[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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