The MM Panhard Bar eliminates the Mustang’s unstable and unpredictable behavior while cornering near its limit. This feeling is caused by the rear of the car steering itself without any input from the driver. Rear end steering is caused by the sideways movement of the rear axle.
    In the Mustang’s 4-link suspension design the lateral location of the rear axle is done by the upper control arms. The axle, however, is not precisely located--it shifts around because of the deflection of the rubber bushings in the control arms. We built a device to measure sideways movement of the axle, and actually recorded up to 2" of sideways movement of the rear axle while cornering.
    In addition, the upper arms are at about a 45 degree angle from the direction of the cornering load. Some binding in these arms is induced during cornering as one arm enters into compression and the other enters into tension.
    Do NOT use urethane in the upper arms to improve the side to side motion of the axle. Because of the three dimensional movement of the upper arms, severe binding will be induced as the suspension moves--with or without a Panhard bar. This effect may not be evident in a drag-only car, but if you plan to drive the car on the street at all, we do not recommend urethane in the upper arms (see the Rear Lower Control Arm section for more information).
    The Panhard bar is a lateral suspension link between the rear axle and the chassis, and is the best method to control the side to side location of the axle relative to the chassis. The Panhard bar is in a direct line with the cornering load, and acts through spherical rod ends, therefore no bind is induced during cornering. By nature of its design, a Panhard bar does a better job of locating the axle than the Mustang’s four-link suspension.