Lighter weight 'convertible' targa top

Thank you, that is perfect. Greatly appreciated.

I do have a extra top courtesy of Jeff VanDyke which I think would be perfect for this.
 
Dr. Jeff, I cut as much as I dared cut out in order to make the conversion worthwhile. Did I cut too much? Only time will tell. I did have a chance to drive the car briefly yesterday and no issues and top worked great. To answer Joe's question, measuring from the side edges I have approximately 4 3/8 inches of 'side' fiberglass until the cut begins. The cut at the back of the targa is approx. 5 1/8 inch. That's about all you can cut in that area. If you are unsure, or want to be safe, I'd say allow 5 inches of material or so. Of course, that will come with a slight added weight penalty. I also forgot to mention, if you look at the thin rib in the photo of the underside of the top, it's actually reinforced with a thin strip of fiberglass I cut from the adjacent panel. That basically doubles it up and adds strength to keep it from bowing inwards too easily. Hopefully that makes sense!

If either of you guys do this conversion, post some pics of your progress. Hopefully you'll be able to figure out some improvements to the process!

Thanks for the tips - I like the addition of the brace rib. I'll copy that.

Started stripping mine today - have to bag all the hardware with each strip so I don't forget

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drilled out all the rivets

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cleaned up the butyl film a few years ago - thankfully I just put batting/padding in there

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Mine needs repair to some of the screw bosses - scraped back to the fiberglass so I can epoxy.

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Thanks again "Cruiser" for all of your added info. I totally agree, how much to cut out is pretty much a guess. With the sides being more contoured, they will have to retain enough of the fiberglass structure to keep the shape as it curves up and across. Otherwise the fabric would pull straight across the void rather than curve. But as you say, strength wise those sides are the strongest portion so they don't seem to need a lot of structure. The front and rear ends are limited by the attachment hearware mounting surfaces. But you managed to cut around them very well. So a good balance between reducing weight and retaining shape and strength.

I'm impressed at how well the fabric retains the overall shape without showing obvious depressions where the cutouts are. That was my hesitation with using a fabric cover. I was thinking of maybe a thin aluminum skin. But I like the fabric much better.

Just for reference. Here is an image of how one of our vendors cut the fiberglass away to use it as a framework for a plexi covered top:
X19 custom targa frame 1.jpg

But in this case it would not need a lot of structure due to the firm cover that will be added over it.

In the past we have thrown around ideas for something like your light weight top. I've always thought it would be nice to have a collapsible framework with cloth cover, like the old Porsche Targa tops had. That way it takes up very little room when removed. You could even store it behind the passenger seat rather than going in/out of the trunk, freeing up all that room and being more convenient.
One possible simple design might be something like those camping tents, with fiberglass poles to stretch the cover tight. The front and rear edges of the original top would be retained to connect it to the car. But the space inbetween could be some removable poles. The fabric is attached to the front/rear end sections so the whole thing could be collapsed by taking the poles out. Sort of looks like this (the red lines are the poles):
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The entire space between the ends of the poles is removed and replaced with fabric. You might be able to carry the poles further rearward, with more area cut out. The major drawback is the lack of side frames and side-window seals. So definitely a summer top. Maybe a good candidate for the mesh fabric I referenced earlier.
 
While you got me thinking about this whole topic ('top'ic) again, I was debating the idea of a mesh cover more. Here is a poor example of a mesh type top for Jeeps, but it illustrates what the fabric is like looking from the inside:
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As you can see it allows some light and air to pass, but reduces UV (sunburn and interior damage), glare, heat, and excessive wind. I'd view it as something you might pop on when going for a drive where you would otherwise go topless. No need for a hat. Making things more pleasurable/comfortable while still able to see the sky and feel the fresh air.
 
Marked up the targa based on pics & suggested measurements. Made some adjustments for convenience sake.

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taped off cut guides

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temp is back in the 30's, so warm enough to work outside for a couple hours

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Used angle grinder for main cuts, wearing a full respirator - this makes a significant dust cloud

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Used a scroll saw to rough radius the corners, then a dremel with barrel grinder to clean up all the radii

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For the cross brace, I cut "T" sections to extend the reinforcement into the side members.

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Sanded and ready for the Fusor epoxy

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Clamped & setup for curing

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This is the StayFast cover material I chose (not quite as red or dark as it appears):

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Looks great Huss.
I'm curious if you noticed how flexible the center rib was before gluing on the reinforcement. And if you can make a comparison of it after adding the reinforcement. Just to somewhat quantify how necessary it is to add it.
Thanks for sharing. I'm looking forward to your installation of the fabric.
 
Looks great Huss.
I'm curious if you noticed how flexible the center rib was before gluing on the reinforcement. And if you can make a comparison of it after adding the reinforcement. Just to somewhat quantify how necessary it is to add it.
Thanks for sharing. I'm looking forward to your installation of the fabric.

It has no real substance without the reinforcement. It will not hold the curve of the top without the brace. It is possible that when the canvas is glued to the rib, that it would retain the arc, but it would still deflect under light pressure. Can't see it not sagging at road speeds.
 
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would two diagonal braces left uncut work better than the brace purely in the lateral direction? I do not know what shear forces go through the top but believe that would provide much more support being triangulated.

I still want to do a top with CFRP at some time. Obviously not top priority though.
 
would two diagonal braces left uncut work better than the brace purely in the lateral direction? I do not know what shear forces go through the top but believe that would provide much more support being triangulated.

I still want to do a top with CFRP at some time. Obviously not top priority though.

I suppose you could do that in the forward section. Probably could go with a thinner brace to offset the reduced amount otherwise removed. You would not want to cut the existing reinforced rear brace, so the tail section would stay rectangular. I'd say the iffy part of that would be how much it shows through the canvas in natural light. I wouldn't want to be looking at an "X" through the cover material. Easier to just brace the center rib, IMO.

I was playing with the idea of sewing a section of auto grade clear sheet, like a convertible rear window, into the top prior to install. That would require a different brace arrangement though, and I don't know how long that holds up with sun beating directly on it. If I had a spare top, I might have played around with that direction.
 
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to me, my question is could you make the frame foldable. that way not only would it be light, but take up less room. I know they made the flimsy snap on tops, but a frame under it would add the stiffness that the snap on version lacked when it rained.

Odie
 
to me, my question is could you make the frame foldable. that way not only would it be light, but take up less room. I know they made the flimsy snap on tops, but a frame under it would add the stiffness that the snap on version lacked when it rained.

Odie

That is what Jeff was discussing - I would say it would take a complete redesign of the side members, to incorporate a functional door seal & drip rail. I don't see any point to it unless it's watertight. If occasional use/dry climate is the goal, I wouldn't think it would be particularly involved to create a simple (locking) scissor hinge. I personally wouldn't spend the energy on figuring that out. Must be someone on here who could though...

Edit: there are cheap easels that have typical collapsible hinged bases. Like this:

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exactly what I was thinking. my thoughts were so I could vent the radiator through the frunk and not have to worry if it rained.

Odie
 
Odie, look at how Porsche did it on some of the old Targa tops. They are a folding frame and weather tight. Sort of a mini convertible top that detaches on both ends so it is completely removable. Hard to make out much in these images:
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two diagonal braces
I like that idea. The triangular cross "X" pattern is stronger and might even help a little to retain the chassis rigidity between the windshield frame and targa bar. And looking up at an "X" while sitting in a "X" sounds cool to me.
 
Marked up & cut the canvas. Left 1" border all around, so I can trim it back as needed. Should NOT have cut the corners diagonally prior to bonding, I've left myself almost no wiggle room by doing that.

EDIT: My plan involves glueing the entire canvas in the areas it contacts the panel. Since the contact cement is sprayed on both surfaces and bonded 'dry', there is no wiggle room for adjustment once the two touch. Most likely I will start on one of the sides, bonding just the area up to the cutouts. I could tack the center of each "rib", then work my way out from center L - R. I'm leaning towards the former. Just have to be careful since I cut the corners.

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I like that idea. The triangular cross "X" pattern is stronger and might even help a little to retain the chassis rigidity between the windshield frame and targa bar. And looking up at an "X" while sitting in a "X" sounds cool to me.
I don't think the targa top provides any real rigidity to the car, only negligible amounts compared to the rest of the frame. I was more thinking keeping the targa itself square and preventing cracks at the corners of the big cut out square if it does indeed see some loads at the corners forcing it into a parallelogram shape. The latches on the top put it in complete tension front to rear. Any chassis deflection between A to B pillars would pull the top even farther into the parallelogram. Thinking through this my bet is the tops will generate cracks at the corners of the cut out rectangles.
 
Although it doesn’t keep the box from bending inward, it does give torsional rigidity. I have noted some motion under load at the A pillar between the glass and the A pillar with the top off which is not evident with the top on.

Personally I have never seen an X bend in half with rust but have certainly seen 850 spiders and Porsche 914s that have collapsed at the back of the doors. Though I clearly have not seen everything :)
 
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