DeTomaso Mailing List: July 97, Message #209
     



From: MikeLDrew@aol.com

Subject: Re: Shocks or sway bar upgrade?

Date: Fri, 11 Jul 1997 16:49:11 -0400 (EDT)



Capt Mike Drew

150 Westgate Dr.

San Francisco, CA  94127



Home:   (415) 334-7860



E-mail:  MikeLDrew@aol.com



Steve,



You mention that your Pantera is fitted with '1-yr old Konis', but you don't

specify which KIND.  That is, your car could have stock springs and Koni

shocks, or you could have the full-on bucks-up Koni shock-and-spring package,

as sold by Pantera Performance Center.



If you've got the former, then merely buying new (and stiffer, i.e. GT5)

springs and using your existing shocks would probably be a smart move.  If

you've got the latter, you're already set.



Assuming the former scenario is true, if I were you, I'd invest in a new set

of GT5 springs.  Regarding sway bars, if you've got a stock-bodied car, then

the stock 7/8" sway bar in the front is fine, just go from the existing 3/4"

bar to a 7/8" bar in the rear.  Supposedly, GT5/Group 4 cars with their wider

stance can use 1" bars at both ends.  



This would allow you to spend considerably less than $1,000 and still get

terrific results.  Can't say whether the Gehling swaybars are worth the huge

investment (I think they're quite a bit more than $1,000 aren't they?); but

personally I can think of any number of different areas of the car where that

money could be spent to greater effect (assuming, of course, that you've

upgraded your stock springs, bars, and shocks already.)



Seemingly eons ago, PCNC had a skidpad day, where we used a Valentine g-meter

to test the effectiveness of various sway bar combos.  We started off with

stock, then changed the rear bar and got an IMMEDIATE jump in max g's.

 Figuring we were on a roll, we then changed the front to a 1" bar, with 7/8"

rear, and actually went backwards a little.  We then put a 1" on the rear as

well, and went FURTHER backwards.



Actually, surfing my hard drive, I've found the original article I wrote for

the PCNC newsletter detailing the results from this tech session (with some

theoretical inputs from Jack DeRyke); I'll reproduce it here and let you see

for yourself:



   The Shade-Tree Mechanic

   

Swaybars: What's Right for the Pantera?



By Mike Drew and Jack DeRyke



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 -1.5x 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, an absolutely huge improvement.  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!









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