Hi - Convertible Frame Stiffening

[Carman 1964]

I discovered a post in which the owner of a Beetle convertible raised the same problem I discuss here. It is this:

https://www.thesamba.com/vw/forum/viewt ... p?t=703855

There he proposes as a solution to the lack of rigidity of the chassis to weld on the central tube another tubes in the rear section that will remain under the rear seat.
For this proposal is inspired by the Lotus backbone chassis, similar to that of the VW (see photo), from which two triangles are welded to the rear suspension bridge.
I thought it was a very good idea, those two triangles seem light and capable of providing great rigidity. The question is, if they were welded to the center tube just under the rear seat, the other end should be welded to the rear torsion tube?

[Ol'fogasaurus]

I looked at the TOS postings, the lower pix that chad1376 posted is very interesting.

IMG_2491 copy.jpg

IMG_2492 copy.jpg

Just for the heck of it this is part of a "can I do it" mockup I was/am playing with. It would end up, in some ways, like chad1376's design. I am not sure if what chad1376 designed is going to be strong enough (complete enough) to support the outer part of the pan or not but it is interesting. The VW (beetle) unibody setup is identified as a tunnel design if I remember correctly.

One thing I think I would do to chad1376's design is to change the lightening holes in the design and use at least two lines of spherical radius flanged holes (done properly though with material thickness and the lightening holes, again done properly, to support load and all directions): it is an interesting idea to say the least. The only thing I am not sure about is structure for "side loads"; e.g., a hit from the side.

Lee

I tried to find pix of the design and the "whys" for it and after several different text-based searches... "zilch" I got nothing. If you need it I can do a quick draw up (think). I got it from a long many-person discussion at my desk many years ago (late 60's or early 70's).

Lee

[Ol'fogasaurus]

i-HCHgnn9-S copy.jpg

This kept me awake this AM as the idea is so interesting.

I would not use the door jams as that doesn't do much for the pan itself but use it more as an outer edge of the pan stiffener. Lengthen it out, add a 90° flange at the bottom, a "C-shaped" stiffening flange at the top and 90° flanges at the holes. A lot of metal stretching would be needed to do it but it still is an interesting concept.

A lot of other things would have to be done but it is interesting idea to contemplate.

Lee

[Carman 1964]

Thank you so much for all these ideas, Lee.

[Ol'fogasaurus]

"I would not use the door jams as that doesn't do much for the pan itself ..."

I kind of goofed on writing this as the side piece (modified) would/could do some stiffening of the pan, but it would/could also dam(N)age the door frames because of the potential flexing or twisting of the pan. It's one of those things where you just don't know for certain as there are so many things that can come into play here. One of the reasons why the roof of a vehicle is so important but... with the convertible top down sometimes it is worth it .

Depending on how you are going to use the "convert" I would think is going to be the big player here. If the pan is in good condition, e.g., no rust or damage to it and you are just cruising around in it, then it should be OK as is (assuming there is no other damage having been done). If you are planning on other "enjoyable things" then a cage and or other stiffening things might start to be worth doing.

"Cause and affect" is something you are going to have to think about also as sometime in the future you might want to sell it and what you do may affect the sale price... plus or minus.

Lee

[Ol'fogasaurus]

More thoughts on this idea (for some reason every time I look at the design I get distracted... which is good and bad )

the holes would have to be flanged for sure which put some weight back in the panel. Besides the "flanging" of the holes and the strengthening of the top and bottom of the panel you could add some stiffening beads like the floors of the Beetle's pan has.

Looking at the frame design that came off TOS I think I see weaknesses or... maybe they aren't. Most of it having to do with twisting and the possibility of a different flanged lightening hole design. Like I said, after just over 20 years of retirement something like this is a "tease" and the partial remembrance of a long-ago discussion at my desk by several types of engineers on flanged hole design criteria.

Have you gone any farther on any design?

Lee

[Carman 1964]

No, Lee.
I have seen what the body reinforcements are like in the Porsche 356 and maybe that could be the line to follow. But I have put this topic on standby for now.

[Ol'fogasaurus]

No, Lee.
I have seen what the body reinforcements are like in the Porsche 356 and maybe that could be the line to follow. But I have put this topic on standby for now.

Sounds like a good idea to me.

Lee

I did a quick search on the Porche 356 pan. I was surprised to see how much it looks like the bug pan. For example: the tunnel is shape differently, not as high but looks to be somewhat wider (optical delusion?) and the floor pans forming is deeper than the bug. I did see some support ideas but some of them were kind of confusing at what they were attempting to do.

Keep us "up-to-date" on your addition of support. "Rag tops" were never that easy to reinforce.

Lee

[P_Vilefort]

A bit of history is needed. Karmann was a custom body builder in Germany/Austria. The Karmann Ghia was a body design by Ghia of Italy. The Beetle was a body-on-frame, not a unibody. The chassis when new is very stiff in bending and stiff in twisting. Adding the bolted on body just made it stiffer. That is why you could jack it up at the rear corner and lift the entire side of the car. If you cannot do that with your Karmann Ghia then you have rust issues. I have seen some information on TheSamba that describes and has pictures of what modifications were done by Karmann to make the Cabriolet much stiffer. The Cabriolet is the soft top Type 1 or Type 3. Type 1 is the Beetle also called Kafer. It is also the Super Beetle. The Karmann Ghia is a Type 3.
Unlike the Beetle or Type 1 the Type 3 is a fully welded body. The fenders are welded to the main body structure. If rust proofing compound was used on them when they were new, there would be many more of the Karmann Ghias around. The paint dipping process sucked. The Cabriolets also have rust issues due to leaking roofs. Usually the entire area under and behind the rear seat is full of holes or completely gone. The material that was used on the floor pans to reduce noise was also responsible for rotting out the floors. I discovered that I had rust on my floors under the sound proofing material because water would get into the low spots where the metal was deformed that makes the floor stronger. I was lucky in that my car was only driven in Germany for about two years and then put in covered storage for 39 years. When it came out of storage it never saw a salted road again and was rarely driven in the rain. The floors in my Super Beetle are intact with no perforations, not even under the battery.
If I use a large piece of wood like a 2X6 or 2X8 about 12 inches long and put it under the tunnel close to where the engine-transmission support forks join the tunnel I can jack the car up and all 4 wheels are in the air. The doors open and close the same as if tires were on the ground. With the wooden block about under the middle of the door, I can jack up the car with a floor jack and lift three wheels off the ground; both front tires and the right rear if I am jacking on the right side.
To make a Cabriolet stiffer, put back all the rusted sheet metal with a minimum of 18 gauge steel. Put a 1 inch square 11 gauge minimum steel tube in the channel where the body is bolted to the pan. Replace the bolts with longer bolts. Put additional stiffening at the B-pillars from the front of the rear seat all the way to the floor and weld it together. Try to find pictures of a restored Cabriolet to see where stiffening was used. If you have access to a CAD program, do what Karmann and VW couldn't do; create a 3D image of the chassis and body correctly dimensioned and load it to see where additional strength should be added. You might be surprised that it will not take much.

[Ol'fogasaurus]

I have to read your post a couple more times but the idea of the 1" is OK but make it a 1" X2" at least.

(For what's it worth) Square tubes, as I remember, bend easier and for the type of loading (transferring) you are doing the 2" side would give more up and downloading to the perimeter of the body (kind of like a reverse 3" body lift in ways).

The VW is a unibody but different as the tunnel is the prime loading part of the assy. The body of the car (not a convertible) does a lot of load transfer from front to rear, side to side and corner to corner while the convertible has no real top on it to "round" the loading.

black buggy rebuild 3-20-08 007 (2).jpg

On my black buggy I did a 3" body lift but then put a 1" square tube in the body mount tunnels. I was checking the seat loading and got lazy (distracted?) hadn't put the bolts in place to connect the body to the pan via the body lift. I put the driver's seat in place then sat on it and... and guess what, the 1" square tube took a downward "set" to it. Wall thickness of the square tube I used I don't remember but I think it may have been 1/8th stock. Again, one reason for the 2" side doing the work rather than a shorter/weaker 1" x 1" side trying to do the work (I hope this makes sense).

Lee

[Ol'fogasaurus]

One thing about the door flanged idea is that everything will need to be formed, e.g., the bottom of the door area has to be (probably double) flanged not only for attaching to the body and pan, but all three edges would work together for strength.

The "J" shaped notch would have to be flanged on that surface then flanged down (double flanging) for strength and the two end pieces would/might need to be double flanged also when connected to the door opening. On the horizontal door area, I don't think a shear type of join would work that well at the door area.

The holes would have to be flanged also and... maybe there would be some beading of the area also.

It would give some additional strength to the pan body combination but still, the cage would have to do that part of load transfer to make things safe.

More thinking about the area.

Lee

[Ol'fogasaurus]

I hope what I posted is somewhat clear. It is a write up based on several years design discussions by specialist engineers at my desk during several new designs in different places on an airplane.

The top picture has some of the discussion in it but, again, it is still based on the tunnel being the main structural component of the pan and it may/can allow torsional movement of the pan by the fore and aft suspensions.

The door thing I posted is lousy with not enough stuff there to make the body work as part of the whole.

The flanged hole's in the top pix is basically correct as the holes are not only flanged but have flanging on the sides of the unit to take care of the lost structure by the lightening holes. Multiple lines of holes (flanged or not) can be very weak without the extra flanging top, bottom and sides. Some of the flanging will not only have to be in one direct (say bend down for example) but that/those flanges may also need to be flanged for more structural support.

Rows of flanged holes say in a flat piece of stock should not be in horizontal and vertical alignment but in staggered lines so the stagger does not allow a place for the flat stock to bend. Minimum spacing of the flanged holes is usually 2 1/2 "D's (diameters) minimum hence aligned to the mid of the line of holes above them (again, getting complicate w/o pix (may be I will have to draw something to make the point easier

Lee