When you turn the steering wheel, the front wheels respond by turning on a pivot attached to the suspension system. Caster is the angle of this steering pivot, measured in degrees, when viewed from the side of the vehicle. If the top of the pivot is leaning toward the rear of the car, then the caster is positive, if it is leaning toward the front, it is negative. If the caster is out of adjustment, it can cause problems in straight line tracking. If the caster is different from side to side, the vehicle will pull to the side with the less positive caster. If the caster is equal but too negative, the steering will be light and the vehicle will wander and be difficult to keep in a straight line. If the caster is equal but too positive, the steering will be heavy and the steering wheel may kick when you hit a bump. Caster has little affect on tire wear.
The best way to visualize caster is
to picture a shopping cart caster. The pivot of this type of caster, while not
at an angle, intersects the ground ahead of the wheel contact patch. When the
wheel is behind the pivot at the point where it contacts the ground, it is in
positive caster. Picture yourself trying to push the cart and keep the wheel
ahead of the pivot. The wheel will continually try to turn from straight ahead.
That is what happens when a car has the caster set too far negative. Like
camber, on many front-wheel-drive vehicles, caster is not adjustable. If the
caster is out on these cars, it indicates that something is worn or bent,
possibly from an accident, and must be repaired or replaced.
Camber
Camber is the angle of the wheel,
measured in degrees, when viewed from the front of the vehicle. If the top of
the wheel is leaning out from the center of the car, then the camber is
positive, if it's leaning in, then the camber is negative. If the camber is out
of adjustment, it will cause tire wear on one side of the tire's tread. If the
camber is too far negative, for instance, then the tire will wear on the inside
of the tread.
Camber wear pattern
If the camber is different from
side to side it can cause a pulling problem. The vehicle will pull to the side
with the more positive camber. On many front-wheel-drive vehicles, camber is not
adjustable. If the camber is out on these cars, it indicates that something is
worn or bent, possibly from an accident and must be repaired or replaced.
Uprights (Wishbone suspension)/Knuckles
The upright or knuckle attaches the wheel, brake rotor, hub, brake caliper and steering arm to the car (of course, the wishbones and control arm(s) do the final attachment to the chassis)
The upright or knuckle determines the king-pin inclination, and the final camber, caster, and toe settings of the wheel and tire. These various factors are demonstrated in the diagram below.
Kingpin Inclination
determines steering feel to a great extent. In the front view above, the red line on the right represents the center line of the tire/wheel. The kingpin inclination is several degrees, the angle between the center line and the line running through the upright or knuckle. Movement of the kingpin increases and decreases weight as it moves. The kingpin inclination determines steering effort, and feedback.Scrub radius is the distance from the centerline of the tire/wheel to where the kingpin line intersects with the road surface. The larger the distance, the more effort is required to turn the wheel, as the wheel has to "scrub" slightly to turn around the kingpin axis. Scrub selection on FIRA cars is limited due to the wheels and brakes used.
Camber is the angle between vertical (perpendicular to a flat road surface) and the "lean" of the tire/wheel. In the diagram above, negative camber of about 2 or 3 degrees is shown. Negative camber is often used to offset the normally positive change in camber as the wheel moves up. The concept of camber is simply to keep the tire contact patch as large as possible through the complete range of suspension motion.
Toe-In/Out is a slight steering angle that is preset into the suspension. Toe-in has the tires pointing slightly toward the center of the car's front. Toe-out has the cars pointing slightly away from the car. In the diagram above, there is zero toe-in/out. Toe-in/out is used to offset the natural change in toe position caused by braking and acceleration. A small amount of toe out is suggested for oval racing (FIRA), while toe in is suggested for road racing.
Caster is the angle from vertical of the upright/knuckle, when viewing the wheel/tire from the side. This angle is used to create a gyroscopic effect on steering. This is easily demonstrated by turning the steering wheel in the car and then letting go of the wheel (Do this in an empty parking lot!). The caster causes the steering to correct itself back to straight ahead, instead of turning, without the need for driver input.
Roll Center
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When car goes round a corner, it is subjected to centrifugal forces, which act in the opposite direction as the turn. These forces act through the center of gravity of the chassis and cause the chassis to rotate along its longitudinal axis. This effect is termed as "roll" as the chassis rolls to compensate for the sideways force that is acting on it. |
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The suspension system of a car plays an important part in both creating roll effects. As the suspension system of a car allows the wheels to move vertically with respect to the chassis, it allows the chassis to roll by having the wheels on one side move up and the opposite side down. At any point of time, an imaginary line can be drawn through the axis of the roll. This line is known as the roll center. It should be noted that the roll center changes correspondingly with the suspension geometry.
As roll usually reduces traction on the wheels on the inside of the turn, attempts have been made to control it. Adding to the suspension stiffness reduces roll to a certain point, after which the car will simply lose traction and slide sideways. The addition of anti-roll bars or sway bars is another measure to counter roll.
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 alot of rake, the rear roll center may be located beyond the panhard's mounting points (as shown in illus.2).
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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.
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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.
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)
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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!
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).
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Illus 4.  A. PANHARD ANGLE AFFECTS LR MOST |
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.
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.
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.
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 points 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!"
Determining the Roll
Center