The antiroll bar which is also called a sway bar or stabilizer bar, controls the
amount of body roll while cornering. The roll resistance provided by the antiroll bar
is added to the roll resistance provided by the springs. The total roll resistance
determines the total amount of body roll for a given situation. The total roll
resistance at the front of the vehicle, compared to the total roll resistance for the
entire vehicle, tells us the roll couple distribution; in other words, the front versus
rear roll resistance. This determines the handling balance of the vehicle.
If we have too much rear roll resistance the car will oversteer. If we have too much front roll resistance, the car will understeer.
For handling improvements, we use the antiroll bar for two purposes: controlling
the amount of body roll, and controlling the roll couple distribution, which
determines where weight is transferred.
How Antiroll Bars Work
The antiroll bar resists body roll while cornering by twisting. When the body
begins to roll, the arms on the antiroll bar will twist the main section of the bar. This
resists additional body roll. The arms are attached to a suspension arm (usually the
lower control arm) on each side of the car. The other end of the arm (the arms are
usually part of the main bar) is attached to the main bar.
On independent suspension systems, the bars link the left and right sides, causing
the suspension to no longer be completely independent. If the wheels encounter
the same bump or dip, then the antiroll bar does not work. But when only one
wheel hits a bump or dip, the antiroll bar adds to the spring rate by resisting,
adding to the suspension frequency. An excessively stiff antiroll bar can cause tire
contact problems over single wheel disturbances.
The stiffness of an antiroll bar is determined by the stiffness of the material the
antiroll bar is made of, the diameter of the main bar, the effective working length of
the main bar and the effective length of the arms.
Almost all antiroll bars are made from materials of similar stiffness. The diameter of
the bar affects its stiffness to the fourth power: if you double the diameter of the
bar, the stiffness will be sixteen times greater.
Small changes in diameter can have
a major effect on roll resistance. The effective length of the main bar is inversely
proportional. To the stiffness, as is the arm length. For most applications, it is
difficult to change the effective length of the bar, but the diameter of the bar can be
altered, and the arm length can be easily adjusted.
Antiroll Bar Pros and Cons
It may seem that the antiroll bar is the answer for tuning the handling of a
vehicle. It is an important factor in handling, but it is not the answer. It can be used
to fine-tune the handling balance, and to limit body roll for improved tire contact
with the road. Antiroll bars allow the springs to do their job, but there are limits.
First, we can have too much roll resistance overall; second, the bars can provide
too large a percentage of the total roll resistance.
Let’s examine how an antiroll bar works. When a turn is initiated, the outside
suspension moves into bump and the inside into rebound. The antiroll bar begins
to twist, with the outside end of the bar lowering and the inside end of the bar
rising. The bar pushes down on the outside suspension, while it tries to lift the
inside suspension. At the inside wheel, this is the opposite of the way a spring
reacts. The spring pushes the inside wheel down, and the bar lifts the inside wheel.
If the bar is too stiff, the inside wheel is unloaded too much and, if it is the drive
wheel, may cause wheel spin as power is applied at the exit of a turn. This is a
serious problem if the vehicle is not equipped with a limited-slip differential, and has high horsepower.
It will be worse in slow turns. The problem is unlikely in low
horsepower circumstances, and less likely with a good limited slip. This is the
reason we have found it best to have a higher suspension frequency at the end of
the car with the drive wheels. The optimum percentage of roll resistance provided
by the bars seems to be between twenty-five and fifty percent of the total roll
resistance. Springs provide the balance of the roll resistance.
Choosing the correct bar rates is a complex process, requiring a significant amount
of data and plotting. To truly calculate the proper bar rates, the center of gravity
height must be known, as well as the roll center locations and camber change
curves. The work involved is extensive, and is best left to an experienced designer
or antiroll bar manufacturer.
Additionally, when complex bends are required in a bar design, the true rate cannot
effectively be calculated. The actual rate of the bar must be measured on heavyduty test equipment. The easiest approach is to purchase antiroll bars from an
experienced manufacturer or to consult with a suspension design expert.
Overview
The antiroll bar is used to limit body roll, allow the springs to keep the tire
contact patches on the ground over disturbances, and to adjust roll couple
distribution.
Antiroll bar rates are easily adjusted to fine-tune roll couple distribution. The major
rate changes are best accomplished with the bar diameter, and fine-tuning
adjustments with the arm length.
The antiroll bar works by loading the outside tire, but lifting the inside. Excessive
antiroll bar stiffness can hurt handling and traction, especially at the drive wheels
during the exit phase of a turn. Excessive roll resistance will cause the inside wheel
to lift off the ground in a turn.
Finding the best compromise can be time consuming, but usually rewards
substantial dividends to those who make the effort.
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