Geometric roll center is dictated by the suspension geometry, while the SAE force-based definition is the point in the vertical plane of the rear axle at which lateral forces are applied to the sprung mass without producing suspension roll. Basically, roll center is the point at which the rear moves side to side as it moves up and down.
If you were the one in the opening paragraph who had to change his underwear after the off-ramp excursion, it's because the Panhard bar and its ability to center the rear while moving up and down was overcome by, say, the right-side tire hitting a bump and causing the rear to move sideways at the same time. With the Watt's link, the roll center is lowered and the rear movement is more fluid. The Watt's link allows the rear to stay centered better under hard cornering, which makes the car's handling much more predictable.
As with everything else, however, both the Panhard bar setup and the Watt's link have their pros and cons. To get a better feel for both rear suspension styles, we sat down with Duran. As we mentioned previously, Saleen offers the Saleen/PJ car with a Watt's link suspension standard, and it's the one we installed on our subject Mustang.
"The Panhard bar is an easy and cheap solution for the large manufacturer, but it greatly compromises the rear axle," Duran says. "The Watt's link allows the cornering forces to be consistent under all circumstances, making the car very predictable as it's mounted in the middle of the axle, offering the same characteristics in either direction, left or right. The difference is big enough to change the car from having an understeer condition to a more neutral one, just by changing to the Watt's link." Saleen also changes the rear sway bar to a stiffer, 24mm-diameter piece for better balance.
"The only disadvantages are that the Panhard bar is a simpler device, and that the Watt's link is heavier than the counterpart it replaces," Duran says. Either way, Saleen feels so confident about the advantages the Watt's link offers, that the company not only offers it on the Saleen/PJ Mustang, but starting this year, the S302E and H302SC cars will be equipped with the Watt's link as well. "We're already seeing more and more cars equipped with the Watt's link," Duran says. "The handling is greatly improved over that seen with the Panhard bar, and it's easy to feel when driving a car after a Watt's link has been installed in it."
To see what all the hub-bub was about, we decided to check out a Watt's link install. Our cruise to JDM Engineering yielded us not only an '06 Mustang for a test subject, but a Saleen Watt's link suspension kit as well. JDM is a Saleen distributor, as well as its East Coast warranty shop, so JDM tech Craig Silverman handled the install for us. The kit replaces the factory setup with a host of upgraded parts and pieces specific to a Watt's link suspension system. It comes with a reinforced rear differential cover, a billet-aluminum pivot, steel control arms, an upgraded sway bar, and high-durometer bushings.
Jim D'Amore of JDM Engineering also clued us in to a couple of other differences between the Saleen kit and the stock rear suspension system. "First of all, Saleen offers the kit powdercoated in red, but we're the only ones that have a black powdercoated version, which Saleen makes for us," he says. "Secondly, the Saleen sway bar is approximately 0.050 inch thicker than the stock rear sway bar. Finally, the rear differential cover is a meatier piece. It's a thicker aluminum casting as opposed to the stamped factory differential cover. It also has the pivot point for the link bars."
The install was a breeze and took less than two hours. We learned a few things along the way, too. First, the hardware for the Panhard bar and the underbody brace were reused. When removing the bolts, however, make sure you either take a digital photo or mark each bolt with tape, identifying where the bolt went and in what direction, as this is a critical part of the install of the Watt's link. Secondly, once all of the components are on, lower the car to the ground and tighten the link arms to their proper specs. Doing so while the car is on the lift without a load on the suspension causes the car to sit awkwardly and, more importantly, not perform well.
Once all was said and done, the car handled much better and the ride was slightly smoother. We say slightly because we did go for a ride on local, pothole-filled New Jersey roads.
Guess you can say that, thanks to the Watt's link, our test car's handling is on the ball.