Dale Amy
December 1, 2004

Horse Sense:
The Watt's link is named for James Watt, the 18th century Scotsman who made steam engines practical and efficient. The center-pivoting crank was a prominent feature of his beam-pump design.

It's generally agreed upon that some of the poorest handling aspects of the pre-'05 Mustang's four-bar-link rear suspension are attributable to its lack of positive lateral axle control, one of the tasks the factory assigns to the splayed upper control arms. While this four-bar design provides efficient packaging at reasonable cost and acceptable performance under light g-loads, the arms' various rubber bushings allow way too much side-to-side axle movement when things become spirited.

Sideways axle movement well in excess of an inch has been reported. The result? A queasy, rubbery feeling out back as the axle meanders aimlessly about under side loads, and a sudden and unnerving transition from under-steer at corner entry to oversteer as power is applied. This can be a particularly unpleasant handful as power levels elevate or as road conditions deteriorate. In short, it's difficult to carve a precise curve with the stock setup.

To date, the aftermarket's typical approach to auxiliary lateral axle restraint has been the familiar Panhard rod-a bar that parallels the axle, bracketing to it at one end and to the chassis at the other. Recently, Evolution Motorsport has introduced a Mustang version of the Watt's link-a form of lateral restraint that has theoretical advantages. As with a Panhard bar, a Watt's link lowers the Mustang's rear roll center, something that tends to provide more rear grip. But, according to Evolution Motorsport President Kevin Gallagher, "With a Watt's link, as you make a right turn or left turn, your roll center height stays constant. With a Panhard bar, depending on which side you mount to the body, the roll center goes up in, say, a right turn, and down in a left turn." In other words, a Watt's link is completely symmetrical in handling response in either direction, whereas a Panhard is slightly asymmetrical.

While a Watt's link's job is to keep the axle laterally centered at all times, it must do so while permitting normal, unrestricted, vertical axle movement. Actually, there are two forms of Watt's link. One has its center pivot-or crank-mounted to the axlehousing and its outboard link connections at the chassis, while the configuration used by Evolution is just the opposite, having its crank chassis-mounted and the link ends axle-mounted. With its cradle and crank affixed to the chassis, a Watt's link is also symmetrical in jounce and rebound, in that the "roll couple"-the distance between the center of gravity and the roll center-remains constant. Roll couple has a direct effect on the amount of roll each end of the vehicle experiences during a turn.

All of this explains why EvM went to the considerable trouble of engineering a Watt's link package for all us handling freaks out here in Mustang land. Installation seems no more complicated than a typical Panhard bar, a situation made even better by EvM's exhaustive 27-page instruction booklet-one of the best we've ever seen. The net effect is that the normally wobbly rearend simply feels more "planted" in any circumstance, contentedly following the nose around without wanting to wander off on its own random tangents. Having the axle laterally locked in place also noticeably sharpens steering response, makes steering with the throttle a much more precise art form, and-as an unexpected fringe benefit-allows use of wider rear tires without rubbing under heavy cornering.

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