The 2005+ Ford Mustang, chassis code S197 by Ford, comes with a proper MacPhearson Strut up front and a 3 link plus Pan Hard Bar, live axle rear suspension. In comparison to previous generation Mustangs the S197 represents a huge technological jump for performance driving.
The previous two generations of Mustangs were based on the aging Fox platform which involved a MacPhearson Strut front suspension with the coil springs mounted inboard of the strut location and a triangulated 4 link in the rear that has zero business in a performance car. Up front the spring location required extremely stiff springs to compensate for the poor motion ratios in the front and rear. The front geometry also has a substandard camber curve which is only made worse with lowering requiring super stiff springs and swaybars to keep the front suspension in an ideal section of the camber curve.
Out back the triangulated 4 link (2 parallel LCA's, 2 triangulated UCA's) would bind during roll which artificially increased the spring rate. The result in performance driving was serious cases of snap oversteer when those bushings bound solid. The other problem was the poor lateral location of the rear axle by the UCA's soft bushings. Firming up the bushings resulted in more bind and snap oversteer. One of the biggest modifications for a Fox Mustang is to go to some form of 3 link (usually a Torque Arm) with a PHB or Watts link to control the lateral location.
The Fox based unibodies tended to be very poorly structured and as a result the body had a lot of flex. This meant that any performance driving required subframe connectors or a roll structure to provide enough reduction in chassis flex to adequately use the contact patch and suspension.
The S197 chassis rectifies all of these short comings in a way that creates a very potent corner carving car. The front suspension moved the springs so that they are springs over strut for a significantly better motion ratio which not only improved ride but decreases the size of springs necessary to control the suspension in performance driving. The swaybar now is attached to the strut as apposed to the lower control arm for better motion ratios and the camber curve is significantly improved with a longer lower control arm and other improvements to the front suspension geometry. Ford is jokingly criticized for ripping off the BMW front strut suspension. The result is being able to run a fairly sizable amount of negative camber in a street driven vehicle without destroying tires and a more usable front suspension. No Chapman's law required here!
The rear suspension is a huge leap forward from the previous Fox and SN95 chassis Mustangs. The three control arms in the rear suspension control fore/aft location of the axle as well as pinion angle change, antisquat and roll steer, but it does so without having to deal with locating the axle laterally as well. A pan hard bar is used instead to locate the axle laterally in a cheap, lightweight, and simple solution to a very complex problem.
The S197 chassis is also significantly stiffer than the Fox/SN95 cars to the point where many enthusiasts feel that sub frame connectors are no longer required. Whether or not that is true has yet to be seen as there is still flex in the unibody from not having connected subframes. There is also a strut tower brace on later cars which may indicate Ford isn't as convinced that the chassis is stiff enough to not require extra bracing for performance driving. Needless to say the S197 chassis represents a leap forward in the Mustang chassis.
So what can be improved upon? Aside from some of the "basics" of shocks/struts, good springs, good swaybars, and stiffer bushings, the rest of the modifications are all resultant of any ride height change. For STX prep, live axle cars have a lot of leeway for modifications. A Torque Arm can be added, a watts link can be used and the UCA can be changed and the axle side can be relocated (a relic of the Steeda 5 link for the Fox/SN95 chassis). The Pan Hard Bar (PHB) geometry leaves a lot to be desired in the transitionally violent autocross setting. The PHB has a tendency to plant and unplant tires depending on the direction of the turn. This can create problems in slaloms or quick lane changes where the car remains planted and then unplants in a somewhat violent fashion when changing directions.
The other problem is the PHB geometry induces a rear suspension arc both in the roll center and the whole rear axle. Not only does this place more wear on the bushings in the control arms it creates a feeling of a rear axle that floats under the car and is easily upset by bumps. Prop's to Ford for not cursing us with triangulated 4 link set ups and using a PHB, but it isn't the perfect solution for a live axle car driven to the limits.
Ultimately the solution is a Watts link which is an elegant design that solves the whole PHB problem. There are two types of Watts linkages with different advantages and disadvantages. There is the chassis mounted and the differential mounted.
Chassis Mounted:
Differential Mounted (Image from Griggs Racing):
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The second set up is a differential mounted Watts link. This type of watts link doesn't cause under or oversteer in throttle on or braking situations and it contributes a progressive amount of body roll as the chassis moves independent of the roll center. This type of Watts link can be more advantageous to cars that see more road course use as the braking induced oversteer of a chassis mounted Watts link is generally unwanted. Stiffer sprung vehicles can take better advantage of the roll steer characteristics of a differential mounted Watts as the added body roll can help the rear axle contribute some roll steer.
This is probably the only type of major geometry change over stock that is necessary to the S197 chassis. There are things that can be done with the front roll center with taller ball joints, and rear lower control arm relocation brackets to help with roll steer and antisquat but neither of those modifications are legal in STX so I wont cover them here.
Now, on the S197 chassis, lowering the front more than 1.5" will cause some camber curve issues as this basically levels the front control arm and it slams the front roll center down. In the rear a drop of 1.5" or more will reduce forward bite from the control arm change and requires shorter bumpstops as Ford decided to give a drastically reduced bump travel on the axle for more cargo space in the rear (gee thanks!). Even a mild drop like the Steeda Sport Springs of a 1.25" drop in the rear gets the chassis plenty close to the bump stops on the axle. Riding on the bump stops is obviously bad for handling. I trimmed my bumpstop down a nub (the factory bumpstop has 3 nubs) to prevent the suspension from over utilizing the bumpstops and this resulted in a improvement in ride quality some.
With my drop of about 1" in the front and 1.25" in the rear the car utilizes a slightly lower center of gravity and doesn't sacrifice the front camber curve in the name of a drop and doesn't toss out all of my antisquat. It still encourages a lot of roll understeer as the control arms are angled slightly up at the axle side. The end result is not needing the massive amounts of negative camber that other cars need to utilize the front suspension.
At -1.7º of camber you can see the wheel is just barely over into positive camber. I think about -2.5º with this ride height and this soft spring rate is going to be more than plenty of negative camber up front. I think with stiffer springs I might be able to run even less static negative camber or no more than -2.5º. Testing will show how much is genuinely necessary but -2.5º isn't much for a strut based car. Some folks in E-Street Prepared running the S197 chassis have run upwards of -4º with more of a drop. I intend to utilize some geometry to maintain a safe ride height for daily driving around here and use less negative camber to allow for better tire wear when daily driving.
Awesome diagrams and information! Learned a lot about where the suspension on my base model SN95 stands in terms of performance driving. Lots of work cut out there! Got the message, though, that a subframe connector would be a good start.
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