[DeTomaso] 1980s Pantera chassis weakness (was: Original Magnesium Wheel Failures?)
MikeLDrew at aol.com
MikeLDrew at aol.com
Tue Mar 6 20:16:04 EST 2012
In a message dated 3/6/12 16 52 12, lashdeep at yahoo.com writes:
> Please tell me more about my dismantling chassis!!
>
>>>Ah yes. By and large the quality of the later-model Panteras was a
mixed bag. On the one hand, the quality of the materials used was far
superior to those in the early Panteras (particularly the interior, leather vs.
naugahyde). But the switch from complex stampings to relatively simple jig
construction also led to some engineering compromises that have bitten more
than a few late-model owners in the years since.
Rather than reinventing the wheel, I'll do some simple cut-and paste from
the Aug 2001 PCNC newsletter, and May 2002 issue. First, Charlie's
experience:
====
A year and a half ago, I issued a warning about the need to check lower
front suspension mounts for potential failure, after discovering a broken lower
A-arm mount on Walter Villere’s car. His car appeared to have suffered a
shunt early in its life, and the front suspension may have taken a hit which
weakened the mounting structure enough to inspire failure ten or twenty
years later.
Recently, Charlie McCall suffered a similar sort of failure on the rear
suspension of his absolutely rust-free 1985 GT5-S. During our recent drive
across Europe, I had noted what appeared to be noticeable rear camber change
on the passenger side of the car, and foolishly urged Charlie to have it
looked at soon, instead of just diving down and looking at it NOW.
The fact that his motor blew up the following day was perhaps a blessing in
disguise, for when the car was towed to a repair shop, the workers there
discovered incipient rear suspension failure; had the motor held together much
longer, it’s quite possible he could have experienced full suspension
separation and consequent loss of control.
Although this was the first I’d heard of this sort of thing, I have
subsequently heard from several other people who had similar failures;
interestingly, they were all confined to late-model cars, i.e. GT5 and GT5-S. It’s
unclear to me whether the failure can be attributed to the larger wheels and
tires fitted to these cars, or to different manufacturing characteristics, but
I believe the latter is more likely the cause.
Apparently, the early Panteras feature a steel tube sandwiched in between
the inner and outer frame rails, through which the lower control arm mounting
yoke passes. This transmits all forces to both the inner and outer frame
rail. In looking at these photos, it appears that this steel tube is not
present on this particular car (it’s somewhat difficult to tell for sure.)
That would mean that all pulling forces were felt only by the inner rail,
and pushing forces by the outer rail.
Eventually, the forces were too much for the mild Italian steel, and the
washer and mounting nut pulled through the inner rail, and came to rest on the
inside edge of the outer rail. It’s also apparent that they then began to
pull their way through the outer rail, and it wouldn’t have been much
longer before they pulled through entirely! The driver’s side of the car showed
deformation and a crack along the bottom edge, indicating that failure was
underway there as well.
This knowledge would seem to demand an immediate inspection of this area on
every Pantera. I feel fairly confident that early cars will be in
reasonable shape, but would caution owners of newer cars to be expecting some
degree of failure here, and would urge anybody driving a GT5 or GT5-S to spend a
few minutes peering underneath before their next drive, spirited or
otherwise.
There are several potential solutions, some of which will come with
additional benefits which makes them doubly worthwhile.
Assuming that these later cars were manufactured without the aforementioned
stress-relieving tube between the inner and outer rails, then external
reinforcement is in order. One solution would be to bend a plate of fairly
thick steel into an “L” shape, drill an appropriate hole, and weld or screw it
to the inside of the frame rail. The idea here would be that the threaded
portion of the yoke would pass not only through the inner and outer chassis
rail, but also through this steel plate. It should be at least three or
four inches wide, enough to transmit the various forces over a larger area of
the chassis. Although the failures I’m aware of were concentrated on the
rear mounts, I’d feel better having reinforcements on all four mounts.
Alternately, the chassis braces sold by Hall Pantera and Precision
Pro-Formance utilize all four A-arm mounts to affix the brace to the car (the
Pantera Parts Connection brace only utilizes the rear a-arm mounts); purchasing
and installing this brace would effectively result in a full-length steel
reinforcement for the A-arms, as well as the primary advertised benefit of
reduced chassis flex.
One area of potential concern would be the parking brake mechanism. Later
Panteras utilized a completely different parking brake scheme, and the
parking brake pulley mechanism is located at the rear of the car, between the
a-arm mounts, instead of in front of the motor as it is on the early cars.
Thus, there could be interference between this mechanism (or at least the
mounting bracket) and the chassis brace. However, the mechanism is thankfully
mounted to a removable bracket, which presumably could be massaged to
ensure adequate clearance.
In light of this potential problem, it should go without saying that it is
important to not over-torque the A-arm mounts when installing them. While
it’s possible that failures of this type are caused by stresses imparted by
the road, there’s no doubt that they can be accelerated by over-enthusiasm
when tightening the nuts down (remember they have to be loosened and
tightened each time the rear wheels are aligned, so one can assume that several
hands have touched them since the cars were first put together.)
We can never forget that we are driving cars which are approaching 30 years
of age, and in most cases, have seen upgrades in terms of horsepower,
braking and traction, sometimes dramatic ones. While one shouldn’t expect these
cars to simply fall apart like a soup sandwich, nevertheless we as owners
can bear a certain responsibility for helping to induce failures of this type
by performing these modifications. It is only through vigilance that we
can safely continue to aggressively drive these cars for the indefinite
future.
====
Then, some destructive testing done on a 1971 Pantera to compare it with a
GT5-S:
====
Several months ago, I wrote a short article detailing the failure of
Charlie McCall’s GT5-S lower rear a-arm mounts and cautioned that this appeared to
be a fundamental design flaw in the later, hand-built Panteras. While
there is plenty of anecdotal evidence supporting the contention that late-model
chassis is dramatically under-engineered, particularly in the rear lower
control arm mounts, some owners of later cars are especially sensitive to the
criticisms heaped upon these cars by the Pantera vendors who work on them
every day. They cite an overall improvement in development, comfort, and
build quality and have big problems with people levelling accusations of
structural inferiority upon their cars without any proof.
So I decided to fly to Detroit and drive down to Kirk Evans' shop, do some
destructive testing on early and late Pantera chassis rails and determine
once and for all if there is a significant design problem.
Well, here is your proof.
Gary Roys' GT5-S (recently sold) was up on jackstands in Kirk's shop, minus
any suspension, so it was a simple matter to bust out the Tool of Justice
and ziz some sheetmetal away to peer inside and see what is going on. Kirk
happened to have a pair of 1971 chassis frame rails literally lying around
which provided an excellent side-by-side comparison.
A peek inside the absolutely rust-free GT5-S chassis was somewhat
horrifying. Kirk has spent more time than just about anybody dismantling and
reassembling early Pantera chassis, but I believe that he hasn't really done too
much with the late cars other than import them and sell them under the aegis
of Amerisport during the late 1990’s. He expressed considerable surprise
at the comparative lack of spot welds on the chassis rail, and the hokey tack
welds used to join the two halves together at the bottom. Upon opening
the rail up, there was no visible reinforcement except the tube, which he was
able to move around by hand. He agreed that this was grossly inadequate
for the task at hand.
A comparison with the 1971 chassis rail showed that somebody was on the
ball back then—a second layer of sheet metal encompasses the inside of the
frame rail and has a good 20 or so spot welds to keep it in place. Inside the
frame rail is an extremely complex reinforcement structure formed from sheet
metal bent into an M shape and placed on end.
Bottom line—in this area, early cars are relatively strong, and late cars
are weak to the point of being potentially dangerous. Evans plans to
engineer a simple bolt-on or weld-on sheetmetal cover (similar to that used on the
early cars) which he will sell at cost to any owner of a post-Ford Pantera.
One side note—both structures feature internal reinforcement of one type or
another that would be virtually impossible to deform simply by
over-tightening the nuts which secure the yokes to the chassis rails. There has long
been speculation that the several failures reported on the GT5-era cars were
caused by over-tightening, but that no longer seems to be a valid theory.
Thanks to Kirk and Gary for helping settle this issue and hopefully solving
this problem.
=======
Photos from the destructive testing can be found here:
http://www.poca.com/index.php/gallery/?g2_itemId=35465&g2_imageViewsIndex=1
Click the 'full size' link for a better view.
While the problem is not terminal, it definitely needs to be addressed if
it hasn't been already!
Mike
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