Sway Bars (aka Anti-roll Bars or Stabilizer Bars)

A Tutorial:   The anti-roll bar is essentially a transverse-mounted torsion bar designed to reduce body-roll during turns.  It exerts no influence on the suspension when wheels bounce in unison.  If vertical movement on one side exceeds the vertical movement on the other, the anti-roll bar exerts an opposing force.  Along with its primary function of reducing body-roll, the anti-roll bar will also reduce the combined cornering force and the adhesion limits of the side-by-side tires that are being acted upon.   Consequently, the location and stiffness of the bar can be modified to influence the oversteering or understeering characteristics of the vehicle.

An oversteering tendency will be reduced by locating the anti-roll bar at the front where it will reduce the cornering force and adhesion of the front tires.   If the vehicle understeers, the anti-roll bar should be located at the rear.  If an anti-roll bar is already required at both ends of the vehicle to achieve adequate roll stiffness, use an anti-roll bar of greater stiffness/diameter at the end of the vehicle where reduced cornering force is desired, and use a less-stiff/smaller-diameter bar at the other end.   (http://www.rqriley.com/suspensn.html)

Sway bars on Mustangs

Because of its rear-wheel drive and front-end weight bias, the Ford Mustang naturally understeers.   It tends to "plow" in turns and resist steering inputs.  Installation of a rear sway bar is an easy way to add some stiffness to the rear suspension and reduce this understeer.   It gives a more "precise" feel to the steering.  It makes the car turn easier and feel more stable.   It makes the front tires wear longer.

Ford has put rear sway bars on Mustang's since at least 1979.   Why did Ford leave the rear sway bar off the '98-'01 V6 Mustang?

An obvious reason is to save money.   But I think Ford's decision was also based on assumptions about the driving skills of the typical V6 Mustang owner.  Understeer is the accepted norm in the U.S.   When the car doesn't seem to be going where the driver wants it to go, he just brakes and turns the steering wheel farther -- an intuitive design considered defensible in court.

Adding a rear sway bar not only reduces understeer, it also makes the car more likely to oversteer. -- for the rear end to lose adhesion and swing out when cornering. That makes the car more fun to drive (it turns quicker), but also more dangerous because it could lead to a spin-out.  To correct for oversteer, the driver must stay off the brakes, add power, and turn the steering wheel in the direction of the slide -- a design considered indefensible in court, and one that could expose Ford to liability.

What thickness rear sway bar do I need?

As a general rule, when a rear-wheel-drive car is equipped with both front and rear sway bars, the rear bar should have less stiffness/diameter than the front sway bar.  Installing too large a rear sway bar can make the rear suspension so stiff that it will cause the inside tire to lift on turns, limiting rear traction. In a high speed corner, it will also increase the likelyhood of "snap" oversteer occuring if the upper control arms bind and cause a sudden increase in the car's wheel rate.  With the exception of the '94-'95 Cobra,   Ford put a smaller sway bar on the rear of all late model Mustangs.. This is illustrated in the following table, which lists some stock Mustang anti-sway bar diameters that I found in various publications. 

Year V6 Front V6 Rear V8 Front V8 Rear Cobra Front Cobra Rear
   1984            28.5mm (1.12")    20mm (0.79")        
   1987            33mm (1.30")    21mm (0.83")        
   1993    24mm (0.94")    none    33mm (1.30")    21mm (0.83")    28.5mm(1.12")    21mm (0.83")
   1994    27mm (1.06")    21mm (0.83")    30mm (1.18")    24mm (0.94")    25mm (0.98")    27mm (1.06")
   1998    27mm (1.06")    none    30mm (1.18")    24mm (0.94")    29mm (1.14")    27mm (1.06")
   1999    27mm (1.06")    none    28mm (1.10")    23mm (0.905")          
   2000    25.4mm (1.00")    none    27mm (1.06")    24mm (0.94")          
   2001    25.4mm (1.00")    none    26.4mm (1.04")    23mm (0.905")          

How much difference will it make if I install a slightly thicker sway bar?

A number of factors determine the stiffness of a sway bar, including the bar's thickness and length,   the length of the swing arm or lever,   the bar material,   and the bar's attachment method.   But if two sway bars differ only in their diameters,   all you need to know is that their relative stiffness (torsional rigidity) is proportional to the 4th power of their diameters.   A small increase in diameter results in a large increase in sway bar stiffness.   You can caclulate the actual percent difference in the stiffness using the following formula:

larger bar diameter4/smaller bar diameter4 * 100 = percent change in stiffness.

What if one or both of the bars is hollow?   That adds one more step to the calculations.   After you have calculated the rate of the hollow bar based on its outside diameter (o.d.), you have to subtract the rate of a hypothetical bar that matches the inside diameter (i.d.) of the space within.  To do that, you need to know the wall thickness of the hollow bar.

I'll illustrate the above by comparing three stock Mustang rear sway bars:   a 21mm solid bar (1996 V6 Mustang),   a 24mm hollow bar with wall thickness of 3.84mm (1998 GT Mustang)   and a 27mm hollow bar with wall thickness of 5.4mm (1998 Cobra).   Here are the calculations:

  • The V6's 21mm solid bar has a relative rate of (21^4) or 194,481

  • The GT's 24mm hollow bar has an i.d. of 24 - 3.84*2 = 16.32mm. and a relative rate of
    (24^4)-(16.32^4) = 260,838

  • The Cobra's 27mm hollow bar has an i.d. of 27 - 5.4*2 = 16.20mm. and a relative rate of
    (27^4)-(16.20^4) = 462,566
Conclusions: The 27mm Cobra bar sway bar has about 2.4 times (462,566/194,481 * 100 = 238%) the stiffness of a 21mm V6 sway bar and about 1.8 times (462,566/260,838 * 100 = 177%) the stiffness of a 24mm GT sway bar.


Disclaimer:

I'm no expert on this topic.  If you think I've got any part of this story wrong, let me know.

miracerros@comcast.net