do i have to get a bigger rollbar if i get the 2Bennett setup?

[Luis Marques <marques@rtis.ray.com>]

From: Michael Williams <daserde@ucla.edu> wrote:

> Hey all, my friend chris morlock (yes, again) is insisting that since
> im upgrading the rest of the suspension, that i MUST increase the size
> of the rollbar and add one in the rear.  I contest that the one up
> front is already huge for the weight of the car and doesnt need to be
> upgraded, and that in the rear the tie rods act as a roll bar.  Im
> getting 350lb springs up front and 270 in the rear.  So do i have to
> upgrade?  He is gettign his info from the chassis teacher at skyline
> college here in the bay area.  They do have an excellent automotive
> program,.  But he wont believe anyone that has done it and says that
> its fine and he wont believe that the car is severely
> overengineered....


Michael:

Yes, the stock front bar is quite big, but no, the rear tie rods don't act as a
sway bar.  They are there for the ride simply controlling toe.

There are so many variables in suspension setup that there is no easy answer to
your question.  Any car will roll no matter how good the suspension is.  The
question is how much (or little) do you want it to roll (usually only enough to
match the camber during cornering).  Spring rates and sway bars should initially
be matched to the weight distribution of the car.  So with a f/r distribution of
about 60/40%, the front springs should be roughly 50% stiffer than the rears -
and with a 350lb front spring that would put the rear spring at about 235lb. 
The front sway bar should provide about 50% more roll stiffness than the rear
one.  However, this roll stiffness is not a direct function of bar diameter, but
pick-up points/geometry and spring rates affect this as well.  With a
front-heavy car, the natural tendency is to understeer and a stiffer rear
suspension can help fix this so, IMHO, your initial spring choice is a good one,
especially since there is no rear sway bar. 

Now, about the sway bars... a car will tend to roll because the center of
gravity is above the tire contact batch.  That creates a torque on the chassis
during cornering that would make your car roll over if not counteracted by
something.  That something are the springs and sway bars.  As the outside spring
compresses, it is applying more force on that corner of the car and as the
inside spring expands, it is applying less force on the inside corner.  If there
were no sway bars, this would continue until the torque resulting from the
spring force mismatch would cancel the tendency of the car to roll due to the
side g-forces.  When you put a sway bar, what it does is transfer some of the
force of the inside spring to the outside, so they don't need to compress/expand
as much to counteract the tendency of the car to roll.  However, if you increase
the spring rates, the sway bars (if unchanged) will be doing less work because
height mismatch form side-to-side during cornering is smaller.  So if you
increase the spring rates enough, the sway bars won't be doing hardly anything
at all and you would need to upgrade them or change the pick-up points if they
are to continue helping you reduce the roll (like your friend Morlock said).

So in the end, everything is a compromise.  You can add bigger sway bars and use
lighter springs, or you can use stiffer springs.  You can also reduce the roll
by reducing the ride height, but at some point you will run out of suspension
travel.  Some race cars have such a low center of gravity and stiff springs that
they don't need any sway bars at all!  Also, with the strut suspension on the
4kq, there is a point where, if you continue compressing the suspension, the
camber begins to change towards positive and you want to avoid this.  If it were
me, I would add a rear sway bar (smaller than the front one), leave the front
one alone, and drive the car and see how it feels.  From there you will have a
baseline from where to make adjustments until YOU are comfortable with the
handling of your car.  What works for some people doesn't do it for others.  HTH

Luis Marques
'87 4kcsq