[DeTomaso] Best all-around rear sway bar size

[MikeLDrew at aol.com]

In a message dated 3/25/06 9:50:55, kenn_green at yahoo.com writes:

<<   You may want to check with others, but one of the most accepted upgrades 
to Pantera suspension is to go to a 7/8 rear sway bar.  I think it's suppose 
to make the handling mroe neutral.  You might want to consider that and just 
get the 7/8 ball ends and a 7/8 rear sway bar.
   
  Can anyone else comment on this? >>

>>>Jack DeRyke and I wrote an article covering this in detail more than ten 
years ago.  Rather than listening to marketing speak which touted the relative 
goodness and betterness of ever-larger sway bars, we decided to scientifically 
test them and collect real data.

Rather than rehashing the contents of the article, I'll just go ahead and 
copy/paste it here for all to enjoy.

Note that this article was written before the various chassis-stiffening kits 
were produced--and in fact was itself the impetus for at least one vendor to 
design and engineer such a kit.

Cheers!

Mike

=====

   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! 

Larry Stock had a new motor built after losing the mostly-stock original at 
Las Vegas ’92 to a spun rod bearing.  We think this occurred due to Larry using 
Goodyear race slicks with a stock oiling system.  The new motor is 
substantially stronger than before, so he really wanted to maximize the handling of his 
now not-so-stock Pantera.  A 100-ft diameter skidpad was laid out in a lot 
across from his business.  The surface was an average asphalt-bonded stone, about 
five years old.  A G-analyst from Valentine Research was solidly mounted in 
his car per directions, on the console back in the knick-knack tray.  This 
position seemed to be very close to the center of balance of the car.  The good 
folks at Pantera Performance Center in Denver, Colorado, provided a variety of 
swaybars for us to test and compare.

The car is a ’72 Pantera, with stock suspension components except for Carello 
shocks, running 15x8 front and 15x10 rear aftermarket wheels with Pirelli 
P7’s in the standard 225/50-15 and 285-50/15 sizes.  The wheels had stock offset 
and required no flares or spacers to fit.  The shocks were adjustable, but 
were not optimized for the various swaybar combinations used.  To obtain the best 
handling characteristics, the ride was lowered, and the front end was set for 
1/8" toe-out and -1.5° camber.  

In Test 1, the car used stock front and rear swaybars; 0.845" front, .750 
rear.  It was able to maintain 0.79 g’s on the described skidpad.

Test 2:  the rear bar was exchanged for a .875" unit.  This is the so-called 
GTS or Group 3 swaybar setup, recommended when running larger-than-stock rear 
wheels/tires.  The car was now able to maintain 0.93 g’s with this single 
change.  Since the bar change took over half an hour, the tires had obviously 
cooled back down to ambient before starting Test 2.

Test 3:  figuring that if some is good, more is better, the front bar was 
exchanged for a 1.0" bar, while the rear stayed at .875".  Now, the car could 
only hold 0.91 g’s — a slight step backwards from Test 2.

Test 4:  the rear bar was increased to 1.0" along with the 1.0" front bar 
from Test 3.  The car could only hold 0.89 g’s while in this configuration, a 
further step backwards.  This is especially confusing, since it is the 
configuration used by some for road racing, and was recommended by the folks at Pantera 
Performance as the “killer setup”.  However, they have used this combination 
mostly on cars with 15x10 front and 15x13 rear wheels, i.e. Group 4 or GT-5 
cars, which not only have much more rubber on the ground, but possess a much 
wider track, as well.  

For Test 5,  the car was returned to the test 2 configuration and Larry’s 
favorite gumball race tires were mounted.  In studying the printouts from the 
G-analyst for this Test, it can be seen that the car now turns left at 1.10 g’s 
and turns right at 1.25 g’s.  Left turns are smooth and predictable while the 
right turn segment was ragged and obviously right on (or over) the edge.  With 
cornering power like this, it is easy to see why Larry had oiling problems on 
the racetrack.  The oil in the stock pan rode right up the side, away from the 
pump pickup.  After a few episodes of the pump sucking air, there were no 
bearings left!

Most people can turn to the left faster than to the right, simply because in 
a hard left turn, the whole car is pivoting around the driver’s seat.  During 
right turns, the driver’s position is also pivoting with the car, so his point 
of reference is constantly changing during the turn.  This introduces another 
variable or two which destabilizes the driver’s reference points.  The result 
is, you tend to slow down (unless you are in a ‘banzai’ mode, but this is 
difficult to maintain for long, and often results in a trip to the tules).

So why didn’t the progressivly stiffer bars give progressivly better 
performance?  The answer probably lies not with the swaybar or suspension, but with 
the chassis itself.  Noted engineer Kevin Cameron recently wrote a piece on the 
relationships between frame and suspension stiffness in motorcycles, and used 
automobiles to explain his theories.  He described the fact that any 
suspension is three springs in series — first is the tire, deflecting to absorb the 
smallest bumps; next is the suspension spring and associated components; behind 
that is the flexibility of the chassis itself, being deflected by the forces 
transmitted through the suspension.

Stock car racers learned years ago that there is no point in putting stiffer 
suspension on a chassis too weak to support it.  As a bump pushes the wheel 
up, the spring, shock absorber and sway bar resist the motion, passing on the 
force to that corner of the vehicle, which bends upward as well.  Once the bump 
has passed, the shock absorber prevents the wheel from snapping back, 
rebounding off the pavement again, and continuing to oscillate.  But the chassis has 
no shock absorber to damp its motion so it continues to vibrate up and down.  
This continuing motion can be just as disturbing to tire grip as running 
without a shock absorber.  The correct response would be to stiffen the chassis 
(notoriously weak in the Pantera, particularly in the rear, although 
extrordinarily strong by street-car standards), to force more of the bending to occur in 
the suspension, less in the chassis.  However, this would require major 
re-engineering of the chassis, and the costs of the design and fabrication would be 
prohibitive to all but the most die-hard Pantera crazies.

It seems that most Pantera owners would be well-served by changing their rear 
bar to a .875" unit and leaving the front bar alone, particularly if the 
tires have been upgraded to at least 1980’s-spec sizes.  Besides increasing the 
overall cornering abilities of the car, the driving characteristics will likely 
become much more neutral, without the plowing of the front end that Pantera 
owners have endured for decades.  However, such a setup will create the need for 
circumspection when driving, for once the car reaches it’s (higher) limit, it 
will probably be much less forgiving!