Fox Mustang Front Suspension upgrade Copyright © 2000 by Eugene Chu

Introduction:

The late model Mustang is built on Ford's "FOX" chassis platform derived from the Fairmont in 1978. This chassis has a front suspension that uses a "modified" MacPherson strut and independent lower control arms. The struts are modified from the traditional design in that the springs are located inboard of the struts, instead of concentrically over them. MacPherson struts were originally used in small cars because they are more compact, and therefore can allow very compact packaging, especially in the engine compartment. The struts perform triple duties as suspension location members, steering pivots, and suspension motion dampers. And, it also provided the location for the springs. This design is adequate for use in most passenger cars but does not have the flexibility needed to provide good handling characteristics. Ford also lost its compactness by putting the springs in another part of the suspension.

The specific problems with Ford's strut suspension start with its static geometry. In order to minimize scrub radius, it uses a lot of kingpin or inclination angle. This is effectively defined by the inboard inclination of the strut. When a car turns and generates lateral forces against the tires, the treads are the first parts that receive the forces. They will attempt to roll inward, lifting off the ground. This reduces the ultimate traction of the tires, regardless of the kind of tires or materials, and puts heavy wear on the outside edges of the tires. What makes this effect worse on the Mustang is that the inclination angle actually forces the wheel to take on a new attitude as it is turned; its top moves out and its bottom moves in, an effect called positive camber, which forces the tire tread to roll inward even more. As an attempt to compensate for this dynamic camber angle gain, Ford specifies a large amount of static negative camber. The problem is that this static negative camber is very quickly offset by the inclination angle as the wheel is turned, and it increases wear on the inside edges of the tires while the car is driving straight. So the result is a front suspension that severely understeers, can not make very good use of available tire traction, and chews up the tires.

My Early Fixes:

The first time I tried to upgrade the suspension on my 1987 Mustang GT, I went with the most popular modifications that everyone else was implementing. I got a complete suspension kit from BBK, which included Monroe Formula GP struts and dampers, BBK's specific rate coil springs, and a number of polyurethane bushings. The new springs were stiffer than stock, and lowered the car about 1.5". Similarly, the dampers were also stiffer than stock. I also installed a set of Global West McCastor upper strut mounts at the same time. This replaced the stock strut mounts, which provided no caster adjustments at all. At this time, I'm not certain how much improvement the new parts netted, but the lower riding car bottomed out a lot. I switched to yet stiffer front springs (800 lbs/in from Global West), which eased the bottoming problem somewhat on the front. I would later find out that I was pursuing a course of locking out the suspension with the stiffer springs and dampers. Since the dynamic characteristics of the stock suspension was detrimental to handling, the quickest fix from most aftermarket "tuners" was to stiffen it up in order to prevent it from doing too much. But this added too much roll stiffness to the suspension, making it behave more like a brick.

The McCastor kit on the other hand, proved itself immediately by allowing increased caster angles. This improved cornering by introducing dynamic negative camber gains when the wheels are turned (or more accurately, counteracting the positive camber gains due to the stock kingpin angle). The additional caster also improved straight line stability by forcing the tires to roll straighter when they are not being turned. This also meant that the steering effort was increased ever so slightly, but the steering also more readily returned to center after the turn was complete. The most cost effective benefit was that now the tire life almost tripled. These effects could be achieved with no other suspension modifications. Indeed, Ford attempted to replicate the effect more economically by redesigning the upper strut mount for more caster in the 1990 and later versions of the Mustang. With a little tuning advice from Doug Nordin at Global West, I was able to solve the bottoming problem as well; the McCastor strut mounts had different spacers that allowed the strut to regain the travel that was lost when the suspension was lowered. However, since the strut is working as a combination of suspension locating device, steering pivot, as well as a motion damper, too much caster angle on the strut will put too much stress on the damper rod, and thus the seals of the damping chambers. The other problem with the strut design is that it can not change tire camber fast enough to match the roll of the body through any given turn.

My Latest Fix:

I was looking at a number of new front suspension designs that replaced the struts with a set of upper and lower control arms, like the classic Mustangs of the 60's and 70's. The double control arm design can be tuned to provide much better camber characteristics with body roll, and with much reduced kingpin angle. (Unfortunately, the stock configurations of the classic Mustangs did not take advantage of this capability, but that's another write-up.) Ideally, the camber should change based on the amount of body roll during cornering to keep the tire's treads on the ground. This would be dictated by a combination of the mass center of the front of the car, its weight relative to the spring rates, and the roll center characteristics. The roll center does not stay put as the body rolls, and in theory, its locus can be optimized with body roll by changing the effective leverage of the suspension components as they move. The exact desired interaction is unfortunately still beyond my understanding, so these are just my guesses. The combination of roll stiffness, roll center, and mass center also affect side to side weight transfer. Any traction lost by a tire on the inside of a turn due to weight transferring off of it is not completely made up by the increased traction of a tire on the outside of the turn from the transferred weight added to it, so there is always a net loss of traction due to weight transfer. So one of the trade offs in tuning roll characteristics (front or rear) is between how much roll and pitch to allow versus how much weight transfer to allow. Then, the roll camber coefficient of the front suspension should be tuned to match. Finally, it is important that the actions of the front suspension are matched to those of the rear suspension so that they can work together. Too much weight transfer from the back to the front in a turn can cause oversteer. Both ends also need to work together to prevent effects such as dive on braking or squat on accelerating. This meant getting a system that was designed as part of a package that included the rear end as well as the front end.

I chose the Contex/IPS components because they had the most intriguing rear suspension design. (See my other write-up on my rear suspension modifications.) My front suspension package is the IPS Grand Sport kit. It includes the upper and lower control arms, mounts for the coil-over spring/damper assemblies, and a new front cross member with engine mount. The coil-over spring and damper assemblies had to be purchased separately. I ended up with HAL adjustable dampers and springs.

Note: These parts are top rate, so they are not cheap. They are also not exactly bolt-in; some cutting was required.

The Installation:

The first step in installing any new suspension components is the removal of the old parts. Removal of the suspension components is pretty straightforward so it will not be covered here. Just take care when removing the springs from their perches. What makes this installation special is that the front cross member and anything attached to it (such as the rack and pinion steering gear, control arms) have to be removed as well. Some shops remove the entire assembly with all parts, including the rack and pinion steering gear, control arms, and spindles attached. This would have been too heavy for me to handle, so I stripped all the parts first, then removed the cross member. To do this, the engine must be suspended by some other means. IPS has a jig that bolts into the sway bar frame mounts and holds the engine up by the oil pan bolts. I used a combination of straps looped around the strut tower brace and a jack with wooden blocks under the transmission. These are the two cross members next to each other.

Actually, a large part of the time taken by this process was cleaning up the greasy gunk that was left by various oil leaks in the engine. With the old K-member removed and most of the gunk cleaned up, the under side of the engine is much nicer to work on.

The new cross member needs to be mounted in conjunction with the upper control arm assemblies. It is possible to mount just the frame bracket at this time, but it would be much more difficult to mount the arms later. But installing the upper control arm assembly also means installing the upper camber adjuster cups. IPS provides a template to guide cutting of the strut tower tops, but I laid it out with a pen and some careful measurements. Then I used a grinder and a body saw to cut out the holes.

The lower control arms were pretty straightforward to install. Some thick powder coating had to be removed from their pivot locations on the cross member.

Here is the new cross member installed in the car. Here is the final assembly of the front suspension with the spindle and brake parts installed. Actually, there were some trouble along the way, including some incorrect parts. The component that attaches the SN/95 spindle to the upper ball joint is called the spindle adapter. IPS has some 35 different versions, depending on the setup of the car. I was initially shipped the version made for a 1987 Fox Mustang, but it didn't work since I added the 1996 Cobra disk brake kit from Ford Motorsport; there was major interference between the spindle adapter and the coil-over assembly. Then they sent me the version for the SN/95 Mustangs, which still didn't work because my car is a hybrid; the interference was gone, but the wheels ended up with 2.5 degrees of positive static camber with the adjusters set to full negative. The third time, they sent something that did not interfere, and provided the correct camber ranges. It looked correct, and it is the set I'm using now. This is critical information when ordering parts.

Other details included properly dressing the brake flex line so that it does not rub against or get pinched by any of the new parts. I also have a slight clearance problem between the left header and the steering shaft, which I haven't solved yet. It's not too critical, as there is only some minor rubbing during some turns.

The other problem is that the springs as supplied by IPS were way too soft. They were 450 pounds per inch by 9 inch long, and the car would bottom out on the smallest of bumps. IPS was working on getting me a new set, probably rated at 550 pounds per inch. IPS claims this may be too stiff for proper suspension functions, but I would prefer it over damaging parts due to bottoming out. The suspension needs to articulate in order to provide the gain in negative camber with compression. If the spring is too stiff, that effect will be reduced.

There is a very important point about IPS service that must be mentioned: it is very slow. After a few months of waiting, I eventually got a set of springs myself, HAL 10-550, through Summit Racing. (Summit had to add the part number to their ordering system and order them from HAL, but they arrived in less than 2 weeks. The price was good as well.) Unfortunately, they seem to be too tall; with the spring perch set to the lowest position on the damper, the front end sits about 1 inch too high. I may have to switch back to 9 inch springs with higher rate to bring the frontend down to a more stable height. That may introduce another problem with suspension travel. The damper may sit too low at static ride height, and would again bottom out too easily unless the spring rate is stiff enough. IPS has a different upper arm frame bracket that relocates the upper strut mount slightly higher. This may be another change I need to make due to the different spindle I'm using. I'll have to check with them, but I'm not expecting any kind of fast response.

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