Maybe, but this isn't a trivial swap...

The sill plates shown in the prior pic are the ones I welded into 5speed's car, not mine. I have a full cage in mine now.

Plus....even if the glass is glued-in and theroretically strengthens things....glass itself is s very poor transmitter of forces across its surface...it gets easily cracked because it cannot resist twist or distortion very well.

All-in-all, I would not put too much faith in counting upon a glued-in windshield to *truly* gain you much...no matter what the factory may have said ( like so many factories did when this contruction became vogue).

- Wil

I wonder how much load the diagnal carries when a prallelogram gets a force at the corner?

The sunroof and rain sills are a weak part of the roof and if cut and welded would strengthen the top and A pillars. Using a harness bar also adds strength. Taking weight away from the ends of the car, exhaust and battery would also reduce twisting. The bushings have some flex and also can be replaced.

It would be good to see what is available without welding and then with welding and then more internal bracing. A welded brace between both rear shock towers would help (I have seen this) and front brace. I think removing 90% of the factory flex would be enough for most...more is good for pure racing or rally cars.

>I wonder how much load the diagonal carries when a parallelogram gets a force at the corner?

Just back of the napkin, I get the windshield needing to take 1000+ lbs in shear force when the chassis is at “max twist”. This is only a rough ballpark, and factors like how rubbery a glue you use to fix the windshield, how much twist, etc are all important players I can only estimate.

>glass gets easily cracked because it cannot resist twist or distortion very well

Yes. But here it doesn't crack, because the force is not high enough to distort it much, and the force is almost all in shear and not bending. Think of it this way: With no windshield you'll get a bit under 1/2" sideways offset between the top and bottom for the windshield frame at max torsional deflection. This is because those 2 thin A pillars form a very springy structure with 1500 or so lbs trying to bend them sideways.

Now with a glued windshield this is impossible- as you say you'd crack the glass. In fact for our purposes the glass barely deflects in sheer at all. You have some deflection in the rubbery glue used to hold the windshield in, etc, but not much. So most of the load goes through the glass, and a little goes through the metal frame.

I do know some 911 guys who do this mod to older 911s to improve structural rigidity- this is not a new idea. Maybe someone who has done it can chime in and say if they felt the difference or not. Clearly all the math, and all the car makers, say this makes a big difference. BTW, this is also the same principal used in monocoque aircraft frame construction- the aluminum skin stretched between the skeletal frame ribs and stringers creates all the stiffness, despite the fact that at times it is not much thicker than a coke can.

I'll go out on a limb and say that the glued windshield by itself, based on the FEA I've done in the past and gut feel, could buy you around a 50% improvement in torsional stiffness of a stock older 911 coupe(!). But what do I know?

Could you get a windshield from a 993/964 (whatever got a glued windshield first) and glue that to an older car, or would you need to change the a-pilars etc?

That's a big statement! I like it!

Actually Randy;
I think that you missed the most obvious way to evaluate the effects -- You measure it!

Here's an example demonstrated on a simple balsawood model of a chassis.
http://forums.pelicanparts.com/uploa...1143725164.jpg

Here's the same style of test done on a real chassis.
http://forums.pelicanparts.com/uploa...1143734824.jpg

It's basically a case of doing a before and after test of the chassis with the modifications. In the absence of a "before" test, substituting a stock model of the same year should suffice. These pictures are from Herb Adams book: Chassis Engineering. It's an OK text on the subject.

A glue in window would make a very big difference as a sheer panel. Glass is used structurally on buildings all the time. As long as it stays in a sheer situation it is pretty robust. Again, the question becomes can the later glass be glued into the earlier car??? If so, I'll do it on my next windshield change.

a good point John - I haven't been able to urge someone else to make those measurements on a 911 tho...

____________________________________
putting thin panels over a skeleton is an old idea - look at a crab's exoskeleton sometime - depending on who you work for, you may be able to write off the cost of dinner...

then, there is also the same principal used in building construction- the plywood "skin" between the skeletal frame of 2x4's creates all the stiffness, despite the fact that the plywood is not very thick

You guys are missing the most important factor here.

You can't use the chrome trim with a 993 windshield!!!! (Gasp!)



BTW, the windshield opening is unchanged up through '98, IIRC, so you should be able to use the glue-in 993 windshield in all earlier cars.

I think expecting a 50% increase in torsional stiffness of the tub just by adding a glue in windshield is very optimistic.

The windshield ( dimensions) changed....slightly....late 89 .....IIRC

That accounts for the later C2 type cars haveing a more flush install, and the rubber trim that is used w/o the interior metal trim bead.....

I saw the text in a factory manual...again IIRC... need to dig this up and repost.

Wil

Anyone thinking a glued in windshield will greatly contribute to torsional rigidity in a porsche is kidding themself. First off I doubt the newer windscreens were designed with the thought of it being a load transferring member. Secondly the ability of said windscreen to transfer loads is limited by the way in which it is attached to the sheel, chemically and also a slight pressure fit. The glass will move about inside its rubber gasket and will also flex slightly at its chemical bond.

I have my 72 shell in the garage and can stare at it all night dreaming up ways of increasing the stiffness of it. The torsional rigidity from front suspension to rear suspension is my main concern in terms of handling. I saw some pictures on this board of someone who had welded the torsion bar tube with plates to the shell, a good start. All cage designs I have seen in porsche's lack something. Everyone is concerned about driver safety (this I do not argue with) and stiffening the car. There are some that are a normal cage with extended legs to tie in the rear suspension and front suspension. Then these cages are simply welded to the inner rockers on the chassis. Why are they not tied together at the base?

How about welding a frame UNDER the car in a large X to tie front and rear suspensions and also give a place for the upper cage to tie into?

On top of that reinforcing sheets can be welded to stresset areas providing a greater surface area to spread loads. All these cages and rear "RSR" suspension mods seem a good start but stop short of something great. Why simply weld a small triangle bracket from shock tower to the rear crossmember? How about large sheets welded and glued on top of the existing structure to strengthen it further?

This might seem over the top at first but there are tube frame cars out there are plenty have had their floor replaced. Why not cut out the entire floor, now you can tie round or square braces from front lower suspension mounts to the rear lower suspension mounts. Top it off with honeycomb plate as your floor, I could go on and on with crazy ideas then realize that it would be simpler (not cheaper I aint thinking price yet) to head off to prodrive, give them a stripped shell and say, look, build me a composite monocoque that I can slap Porsche bodywork onto and fiddle around with suspension heights as well. Better yet give me unequal length and non parallel A arms all around with push or pullrod activated dampers...........see how slippery this all is?

I am just beggin for some welding equipment, raw material, a couple tools and to see what I can come up with. Silly in the long run I know but we like these silly cars

Ha, interesting thread... Which brings up a very personal problem ...
I've found out my 73 was made from 2 cars spliced together (72S nose, 73.5 T ass), welded at the shifter level, more or less... The weld was staggered, made long on the longitudinals, short on the floor, long on the tunnel tunnel, etc.... Not sure where the roof was done but I am guessing bottom of A pillars... It's been like that since the early 70s apparently, but it's too late to throw it away now - I did not find out until too late - and it's a great rust-free commuter. It was pronounced pretty straight on a celette bench - supposedly a good welding job.

What would you do to such a frankencar to stiffen it up enough so that you could track it occasionally ? Or stop worrying about a titanic style breakup ? I'm wondering about a roll cage that would unite the suspension points but not be too intrusive... ???

Yes, the later windshields are larger, and the 964 used a smaller rubber seal that sat more flush. The 993 windshield may be even larger still, and the trim is nothing more than a small flat piece of rubber that is glued in to ccover the seam.

But I'm pretty sure the chassis' winshield opening is the same. I know people have used a 964 windshield and rubber in earlier cars without issue.

Tyson:

That's my point and I'm mad ( at myself!) for nor knowing this more accurately...I'm speaking specifically of a "minor" dimensional change made by the factoy of the "opening"...to coincide with the glass / trim changes we both seem to agree upon that were made around this time.....

Hafta find the source !!! ......

-Wil

Porschekid;
I think I got you. What you'll wind up with is something like this...

http://hudsonhistorics.com/935MobyDi...03 013_jpg.jpg

http://hudsonhistorics.com/935MobyDi...03 010_jpg.jpg

http://hudsonhistorics.com/935MobyDi...03 003_jpg.jpg

Oh yeah -- that's what the factory did. They called it a 935/78, aka: "Moby Dick"! SmileWavy

HAHA, something like that. I was thinking of mods that were a little less invasive in order to keep things streetable(to a point of course). I have some pictures printed out and will draw out my thoughts and scan them to post them back up. I know that everything has been tried at one time or another I was merely thinking of the easiest way to tie all this stuff together without going way over the top.

Brackets like the RSR type rear shock tower braces I dont really see as a huge improvement. Just strapping things to other things to spread the load a bit but its almost like duct tape in my opinion. You will end up finding cracks and wear in new spots. I cant find it now but at one point happened across a thread wherein the rear shock tower mounts were removed completely and replaced with pieces that moved the mount points upwards. This was then tied into the cage and gusseted. Looked nice but lots of work.

I will post what I come up with in a day or so. Cheers!

That's what the factory did 30 years ago - by todays standards that aluminum tubing and single door bar are downright scary!