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Panhard Bars: The Rest of the Story!
Most racers think of the panhard bar simply as a lateral locating device. Here is the rest of the story! NOTE: This article refers to panhard bars mounted to the rear suspension. You can apply this information to panhard bars mounted to a front axle, however, the handling effects will be opposite to those described in this article.
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Height

The height of the panhard barhelps 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 alot 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").

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!

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. 

FRONT/REAR LOCATION

For clearance reasons, the panhard bar is generally mounted behind the rear axle whenever a long panhard bar is desired. Also, a rear mounting provides more potential mounting positions than a front mounting. Keep in mind that since the roll axis (an imaginary line connecting the front and rear roll centers) is usually inclined to the rear, a rear mounted panhard bar must be positioned higher than a front mounted panhard bar in order to maintain a given roll center height. 

A front or rear location of the panhard bar makes little difference on cars equipped with solid rear suspension linkages. However, this is not the case for cars equipped with torque absorbing devices (5th coils, 6th coils, etc.) These devices allow the axle (pinion side) to wrap (rotate) downward during deceleration and upward during acceleration. During axle wrap, the height of the panhard's axle mount point changes. Consequently, both the angle and height of the panhard bar change during axle wrap and handling is affected. 

The location of the panhard's axle mount (ahead of or behind the axle), determines whether the mount will move up or down during deceleration or acceleration. During deceleration, the panhard's axle mount point drops if the mount is ahead of the axle but raises if the mount is behind the axle. During acceleration the height of the panhard's axle mount point changes opposite to those listed above.

For tight corner entry handling, the panhard bar should be mounted to the front of the axle. This arrangement causes the panhard's axle mount point to drop during deceleration. Consequently, the height of the rear roll center drops & rear side bite is enhanced. Also, if the panhard is attached to the left side of the chassis the angle of the panhard bar changes during chassis roll so to increase rear traction and further tighten handling during deceleration.

However, if the panhard bar is attached to the rear of the axle the panhard's axle mount point raises during deceleration. Consequently, the rear roll center raises and corner entry handling tends to loosen as a result. Also, if the panhard is attached to the rear of the axle and the left side of the frame, the angle of the panhard bar changes during chassis roll so to reduce rear tire loadings and further loosen handling during deceleration.

The effects of axle wrap on the panhard bar appear to influence corner exit handling to a lesser degree than corner entry handling. At this point, you should be able to analyze any effects the panhard bar may have on corner exit handling.

It should be mentioned that during cornering, a front mounted panhard bar resists axle wrap-up (acceleration) and enhances axle wrap-down (deceleration) whenever the panhard bar is angled downward to the right. A rear mounted panhard bar gives opposite effects. However, the effects to handling appear to be minimal.

AFCO manufactures a complete line of high quality panhard bars and mounts that are designed to provide the geometry needed for superior handling. Hopefully, the parts that we build and the information that we provide will enable you to "Experience the AFCO Advantage!" 
 
TUNING TIPS

-- Recommended starting points

Height : Use chassis builders recommendation or start with the panhard mounted at, or up to 2" below, the centerline of the axle.

Angle : Dirt - 1" downhill to the right.

Asphalt - level.

-- A high panhard bar works best on fast heavy race tracks & on heavy race cars.

-- If your panhard bar is mounted too high you can expect: 

  • Loose corner handling (usually) 
  • Little chassis roll 
  • The chassis to be unforgiving and overly sensitive to the driver. 
-- A low panhard bar works best on slick race tracks and lighter race cars.

-- If your panhard bar is mounted too low you can expect: 

  • Tight corner handling (usually) 
  • Excessive chassis roll 
  • The chassis to be sluggish and unresponsive to the driver. 
-- If adjusting the panhard bar angle to the maximum recommended rake (10% of the panhard's length) does not completely cure a handling problem, lower the whole bar to tighten handling or raise the whole bar to loosen handling.

-- If your rear axle steers towards the outside of the race track during chassis roll (loose steer), a lowered panhard bar (which increases chassis roll and normally tightens handling) may actually loosen handling. Conversely, a raised panhard bar (which decreases chassis roll and normally loosens handling) may actually tighten handling. 

-- The shape of a panhard bar has no effect on performance. The mount locations are what makes a difference.


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