exactly Sherwood an over the top strut bar!!! Obviously only for the hard core, those that wanted a spare of carrying capacity would not want that.

I am suprised that something like that is not availble off the shelf. Sherwood I am thinking you should add it to your product line :-))

Dunno, but AWAG is, not. You might pm John Colesante I am sure that he would know.

bump...my previous question ....

" However, this question : Is there a common "test" set-up for this ? Meaning:.... where is the load applied and how is the chassis braced to counter-act to the applied load? Is it at the point where the wheels intersect the body, and if so , is the other-axle's location used a reference point, with that area blocked or braced ? ( This, in my mind ...would mimic the real-world better than some other locations where the force may be applied).

Is it therefore possible, we're quoting different numbers and the point where the applied force is applied is different...thereby making comparisons invalid ??? Example: the longer the "chassis"..and the further apart the two locations are ( applied force vs. braced area)..the "weaker" the body will appear to be, since it will twist more.


Plus this : if the 996 is 19,861....and the 993 is 13,700... I still think the TB 911 is more than 2,000...consider how similar the bodies for all the varioius "911's" are....

Wil - It is a good question, but I don't think we have the info to answer it.

Race Car Vehicle dynamics is one place to look - but it is very Americo-centric, SAE based stuff.

Anybody want to buy Frere a beer and ask him??

"Unless the suspension points are fastened to the engine/drivetrain (as some race cars are), I don't know if makes any difference whether the engine/drivetrain is in place. 'Torsional rigidity' means the relationship between the front and the rear. The less twisting between the two during cornering the better."

Some of the sources respond with that caveat. Something like, "if the engine/gearbox is in place, then the torsional rigidity becomes xxxxxx." (usually a higher number). Since the engine is usually pretty rigid, it's installation in a chassis opening should enhance the flex numbers. However, I don't know what the SAE/DIN specs are to calculate the numbers.

"Is it therefore possible, we're quoting different numbers and the point where the applied force is applied is different...thereby making comparisons invalid ??? Example: the longer the "chassis"..and the further apart the two locations are ( applied force vs. braced area)..the "weaker" the body will appear to be, since it will twist more."

A shorter chassis should have an inherent advantage in this respect just because everything is shorter and more compact. Torsional rigidity is measured between the front and rear, so everything inbetween is included - including large voids in the trunk area.

A modern chassis design includes a combination of medium, high and very high strength steels (in some cases aluminum, in a few cases CF), in single wall construction, in layers and/or with variations in gauge thickness to reduce weight where rigidity isn't as important, then spot welded and/or otherwise joined to maximize strength and rigidity. As advanced as the 911 was for that era, chassis design wasn't as sophisticated then as it is now, even when compared to todays run-of-the mill passenger cars.

Let's hope manufacturers use the same procedures for measurement. As these numbers become more of a sales tool, I'm sure they'll begin to fudge the numbers more; like aero specs.

One short story. Many years ago, a friend with a sagging 914 decided to install an aftermarket chassis stiffening kit, a series of precut metal pieces designed to be welded, riveting or otherwise attached to the known weaker areas of the unit-body chassis. The kit was successful enough in that after he had pop-riveted the pieces in place, the resting chassis maintained the same position it was in when supported by jack stands. Oops.

Sherwood

"As advanced as the 911 was for that era, chassis design wasn't as sophisticated then as it is now..."

- In particular, finite element analysis, which uses a grid of points to calculate distributed forces over a body, was not in existence. These numerical models have had large effects on a variety of products. Not just unit bodies of cars, but also our understanding of heat exchange, of fluid flow, and of lens design for cameras and other optical devices.

sophisticated tools do not always (or even usually) lead to sophisticated product -- they just make getting there easier.

I agree, just seam welding should yield great results for very little weight added.

The crossbrace over the gas tank would seem most effective for a torsion bar based car where the loading is along the longitude and less effective for a coilover based car. I would think that the coilover car benifits most from the bracing between the shock towers.

I think reinforcing the A and B pillars would help greatly on a coupe. Seam welding the roof to the side panels should help as well.

How about installing a high center tunnel from a late car in an early car for extra stiffening? It should help but pmaybe not worth the effort compared to other areas.

"The crossbrace over the gas tank would seem most effective for a torsion bar based car where the loading is along the longitude and less effective for a coilover based car. I would think that the coilover car benifits most from the bracing between the shock towers."

Thanks Bobby,
Can you insert some art that illustrates this?

"I think reinforcing the A and B pillars would help greatly on a coupe."

How? By adding metal via welding?

"Seam welding the roof to the side panels should help as well."

The area between rear window and rear quarter window? Sounds like some paint is jeopardized in this operation.

"How about installing a high center tunnel from a late car in an early car for extra stiffening? It should help but maybe not worth the effort compared to other areas."

That would do it. I realize a full cage accomplishes the objective as well, but for this exercise can we do this as unobtrusively as possible.

Seems to me a good place to increase chassis rigidity is to add a partial layer of floor material, extending from left interior rocker, over the tunnel, then to the right side interior rocker area. Added weight here is in a low location in the chassis. That and some stiffening/bracing in the trunk area. Any thoughts?

Sherwood

934-1/2

http://forums.pelicanparts.com/uploa...1136816488.jpg

http://forums.pelicanparts.com/uploa...1136816548.jpg

I think if your chassis is sagging on jack stands then you have some issues. I've had two open top 911's - a 1990 c2 Targa and a 1987 cabriolet. The 964 Targa was stiffer - one time I put one jack stand on piece of cardboard by mistake and the other on the bare concrete floor. The jack stand on the concrete floor had a gap between the chassis and jack stand that was the difference in the cardboard. I couldn't believe it so I switched out the cardboard to the other side with the same result...

with my cabriolet - I can still easily open the door while on jack stands and have measured the door gap and have found no difference.

I had an alfa romeo spider that when up on jack stands you could not open the doors!

John Cramer, do you have a shot of how those attach at the front of the car? That is a great design, quick in and out, might even clear a temp spare?

Truly poor cars in this respect ( that immediately come to mind) are the 80's Mustang convertibles and the C4 Covette. The Corvette could not get the roof panel back in-place unless it sits on a level surface !!

I wonder what the "specs" are for these cars?...if they show >2000 ft-lbs/ degree, I'll eat my hat !!

- Wil

I believe each manufacturer has there own testing methods and there isn't really a standardized testing procedure (correct me if I'm wrong here). I mentioned seam welding on the first page of this post, this is common practice for touring cars and rally cars, it stiffens and strengthens the chassis and doesn't add much weight. It's also pretty easy to do.

There are a few pretty good SAE technical papers on chassis stiffening and stiffness testing. Paper #2002-01-3300 was put out by members of the Cornel Formula SAE team and it covers their testing procedure in detail. It is a good read. There are also a couple of SAE papers on Winston Cup cars and how their cage designs contribute to stiffness. There may be more SAE papers that are based on street cars, all of my research has been for racecars.

Anyone have measurements - or driving impressions - from seam welding?

What is AWAG?

Yes, I would be very interested in that too!

AWAG (a wild ass guess).

I happened onto this thread on Rennlist about seam welding. Here on a GT3 tub. Thanks to Addict (Geoffrey) for sharing these pics:

http://forums.pelicanparts.com/uploa...1136840727.jpg
http://forums.pelicanparts.com/uploa...1136840743.jpg
http://forums.pelicanparts.com/uploa...1136840758.jpg

He further continues to say:
"......the factory service manuals have an entire section on the Carrera RS (Ed. Carrera RSA) including what to remove, and where to seam weld. I'm going throught that process now but decided to seam weld every seam lik the newer 996GT3 RS cars. It really isn't seam welding but stitch welding where every 25mm is welded. This allows for the panel to be disassembled in the even of damage, and it doesn't warp the chassis. Also, if you want to seam weld the chassis, you'll need to remove all of the seam sealer. I had my chassis baked and acid dipped to remove it. If you choose to use a torch, be aware that the fumes are toxic."

All you have to do is dip the chassis and you're ready to start.

From the comments offered on this thread thus far, it seems the 911 chassis became stronger with every passing year; either through better manufacturing methods and/or increased use of high strength/thicker gauge steel where it counts. And with each new platform, the engineers are able to create even more rigid designs yet provide the desired crush and safety margins in front, side and rear end collisions.

Sherwood

Dipping the chassis makes the welding easy but there are other ways to remove the paint and seam sealer. It can removed with a grinder, bead blast, chemical stripper, etc. You don't have to have the car fully stripped on a rotisserie, just strip out the interior and clean the areas that you're going to weld.

Here's a pretty cool slideshow of a rally car build up with some close up pics of the seam welding process:

http://www.rally.subaru.com/rally/servlet/CarBuildingDetail?part=1&imageId=5

http://forums.pelicanparts.com/uploa...1136846095.jpg

They also added some flat panels in for stiffening (lower B pillar)

http://forums.pelicanparts.com/uploa...1136846357.jpg http://forums.pelicanparts.com/uploa...1136846367.jpg