[DeTomaso] Pantera Si

[MikeLDrew at aol.com]

In a message dated 12/19/13 8 38 2, timepiecepr2 at yahoo.com writes:


> 
> Can you summarize the upgrades that made the car so much more modern and 
> civilized. I am just curious.
> 
> 
>>>The entire suspension system was redesigned in keeping with modern 
practice.   The standard Pantera was state-of-the-art at the time, but then 
again, a horse and buggy was state of the art in ITS time too.   The early 
Pantera uses control arms that are extremely short by modern standards, and wheels 
with a negative offset (and in the case of the GT5, an extremely negative 
offset).   This means that for a given amount of wheel travel in the vertical 
plane, there is a relatively large amount of angular change in the 
suspension arm.

Consider a standard 12-inch ruler.   Hold it horizontally, then move one 
end of it up and down six inches.   Now consider a 36-inch yardstick.   If you 
do the same thing, six inches of vertical movement at one end results in 
much less angular difference.

The same holds true with auto suspension design.   The Pantera Si uses 
control arms that are substantially longer, which spaces the hubs much further 
away from the centerline of the car.   If you bolted standard Pantera wheels 
on, the tires would stick out about a week.   So instead, it has wheels with 
fairly pronounced positive offset--the vertical surface of the wheel is 
relatively flat (particularly in front) and the rim extends inwards and 
envelops the hubs/brakes.

If you look at a normal Pantera suspension and compare the kingpin 
centerline (the imaginary line between the upper and lower ball joint, where the 
steering spindle rotates) and the centerline of the tire contact patch, where 
both touch the ground, you'll see that there is a pronounced difference, with 
the contact patch centerline being several inches outboard of the kingpin 
centerline.   The difference between where these two lines touch the ground 
is called the scrub radius.   The Pantera has a fairly large scrub radius, 
which raises steering effort and magnifies road shock, as an impact to one 
wheel has a leverage effect.   That's why when you hit a bump with your right 
tire, for instance, the steering wheel tugs to the right and you have to 
correct it.

(Replacing the stock 7-inch front wheels with 8-inch wheels helps correct 
this, as doing so reduces the scrub radius a half inch because the extra 
wheel width is all inboard of the wheel centerline).

The Pantera Si with its longer A-arms has much less scrub radius, so the 
suspension is much more immune to unwanted feedback transmitted by bumps.   I 
don't know for a fact, but I suspect the steering axis inclination (SAI) 
figures are better as well.   SAI is the angle between the kingpin line and the 
imaginary vertical line of the center of the contact patch.   Angling the 
SAI inwards causes the spindle to rise and fall as the wheels are turned one 
direction or the other, which in turn raises the suspension slightly.  This 
causes the suspension to attempt to seek its low (center) position 
naturally, which allows the design to rely less on positive caster to keep 
directional stability.   Positive caster delivers stability at the expense of higher 
steering effort; achieving stability through more SAI and less caster allows 
the same stability with less steering effort.

What this all means from behind the wheel of the Pantera Si is that it just 
drives beautifully, like a completely modern car compared to a standard 
Pantera.   Driving a regular Pantera on a bumpy road requires a degree of 
concentration, because even with the bump steer optimized by shimming the rack 
downwards (which De Tomaso/Ford did starting in late '72-early '73), the bump 
steer curve is still there, and the scrub radius means the steering wheel is 
tugging at your hands constantly.   By contrast, a Pantera Si just glides 
across those same road imperfections, with the suspension moving up and down 
to absorb bumps, and delivering only wanted feedback through to the wheel.   
Steering effort is probably about the same as a regular Pantera, or perhaps 
a bit less.

Another terrific Si improvement is the brakes.   It wears the same Brembo 
brakes as the Ferrari F40, and they are simply leagues ahead of the stock 
Pantera brakes, or even most aftermarket systems that I've tried, in terms of 
power and feedback.   There is NO power assist (no brake booster at all), yet 
due to changed brake pedal geometry, pedal effort is about the same as with 
a normal Pantera, but ultimate power and feedback and control are much 
greater.   The pedal feels nice and firm, very mechanical.

The cabin is beautifully trimmed, and things like wind noise are controlled 
a bit better as well.   Overall the car is much quieter going down the 
road, due in no small part to the engine which is as weak as a kitten, and the 
EU-compliant exhaust system which mutes it even further.

Most of us know the feeling after driving a Pantera for many hours at a 
stretch.   You get out of the car and you are pretty tired.   I'm confident the 
same would not be true after making that same drive in a Pantera Si.

FWIW I've heard that Pat Mical has developed a bolt-on suspension package 
that does away with all the stock control arms, hubs and spindles and 
emulates the Si design; it necessarily requires new wheels as well.   Obviously 
that is not a conversion for the faint of wallet, but I imagine it would 
improve the car's handling and driving characteristics immensely.   I don't know 
if any cars have been modified other than the single example that he did for 
himself when he converted a stock Pantera into an Si clone, which is a MUCH 
larger undertaking than you would imagine!

Mike
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