Late Model Mustang Suspension Basics


Rear Suspension (continued)


You can also install a torque arm.   It consists of a beam that bolts to the differential/rear axle, runs parallel to the driveshaft, and pivots at a point near the transmission output shaft.   It prevents pinion angle changes (axle wide-up).   When you install a torque arm in combination with a panhard bar or a watts link, they take over both functions of the Mustang's upper control arms (side-to-side axle location and prevention of pinion angle changes).   Both of the stock upper control arms can be removed and discarded.   Doing that gives you a 3-link rear (shown above).   It exhibits almost no bind and performs much better than the stock system during extreme cornering, acceleration, and braking.

An early example of a 3-link rear could be found on the 1951 C-type Jaguar (illustrated above).   It consisted of a panhard bar and a single, asymmetrical upper control arm.   It's significant feature was that it equalized rear tire loading on forward acceleration.   If done right (correct location and geometry of the single control arm), this design produces the same results as a torque arm/panhard bar combination, but with less weight.

Shown above is the 3-link rear that Ford put on the 2005 Mustang.   It has a panhard bar, two lower control arms, and a single upper control arm attached to the differential case.   Note also the new method of mounting the rear sway bar.

Evolution Motorsport makes a 3-link rear for retrofit to 1979-2004 Mustangs.   Shown above are the main components of the EVM Tri-link.   One end is bolted to the chassis and the other attaches to the differential case and the upper control arm bushing ears on the rear axle.   The upper control arms, quad shocks, and bumpstop are discarded.   Use of this kit requires the installation of an axle locating device (panhard bar or watts link) and upgraded lower contol arms.  

Another improved rear suspension that Ford could have put on the Mustang is found on the 2000 Grand Marquis, Town Car, and Crown Victoria (illustrated above). It consists of parallel upper and lower control arms (reduced bind) and a watts link located on the front of the differential.

     

Steeda Autosports makes a 5-link rear suspension using a similar design.   The 1979-2004 Mustang's 4-link rear suspension is modified by replacing the converging upper control arms with parallel upper control arms (reduced bind, longer effective length).   The new upper control arms use the stock mounting points on the chassis end and attach to the axle with clamped rings.   A panhard bar locates the rear axle side-to-side.   On the race track, Steeda's 5-link rear is proving to be competitive with the torque-arm/panhard bar rear suspension.

Finally, there is the Mustang Cobra's independent rear suspension (IRS).   Ford designed this IRS so that it attaches to the 1979-2004 Mustang's stock suspension mounting points.   That somewhat compromised the performance of the IRS, but it means that the unit can be bolted into any late model Mustang chassis.   The IRS subsystem is about 70 lbs. heavier than a solid rear axle, which contributes a better front/rear weight distribution, and it reduces the car's unsprung weight by about 125 lbs.


Weight Distribution

Mustang ModelWeightPercent
FrontRearTotalFrontRear
1999 4.6L SVT CobraCoupe 5spdIRS19031527343055.544.5
1998 3.8L V6 Convertible5spd 17951421321655.844.2
1998 4.6L GT Convertible5spd 19051489339456.143.9
1998 3.8L V6 Coupe 5spd 17311340307156.443.6
2000 5.4L SVT Cobra RCoupe5spdIRS20281562359056.543.5
1998 3.8L V6 ConvertibleAuto 18761424330056.843.2
1998 4.6L GT ConvertibleAuto 19821496347857.043.0
1998 4.6L GT Coupe 5spd 18401387322757.043.0
1998 3.8L V6 Coupe Auto 18061348315457.342.7
1998 4.6L SVT CobraConvertible5spd 20351484351957.842.2
1998 4.6L SVT CobraCoupe 5spd 19501425337557.842.2
1998 4.6L GT Coupe Auto 19171395331257.942.1

This table shows the front/rear weight distributions of some late model Mustangs. The least front-heavy are at the top, and the most front-heavy at the bottom.  Note how the installation of an IRS on the Cobra in 1999 added weight in the rear and contributed to a great improvement in weight distribution compared to the 1998 solid-axle Cobra.   Convertibles (more weight in the rear) and cars with standard transmissions (less weight in front) also do well in this comparison.


Sway bars

Front and rear sway bars or anti-roll bars are designed to reduce body-roll during turns.  They have no influence on the suspension when wheels bounce in unison.   They only come into play when they are twisted, that is, when vertical movement on one side of the bar exceeds the vertical movement on the other side, as when the body rolls or only one wheel goes over a bump.   In addition to reducing body-roll, sway bars also reduce the adhesion limits of the side-by-side wheels that are being acted upon.   This allows sway bars to be used as suspension tuning devices.   By varying the stiffness of the front and rear sway bars you can influence the oversteering or understeering characteristics of a car.

Firms such as Steeda and Maximum Motorsports sell adjustable rear sway bars that make this suspension tuning a simple operation.   The Steeda bar is an add-on to the stock sway bar.   The Maximum Motorsports unit replaces the stock rear sway bar.

Adding a rear sway bar to a V6 Mustang that never had one will reduce the car's inherent understeer and make it more fun to drive.   But you should use moderation if you decide to upgrade your Mustang's existing sway bars.   If too stiff a sway bar is installed on either axle, it can cause the inside wheel to lift completely off the ground in hard turns.   And if the rear sway bar is made too stiff relative to the front bar, it can produce oversteer and dangerous handling.   As general rules of thumb, on a street-driven Mustang it is best to (1) stick with sway bars that are close to the stock sizes produced by Ford and (2) don't install a rear sway bar that is thicker/stiffer than the front sway bar.  

You can find more detailed discussion of sway bars here and understeer and oversteer here .


Springs, Shocks & Struts

The coil springs in a Mustang's suspension can be replaced with aftermarket springs that lower the car and/or increase the stiffness of the suspension.   When that is done, it may also be necessary to replace the shocks and struts with stiffer units that match the stiffness of the new springs.   The purpose of shocks/struts is to dampen the oscillations of the springs after the car travels over a bump or dip.   Ideally, the oscillation will be limited to just one cycle.  If the shock/struts are too soft in relation to the springs, the oscillations will continue for more than one cycle and the car will bounce repeatedly.   If they are too stiff, the spring will not be able to complete a full cycle and the tires will not be able to follow changes in the road surface.   Either situation will have a negative effect on traction, handling, and ride quality.

    

You can also replace the stock springs with coilover springs which are mounted on the front struts and the rear shocks.   The animation above shows the Mustang front suspension with a coilover strut on the left and a stock spring/strut combination on the right.   Coilovers have a mechanical advantage over the stock springs because they are located farther out on the control arm; and for that reason, they are smaller and lighter than stock springs with the same effective wheel rate.   The more efficient coilover springs permit the use of very high wheel rates to reduce body roll and brake dive without hurting ride quality.   Coilovers also feature threaded collars on the shocks/struts that permit easy adjustment of ride height - a useful feature on a competition car.


The Unibody Chassis

The Mustang's chassis is constructed of sheet metal which is formed into channels and box sections to give it strength.

Sheet metal subframes on the underside of this assembly support the front and rear suspension components.   Box section rocker panels on each side provide longitudinal rigidity.   This produces a strong, lightweight chassis, but one that has a tendency to flex.   It is sometimes referred to as the 5th spring in the Mustang's suspension.

Chassis stiffness can be significantly improved by installing subframe connectors, consisting of metal bars or tubes that are bolted or welded across the gap between the front and rear subframes.


Last Updated: 13 Jan 08 http://miracerros.com/mustang