Lightweight replacement K-frames (fabricated from steel tubing and weighing approximately 19 pounds) are available from aftermarket sources like Griggs Racing (800-655-0336). The Griggs K-member also carries FRPP part number M-5025-R1. FRPP carries a full line of lightweight Griggs suspension components for off-road Fox-body Mustangs. Lightweight replacement frame sections are regarded as being strictly competition-oriented.

Although the Mustang is not designed with any provisions to adjust front wheel caster, it is pos-sible to increase positive caster slightly by loosening the eight attaching bolts and moving the entire K-frame forward. This can allow as much as 1/4 inch of movement. But to be truly effective, all the mounting holes have to be elongated, and in all cases, the K-member must be precisely realigned. (Some of the attaching holes are already oval; others are round.) Relocating the K-frame in this manner pulls the front ball joints forward and improves static caster. It has the same effect as moving the struts" upper mounting points rearward. This modification also dictates revisions to the K-frame"s motor mount provisions to maintain the engine"s OE positioning.

Due mainly to the low-end torque output of the production five-oh, extended or severe service is hard on motor mounts. The V-8 rubber biscuit mounts deteriorate and lose their insulating and locating properties. Worn and/or separated mounts can cause problems with vibration, fan-to-radiator and fan-to-shroud clearance, operating linkages (including throttle, shifter, clutch, and AOD throttle valve cable action), over-angled driveline joints and splines, stressed wiring connections, exhaust leaks, underhood component interference, and even hood panel clearance problems.

To help limit excess engine movement and relieve the load on the production mounts, a solid torque strap (a cable and/or chain with an adjustable turnbuckle) can be used to secure the left side of the block to the frame. In street use, this competition-oriented arrangement will transmit considerable noise, vibration, and harshness to the rest of the vehicle but it takes a big load off the mounts.

Replacement motor mounts that have harder (and/or smaller) rubber biscuits or even solid aluminum blocks in place of conventional rubber biscuits are available from the aftermarket. One-piece solid mounts are also available from some of these same sources. The more rigid provisions will prevent excessive engine movement, particularly in severe service and/or competition applications. In exchange, rigid and solid mounts transfer more NVH from the powertrain to the cabin than production mounts do.

During the mid-1980s, Ford produced advantageous lower control arms for Fairmont police and taxi applications. These arms were stamped from thicker-gauge steel and are considerably stronger than their Mustang OE counterparts. Unfortunately, new replacements for these unique arms are no longer serviced by Ford.

Replacement control arms are available in kit form (including new bushings and low-friction ball joints) from FRPP under part number M-3075-A. Although some of Ford"s recent parts catalogs indicate different original equipment control arms for early and late Fox cars, the late version and the FRPP arms will fit all Mustangs except the (1984-86) SVO models.

For reference, an SVO Mustang front control arm is 1 inch longer than a Mustang arm (measured from the arm pivot center to the ball joint center). The physically interchangeable A-arm used on a 1987-88 V-8 Thunderbird measures 13.75 inches between its corresponding axes. (The slightly longer than OE T-Bird arms offer additional negative camber, but they also require more tire clearance. Although using T-Bird control arms does not require K-frame relocation or revisions to move the lower pivots inboard, these modifications are suggested.)

Ford"s 1979 Fox-body platform became the first production Mustang to use a strut-based suspension system. The front suspension is designed around a modified MacPherson strut that is less complex and less expensive to produce than a double-arm coil spring, leaf spring, or torsion bar arrangement. In the Fox MacPherson-based system, the shock/strut guides, damps, and limits wheel motion. The tubular strut cartridges also function as dampeners; the front shock absorbers are integral with the strut assembly. In its free state (out of a vehicle), the shock/strut has relatively low damping characteristics. But because these units are not vertical when installed in a car, they also carry side loads. Side loading increases damping resistance, because internal friction (sometimes referred to as "sticktion") is increased. The shock/strut"s outer housing (can) is free to rotate (relative to its internal piston"s connecting rod).

The Mustang"s strut design is referred to as "modified" because the coil springs are mounted separately from the struts. This allows more room in the engine bay around suspension and brake components. (In most common MacPherson layouts, the coil springs are wrapped around the strut cartridges.)

For 1984 1/2, low-pressure, gas-charged (nitrogen over oil) strut cartridges were put into Mustang service. Ford"s 1982 Lincoln Continental was the first domestic production passenger car model to use nitrogen-gas-filled shocks and MacPherson struts. In Lincoln-Mercury Capri applications, this arrangement was referred to as the Nitra-Cushion suspension.

Because these units apply a 100 psi nitrogen gas preload to their hydraulic fluid, they automatically extend to their full length when not installed in a vehicle. Approximately 50 pounds of force is required to manually compress the strut rod into the canister, which should not be confused with internal binding. The front cartridge struts are not serviceable; each must be replaced as a unit, and preferably in pairs.

Over the Fox Mustang"s production run, its struts" internal valving (mainly the metering orifices) was continuously retuned in efforts to optimize the cars" ride and handling. The strut tuning process also involved internal clearance settings, seal qualities, and bore and rod finishes.

The top mounts on the struts are carried by tower sections formed in the car"s stamped-steel unitbody shell. They are located underhood, in the engine bay"s fender aprons. The bottoms of the struts are attached to the front spindle carriers where they meet the outboard ends of the lower control arms. Both the struts and lower control arms are rubber-bushed at their attachment points. A rubber insulator and jounce bumper are used at the upper spring and strut attachment points.

Beginning in 1984 1/2, multipiece upper front strut mounts were employed (except in SVO
models). Although this later arrangement provides considerably more cushioning (to reduce noise and harshness) than the earlier one-piece "onion head" mounts, the multipiece mount is more flexible. In hard cornering, this excess play results in undesirable camber changes and more rapid tire-edge wear. For performance service, the early onion-type upper strut mount is considered superior to the late (1984 1/2-93) multipiece mount.

The 1987-93 struts" mounting flange ears at the bottom of the canister are approximately 1/8 inch closer together than on earlier struts.

For 1990, the tops of the front strut mounts were relocated slightly closer to the cowl. This revision increases the 1990-and-later Mustangs" nonadjustable caster setting by .63°, slightly reducing understeer and effecting more favorable wheel camber through a wider range of suspension travel.

Replacing OE and OE-like front struts with specially valved struts can improve a Mustang"s cornering characteristics or its ability to transfer weight rearward for maximum traction on hard acceleration. Because the mounting flange ears at the bottom of the early struts are approximately 1/8 inch further apart than on late struts, installing early (pre-1987) struts on a 1987-and-later Mustang requires aftermarket or custom spacers. Conversely, however, late (1987-93) struts will not work on a 1979-86 model. Some manufacturers include lower mount spacers with their replacement struts to give the units a complete range of Fox Mustang interchangeability. Using early struts on a late car will not have any effect on wheel camber, but the swap will reduce available suspension travel by 3/4 inch.

Helical-coil springs are used (front and rear) on all Fox-platform vehicles. Coil springs in a production Mustang"s suspension system are computer selected according to the specific equipment factory-fitted to each vehicle and how that equipment affects the car"s weight distribution.

Variable-rate coil springs become stiffer as they are compressed. Expressed in pounds per inch, the spring rate (deflection travel under a specific load or loads) increases progressively as applied loads increase. The more this type of coil is compressed, the higher the load required to compress it further. (In contrast, a constant-rate spring will compress the same distance under the same loads at all points in its normal operating range of travel.)

The Mustang"s progressive-rate springs are designed to compress relatively easily for their first 3/4 inch of travel. They then become progressively stiffer as they are compressed further. In an appropriately tuned system, this allows the suspension to deal best with conditions most commonly encountered in normal operation (on streets, roads, and interstates).

Compared to stiff (high-rate) constant-rate springs, variable-rate coils allow a controlled ride on rough roads and a smoother ride on smooth roads. Because of their progressive resistance to compression (and their relative ease of deflection during their initial segment of travel), variable-rate springs can produce a "jacking" (loading and unloading) effect during hard cornering. This means that ride quality aside, progressive-rate springs are not as effective for off-road performance service or competition as constant-rate springs.

Visually, the coils that form a constant-rate spring are equally spaced (vertically) along the entire height of the spring. A variable-rate coil is wound with varying coil-to-coil spacing. A front spring"s topmost coil is ground flat (to about one-half the wire"s original diameter) where its insulator seats in the K-frame. The bottom of a front spring"s coil terminates in a pronounced pigtail (wire end). On installation, the pigtail is positioned with its end between the two holes in the lower control arm"s spring pocket.

Most Mustang coil springs are wound from SAE 5160 steel. The wire from which a front coil is wound is much larger in diameter than the wire used to form a rear coil. Fox car front coil springs all use upper and lower rubber perch insulators to help reduce road noise transfer to the vehicle body and interior. Each spring uses a 1/2-inch-thick rubber biscuit between its top coil and the K-frame pocket and a split-tube rubber (wraparound) insulator between the bottom coil and the control arm perch.

Rubber isolator blocks (also referred to as internal rubber dampening sleeves or oscillation dampeners) are used inside the front and rear coil springs on 1984 1/2-and later 5-liter Mustangs as well as on those equipped with Handling suspension. These sleeves should be retained or replaced (but not eliminated) whenever the springs are changed.

Fig. 29-5. Front springs

The lower right corner of a Mustang"s "data plate" door sticker contains a series of four single-digit code characters directly following a single-character transmission code. Typically expressed separately as front-and-rear, right-and-left designations, each character is keyed to the coil spring"s part number and spring rates. Translation keys to this coded data are published in each 1980-and-later Mustang"s service manual; some are explained further in dealer literature.

It is important to note that removing a coil spring from a Mustang requires special equipment and techniques and should be attempted only by those with the proper tools and training. Appropriate cautions should be taken whether a spring is being removed for modification or for replacement.

Coil springs can be shortened in low-budget fashion by cutting winding sections from the coil stack. This can be accomplished by using a premium-quality hacksaw blade or a high-speed cutoff wheel (while the spring is off the car). This cut-and-try approach can be used to tune a spring/suspension system by experimentation. Shortening a coil spring"s height also increases its effective rate. The spring-shortening approach to lowering also allows the upper and lower spring insulators to be retained. (Front springs should be always be cut at the bottom, and rear springs at the top.)

Removing a spring"s rubber insulator biscuits can lower the front of the car (1/4-3/8 inch), but ride harshness, vibration, and metal-to-metal component wear will increase.

Nearly all Mustang coil springs are physically interchangeable within the 1979-93 Fox model line (although not front-to-rear), but not all springs are suitable for all mechanical combinations. The major exception to this situation involves the front springs in the 1984-86 Mustang SVO models. These unique coils are designed to suit those cars" longer lower control arms and compatibly canted upper spring pockets in the K-frame. Therefore, installing SVO springs in other Mustang models (and vice versa) is not a viable idea.

Lowering a Mustang by installing shorter springs will lower the car"s center of gravity and improve cornering ability, but this also limits weight transfer on acceleration and can hurt traction and straightaway elapsed times.

Replacement coils springs are available from aftermarket sources that include Baer Racing, Kenny Brown Performance, Central Coast Mustang, Eibach, Global West, JBA, FRPP, Steeda, and Suspension Techniques, among others. Note that many of these outlets also supply coil springs packaged with other select suspension components.

Front stabilizer bars have early and late end-link designs. Early (1979-84) bars used rubber-bushed straight links with a spacer between each bar end and control arm. Later (1985-93) bars used shouldered end links (without spacers) between each bar end and control arm. The later links were also upgraded with Teflon-sleeved polyurethane bushings-inner, outer, top, and bottom. With a durometer rating of 82-85, these blue/green urethane bushings are the most favorable OE units to use with a 1.30-inch Mustang front bar. The Teflon sleeves reduce both friction and noise and are less compliant than the rubber units used previously. Although the late bushings" Teflon linings deteriorate in harsh and/or extended service, they are regarded as very effective handling enhancers.

A pair of the late links and their eight isolator bushings were specified under part number E6ZZ-5A486-A. Although this link-and-isolator package was revised after March 1991 (as part number F1ZZ-5A486-A), the earlier units will interchange.

It is generally agreed that little is gained in Mustang handling performance by installing larger-than-OE GT-diameter stabilizer bars. The 1993 Cobra"s smaller-than-H.O.-diameter front bar has proven effective with that car"s combination of more compliant suspension components and wider tires. The stabilizer bar kit listed in the FRPP catalog (under part number M-5627-A) includes an OE-type 1.30-inch front bar, Teflon-sleeved bushings, urethane link bushings, and an unbushed .79-inch rear stabilizer bar. For reference, 1994-and-later Fox-4 Mustangs are fitted with slightly lighter tubular steel stabilizer bars that are not directly interchangeable with the bars used on the earlier, Fox-body models.

Two longer lower trailing arms are positioned in brackets under the outboard ends of the axle tubes to manage the axle housing"s forward and rearward movement. The front pivots on the arms are carried in torque box structures tied to the outboard faces of the subframe rails just forward of the rear wheels. The Mustang"s standard rear stabilizer bar also functions to keep the lower control arms parallel, especially under hard acceleration.

Replacements and Upgrades

Fig. 29-7. Replacement lower rear control arms.

All 1979-93 production Mustang lower rear control arms are interchangeable. Super Duty replacement lower control arms from FRPP (part number M-5649-A) have spherical, Teflon-lined front bushings and grease fittings for the bushings in the rear end housing. Aftermarket replacement control arms (from sources like Griggs, Chassis Engineering, Edelbrock, HP Performance Products, Maximum Motorsports, Southside Machine, and Steeda) offer features like boxed channels, an adjustable spring seat, polyurethane bushings, and adjustable ends. Replacement lower arms fabricated from boxed aluminum are also available from the aftermarket, for obvious structural advantages. Boxed aluminum arms also offer an unsprung weight saving of approximately 2.5 pounds (per pair). Both types are typically packaged with extra-duty bushings. Some replacement arms are configured to allow more room than stock arms for offset wheels and wide tires.

Access to the arms" forward pivot bolts in the torque boxes may be obstructed by exhaust system components in some applications.

A production Fox Mustang"s four-link rear suspension system contains eight rubber pivot-point bushings. Steel pins inserted in the bushings at the factory are used to tune each rubber mounting cushion for its specific application. The factory bushings used in rear control arms (and stabilizer bars) are made of rubber that is too soft for serious performance service. This excessive compliance allows them to compress too easily under load to properly locate and control the rear axle assembly during hard driving maneuvers. In contrast, a bushing that is too hard can cause suspension links to bind and limit suspension action and control.

Some of these OE bushings" deflection load ratings are shown on the accompanying chart. Complete production data on the subject of OE suspension bushings remains elusive. Until more complete specifications are uncovered, the numbers shown should be regarded as reference figures only.

The sources and numerous details and precautions in the Front Springs section also apply to identifying, modifying and replacing a Mustang"s rear springs.

For 1982-83 and part of 1984, pre-Quadra-Shock Mustangs in GT (and SVO) trim or those equipped with the GT package and the 5-liter H.O. V-8 were assembled with two extra gusseted rear suspension links. These members were designed to act as slapper-type traction (anti-tramp) arms and are also referred to as "traction bars" and "hop bars." U-bolted to the outboard bottom of the rear-end housing, each link (one per side) carries a cylindrical rubber sleeve. This bumper is positioned to contact a striker plate (bolted to the underside of the lower control arm) when the rear-end housing rotates (as the suspension compresses during acceleration, cornering, etc.).

Ford"s Quadra-Shock rear dampener arrangement was initially developed as part of an upgraded suspension system to support an "improved-performance 5-liter engine" program. A complete upgrade package had originally been planned for release at mid-year 1984. But when that engine-based equipment group was delayed until the 1985 model year, only some late-1984 cars were fitted with all the suspension upgrades that had been planned. As a result, Ford"s Quadra-Shock rear suspension arrangement was fitted to the limited-production late-1984 Mustang SVOs and many standard production V-8 (and turbo-four GT) Mustangs built after mid-year 1984. For 1985, the Quadra-Shock system entered full production on all V-8 Mustangs as well as on those equipped with the GT/Handling suspension option. (Following their Fall 1984 intro, delivery of 1985 V-8 Mustangs to dealers and their customers was delayed.)

The four-rear-shock-absorber Quadra-Shock arrangement features two dampeners at each end of the rear axle. One shock per side is oriented vertically, and the other horizontally. The multi-shock setup was designed to help control axle hop (torsional axle oscillation) on hard acceleration and to stabilize transitory handling by absorbing the twisting action of the rear-end housing without restricting suspension travel.

The Quadra-Shock system replaced the dual traction-bar setup used on 1982-84 Mustangs equipped with the 5-liter V-8 (and on early-1984 SVOs). The Quadra-Shock arrangement weighs slightly less than the traction bar system. The horizontal rear shock arrangement was also used on 1985-86 SVO models. These handling-oriented Mustangs came with Koni dampeners exclusively. (Koni"s were also fitted to the 1993 Mustang Cobra R-Model.)

Thunderbirds built for 1987-88 and equipped with the Sport Handling package and non-IRS Turbo Coupes also came with a Quadra-Shock rear suspension system.

Quadra Shock Modifications

Fig. 29-9. The components in this FRPP bolt-on quad-shock kit help illustrate the OE system's basic layout.

In acceleration-oriented straightaway applications, large tires can create clearance problems between the tires and the horizontal pair of Quadra-Shocks. Although these kickers can be moved inboard slightly for extra tire clearance, many competitors remove them completely to make room for 9.5- and 10.5-inch extra-wide race-type rubber. In extra-high-horsepower applications, removing the horizontal shocks dictates the addition of special locating/control members (and/or bushings) to help manage the Mustang"s live rear axle.

Quadra-Shocks can be retrofitted to 1979-84 V-8 Mustangs and to later models built without them. Brackets are required on the rear axle and frame to accept Quadra-Shocks. Starting in early 1984, the four-shock-absorber arrangement was offered in bolt-on kit form by FRPP under part number M-4263-C. The FRPP kit includes a template for proper positioning, but drilling access holes in the trunk and bolt holes in the frame to accept the shock mounts can be difficult and time-consuming. The axle housing brackets should also be tack-welded in place.

In service, Mustangs that were not Quadra-Shock-equipped tend to develop sloppy lower shock mounts. These cars can be retrofitted with the sturdier lower mounting bracket and pivot arrangement that came on Quadra-Shock models (fastened to the rear faces of the lower control arm mounts on the axle tubes). The Quadra-Shock mount also offers a more advantageously oriented lateral pivot layout, but this later mounting setup also requires appropriately configured vertical shocks. A number of different lower shock bushing eye widths (on the vertical dampers) were used in Mustang production.

Replacement shock absorbers from the aftermarket are offered for both vertical and horizontal dampers.

Because the 1979-81 stabilizer rear bar was redesigned for 1982, fitting a 1982-and-later rear bar to a 1979-81 Fox car requires redrilling the mounting holes in the lower control arms. The 1994-and-later Fox-4 Mustangs are fitted with slightly lighter tubular steel stabilizer bars that will not interchange directly with earlier, Fox-body models. Larger (.9375-inch and 1.00-inch-diameter) rear stabilizer bars are available from the aftermarket.

Adding a 1993 R-Model-style triangulating strut-tower-to-cowl "export" brace can help stiffen the front-end structure as well as the entire car. Since the Mustang"s cowl/firewall area is not particularly sturdy, a triangulating engine-bay brace should be further supported with a base plate attached at its mounting point on the cowl/firewall structure. Similar base plates are also suggested at both strut tower connections.

A strut tower (upper) crossbrace for 1979-93 Mustangs is listed in the FRPP catalog under part number M-20201-A50. These braces are also available from other aftermarket sources, including BBK, Jamex, and Maier Racing.

Adding a transverse member (a lateral steel tube commonly referred to as a G-load brace or lower chassis brace) across the bottom of the K-frame legs (near the rear of the lower control arm pivots) can also help stabilize a Fox car"s heavily loaded suspension and engine mounting structures. The most common type of these aftermarket lower chassis braces is a two-point member. Other aftermarket setups connect to the car at four points. Adding a transverse member between the front frame rail stubs helps tie the substructures together and also helps support the K-frame itself. Maximum Motorsport"s lower brace provides a direct path for suspension loads.

Starting in 1983, production convertibles were fitted with an undercar crossbrace that ties the K-frame to the subframe at points fore and aft of that number two crossmember. This bracing is also standard in the 1993 Cobra R-Model. A pair of OE-style V-braces are included as part of FRPP"s Chassis Stiffening Kit (part number M-5024-A). The FRPP kit contains the same basic braces, reinforcing plates, and fasteners (bolts and rivets) used by Cars and Concepts to finish the Mustang convertibles they produced for Ford.

The aftermarket offers a steel-tube lateral brace that connects the rear shock towers to help stiffen that section of the Mustang"s stamped unitbody. In a hatchback, this arrangement intrudes on interior cargo space.

Installing welded-in subframe connectors is one of the most effective changes that can be made to any Fox-body Mustang, so much so that adding connectors is strongly suggested, even before any major powertrain upgrades are made. Adding longitudinal braces along the rocker panels between the front and rear subframe stubs will help those stamped structural sections develop added stiffness, especially when the engine"s output levels are elevated.

Although bolt-in subframe connectors for 1979-93 Mustangs are available from FRPP under part number M-5478-B, it is strongly suggested that these members be welded to the subframe stubs. Attaching them by drilling holes and using threaded fasteners can, in some installations, hurt overall structural integrity rather than help it. FRPP also offers "double-cross" subframe connectors under part number M-5478-J. Configured to distribute dynamic loads over more of the body structure, these braces are designed to be welded in place.

Ford issued Technical Service Bulletins (in May 1990 and April 1991) advising owners and service shops about potential problems with the EEC processor due to arc welding. These notifications suggest that the battery"s ground cable be disconnected whenever arc welding is done on a car with computer-managed engine controls. Ford had also released a Technical Service Bulletin in June 1983 covering specific procedures for welding the Mustang"s high-strength, low-alloy steel (HSLA) chassis and body sections. Always follow common sense safety precautions when welding, especially around fuel and electrical equipment as well as near trim, bushings and other structural software.