
c. The angle at which the front A-arms are mounted promotes brake dive.
d. Bumpsteer-The stock suspension has too much bumpsteer.
e. Ackerman-The steering rack is not positioned correctly, and does not provide enough toe out while turning, causing the outer edge of the outside tire to drag through the turn. [...]
Solutions:
Most others throw stiff springs, shocks, and bushings at the car, limiting suspension travel in an attempt to delay the manifestation of the undesirable handling characteristics caused by the stock suspension geometry. This has the added effect of making the car very skittish over uneven road surfaces as well as degrading the overall ride quality. Unfortunately, many people believe that in order to handle well the car must ride this way, and that this is a natural trade-off.
[In order to solve these problems it's important to] start at the root of the problem: The geometry.
The first thing [is] to address the rear suspension. With the TorqueArm & Panhard bar relieving the upper control arms of their duty, [you can] eliminate[...] the bind-induced snap-oversteer and lowered the rear roll center. At this time [you can] also install the subframe connectors, significantly reducing the flex through the unibody.
In front the K-member relocates the A-arm pickup points, increasing caster for better high speed stability, decreasing brake dive, raising the front roll center, and relocating the steering rack to correct Ackerman. Bumpsteer [can also be] addressed at this time with the installation of [a] bumpsteer kit.
[O]f course the car will still need a good set of springs & shocks, but with the corrected geometry the relationship between the CG and the roll axis is significantly improved, which means [you] can use much softer springs and still keep the car flat in a corner.
The end result is a car that is predictable and easy to drive at the limit, without compromising the ride quality.
FYI: analysis of Mustang suspension weaknesses/fixes 
