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Nutrition's Avatar
 
Join Date: Jun 2002
Location: on the run from the looney bin
Posts: 24
Targa to Cab conversion people

I've done a couple of searches on this topic (and a couple with similar topics - the targa vs. coupe for example), but I haven't heard much mentioned in terms of changes in the "torsional rigidity" or specifially if any change was noticed after the conversion.

Some things I've seen mentioned - in no specific order:

From Jack Olsen:
Tyson's car is the kind of 911 that shouldn't be competitive on any track. It's a 72 T with stock torsion bars and sways. It's a Targa converted into a cab, so it's chassis should be the least rigid of any car. It's light, but not particularly light -- with electric windows, a full-size passenger seat, stereo, sound deadening and carpet. It's even got a heavier-than-most roll bar in it.

Further:
I drove the car also (at a pace 2 seconds off of Tyson's), and it's really amazing how a well-set-up, light car handles. It's completely tossable -- you overcook a big sweeper and you can goose a little throttle, or ease back, or brush the brakes, and bring the car right back onto the line. Its balance really spoils you. But you can learn more, because bad moves are (usually) fully recoverable, and there's no cheating with the go pedal to make up for a bad line through a corner.

Question #1

A) Jack, did you notice much difference, torsionally, between your car and Tyson's?
B) Tyson, have you noticed much difference since the cab conversion?
C) In reference to the further section, do you think that those characteristics might be influenced somewhat to a possible loss of rigidity from the conversion - the A++ set-up aside?

From the Massachusetts Maestro, the Westford Wizard - Jluetjen:
As far as the observation of the impact of weight on "chuckability" and fun factor - I've heard from a number of people that driving a FF (105 HP and 1000+/- lbs) is a lot more fun then driving cars with more weight and HP (FA, GT1/TransAm, etc). The precision and demands of a higher weight vehicle increase the challenge and work involved, but also reduce the fun a lot.

An interesting question on your original conclusion would be to also weight (of) the different items. (Nutrition adds: say between Tyson's 72t and Widebody911's 3.6 conversion - signiicant flex differential?)

To expand on James' thought, keep in mind that all chassis flex. The question is : How much? Mark Donahue mentions in his book "The Unfair Advantage" that the 911's that they worked on flexed at a rate 2000 ft-lbs/degree, which he considered plenty stiff. Keep in mind what this means to your alignment. For every 2000 ft-lbs of torque you apply to the chassis, it will twist 1 degree. Imagaine that you have 500 lb springs on the front and that they are about 2 feet from the car's centerline. If you corner such that you compress the outside spring by 1 inch, this is 500 lbs at 2 feet or 2000 ft-lbs or torque. Add on a stiff roll bar and you most likely have even more torque being applied to the frame. Now your rear suspension is rolled 1+ degrees further then your front. This additional roll at the rear basically dilutes the affects of your sway bars. As a rule of thumb I seen that your chassis stiffness should be about 20x stiffer then your spring stiffness if you want to keep it's contribution to the cars total movement to less then 5%.

Question #2

John, do you think a conversion like this would be above the 5% you suggested, assuming a stock suspension set-up on an early car?


And Finally,

Widebody911:
Here's an idea I have for a removable targa/cab stiffening member (hehe - I said stiff member!) which would be removable for normal street use. In hindsight, it's funny how I said 'removable for street use' rather than 'bolt in for track days.'

My idea is to weld metal plates into the door jambs, some arbitrary distance (to be determined later) up from the sill. The bar would bolt to that.

Question #3

Widebody911, A) have you noticed any loss of rigidity since your conversion and B) if so, is that why you're considering the above?

As Always, thanks in advance.


PS - I would've put links for the aforementioned threads but i wasn't positive it would come out right, as in I'm not sure how to do it.

Old 03-07-2003, 08:14 PM
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I noticed a very slight loss of stiffness after the conversion. I had a very wiggly Targa on my hands to start with. Some are stiffer than others. Mine was horrible.

I installed the inner rocker stiffening panels, and it helped immensely. When I would put the car up on a hoist, the door gap would open up about 30% as much as it used to prior to the rocker stiffeners.

But, I still had that annoying cowl shake, and the chassis still felt like it was twisting when cornered really hard. The next thing I did was weld in a strut tower brace just for testing. This helped a little more. But, on one wheel bumps you could still visibly recognize that that corner of the car was moving independently from the rest of the car. You could also feel the wing window move separately from the A-pillar.

The next move was a drastic one. I wanted to eliminate as much twist as possible. This has the most benefit to handling for the reasons you stated above. My solution was to tie the upper strut mounts to the center tunnel (Right next to the drivers feet.) with 1" tubing. I cut holes in the trunk floor, reinforced the center tunnel, and welded in the tubing. (There were many many obstacles to overcome, including some serious throttle pedal rearranging.)

The results were astounding! My girlfriend at the time actually commented to me that the car felt different as we drove over a familiar set of potholes and road irregularities. The difference was night and day.

As you would sit in the drivers seat and go over rough road, the car used to twist so that the steering wheel and dash seemed to move in the opposite direction as the back half of the car. So the drivers seat and steering wheel were moving in different directions as the car twisted. Mind you, this is subtle, and you don't realize it until it's gone. After the diagonal bracing was installed, it was GONE!!!!

With the last set of suspension tuning I did to the car, it really made the chassis stiffness show it's full benefit. I swear to god that this car is nearly as stiff as a good coupe. It is probably as stiff or stiffer than a tired coupe. Others have driven the car and said the same thing. And these are guys who have driven hundreds of 911's.
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'72 911T Targa (Formerly "Scruffy")
2004 GT3
Old 03-07-2003, 08:44 PM
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Tyson, did the end result create a triangulation (as I was researching past posts I saw this term mentioned often) between the strut mounts and the center tunnel (in conjunction with the strut tower brace)? Iím trying to visualize what youíve said (Iíll try to figure out what exactly is involved later). What were your considerations while reinforcing the center tunnel?

Very clever!!
Old 03-07-2003, 09:26 PM
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Yes, exactly. It triangulated the center tunnel with the upper strut mounts.

The center tunnel reinforcement was simply an overlay of sheet metal. Kind of like a cap that folded over the sides so that it had some vertical strength. I made sure to leave the original tunnel openings, so that it wouldn't hinder any future access to clutch cable, etc. My original plan was to continue the reinforcement all the way down the center tunnel to the torque tube, but it became obvious that this wasn't necessary.
If I ever get crazy with horsepower and stiffer torsion bars, I may see the need to do this. But right now I'm completely happy with the car's structural integrity. I no longer long for a coupe.

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'69 911E coupe' 2.7E MFI (retired 911-Spec racer)
'72 911T Targa (Formerly "Scruffy")
2004 GT3
Old 03-07-2003, 09:39 PM
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