Strengthening body panels with carbon fiber

[Hindsight]

I'm looking to strengthen, and help hold the curvature I want in my hood and engine cover. I know a number of folks have done this with various things that they then over-laminate into place with fiberglass cloth. I think so far I have seen balsa (similar to the old 914-6), PVC, and aluminum.

I'm thinking about using carbon fiber. A fairly inexpensive option would be to use solid carbon fiber rod. It's available in many diameters and not too expensive (~$25 for a 3/8" diameter, 4' long piece). The only problem for me is that, without several diameters of carbon fiber rods on-hand, it is difficult for me to determine which diameter I should go with. It would obviously need to be flexible enough to conform to the slight bowed shape I would need, yet stiff enough to provide rigidity once glued in place.

Right now I'm guessing that 3/8" or 1/2" diameter solid rod would be what I'm after, but thought I'd pose this question to everyone else to get input. Maybe there are some die-hard composite gurus on here?

[Mechie3]

Not really what you're looking for, but nkw (nolan) used balsa covered with carbon cloth. He posted a good source of 2nd run cloth. I bought a big roll of it and found no defects personally.

[Rasmus]

Use foam for a core like the boat guys use. Cheap, light, easy to cut, easy to form.



1/4" thick would be plenty strong for your needs. Use the coarsest weave fiberglass you can still get to conform around your corners.

[RM1SepEx]

make sure the foam is matched to the resin. Polyester resin dissolves many foams

[ssssly]

You would probably be better off with carbon tube.

I get the CF tube for my 3d printers from Dragonplate. They will cut to length as well.

I am going to be designing some 3d printed, straight epoxy or glass in, PETG and CF reinforcements.
But probably won't be for about 6 months on the low side.

And the foam method Rasmus posted works very well for making rigid light objects. You can also use expandable spray foam for doing the same thing with preformed objects with complex shapes.

[mikeinatlanta]

I would not recommend going with tube, or carbon for that matter. You will get greatest effect by adding material separated from the original laminate as far as you can reasonably make it. For hypothetical example: If you had a solid laminate .2" thick and compared it to a honeycomb cored laminate that was ten times thicker, but with .1" thick laminate on each face sheet, the honeycomb cored part will be approximately 3,000% stiffer, even though it had an identical amount of resin and fabric. Basic rule of thumb, 10X thickness gains 3,000% stiffness. Of course what materials you use will make a difference, but that number will get you close.

Now, let's compare that to simply changing the resin or fabric. Going with a stiffer resin and from glass mat to carbon fabric, you may gain a couple hundred percent stiffness, which doesn't begin to compare to the stiffness gained with a core, and that would going from cheap materials to top grade big buck materials.

End story is that you would be best bang for the buck using high quality glass cloth (like 7781), the same resin as original (vinylester), and separating the new material with the thickest core you can tolerate.

Now down to which core to use. If working in an area with complex uneven surfaces, you may be best to use a syntactic core or pourable urethane foam. For large flat areas a sheet foam core works well, but you need to study up on how to achieve good adhesion. I tend to use a combination of both. Whatever you do, do not use spray insulating foam unless you are insulating, never for composites.

Here are some pictures of my hood. Even with all the material removed from the center it is stiffer than a normal MKII unlined hood. It uses a combination of 7781 glass, foam, syntactic core and vinylester resin. If I were only wanting to make a hood stiff, I would simply add the blue foam around the perimeter.

DSC00107-600x450.jpg
Yellow is eight pound density urethane, blue is six pound urethane sheet 1" thick, white is syntactic.
DSC00118-600x450.jpg
All covered with 7781 glass, prior to saturation
DSC00119-600x450.jpg
Finished bottom prior to prime. Keep in mind that this hood is both lighter and stiffer than the original solid laminate.
DSC00176-600x450.jpg

Similar treatment on a trunk lid
Copy_of_DSC000013-600x450.jpg

[Hindsight]

Great info thank you!

So FFR uses vinylester and not epoxy or polyester? How can you tell?

On the hood, how would you hold the supports in place while also keeping the hood pre-curved to the desired shape when using those kinds of foam supports?

Also, is there a good source around Atlanta for the blue foam board?

[BC Huselton]

I used Divinicell from Fiberglast

http://www.fibreglast.com/product/Vi...b_Density/Foam

Check out their site. Also used their cross hatched fabric to make my front splitter and rear difusser with the divinicell core.

It is very strong, light and shapable.

Put the cell 1/8' foam on the back of the hood with 3" holes then toped it off with light weight fabric...stiffened it right up.

They have great application videos on the site.

[RM1SepEx]

The problem that we have with the 818 is holding the body panel's desired curve when we remove it and turn it upside down, they are so flimsy we are more likely to stiffen it to the wrong profile... We need to make a cradle fit from the top that matches the desired profile, supports the curve that we want. Right now my hinges, locking pins and center support force it to fit and curve in 3 dimensions, release the pins and the correct curves are lost

[flynntuna]

Thanks Dan, making a cradle can serve two purposes, to screw down the hood while reinforcing the inside of the hood. And as a frame to make a mold to make duplicate hoods to practice on for making mods to the original hood.

[RM1SepEx]

The hood, engine cover and trunk all need to be made with some sort of support to hold their shape from FFR. When in the original mold it would be easy to lay in a folded over piece of that plastic cardboard, foam, aluminum etc... something to add another dimension to the molding to stiffen it. My trunk took forever to shape after it sat for months. Getting the hood to sit right is taking a long time as well. The engine cover just plain fits too tight on the roll bar and getting it to sit down on the side panels was just about impossible. They need to be molded with stiffening features.

[ben1272]

Thanks Dan, making a cradle can serve two purposes, to screw down the hood while reinforcing the inside of the hood. And as a frame to make a mold to make duplicate hoods to practice on for making mods to the original hood.

To accomplish what Dan suggests, Is there a way to mount the panels to the car in the shape that you want and then temporarily stiffen them from the Outside such that you can remove them without them losing shape? It would be a pain, but you could mask off the body panels and then lay-up a fiberglass/etc structure, as if you were building a mold, and then use the 'mold' to hold the shape of the panel while you reinforce it form the inside. Is this a situation where everyones panel is going to need to be slightly different owing to variations in how nose, side sails, etc, are all mounted? If this is the case, even having the original FFR molds wouldnt help much because they would hold the parts in the wrong shape.

Obviously that is a lot of work, but so is stiffening the parts.

Second idea would be to apply some stiffening members as you propose, add resin, and use vacuum assisted molding components to suck them tight while you replace the panels on the vehicle in desired location. Once resin sets up you can remove from frame and finalize remaining stiffening if needed. The vacuum bagging equipment I have seen amounts to a sheet of thin plastic that drapes over the glass and resin along with a vacuum port. Fairly low profile stuff that should probably fit in the space allowed. If you google 'vacuum assisted fiberglass' use image search and you'll see all sorts of setups.

Sorry if rambling....think of it as a stream of consciousness!

[ssssly]

The problem that we have with the 818 is holding the body panel's desired curve when we remove it and turn it upside down, they are so flimsy we are more likely to stiffen it to the wrong profile... We need to make a cradle fit from the top that matches the desired profile, supports the curve that we want. Right now my hinges, locking pins and center support force it to fit and curve in 3 dimensions, release the pins and the correct curves are lost

Hence why my ultimate goal is to 3d scan the body panels and design 3d printable honeycomb core inserts. That way you don't have to worry about adopting the wrong profile while adding stiffening. The inserts will dictate the shape.

[UnhipPopano]

As you add stiffening, you are also adding weight. This weight, wile the car is going over bumps, will increase the stress of the points where the body connects to the steal. IMHO these points where the fiberglass is bolted represent a point of failure for the panels.

Also, where the fiberglass needs to follow a particular curvature, an aluminum panel cemented to the fiberglass may be a better solution.

[Hindsight]

Itvmay be easy to get carried away with strengthening panels. I think the critical things are doing something for the engine cover so it hokds the proper curve/bow, and possibly something for the hood so those of us with hinges dont crack the hood while opening it.

[mikeinatlanta]

I am not familiar with the 818, but do know that FFR uses vinylester resin for their bodies.

Question: Is the hood a single layer or does it have a fiberglass liner like the MKIV? This will completely change your options.

Regarding printing honeycomb: Honeycomb is quite flexible and if you go with bell core it is exceptionally flexible. The printed core will not be able to match any commercially available core for strength, weight, and I suspect cost. If you want to bond a rigid core structure to the back of a panel, you will have a difficult time with the normal unevenness on the back of the laminate. Compensating for the uneven back side of a panel will require adding an excessive amount of filler to compensate for the uneven bond line thickness. You are much better off to use a pourable foam of an appropriate thickness.

Regarding vacuum bagging: Bonding with vacuum puts an exceptional amount of stress on a part. Best stay away unless you have the part in a mold or are very experienced.

One very good point made is that rigidity will cause stress points. Even if you keep the weight down, making panels more rigid without properly engineering the panel mounting points is a recipe for cracks. One relevant exception is movable panels such as the hood or doors.

Regarding the entire subject: Exactly what problem are you attempting to solve? So far, the discussion is entirely too general to provide useful advice.

[mikeinatlanta]

Itvmay be easy to get carried away with strengthening panels. I think the critical things are doing something for the engine cover so it hokds the proper curve/bow, and possibly something for the hood so those of us with hinges dont crack the hood while opening it.

Hood hinges really only stress a hood when they are spring loaded. I know some here won't care for this comment, but you should never use sprung hinges or gas struts on a composite hood unless it is specifically engineered for it. And by engineered, I mean engineered well beyond the engineering capabilities of FFR. If you are truly concerned about hood stress, properly support the perimeter and use a prop rod.

[Rasmus]

Exactly what problem are you attempting to solve? So far, the discussion is entirely too general to provide useful advice.

I'm looking to strengthen, and help hold the curvature I want in my hood and engine cover.

I don't think I'd be far off base by typing he wants the panel to have the same curve when open as it has when it sits static on the car lined up with all the other panels. Also he'd like it not to flap in the wind @ 80+mph.

[Hindsight]

Yup, Rasmus nailed it. Without that, some of the 818 panels will flatten out and cause fitment/appearance issues to arise which weren't there when the panel had the proper curvature. In addition, some of the panels flatten out during storage and when mounting them to the car, you need to PUT the curve back into them.

Putting the proper curves in, and ensuring the curve won't go away is what I'm after.

The hinges many of us are using aren't gas strut or spring. They are just a swing hinge with nothing to hold them open other than gravity (forward opening hood that goes beyond vertical).

[mikeinatlanta]

Does the 818 hood have a liner, or is it a single layer? When you say "putting the proper curves in", is this a matter of stressing the part a bit to install or is it more significant? I can help, just trying to get a very clear picture.

For body panels, and generally speaking, it's best to stress the part upon installation and give it some time in the sun to relax, or heat each panel to about 160 for a few hours under heat lamps while installed to accelerate the stress relieving. It's normally not productive to reshape a part prior to installing, especially if you end up making it more rigid. The hood depends entirely on the liner question.

[Hindsight]

No liner on the 818 hood or any other panels. Just a piece of thin fiberglass. The only strength it has comes from the returns on the edges, and a little from the angles associated with the hood vents, but it isn't enough to provide good strength and most of the returns have to be ground way down when fitting the body.

And yes, this is a matter of stressing the part to install it. Perhaps the parts would fit perfectly right out of the mold but become misshapen after storing them... no way to know for sure. I just know that when it comes time to install, most of us have the same issues: The front hood needs a little bowing to fit between the cutout in the front bumper, the rear engine cover needs a ton of bow bent into it so it will match the adjacent panel, clear the engine, and conform to the rear quarter panels.

[RM1SepEx]

And you have to cut the returns to about 3/8 inch to get alignment, reducing the ability to hold the curve. They need a liner. Don't forget the engine cover, roll bar, side sail issues

[mikeinatlanta]

Provided you have room under the hood, I would suggest reinforcing with the blue-green foam as shown in the photos I posted of the cobra hood and trunk lid. You will want to sand a good 1" beyond the foam area so that you can have a good area for the reinforcing glass. The thicker and wider the reinforcing foam the stiffer you will make the hood. If you wanted to get really custom, you can sand the back side and do 6-8 pound density pour foam sanded to whatever shape you want, essentially making a liner for the hood.

Regarding getting the correct shape. The hood is shimmed into the correct place and held with long reach draw clecos. At this point you make sure the shape is how you want, or moved to how you want by forcing it into shape with whatever you want held in place with pk screws. Then remove the hood and install the foam and glass. After installing the foam and glass, and before the glass sets up, you reinstall the hood and secure with the clecos until it firmly set, and then give it a few hours at 150 under heat to fully set. This will permanently reshape the panel. After fully curing, fill all the holes made for the pk screws and clecos.

Caution: I would not recommend this process for fenders. I would also recommend that you do some practice runs with bonding the foam, and glassing over the foam. It can be tricky getting it to stick and google is your friend.

Sheet foam comes from Aircraft Spruce, pour foam from US Composites.

Here is a pic of a trunk lid showing cleco install.
Copy_of_3_4_-600x450.jpg

[Hindsight]

Great, thanks for the tips. Sounds like you and Rasmus are speaking the same language.

[RM1SepEx]

Doesn't help if you don't want to refinish/paint the gel coat on our "paint free" cars...

[mikeinatlanta]

Doesn't help if you don't want to refinish/paint the gel coat on our "paint free" cars...

Seriously? You are saying that a thread all about modifying composite body panels isn't helpful to someone who doesn't want to modify their panel?

[Bob_n_Cincy]

Doesn't help if you don't want to refinish/paint the gel coat on our "paint free" cars...

Seriously? You are saying that a thread all about modifying composite body panels isn't helpful to someone who doesn't want to modify their panel?

Mike,
All your post have been great. I'm learning. Thanks.
Like Dan, I don't want to drill holes through my gel coat because I won't be painting.
For the 50% that will be painting. The cleco method will work great.
Bob

[RM1SepEx]

Thanks Bob, I'm experienced with the foam core process, my son and I used it to build the reverse trike body. The difficulty is building up stuff underneath when there is no way to hold it in shape. When it was molded, the shape was correct. It would be easy to lay up some support before pulling the part from the mold.

[bigAl]

I hate to say it but this proves that the optimistic B...S.... that FFR was a capable, equal to mainstream manufacturers operation (which we heard endlessly back when they were originally promoting this product) was simply not true. Your average Toyota, GM or whatever would never have this kind of problem with panel fits. FFR is a frontline kit manufacturer, with pretty-good-support, but not more than that.

[mikeinatlanta]

I've probably offered harsher online criticism of FFR's composites than anyone (just ask Dave). That said: I believe that their product has improved to the point that people need to consider if expectations are reasonable or realistic when considering the price point.

You get an awful lot for the money, and if given the choice, I would take lightweight flexible parts over rigid heavyweight parts any day. To expect both lightweight and rigid you need to expect to pay in the range of $1,000 per panel for a reasonably high quality composite part. Double or triple that for race level top quality parts.

[UnhipPopano]

Other than the reported information that FF has the panels made by a different company, being able to make large lightweight fiberglass panels for a car is quite an achievement. For example, one of the vendors offers a different back panel and it is reported to be noticeably heavier in weight. Mikeinatlanta is correct that the same molds could also be used for graphite composite panels, and maybe some day there will be enough of a demand for this option to be offered. Also remember that they also could have chosen to use injected molded plastic for the panels and saved themselves a lot of grief and money. So remember that the 818 is just one of the models that you could have selected. In reading some of the threads for other models, fiberglass issues are to be expected no mater who makes them, and I do not expect any company to make quality control problems that they are dealing with public.

On the other hand, I know from experience that the itchy feeling you have after working with fiberglass is bound to leave some people in a bad mood, so go ahead and get it out of your system.

[bigAl]

Also remember that they also could have chosen to use injected molded plastic for the panels and saved themselves a lot of grief and money. So remember that the 818 is just one of the models that you could have selected. In reading some of the threads for other models, fiberglass issues are to be expected no mater who makes them, and I do not expect any company to make quality control problems that they are dealing with public.

On the other hand, I know from experience that the itchy feeling you have after working with fiberglass is bound to leave some people in a bad mood, so go ahead and get it out of your system.

Injected molded plastic tooling would have cost a tremendous amount, way beyond any reasonable consideration (you are exposing your production "expertise"). This was probably never a valid possibility at these production numbers and would not have necessarily solved the distortion issue, minus a separate inner panel. Some other kit manufactures (i.e. Meyers Manx) managed to make very high quality parts, albeit at a slight weight premium.

[mikeinatlanta]

Some other kit manufactures (i.e. Meyers Manx) managed to make very high quality parts, albeit at a slight weight premium.

Please tell me this was a joke. Nah, what was I thinking, of course it was. Good one, you almost had me there.

[bigAl]

There are lots of knock-off dune buggies and I am not going to defend their quality. As far as the Meyers Manx products (the originals), every one I have seen is strong, accurate and high quality (surface wise). They probably are relatively heavy... but they FIT!!
So no, it was not a joke.

[UnhipPopano]

Perhaps racing sailboats would be a better analogy. There hulls are of high quality fiberglass and as light as possible. They are also extremely expensive. If I remember correctly in what I read about the project, affordability was given a high priority.

[RM1SepEx]

I just bought a 38 year old Kelmark GT, the Fiberglass is as good as my fiberglass tub, old boat everything else that I have ever bought in Fiberglass... No voids, waves, cracked gel coat, unfilled corners etc...

P1130008.JPG P1130009.JPG P1130010.JPG P1130011.JPG

This kit was placed on a VW tub and stored in a garage, not she sits in one of my portable garages in the "project que) All of the opening parts have bonded liners or other supports.