Torsional Rigidity

[1967 R50/2]

Just wandered in from the motorcycle board...

Can someone give me the torsional ridgidty number in Nm for a 2004 911 Carrera soft top? BTW: What is your source? I've looked all over the web and found nada.

Thank in advance.

[jluetjen]

I've never heard or read of anyone who's measured either a convertable nor a Targa. In both cases the number is significantly softer then the coupes which are at about 2000 lb-ft per degree per Mark Donohue in the "Unfair Advantage"

[1967 R50/2]

Thanks...do we know exactly what the torsional rigidity of the hardtop 911 is? I can use that info too...but haven't been able to find an exact number.

[jluetjen]

Uhhh -- 2000 lb-ft per degree. My coupe was a hardtop the last time I check the garage. If you're off by a couple of lb-ft, it won't make much difference.

[masraum]

I thought the newer cabs and targa's were much closer to the hardtop than the older models. And the numbers from the Mark Donohue book will be for much earlier models than 2004, so I wouldn't go by that. The current cars are much stiffer than the older models.

[304065]

How would you tell? Strap the thing to a frame jig and apply a hydraulic ram to one corner with a strain gauge/scale in between and note the reading when the chassis bent 1 degree? From where?

[jluetjen]

Good Point John, I missed the 2004 reference. The new car is very different from the older car.

I humbly stand corrected.

[Grady Clay]

I think it is much easier than that.

First get the car appropriately balanced on some concrete and measure the fixed suspension points. I would choose the two bolts that hold the front cross member to the chassis and the rear torsion cover bolts.

Second, add some known shims (like vinyl tile) to two diagonally opposite corners.

Next, measure the corner weights (now all screwed up) and the suspension points as before.

From these numbers and the dimensions between suspension points and tires, you can calculate torsional rigidity numbers in units of [foot x pounds per degree] or other.

Best,
Grady

[cegerer]

Sounds like a good method Grady, but unless there is a standard that all cars are measured by, it seems like any 'rigidity' number would be meaningless. Unless one were attempting to set a baseline on an individual car for measuring modifications to improve rigidty. There seems to be an incredible number of variables ........

[JMar]

Originally posted by jluetjen
Uhhh -- 2000 lb-ft per degree. My coupe was a hardtop the last time I check the garage. If you're off by a couple of lb-ft, it won't make much difference.

Uh, that's for an old 70's car...I think he asked about a 996 vintage which a lot stiffer

[304065]

Well, why would you want that number? I can tell you why I think I would want it. The old "front strut bar" debate-- if you visualize the front structure of the car as a box-- set up, do your first pull, then bolt the strut bar in and do the second. Everybody talks about the importance of triangulation there, this would test the effect of the hypotenuse piece on the torsional ridgidity of the car. I suppose you could arrive at the same conclusion by measuring the distance between the front strut towers and then jacking the car way up on one side to a 45 degree angle (pretty damn high) and measuring the distance between the front strut towers again to see how much the lower one bends in with and without the strut bar in place.

So this would be a control/experimental test. If you were doing chassis simulation you would probably want it also, I would assume it's a variable in the movement of the suspension under load. . . but that's light years ahead of where I'm at . . .

[Randy Webb]

see Race Car Vehicle Dynamics -- I think they have some info on how to measure this.

As to the strut bar -- I would just hook a strain gage up to a data logger and head for the track....

[c2 rower]

All I know is when you jack a corner of most cars, the door does not shut right. Not so on a 911 coupe; it's like a bank vault.

[1967 R50/2]

Erhh..really didn't want to start such a debate...I just wanted the Nm number. The conversation kinda confirms to me though that no one really knows. Porsche MUST know, but I find it strange that they don't make the number public. Many other companies do. Perhaps the don't want the Cabrio to be compared to the hardtop because the rigidity number on the cab could be comparatively anemic. Don't know, but I'd be interested in finding out.

The highest figure I've been able to find yet is 38,000 Nm per degree for the VW Phaeton. In comparison, the Lambo Murcielago has "in excess of 20K Nm per degree" while the Lotus Elise has a suprisingly low number of around 10K Nm per degree. Suprising, because the Lotus is known for it's ultimate handling. Maybe it has something to do with the Lotus frame being aluminum.

[Randy Webb]

I swear I saw th real number for the 996 (hardtop) somewhere in a book. If you want to dig start with the books on the 996 since it came out -- I did not see it in the "evolution" book as I've not read it yet (tho it may be in there). Another place it might be is in the Pano article on the 'new' 996 - the one with the drawing showing the space efficiency of the cabin.

Good luck and post the value if you find it.

[jluetjen]

According to Paul Frere's "Porsche 911 Story" the 996 Coupe's body is 45% stiffer in torsion and 50% stiffer in flexion than its predecssor. The Carrera 4's body (on which the GT3, GT2 and Turbo body is based) are 49% and 82% stiffer respectively. The 996 Cab is 25% stiffer in torsion and 15% more rigid in flexion then the 993 Cab, and 33% and 17% stiffer then the earlier C4 cab.

I couldn't find any specific information about the 993's or the 964's.

[Early_S_Man]

Paul Frere discusses the rigidity numbers, and improvements, over the years in Porsche 911 Story!

Of course, I don't have the latest edition, so I can't address the 996, and don't care, either!!!