Panhard Bars; A brief tutorial

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Panhard Bars: A brief tutorial.

Height
The height of the panhard bar helps to determine the height of the rear roll center(see illus. 1). The roll center is an imaginary point around which the rear of the race car rolls. The height of the rear roll center (and the front also) is critical to handling. When you lower the panhard bar the rear roll center drops. A lowered rear roll center promotes side bite at the rear which tends to tighten corner handling. However, an extremely low roll center can generate excessive chassis roll which can cause suspension geometry problems. Also, excessive roll can delay corner exit acceleration.


Illus 1.






The roll center moves downhill whenever the angle of the panhard bar is increased and uphill under opposite conditions.

The roll center moves towards a stiffened and away from a softened side of the rear suspension.

NOTE: When a short panhard bar is used and installed with allot of rake, the rear roll center may be located beyond the panhard's mounting points (as shown in illus.2).


Raising the panhard bar raises the rear roll center. Generally, this adjustment causes corner entry handling to loosen and chassis roll to lessen. You can learn the "tuning range" for heights of your panhard bar by testing at the race track and taking good notes!

When adjusting for height, change both ends of the panhard bar. Otherwise you may introduce another handling effect by changing the angle of the panhard bar (more later). Also, if you adjust the height of the panhard bar just at the chassis, the rear roll center may move in the opposite direction(see illus. 1&2). Generally, a 1" change to the height of a panhard bar makes a noticeable change in handling on dirt race cars (asphalt cars = 1/2").

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Illus 2.





Example "a" is typical of many dirt cars using a panhard bar mounted to the axle near the pinion gear. if the rear roll center occurs outboard of the panhard's mounts (as shown above), raising the panhard at the chassis will lower the rear roll centers and tighten corner handling! the opposite adjustment gives opposite results.



Angle
During cornering the chassis exerts a side force on the rear axle and tires through the panhard bar (see illus. 3). When the panhard bar is level, it transmits a wholly lateral force to the rear tires. However, when the panhard bar is angled downward to the right, it transmits a partially downward force to the rear tires and rear traction is enhanced. Conversely, when the panhard bar is angled upward to the right, it transmits a partially upward force to the rear tires and rear traction is lessened. The effect of an angled panhard bar on rear tire loadings is brief but very important handling.

Illus 3.








During cornering, the chassis exerts a force to the axle through the panhard bar. The load effect on the axle is dependent on the angle of the panhard bar.

position "a" - axle is loaded (briefly)
position "b" - no affect on axle's load
position "c" - axle is unloaded (briefly)
NOTE: the effects of angle are the same for panhard bars mounted to left or right side of the chassis. each rear tire is affected in proportion to each tire's distance from the panhard's axle mount points.(see illus. 4)

You can increase the panhard bar's effect on tire loadings by increasing the static upward or downward angle of the panhard bar. However, too much panhard bar angle can cause drastic changes in rear tire loadings during cornering and handling may become erratic as a result. Generally, a 1" change to the difference in mounting point heights of the panhard bar makes a noticeable handling change on dirt race cars (asphalt cars = 1/2"). A good rule of thumb is to keep the height difference of the panhard bar mounts to within 10% of the panhard bar's length (for example: 20" panhard bar = 2" maximum mounting height difference). When making changes to the angle of your panhard bar, be aware of any effects to the height of the rear roll center.

In order to determine the ultimate handling effects of the panhard bar's angle, one must consider where the panhard bar is attached to the rear axle--Read on!
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Axle Mount Location
The forces transmitted through by the panhard bar are applied to the rear axle at the panhard's axle mount point. The lateral location of the mount on the axle determines how much each associated rear tire is loaded or unloaded by the panhard bar during cornering (see illus. 4).

Illus 4.





A. PANHARD ANGLE AFFECTS LR MOST

B. PANHARD ANGLE AFFECTS LR & RR EQUALLY

C. PANHARD ANGLE AFFECTS RR MOST

DURING CORNERING, THE ANGLE OF THE PANHARD BAR CAN AFFECT THE INITIAL LOADINGS OF THE REAR TIRES (SEE TEXT). THE DEGREE OF THE EFFECT ON EACH TIRE IS DEPENDENT ON THE LATERAL LOCATION OF THE PANHARD'S AXLE MOUNT (SEE ABOVE).


If the panhard bar is attached to the rear axle near the center of the rear tire track*, the panhard bar will load or unload both rear tires by a similar amount during cornering. With this arrangement you can increase rear traction, hence tighten handling throughout the corner, by increasing the downward or decreasing the upward (to the right) angle of the panhard bar. You can loosen handling throughout the corner by making adjustments opposite to those listed above.

If the panhard bar is not attached to the rear axle near the center of the rear track, the panhard bar will load or unload the rear tires unevenly during cornering. The closer a tire is to the panhard bar's axle mount the greater the tire is affected by the angle of the panhard bar. Conversely, a distant tire is affected less by the angle of the panhard bar.

Generally, a 6" change in the lateral location of the panhard's axle mount point makes a noticeable handling change. You should keep the following in mind when adjusting the panhard's angle or its lateral location on the rear axle: *Any increase in the load of the RR tire and/or decrease in the load of the LR tire tends to tighten corner entry and loosen corner exit handling. *Any decrease in the load of the RR tire and/or increase in the load of the LR tire tends to loosen corner entry and tighten corner exit handling. *Increasing the load of the rear tires equally tends to tighten overall corner handling. *Decreasing the load of the rear tires equally tends to loosen overall corner handling. *Adjustments to the panhard bar primarily affect corner entry and mid-corner handling.

By now you should have a good understanding of how some of the design elements and tuning adjustments of a panhard bar have a collective effect on handling. If anything you have read is unclear, go back and reread the article before progressing.


Frame Mount Location
A panhard bar that is attached to the right side of the frame lowers during chassis roll. However, a panhard bar that is attached to the left side of the frame raises during chassis roll. However, the effects on handling of a right side versus a left side frame mounting are not always predictable. The location of the panhard's axle mount can counteract any predictable handling effects. The current tendency is to mount the panhard to the left side of the dirt car chassis and to the right side of asphalt chassis.


Length
During chassis roll a short panhard bar changes its angle, hence handling, more radically than a long panhard bar. Consequently, handling can become inconsistent if the panhard bar is too short (20" minimum length is recommended). Generally the length of the panhard bar is determined by the desired location of the panhard's axle mount.
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