A panhard bar is also known as a track bar. It’s purpose is to locate the rear axle along the centerline of the chassis. The current chassis manufactuers right side mounting bracket has a series of vertical holes so that the bar can be unbolted and moved up or down as required and then rebolted. Adjusting the right side track bar end upward moves the rear axle to the right, and therefore both rear tires. Adjusting it downward moves the rear axle to the left, and therefore both rear tires. Raising the track bar makes the car looser (oversteer) and lowering the track bar makes the car tighter (understeer).
In a turn, a car transfers weight from the inside of the car to the outside of the car due to centrifugal force. On an oval track that means more weight moves from the left side to the right side in a turn. When the rear axle is moved to the right, more weight is moved to the left of the centerline. This allows more weight to transfer during cornering (more body roll). The opposite is true if the rear axle is shifted to the left, more weight is then moved to the right of the centerline and less weight is available for weight transfer to the right side during cornering (less body roll). In conjunction with all this and because of the trailing arm geometry on quarter midgets, moving the axle to the right shortens the right side wheel base, which results in loosening the car (oversteer) and shifting the axle to the left lengthens the right side wheel base, which results in tightening the car (understeer). Now that we understand that adjusting the track bar shifts the rear end right or left and also creates rear stear, it is easy to imagine the affect on the cars handling.
The The length of the track bar is also adjustable because of the left hand and right hand threaded rod end on opposing ends of the radius rod. There are different approaches to the initial set-up of a track bar. One common approach is to set the length of the track bar such that the rear axle is square to the front end and then the track bar is set mid-point in the series of vertical holes (mentioned above) so that lowering the track bar moves the axle to the left and raising the track bar moves the axle to the right. Again, there are many different approaches to the initial set up dependant on driver preference and ability.
Roll Center Explained:
Roll center is the point at which the sprung weight of the car rolls from side to side. It is a rather complicated concept that includes lateral forces, rotational forces, center of gravity location, raising and lowering of roll centers and roll couple, etc. Pretend that a car has had it’s tires and wheels removed and is only supported by two jackstands, one in the center of the front of the car and one in the center of the rear of the car. These two jackstands now represent the roll center of the car and it would be at this point that the weight would balance. The car could easily be rocked from side to side (weight would roll from one side to the other). In the rear of the car, if the jackstand were moved to the right then more of the cars weight would be placed on the left side of the jackstand. If the jackstand were moved to the left then more of the cars weight would be shifted to the right of the jackstand.
In the track bar discussion we determined that the more the body rolls, the more the right side spring is compressed and the more the left side spring expands. The more the left side expands, less force is placed on the tire, decreasing the contact patch and less traction is available, loosening the race car. Conversely, the lower the body roll, more traction is maintained by both tires and the tighter the car becomes. Moving the track bar up on the right side puts a greater percentage of weight on the left side but in a corner you’ll actually end up with less weight on the left side because that greater percentage is now available to be thrown right.