Late Model Mustang Suspension Basics

Front Suspension Weight Distribution Wheels
The Mustang's Camber Curve Sway Bars Wheel Alignment
Lowering a Mustang Springs, Shocks & Struts Books
Rear suspension The Unibody Chassis Vendor Web Sites


Front Suspension

The 1979-2004 Mustang front suspension consists of modified MacPherson struts with lower control arms (A-arms).   The term modified refers to the fact that the coil springs are inboard of the struts.   Normally, MacPherson suspensions have coil springs that surround the struts.   This suspension is compact, cheap to build and easy to service, but it is often criticized because (a) its geometry produces camber changes during vertical suspension movement and body roll that adversely affect handling and (b) the strut towers prevent designers from lowering the car's profile.   The performance of the front suspension can be improved by installing stiffer springs and struts and by replacing rubber bushings in several locations with less compliant urethane bushings.

In the 2005 model year the Mustang got true MacPherson struts.

The Mustang's Camber Curve

     

The above animations compare the camber changes of the Mustang's MacPherson strut suspension (left) with those of the SLA (short-long-arm) or unequal length A-arm or double wishbone suspension (right) found on many high performance cars.   When negotiating a fast turn, you want negative camber gain on the inside front wheel to counteract the tendancy of that tire to roll under. But the fixed location of the strut mounting point on the Mustang suspension limits negative camber gain during jounce and rebound.   At the extreme limits of suspension travel it adds positive camber.   In contrast, the unequal length A-arm suspension provides consistent negative camber gain throughout the full range of jounce and rebound.   You can compensate for the Mustang's nasty camber curve by adding negative camber to your static wheel alignment.   But that has drawbacks, as explained below under "wheel alignment".   Another approach is to install stiff springs and struts that limit the suspension's travel to a very narrow range in which camber change is insignificant.

  

It is also possible convert a Mustang's modified MacPherson strut front suspension to an SLA setup.   In August 2004, Griggs Racing announced the availability of the conversion kit shown on the left above.   A similar kit (right) has been available for some time from HP Motorsports.

Lowering a Mustang

The picture above shows a 1979-2004 Mustang that has been lowered about 1.5" by installing aftermarket (shorter/stiffer) springs.   Before it was lowered, this car's front control arms were parallel to the ground.   Lowering the ride height by changing the springs caused the control arms to angle upward.   This has a negative effect on front suspension performance.   It lowers the front roll center, which increases the car's natural tendancy to understeer.   It reduces the range of strut and control arm compression travel.   It changes the alignment of the sway bar with the lower control arm.   And it changes the alignment of the tie rod with the lower control arm.   If the car is lowered much more than 1.5", this misalignment may produce bumpsteer, which occurs when the tie rod pulls the steering arm left or right as you go over a bump.

The steering geometry can be restored on severely dropped cars by installing a bumpsteer kit that lowers the tie-rod ends slightly relative to the spindle, or by raising the steering rack with offset rack bushings.   Sway bar misalignment can be corrected by installing shorter sway bar end links.   The roll center can be raised back up by installing taller ball joints.

If the car had been lowered by installing drop spindles (rather than springs) there would have been minimal changes in its front suspension geometry.   Drop spindles have been around for a long time, but they have only recently become available for the late model Mustangs.   Some people have expressed concerns about the strength of such aftermarket units, but the Mustang drop spindles produced by Racecraft Inc. (shown above) are proving to be able to stand up to race conditions.


An alternative that accomodates both lowering springs and fine tuning of the Mustang's front end geometry is to replace the K-member and control arms with aftermarket units.   Typically, these units have mounting points that can be changed to provide for correct control arm alignment at different ride heights. They are also lighter weight than the stock units, provide for an increase in wheelbase, and improve the steering geometry.   Some of these aftermarket units can be used with springs in the stock position.   The K-member and control arms pictured above require the installation of front coilover springs (described below).


Rear Suspension

The 1979-2004 Mustang's 4-link rear suspension consists of a live rear axle held in position by upper and lower control arms, coil springs, and gas-filled shock absorbers.   Pictured above is the rear suspension on a Mustang GT.   It includes an 8.8" traction lock rear, quad shocks (shocks & dampers), and a rear sway bar.   The base Mustang currently comes from the factory with a 7.5" open rear and without axle dampers or a rear sway bar.

The upper and lower rear control arms constitute the four links in this rear suspension.   The upper arms locate the axle side-to-side and prevent pinion angle changes (axle wind-up).   The lower arms locate the axle front-to-back and and transmit the wheels' thrust to the chassis.  Note that the control arms are not parallel to each other.

When the car leans (rolls) in a turn, one side of the chassis moves upward relative to the rear axle, the other side moves downward, and these non-parallel control arms must twist and change length axially to allow the axle to articulate.   This causes the control arm bushings to bind.   If this bind becomes excessive, it can raise the rear wheel rate and produce sudden, undesireable changes in handling (e.g., snap oversteer).

Ford minimizes this suspension bind by using compliant rubber bushings in both the upper and lower control arms.   These relatively "soft" bushings acommodate the necessary motion of the control arms during body roll.   However, they also permit wheel hop on hard launches and horizontal axle deflection in aggressive turns.   Axle dampers (quad shocks) were installed on V8 Mustangs to eliminate this wheel hop, but nothing was done by Ford to cure the horizontal axle deflection.   That deflection is responsible for the "tail-wagging" sensation well known to anyone who has driven a late model Mustang aggressively.

If improved straight-line (dragstrip) performance is your primary goal, the stock rubber upper & lower control arm bushings can be replaced with solid polyurethane bushings or spherical bearings.   Hard bushings/bearings eliminate wheel hop, reduce axle deflection, and improve rear end grip.   The downside of solid poly bushings is that they prevent the necessary movement of control arms during body roll, which in turn produces significant suspension bind in turns.   Spherical bearings do a better (but still imperfect) job of handling control arm motion during turns. And they also tend to transmit a lot of road noise and vibration into the car.  

If you use your car in open track or autocross competition, you'll probably want to leave the stock upper control arms alone and install only new lower control arms. The stock rubber bushings in the upper arms continue to allow the axle to articulate through its full range of motion in turns, while the new lower arms improve axle location. One popular lower arm design (illustrated below) uses segmented poly bushings on the chassis end and spherical bearings on the axle end.   This combination has been found to improve rear axle location without any significant increase in suspension bind.

A further improvement to the Mustang's rear suspension can be made by installing a panhard bar or a watts link.   Either one will do an excellent job of locating the rear axle.   By selecting different mounting points for these devices, you can also adjust the car's rear roll center height.



A panhard bar locates the rear axle side-to-side.   Note how the axle end of the bar moves in a slight arc.


A watts link locates the rear axle more precisely side-to-side.   It uses a central bellcrank to eliminate any arc from its up-down motion.   The bellcrank can be mounted either on the differential cover (left) or on the chassis (right).


Last Updated: 20 Feb 08 http://miracerros.com/mustang