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Make Driving Fun! The Magical MM Panhard Bar

Topic #7 from our series.

Read on for more expert advice on transforming your Mustang into a high-performance handling machine. This week's topic is all about the Panhard Bar. (If you missed the previous episodes, they're posted on our website in the FAQs & Tech Tips section.)

You've already learned that making your Mustang's chassis stiffer is the first modification you should do to improve handling. But what comes next? For Fox and SN95 Mustangs, our Top 5 list of mods includes adding a Panhard Bar. Got an S197? Even though it came from the Ford assembly line with a Panhard Bar, MM makes a much better one.

What's a Panhard Bar?
In a nutshell, it's a rear suspension link that locates the chassis laterally, relative to the rear axle. All solid axle-equipped Mustangs require some way to keep the body centered over the axle. The original rear suspension design of Fox/SN95 Mustangs does this primarily through the significant opposing angles of the upper control arms. The rear lower control arms are also at opposing angles, but they only provide a very small centering effect because their angles are much smaller than the upper arms' angles.

Ford, we have a problem.
Fox/SN95 Mustangs were designed with a 4-link rear suspension that makes the rear upper control arms do two jobs. One job is to locate the axle laterally, which is done by the opposing angles of the two upper arms. The opposing angles tend to center the body sideways over the rear axle.

Unfortunately, compromises in the Mustang 4-link design prevent the chassis from being precisely located over the rear axle. This design lowers manufacturing costs, but doesn't improve performance. Between the odd geometry caused by the opposing angles during suspension movement, and the bushing designs required for it all to work without the suspension locking up, the result is very poor lateral location of the body over the axle. In fact, the chassis shifts from side to side, relative to the axle, by as much as 2 inches! The rear axle's inconsistent movement also causes the Mustang's notoriously unpredictable rear-steer effect. As much as we love Mustangs, this whole dynamic stinks for anyone who appreciates consistent, safe, and precise handling.

Why does lateral location matter?
When the rear of your Mustang's body shifts sideways relative to the rear axle, it points the car in a new direction—hint: not the way you were trying to steer it. Surprise! "Rear-steer" means the rear of the car is steering itself, without any input from you. As the rear of the chassis moves sideways relative to the tire footprint, the geometry of the rear suspension causes the toe angle of both rear tires to change, which alters the car's cornering path. This is always bad, but especially if you're racing at high speed or doing a little spirited street driving, where the effects of poor geometry are magnified.

To get the car pointed back in the intended direction, you have to correct course by steering the front tires. As soon as your brain senses you're no longer going where you want to go, it signals your arms to rotate the steering wheel back toward the intended target. Unless the change of direction is significant, you don't even consciously notice you're rotating the steering wheel, but the rear-steer makes the car unstable and unpredictable. It requires continual steering corrections, making you feel subtly out of control (or almost completely, if you're going fast). Instability is uncomfortable, and it prevents you from driving as well or as fast as you could otherwise.

While the stock Mustang 4-link design might be all right for a commuter car, it cannot provide the handling ability expected of a high-performance vehicle. To make your Mustang handle well, you need a much better way to locate the chassis laterally over the rear axle.

I see. How do I fix this terrible situation?
Simple. Improve the rear lateral location of the body relative to the rear axle. While there's more than one method to do this, we prefer adding a Panhard Bar to the rear suspension. Ford's engineers had the same idea; in 2005 a Panhard Bar became standard equipment on all Mustangs until an IRS became standard with the 2015 model year.

If you have an S197
Be happy, you already have a Panhard Bar! But also be warned: the magic doesn't happen until you get a well-made Panhard Bar, which significantly improves handling. What makes for a better Panhard Bar? Mainly, bushings with less yield. The stock Panhard Bar has rubber bushings at each end that deflect enough to affect the car's stability. Change to a Panhard that has bushings that don't deflect in unwanted directions and enjoy the new stability of your S197. Fortunately, of course, MM has just the Panhard Bar you need!

Enter the MM Panhard Bar
The MM Panhard Bar for Fox/SN95 Mustangs uses an aluminum rod as a lateral suspension link between the rear axle and the chassis. This simple design precisely controls the side-to-side location of the axle to eliminate dreaded rear-steer. The unstable and unpredictable feeling typically associated with the stock 4-link suspension vanishes, making your Mustang easier and safer to drive!

Why not a Watts Link?
  • There are two good methods of controlling the rear axle's side-to-side location: a Panhard Bar and a Watts Link. MM's Engineering Team chose the Panhard Bar because it provides a much lower roll center than is practical for a Watts Link design. More rear cornering grip comes from a lower roll center than a higher roll center. And you know we love rear cornering grip!
  • A Watts link is significantly heavier and more expensive to build.
  • Our Tech page Busting the Myth of the Watts Link explains in detail why the MM Panhard Bar is the best choice for your Mustang. Hey, it's got video!

How it works: The interactions of a 4-link rear suspension plus a Panhard Bar
As the suspension allows the chassis to move when the car goes over bumps and rolls during cornering, the rigid Panhard Bar keeps the chassis centered over the rear axle—something the stock 4-link design thoroughly fails to do. While the opposing angles of the Mustang upper control arms try to move the chassis in an odd, sideways-shifting path as the body rolls, the rubber upper control arm bushings deflect, allowing the chassis to follow the path determined by the Panhard Bar. This deflection of the upper control arm bushings is a necessary compromise for the suspension to move, rather than binding and locking up.

How well does the Panhard Bar work?
Our unbiased opinion: it's phenomenal! Seriously, you'll feel major improvement the first time you round a corner aggressively. Just how major is it? We've had customers call and tell us their wife, in the passenger seat, noticed the car felt much better, more in control, more sure-footed. So, add a Panhard Bar and impress your passengers with your new-found cornering ability. Just tell them you're picking up some techniques at the track—we got your back, Speed Racer.

Obviously, as we've discussed in this article, when it comes to solid-axle handling, your Mustang needs an MM Panhard Bar. But the magic doesn't operate in a vacuum! Read on for some brief tech about a critical factor that makes our Panhard Bar work so well.

The Importance of Upper Control Arm Bushings
When your Mustang's equipped with an MM Panhard Bar, it's very important to have rubber upper control arm bushings. This is one application where the compliance of a rubber bushing is a benefit.

Why is compliance a good thing for the upper control arm bushings?
In engineering terms, a 4-link suspension (we're talking a 4-link with NO Panhard bar) is "over-constrained." For the suspension to move, the upper control arms must physically change length. The metal control arm certainly can't change length. But its effective length, i.e., the distance between the control arm's two pivot points, can change because of the inherent compliance of a rubber bushing. If the upper control arms can't change their effective length because of an unyielding bushing material, the suspension will bind up. That's as bad as it sounds, by the way. Here's the picture: with a non-deflecting upper arm bushing material, the only way for the suspension to move freely is for the metal control arm mounting brackets to bend or tear. The resulting restriction in the rear suspension's ability to articulate will cause poor handling and the car will tend to snap-oversteer. Eventually the torque-boxes will fatigue-crack and tear out. Nobody wants that.

Retaining the rubber upper control arm bushings is a necessary compromise for good handling with a Panhard Bar. The Panhard bar prevents the chassis from shifting far enough sideways to reach the limit of the rubber bushings' compliance, which would cause them to bind and lock up. But the bushings do deflect just enough for the Panhard bar to keep the chassis centered over the axle. In the vast majority of applications there's no noticeable performance detriment from that compromise. (MM has stock replacement upper control arms and axle-end rubber bushings, handily enough!)

Installing stiff bushings or bearings in the upper control arms only works for a car that always accelerates in a perfectly straight line, like on a drag strip; and a perfectly smooth one, at that.

Thanks for reading! Are you convinced yet of the benefits of a good Panhard Bar? Watch our under-car video for more proof. We'll see you again next week for our exciting discussion about the MM Torque-arm rear suspension. Week by week, we're showing you the smartest steps to building your dream Mustang!