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RE: Sway bar ratings

To: "'VintageRace'" <vintage-race@autox.team.net>
Subject: RE: Sway bar ratings
From: "Larry list account" <list@marketvalue.net>
Date: Sat, 13 Dec 2003 09:52:50 -0700
No, I didn't write this, something I found in my personal archives.

Larry Hoy
==================================

There are two primary factors that determine an anti-roll bar's torsional
stiffness: the diameter of the bar and the length of the bar's moment arm.
Diameter is generally the easiest concept to grasp, as it is somewhat
intuitive that a larger diameter bar would have greater torsional rigidity.

Torsional (or twisting) motion of the bar is actually governed by the
equation:

twist = (2 x torque x length)/(p x diam4 x material modulus.) And since the
diameter is in the denominator, as diameter gets larger, the amount of twist
gets smaller. Which, in a nutshell, means that torsional rigidity is a
function of the diameter to the fourth power. This is why a very small
increase in diameter makes a large increase in torsional rigidity.

For example, to compare the rigidity of a stock 15mm bar to an aftermarket,
16.5mm one, simply use the equation 16.54/154. Some quick math yields the
figure of 1.46. In other words, a 16.5mm bar is 1.46 times as stiff-or 46
percent stiffer-than a 15mm bar of the same design.

Add just one more millimeter to the diameter of the bar-for a total of
17.5mm-and the torsional strength skyrockets to 85 percent stiffer than the
stock 15mm bar (17.54/15.04 = 1.85).

However, in addition to the diameter of a bar, there is another very
important factor that determines an anti-roll bar's torsional rigidity. This
factor is known as the length of the moment arm-or in common terms, the
amount of leverage between the vehicle and the bar.

As with anything, an increased amount of leverage makes it easier to do
work. This is governed by the lever law:

force x distance = torque
As distance-or the length of the lever-increases, the resulting amount of
torque also increases. (This is why it was easier to move your big brother
on the teeter-totter when he moved towards the middle and you stayed out on
the end. You enjoyed increased leverage at the end, while he suffered from
reduced leverage near the middle.)

Because an anti-roll bar is shaped as a "U," the ends of the bar that lead
from the center of the bar to the end-link attachment serve as a lever. As
the distance from the straight part of the bar to the attachment at the end
link becomes longer, the torque applied against the bar increases-making it
easier for a given amount of energy to twist the anti-roll bar. As this
distance is reduced, torque is reduced-making it more difficult for a given
amount of energy to twist the anti-roll bar.

It is this lever law that is applied during the design of an adjustable
anti-roll bar. By using multiple end link locations, the distance from the
point of attachment to the straight part of the bar can be altered. Or, in
engineers' terms, the length of the moment arm can be increased or reduced
in order to make more or less torque against the bar.

Using a setting farther from the center of the bar increases the length of
the moment arm, resulting in more torque against the bar, allowing more
twisting motion of the bar, creating more body roll. Using a setting closer
to the center of the bar reduces the length of the moment arm, resulting in
less torque against the bar, allowing less twisting motion of the bar,
creating less body roll.

The actual impact upon torque can be compared by dividing the
center-to-center distances of the end-link attachment points. For example,
say the center-to-center distance of the stock rear anti-roll bar is 200mm.
We can compare this to the 160mm distance of the firmest setting of a
four-way adjustable 17.5mm bar by simply dividing the distances (160/200 =
.8). In other words, a 160mm center-to-center bar produces only 80-percent
of the torque that would be produced by a 200mm center-to-center bar of the
same diameter. Or simpler yet, by using the 160mm end-link attachment
points, we increase the stiffness of the anti-roll bar by an extra 20
percent.

> -----Original Message-----
> From: owner-vintage-race@autox.team.net
> [mailto:owner-vintage-race@autox.team.net] On Behalf Of 
> Robert L Marcum
> Sent: Saturday, December 13, 2003 6:59 AM
> To: VintageRace
> Subject: Sway bar ratings
> 
> 
> I am looking for a source to calculate the ratings of sway bars.
> 
> Thanks for any assistance.
> 
> Bob Marcum




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