Long trailing arms question

[georgeramos]

Has anyone doubled up their long arms? I bought a new set that Ill be swapping in this weekend and am considering pressing new bushings into the originals and adding them back in, A friend said it was similar to adding a sway bar. Does anyone have first hand experience with this?

[124JOE]

not me
i bought a rear swaybar years ago

[opus10583]

Has anyone doubled up their long arms? I bought a new set that Ill be swapping in this weekend and am considering pressing new bushings into the originals and adding them back in, A friend said it was similar to adding a sway bar. Does anyone have first hand experience with this?

Hi George,

While I'm not positive I envision what you intend; how could that be similar to adding a sway bar?

Roll stiffness might be increased by the binding of unaligned links, but a sway bar works by increasing spring rate only when deflected.

...I'd worry about twisting the mounting brackets off the car and axle.

Mark

[coopertl]

George, i recently added a second set of lower (long) arms to the rear of my '75 spider. Obtained two used arms and pressed in new rubber bushings. Elegant and effective way to increase rear roll stiffness. Described at length by Dave Voss over on MiraFiori forum. terry

[georgeramos]

Thanks, found the following info from Dave Voss:

Doubling the lower trailing arms will definitely reduce rear body roll, though
I can't say how that might compare to a 5/8" bar since many factors affect the
stiffness of an anti-roll bar, including the width, arm length, and method of
mounting. I would bet that doubling the lower trailing arms eliminates much
more body roll than an anti-roll bar for the following reasons:

First, the rubber bushings in the trailing arms do a lot to absorb cornering
forces while the car is turning. Doubling the trailing arms means doubling
those bushings, which (in total) will absorb the same energy with less flex at
each bushing (since there are more of them) and this reduces the body roll.

The major benefit, and therefore the main reason why doubling the lowers arms
is so effective, is that when the body leans in relation to the axle housing,
each pair of doubled trailing arms (which are parallel to each other at rest)
become misaligned in relation to each other as they attempt to twist slightly
around each other. For a quick and easy demonstration of this, hold 2 pencils
parallel to each other in your hands, and then rotate each hand in the opposite
direction. Notice how the pencils are no longer parallel? This happens with
each pair of doubled trailing arms each time the body rolls.

Unlike the pencils in your hands, the ends of the trailing arms MUST remain
parallel since they are attached by a single pivot bolt, and so something in
between the ends MUST flex. The bushings absorb much of the energy by also
distorting along their axis in addition to flexing across their axis, but as
the roll increases, some flexing of the trailing arms also occurs. Combined,
the energy absorbed by twice as many bushings that are flexing in twice as many
directions, added to the flexing of four trailing arms, significantly reduces
the body roll angle for a given amount of cornering force that is absorbed.

Another nice benefit of doubling the lower trailing arms is better support and
positive location of the rear axle housing, which is great when traveling over
bumps, rough pavement, driveways, etc. due to the decreased tendancy for the
rear axle to wag sideways as the bushings in the trailing arms and panhard rod
flex to absorb forces. As above, more bushings provide less movement.

I've been asked about trailing arm bushings wearing out with this arrangement,
and so far I haven't worn any out at all in over 15 years of driving various
cars totaling close to 200k miles! I only use the later style arms with the
round ends and pressed-in bushing assemblies, not the early style arms with the
hour glass shaped rubbers and center pins. If there is a weak link to this
setup it would be the right side bracket on the rear axle housing. Unlike the
left side bracket which is pretty stout to handle the forces of the trailing
arm and the panhard rod, the right side bracket is rather light and can develop
cracks in the original welds from the flexing that occurs. Have a welder run a
second pass along each side to strengthen the joint, otherwise wait to see if
yours cracks later. I've had to weld repair a few 131 housings, but these cars
can allow more weight over the rear axle (in the trunk) and I tend to ask a lot
of them when cornering!

I'm not sure I answered the question, but I hope you may be willing to try the
double arm setup, especially since it's an easy installation without any need
to fuss with custom brackets, etc. I initially did this back when the only
anti-roll bar kits available for 131s required awkward clamping of components
to the axle housing and trailing arms, which just didn't seem like a good idea.

-Dave Voss

[opus10583]

...like I said.

Mark