As I have a better understanding of how flexible the 914 is, I'm planning on building a C/F targa top and rigidly mounting it to the chassis. My question is: how many layers do you think I need to get a reasonable amount of structural stiffness. I've ordered some 7oz plain weave that is about .015" thick. Right now I'm thinking of 5 layers: 3 layers with the weave aligned with the sides and 2 layers with the weave at a 45* angle, alternating layers. This should yield a thickness of over .075" which I would think would be a good start. As this is my first attempt at C/F and wanting to achieve strength and stiffness, I would like some advice from someone with C/F experience.....

you will need some bracing either carbon fiber or something else. even the flimsy stock one has bracing on the edges. if your going a flat sheet route, it would have to be pretty thick, and weigh much too much,

since this is to stiffen your car, you need maybe to firmly attach an "X" brace of say steel tubing formt eh windshield frame to the roll bar, then make some light wieght carbon fiber skin for that. Maybe an 1/8th inch thick would be nice, but I am sure you could get away with less as the strngth is in the metal

My experience with it comes from building prosthetics and large model sailplanes (vacuum bagging). I can give a brief outline from my perspective.
1. consider your resin very carefully as it will get alot of UV exposure and heat if not painted. 2. Carbon does not like to be flexed in a thin gauge lay up as it can fatigue and the fibers will fail in a domino like effect. 3. To get the maximum rigidity, you'll want to design and mold in spars, I-beam like structures into the top to cover the spance of the surface area. 4. The real trick structurally will be to get the attatchment to the windshield frame correct since this is where all the flex comes to meet a less flexing structure. 5. Carbon can be fun to engineer and build with, but with engineering, you have to take into account ALL the loads and shear forces. Do you any fabrication facilities to work in? Vaccuum bagging can be your friend to reduce material costs and weight. This stuff can get expensive.

Bill I cannot help but if anyone can do this you probably can... Mark Debernardi uses a vacuum fiberglass system which is very intriguing. Isn't the strength in carbon fiber in the layering? Not sure why you would need something to brace it if it is properly layered.

The weakest point will be where it attaches to the car. If you don't get this right the rest is a waste of time.

For any structural intergrity to provide stiffness, the part would have to be vaccuum bagged, and layed up with quality resins. The problem you are going to find, is that carbon fibre flexes. It is not a super rigid, cure all material... It would need steel inserts to provide any real noticable difference from the fiberglass original top.

I say, if you're looking for a stiffener, find a steel roof, or put in a full cage and tie into the rear sail panel and front A pilar..

I'm planning to bond the new C/F top to the windshield frame and roll hoop with epoxy and back it up with a few pop rivets. Then as the surface area on the top windshield frame is pretty small, I'll add a couple strips of f/g to the bottom side of the flange. I will eventually paint it, but will wait a while and let the resin cure fully. My front and rear deck lids were painted too soon and now you can see the cloth weave in the paint. I believe this to be from the epoxy resin curing and shrinking over time.

IMHO, you'd be better off making a steel structure that still accommodates the stock roof.

T.C. show the picture of the cool X brace you have

The brace T.C. has would make a good addition with a CF top if you could do it.