Of all adjustments acknowledged by NASCAR commentators, the track bar (or Panhard bar) is the most discussed but least explained. That is disappointing compared to the highly visible nature of often discussed camber. Here is my chance to write something boring and technical again! I'll explain the Panhard bar's location, function, and potential modifications.
If you read my writings on wheel hop, then you already know the trailing arms control axle windup (a rolling of the axle housing) as well as vertical motion. Trailing arms cannot control the lateral motions of a rear axle. A track bar is essentially a transversely mounted trailing arm (not to be confused with a control arm); one end mounts to the chassis, the other mounts to the axle, both ends pivot, and the bar itself is parallel to the axle. This arrangement is depicted in the photograph below: the track bar (white) runs from left to right along the rear axle; however, it is not perfectly parallel to the axle because the vehicle is on a lift.
This photograph also introduces the shortcoming of a track bar. Once this vehicle is lowered to the ground, the axle will shift to the left. The bar connects two pivot points and it cannot change length when traveling over a bump. This necessitates a controlled amount of lateral motion from the axle: to the left when the suspension compresses, and to the right under rebound. On an oval track, all the turns are in the same direction, making a controlled pivot of the axle advantageous (provided the turns are to the axle connection side). This advantage is eliminated at a road course; however, NASCAR teams usually make a point of swapping the chassis and axle ends of the track bar to best attack the right turns of a clockwise track. Street cars are seldom driven aggressively enough to reap advantages of mounting the bar to a particular side of the chassis, and practical concerns like fuel tank location and exhaust routing come into play. Road racers with live axles would sooner use a Watt's Linkage, which serves the purpose of a track bar while eliminating lateral pivoting thanks to its self-contained pivoting link.
Anyone who has watched a NASCAR race in the last 10 years, which probably means everyone bothering to read something so NASCAR specific, has noticed teams making track bar adjustments during pit stops. What those teams are actually adjusting is the height of the track bar's chassis mount. To raise the track bar is to raise the mounting point and vice versa. A higher mounting point gives the axle greater leverage on the chassis. The effect of that leverage is to correct a tight (understeer) condition from the center to the exit of a turn.
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