DeTomaso Mailing List: January 98, Message #144
     



From: 	MikeLDrew@aol.com

Subject: Sway Bars [part 2]

Date: 	Tue, 6 Jan 1998 00:33:20 +0000



Well, the List barfed back this article I posted last night, saying it's too

long!  So now I'm sending it as parts 2 and 3.



This thing is only two pages long-Andy, what gives?

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  The Shade-Tree Mechanic   

Swaybars: What's Right for the Pantera?

By Jack DeRyke and Mike Drew

What is a sway-bar, really?  A swaybar is a piece of steel barstock that

connects the suspension together on opposite sides of a car that has

independent suspension.  The bar can be on either the front or rear

suspension.  Fastened between the left and right side suspension pieces, the

bar acts as a twisted torsion-bar when one of the wheels hits a bump or the

vehicle leans in a corner.  The spring-force of one bar end twisting up tends

to force up the opposite side wheel, which is being extended by the body

rolling away from it.  In this way, the suspensions for the two sides of the

car are "evened out" as far as the forces acting on them, but more

importantly, the "light side" wheel is forced up while the car body is

forcing it down. 



This adds cornering force to the (relatively) lightly-loaded inside wheel and

removes some of that same force from the heavily-loaded outside wheel.  It

also tends to keep the wheels and tires more straight-up-and-down so the

tire's traction-footprint is maximized. Too, the reaction force from the bar

to the body of the car will affect the "lean angle" of the body, which

affects the driver's perception of how things are going down under him, as

well as preventing the tires from tipping over onto their low traction

sidewall areas.  A drastically-tilted body and driver's seat, together with

reduced tire traction, is not a confidence-builder for speeding up your

cornering!



A high-performance car like the Pantera has anti-sway bars at both ends of

the car.  There is a relationship between the two bars that affects the

cornering, called "roll distribution".  Increasing the front roll stiffness

loads the outside front tire and unloads the inside rear tire.  This tends to

cause understeer by increasing the front-tire "slip angle".  This is done by

increasing the stiffness of the front anti-sway bar.  Conversely, increasing

the stiffness of the rear bar will unload the front tire, increase the load

on the rear tire and the car will "oversteer".  Understeer is when the

steering wheel has no effect on the direction of the car, which goes off the

outside of a turn.  Oversteer occurs when the rear attempts to pass the front

in a turn, and the car goes off backwards.  It has been described thusly:

"Understeer is when you see what you're gonna hit, while with oversteer you

don't." 



Mid-engine cars are often designed to be neutral, neither over- nor

under-steering.  Unfortunately for those with racing aspirations, when Ford

became involved with the Pantera, they redesigned the front suspension for

understeer.  Understeer is what nearly all street cars have straight from the

factory, because it's self-correcting: if you find yourself too fast in a

corner, slow down a bit and the steering works okay again.  But what they did

not design out is another characteristic of mid-engine cars: ultra-quick

steering response.  Because the main mass of the car is very near the center

of balance, things happen extremely fast in a mid-engined auto when one end

or the other begins sliding.  My experience in autocrossing Panteras leads me

to say that, if the front or rear tires break loose in a turn, it's very

difficult to catch the car without what a one writer called "a lot of untidy

elbow-flailing from the driver".  Both the Porsche 928 and 944 were designed

from a "clean sheet of paper", but this "twitchiness" led the Germans to

design them both with the main masses at each end rather than concentrated in

the middle, just to slow down their handling for the benefit of the average

driver. 



A complicating factor in this balancing act between front and rear bar

stiffness, front and rear tire sizes and steering response time, is the

limited-slip differential.  The tighter your "posi" unit is, the more

tendency there is for oversteer, because the rear axle acts like it has no

differential, or like what dirt-track guys called a "Lincoln-locker", from

their practice of using a (Lincoln) arc-welder to weld the spider gears

together for racing.  This is particularly troublesome because the degree of

"lock" from a posi-unit changes with gear lube temperature, confusing your

diagnosis of handling woes.  A quick check of the amount of traction one gets

from a limited-slip: GM's test is to wet down the concrete under one tire,

then lay a two-by-four in front of the other wheel.  If the rear wheel can

barely climb the two-by-four, a brand-new limited-slip is set just right!

 But, by this time in our 20-plus-year-old Panteras, we probably don't have

to worry about the posi being too tight!



OK, so what's the effect of changing swaybar sizes a bit?  A stock Pantera

has a .750" bar in the rear and a .845" one in the front; the lever arm on

the front bar is shorter, so it is stiffer-acting than the longer rear as

well.  When 10" rear wheels are mounted, the factory recommended increasing

the rear bar to .875".  That 1/8" increase raised the bar stiffness by

roughly 50%.  A 1" rear bar would be 3.15X stiffer. When Rich Agiorni and I

autocrossed his '71 back in the '80s, we ran a 1-1/8" rear bar and decreased

the front to .675"!  For autocross/solo-II's fairly low-speed turns, this

worked great.  The hollow bars we used were 5% less stiff than solid bars

would be, but 40% lighter!  I'm not sure what the car would've done with this

chassis setup at real high speeds, either.  I suspect it might've been a real

handful, though, because it was so super-quick-handling in turns!

 Interestingly, a noted midwestern Pantera owner has terrific success

autocrossing his car with a stock rear bar and no front bar at all! 



[continued in part 3]