Torsional Rigidity Measurement...warning pseudoscience involved

[Walt Fricke]

For what it is worth, Porsche's Targas and Cabs, at least through the 1989 body style, used an extra layer of sheet metal in the sandwich which makes up the upper A pillar (holds windshield), and in the sills under the doors. They also have an extra layer of sheet metal over the front cockpit side of the fenderwells - the curved structures which intrude into the front footwells on the outer side of each footwell.

A coupe with these additions should be more rigid than one without them.

[Iciclehead]

Very interesting thread, particularly as I am at the point where I could add stuff to stiffen the chassis. I have already welded the drip rails, removed the sunroof, am fabricating an "X" brace for the frunk and have inspected the '89 torsion tube and figure it is pretty good the way it is when compared to the loosy goosy '75 set up.

Anyway, a thought about how to stiffen it further. I am considering laminating some steel over the A pillars, perhaps making them a little stiffer. I am putting a cross-roof brace, structurally bonded to the roof to give a bit more stiffness there.

But now I am looking at the inner roof structure as per below....not a photo of my car by the way,



and noting the area by the red arrow. Would it make sense to tie that most inner flange into the roof and effectively make a tube, which ought to be stiffer and form a kind of roll cage effect. Also wondering if the way to do it is to build a small "U" section that I could use structural adhesive to tie into the outer roof skin on one side of the "U" and similarly tie the other side of the "U" into that inner roof flange.

Couple that with a couple of cross roof braces, tied into that tube structure....well....would it help?

Thoughts on the matter from this brain trust?

Dennis

[Elombard]

Did the factory do that on the race cars? seems like a really simple way to increase stiffness. Maybe if you have to add a cage per the sanctioning body rules it would not be worth the effort or weight.

That tub is a freaking work of art!!! I cant stop staring at it.

[patkeefe]

Why not just bond some strain gauges to wherever you want to get a relative measurement? Leave them there through the build.
The Strain Gage

[Iciclehead]

Originally Posted by Elombard

Did the factory do that on the race cars? seems like a really simple way to increase stiffness. Maybe if you have to add a cage per the sanctioning body rules it would not be worth the effort or weight.

That tub is a freaking work of art!!! I cant stop staring at it.

yeah, it is out of the UK, they did a fabulous job of it.

My only possible explanation as to why the idea I have would not have been used is that prior to good structural adhesives you would be stuck with a bunch of welding on the low crown roof, which, over time would crack bondo and thus would require lead filling.,

Or...perhaps it is just easier to do the roll cage for the racing application as you need one anyway?

Dennis

[Discseven]

Nice study John. Looking forward to see where this goes.

[911pcars]

Originally Posted by gdfgdsgfs

I'll be following this to see what mods will make a noticeable improvement.

This isn't the first time this subject was discussed on a car over 30 years old. Latest improvements are incremental, if any. Go back to page 1 for a link to a pretty extensive thread.

Sherwood

[Walt Fricke]

I think us racers haven't looked at any of this because we have to have a full roll cage. And, where our rules allow it, we can tie the cage to the body here and there along the way. The incremental strengthening over tying the edge part of the roof understructure back into the roof would probably be kind of small. Especially compared with welding it to the bar structure if that could be put in suitably close. You've probably seen those steel panels with formed holes in them connecting the front hoop uprights to the A pillar.

It would be interesting to see if there is a picture of a cut crosswise through that roof structure, though. Porsche has an extra layer in the A pillar/windshield sheet metal layer cake for cabs and targas. I'm guessing that somewhere in that area over the doors there already is a sheet metal box, but maybe not. And a second one might help, especially if no roll cage is in the plan.

And look what Porsche is doing with different strength steel alloys in their new cars.

[JohnJL]

Here are a couple of pics of the b-pillar to sill plates I made, I also have some across the top of the b-pillar and a few more cut up for the a-pillar to windshield frame to the roof sill and a-pillar to the front sills.

In the background is a box I made up to move the shifter up and back 4", and to mount the hydraulic handbrake.

Once I get those braces welded in I will start with the rust repairs on the body and shaving the gutters....

[JohnJL]

I work slowly so it doesn't look like 12 hours but it is...lots of bracing and gusseting and dimple dies...

[RichardNew]

This is all very interesting. Years ago I was running a C4 Corvette. That particular car is a true flex flyer. The targa roof is actually a structural member. GM racing was doing some testing for their Solo 2 program.

On the chassis rig there was a huge difference in stiffness between roof off and roof on. The installation of the roof made a huge difference.

We then took the car out to black lake and actually drove it. Amazingly there was no difference in lap times between roof on and roof off.

I just raise this because lab numbers don't always match up with what happens in the real world.

The nice thing about strain gauges is that you can install them on a car that is driven on the road. That would be very interesting.

Richard Newton
Historic Racing Pictures

[RichardNew]

This is all very interesting. Years ago I was running a C4 Corvette. That particular car is a true flex flyer. The targa roof is actually a structural member. GM racing was doing some testing for their Solo 2 program.

On the chassis rig there was a huge difference in stiffness between roof off and roof on. The installation of the roof made a huge difference.

We then took the car out to black lake and actually drove it. Amazingly there was no difference in lap times between roof on and roof off.

I just raise this because lab numbers don't always match up with what happens in the real world.

The nice thing about strain gauges is that you can install them on a car that is driven on the road. That would be very interesting.

Richard Newton
Historic Racing Pictures

[LJ851]

Your large gussets would be a lot stronger if they had a folded top edge or a small piece of tubing welded on the edge.






..

[911pcars]

I'm somehow not convinced these corner gussets by themselves are the most effective solution to increase torsional rigidity. What other chassis mods are done/planned?

Sherwood

[JohnJL]

So here are some progress pics. I have...
- tacked in the corner gussets and started welding them in for real
- stitch welded the front end of the car along the spot-welded seams
- tacked in the rear shock tower gussets. I will make new braces, the ones I got from Elephant weren't a good fit to my towers.
- removed the gas tank support.
- began removing the front suspension pan
- drilled out the front sway bar holes and bracket bolt an a support flange
cut out much of the rust along the bottom/rear of the doors.
- welded in the harness/seat belt brackets
- prepared the chassis for welding in a knee bar and a harness bar/brace behind the driver/passenger shoulders
- measured the cutout for the front cooler and ducting










Look what I found in a pile of rust UNDER the gas tank support, above the front pan...very interesting...doesn't look like its ever been wet, so must have found its way down there after the car was off the road? I bought the shell from another Pelican, maybe he wants to volunteer an explanation?

[JohnJL]

more

[JohnJL]

I picked up the replacement sheet metal from Restoration Design. They are about 45 minutes from me. I called them at 9:30 AM of New Years Eve, they had it all boxed up an ready to pick up at 11 AM same morning. Nice guys and girls in the office.

[JohnJL]

I have seen a comment here along the lines of "if the front suspension mount was under a lot of torque then you would see more of them fail as they rusted."

Well here you are....there are no apparent marks from a strike with any hard objects on the mount or elsewhere I can find...

[safe]

Originally Posted by JohnJL

I have seen a comment here along the lines of "if the front suspension mount was under a lot of torque then you would see more of them fail as they rusted."

Well here you are....there are no apparent marks from a strike with any hard objects on the mount or elsewhere I can find...

The only thing connected to the front bolt is the "skid plate" and/or the tow hitch.
So your car has probably hit a curb with some force.

[JohnJL]

Magnus, it looks like it was pulled forward though?

I got the bad sections out of the old pan and started clamping it in place to measure and trim...