Sway bar, aka anti-roll bar, aka stabilizer
bar – they may have many names but they have one purpose and that is to reduce
body roll. There are ride comfort reasons to reduce body roll but you are
probably reading this article to learn more about the performance benefits. In
short, the sway bar works by tying the left and right suspension together and as
the car leans into a turn the body leans and pushes down on the sway bar at one
side. This causes the sway bar to try and lift the inside tire and causes a torsional
force (twisting force) on the bar. The bar resists this torsional force and so
reduces how much the body can lean. The best way to imagine how a sway bar
works is to think of it as a long spring. The thicker the bar the more it
resists twisting and therefore the stiffer it is. Now I am sure this
description isn’t sufficient so I will walk through it with some diagrams below.
First, let’s look at how the sway bars are designed because
this is critical to understanding how they work. The most simplified version of
a sway bar is a straight bar with two arms that project at 90° from each end
forming a very wide but short U shape. Somewhere near the arms is where the
sway bar will connect to the body usually with a rubber bushing – higher performance
bushings will be made from polyurethane or some other very rigid plastic. The ends
of the arms are connected to the control arm or strut tower through end links. Below is a picture of a typical set of sway bars you
will find in a car – these are a performance upgrade for a BMW e46.
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| Eibach front and rear sway bars for a BMW e46 |
I have attempted to draw up a diagram showing how the sway
bar works in a turn. The first diagram is the car going straight. I tried to
represent the 90° bend, and its relation to the body of the car because this is a critical feature of how the sway bar works.
In the diagram below the car is in the middle of a right
hand turn. When turning right the car will lean to the “outside” of the turn –
which in this case is to the left. Now, looking at number 1, the body of the
car pushes down on the sway bar mount. This changes the relative position of
the sway bar end (#2), which goes up in relation to sway bar mount (#1). This
causes a twisting force on the bar (#3) and also tries to lift the opposite end
of the bar (#4). It’s this resistance to the torsional force that reduces the
body roll. Now what is keeping the opposite end of the sway bar (#4) from going up? The weight of the wheel/hub assembly, and the stiffness of the spring.
So a stiffer sway bar is harder to twist than a thinner bar
so therefore is resists the body roll more effectively. This in turn keeps more
weight on the inside tire which means it can help increase the total amount of
traction. Now there is an upper limit to how stiff of a bar you can throw on
any car. Once the you start lifting the inside tire it is no longer able to
help add additional traction. Another way to address this is to increase the stiffness
of your springs which will also help reduce body roll and help keep the inside
tire planted.
You may want to read my article on suspension tuning to get a better idea of how sway bars will effect the balance of the vehicle HERE.
Now I demonstrate this in a video that I made a number of years ago and I won't say is a spectacular video but it helps show how these forces are working in real time and models the diagram I show above.
If you have any questions please ask them in the comments section and I will try and answer them when I can. Thank you.
You may want to read my article on suspension tuning to get a better idea of how sway bars will effect the balance of the vehicle HERE.
Now I demonstrate this in a video that I made a number of years ago and I won't say is a spectacular video but it helps show how these forces are working in real time and models the diagram I show above.
If you have any questions please ask them in the comments section and I will try and answer them when I can. Thank you.
