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Strut brace and body roll (was Spring/Shock Combo's)
Huw Powell <human@nh.ultranet.com> wrote:
> OK, let me first point out that I don't know what the heck I'm talking
> about here...
>
> My observation is mostly based on what changed about my car when I
> bolted that sucker on. It stayed level while turning, radically so.
> On little turns, low speed stuff, it was like a lawnmower or something
> - it just stayed level. On harder turns, it dips a whole lot less
> than it used to. Is this body roll that has been lessened?
Did you change anything else when you installed the strut brace, Huw?
By stiffening the front end the car is likely to respond quicker to steering
inputs and subjectively it may "feel" like flatter cornerring, but I bet the
actual roll for a given lateral acceleration is virtually the same (unless you
changed springs/swaybars at the same time).
> How does it do it? I think the sway bar is doing as much as it can
> from below, to apply opposing force to the control arms, trying to
> keep them level. (deep water warning - I can't swim!) Since the
> suspension seems to me to be essentially a parallelogram (or
> trapezoid?) in the lateral cross section of the car pair of
> dimensions, stiffening the other long side of the figure (the top)
> aids in the process. Of course what this "stiffening" amounts to
> dynamically is that as the parallelogram (trapezoid!) is being pushed
> out of shape at one corner, the sway and stress bars are transferring
> the force across to the opposite side, even the opposite corner, to
> attempt to correct this distortion.
I'm not sure I follow what you are saying, but yes, there is some distortion of
the uni-body sheet metal - even though it's very small relative to how much a
sring compresses on the outside and extends on the inside.
> This all depends on the tendency of the strut towers to be able
> (undesirably) to move relative to each other without something taking
> corrective action. The stress bar does this. This relative motion I
> would presume to be in all three dimensions, affecting caster, camber
> and toe in. Lessening this makes the car behave better while
> cornering, by keeping the alignment of the tires where it is supposed
> to be, relative to each other and the body.
>
> Have I rented a clue yet?
Yes, you have a clue :) The small movement of the strut towers translates in
small changes in the suspension geometry. The towers may move in at the top, or
they may move out, or one may want to go up and the other may want to go down.
Minimizing these deflections gives you a stiffer, more responsive, perhaps less
understeering chassis. But let's be realistic. How much can these strut
towers really move? 0.1"? (educated guess) That translates into about 0.1
degrees of body roll. Compared to the 5-7 degrees (or more) or body roll due to
how much the suspension compresses on one side and extends in the other, that's
negligible. Even the best track suspensions will roll 2-3 degrees.
> Love to hear how close I am from someone who actually knows about this
> stuff.
I would also like to hear from someone who really know this tuff, as I am not a
chassis expert either ;)
Luis Marques