[DeTomaso] looking for 7/8" rear bar/skid pad day

[MikeLDrew at aol.com]

In a message dated 10/1/10 11 05 42, JDeRyke at aol.com writes:


> PCNC did this decades ago with a clone GT-5, but its certainly worth
> repeating to see if this one-shot test repeats. In this basic test 
> attended by
> myself, Larry Stock, JIm Kuehne and Terry Aultman, we laid out a 100 ft 
> dia
> circle in an industrial parking lot on Sunday, and with a stopwatch, we 
> had
> Terry drive his GT-5 around the circle as fast as he could without sliding 
> off
> the chalked circle.
> 

I was there too--the car in question was Larry's black '72L fitted with 8s 
and 10s, not a GT5 with 10s and 13s.   Terry's GT5 wasn't even a gleam in 
Steve Mooney's eye at the time; Hall built that car for Steve a few years 
later, and he sold it to Terry a few years later still.

We also changed both front and rear bars, not just the rear bars.

Your recollection of Jim's dune buggy is pretty close; I seem to remember 
that its cornering performance was stronger when he ran the skidpad going 
backwards! :>)

Here's the text from the subsequent article:

=====

    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!