Core materials are the fillings in the sandwich structures that comprise most composite structures. By sandwich structure, I’m referring to a structure that has a core material (the bologna) covered by two facings (the bread). Most people would conclude that the facings are where the strength of the structure originates, but this is only partially true. The strength also comes from the geometric arrangement of the facings — in particular, the distance between them — and that distance is established by the core materials.
Think of an I-beam. It’s composed of a web and two flanges. When the beam is put under a bending load, the web gives it a certain amount of strength. One flange is put into compression and the other flange is put into tension. The joints between the web and flanges keep the three elements aligned and from buckling. A sandwich structure works pretty much the same way. The core material takes the place of the web and the facings take the place of the flanges. The larger the web (i.e., the thicker the core material), the stronger the structure — even though the core material itself (foam) is fairly weak.
Most core materials are made from some form of foam and those are the ones we’re going to be looking at today. However, the aircraft industry uses some pretty high-tech core materials now-a-days, including aluminum or cardboard honeycomb. Check out the Aircraft Spruce website for more information on core materials.
Polystyrene is used to form airfoils or other shapes that need to be cut precisely with a hot wire saw or water jet. More importantly, it is the ONLY foam that you should cut with a hot wire saw. All other foams emit a poisonous gas if subjected to heat! The aircraft folks use it to build their wings, canards, elevators, rudders, etc. because it can be cut into complex airfoil shapes with templates and a hot wire saw. If you want to save some money in buying polystyrene, look for “dock billets” at your local boating supply store. It’s the same stuff — the large blue rectangular blocks of foam that are used to float boat docks. We won’t be using any polystyrene in the reconstruction of the Bradley, because we have no need to do work with a hot wire saw.
Polyurethane foam is the foam you want to use when you’re going to carve complex shapes by hand. It’s a wonderful material for carving (with a butcher knife) and sanding. The best tool to use in carving urethane foam is another piece of urethane foam. If you use one block to shape another, the two pieces will take on mirror images of each other. If you’re sanding a curved surface, your sanding block matches the curvature you’re trying to create! Folks who like to create exotic car bodies use a lot of urethane foam for this reason. However, always use a dust mask and wear a long-sleeved shirt when working with urethane foam. The small flecks of foam that you sand off are as sharp as glass. You don’t want to get them on your skin or you’ll itch as if you had been working with fiberglass insulation and you definitely do not want to breath them into your lungs.
PVC (polyvinyl chloride) foam comes in generic forms (H160, for example) and proprietary forms such as Divinycell Foam. These foams are used where large, flat sheets need to be faced with fiberglass. The airplane folks use it for parts like landing gear and fuselage bulkheads, where high strength is important. The bond strength (peel resistance) between the facings and foam is very good, and the material is easy to cut and shape. However, it’s not available in thick sheets so urethane is preferred when carving complex three-dimensional shapes. We’ll definitely be using PVC foam in repairing the Bradley. The custom overhead console that we’re going to build will be made from PVC foam.
Proprietary Foams include things like Last-A-Foam and Clark Foam (no longer available). These foams are available in sheets similar to PVC, however their chemical formulation is different. Last-A-Foam is what the aircraft folks use to form the structures around their fuel tanks because it is one of the few core materials that isn’t dissolved by aviation fuels.
One final caution about foams. The white beadboard foam that you can find in the insulaton section of your local big box hardware store is not appropriate for composite work. It’s just too weak to act as a core material.
There are a couple of ways that foam can be bonded together if the pieces that you have aren’t large enough to do the job that you need. For sheet materials that need to be bonded edge-to-edge, we’ll use 5-minute epoxy. While not nearly as strong as the epoxy that we’ll be using for the fiberglass facings, it’s still stronger than the foam itself and that’s what counts. To bond sheets together, we use the hinge method. Butt the two pieces to be joined together and run a length of 2-inch-wide masking tape along the seam. Then, bend the two pieces of foam 180-degrees back along the taped seam to open the joint. Apply 5-minute epoxy to the edges of both pieces of foam, close the hinge, wipe off any excess epoxy that squeezes out (a popsicle stick works well here) and tape the other side of the joint. Watch the epoxy in your mixing cup and when it starts to thicken (but before it gels) remove the masking tape and scrape off any excess squeeze-out. By following this procedure, you’ll get a perfect joint between the foam sheets.
If you’re bonding blocks of foam, you’re going to mix up a material that we call micro slurry. Slurry is a mixture of pure epoxy and microballoons. The amount of microballoons that you’re going to add is not critical — just add enough to get the mixture to the consistency of pancake syrup — give-or-take. Microballoons, if you’re not familiar with them, are microscopic hollow glass spheres. In bulk, they look like flour. Wear a dust mask when working with them because, if they get into your lungs, they’ll stay there for a long time. Being made of glass, they’re inert and won’t dissolve. To make slurry, first mix up some pure epoxy in a cup. Stir it for the required 2 minutes, scraping the side of the cup every 15 seconds. Only after the epoxy is mixed should you add the microballoons. Add enough to make a substance that has the viscosity of pancake syrup.
To bond foam blocks, apply the slurry to both sides of the joint with a mixing stick. Add enough slurry to fill all of the voids in the foam. Then, mound some additional slurry in the center of one of the blocks and press the two blocks together, twisting the blocks against each other to distribute the mound of slurry evenly along the joint. The idea here is to make sure that the slurry squeezes out of the joint on all sides. If you see squeeze-out along all sides of the joint, you know that you used enough slurry; if you don’t, you may have some dry areas in the joint. Separate the blocks and add more slurry, if need be. Also, make sure that the two pieces of foam are in contact with one another. You want a joint where the two pieces of foam touch — not one where they’re seperated by a zone of slurry.
To hold the blocks in position as the slurry cures, just place nails thru the foam blocks. Use just enough nails to hold the blocks in position so that you don’t have too many nail holes to fill later. Leave the heads of the nails protruding so that you can grab them with pliers to extract them after the slurry cures overnight. If you can position the blocks so that you can weight the joint, do so. Just remember that some foams crush easily. If you use a set of barbells for weights, put a board or something that won’t give between the foam and the weights.
Slurry cures to a hardness resembling plaster. The nails may be hard to extract and may pull out chunks of foam along with them. That’s OK — we’ll show you how to make repairs at the appropriate time.
Carving foam along a slurry joint is problematic. Because the slurry is hard and the foam is soft, you tend to leave slurry ridges in the carved parts. The way to get around this problem is to dig the hardened slurry out of the joint to a depth of around a quarter inch or so, do your carving, and then fill in the joint when you apply the fiberglass facings. Again, we’ll teach you the technique when we have an example that we can photograph.
Enough for now. Our next class will go into structrual and non-structural fillers and the techniques that we’ll use for bonding structures together.
I’m still cleaning up all of the old fiberglass pieces. I have the main body, the T-top and the two gull-wing doors left to work on. After that, I’ll start repairing all of the damaged areas, make any modifications that I’d like and scratch-build some additional parts.
Today, I’d like to continue on with the introductory discussion about Moldless Composite Construction, according to the School of Burt (Rutan, that is…). Last time, we talked about adhesives, today we’ll look at fabrics and next time we’ll explore core materials.
Composite fabrics include fiberglass, carbon fiber and Aramid fibers such as Kevlar. For this project, I’ll be using fiberglass exclusively. There are literally thousands of types of fiberglass with different weave patterns, weights, yarn sizes, sizings (coatings that make it easier to wet out the glass), etc. There are also materials called prepregs — fabrics that are pre-impregnated with an adhesive and stored at cold temperatures to keep the adhesive from curing. We’ll be using three or four types of fiberglass in our work, including bidirectional fiberglass (BID), unidirectional fiberglass (UND), deck cloth and perhaps unidirectional fiberglass tape. You can learn more about the properties of these materials here:
Bidirectional fiberglass cloth (known as BID or RA7725, where RA stands for Rutan Aircraft) has half of the fibers woven parallel to the selvage edge of the cloth and the other half at right angles to the selvage, giving the cloth the same strength in both directions. The cloth is almost always cut “on the bias,” meaning at 45-degrees to the selvage edge. To understand why this is done, imagine a crack in a part that you want to reinforce with fiberglass. If you apply the fiberglass such that one set of fibers runs parallel to the crack and one set runs across the crack, then you have reduced the strength of the repair by 50 percent. The fibers running parallel to the crack aren’t reinforcing the damaged area — they’re just holding the cloth together. By applying the cloth on the bias — so that both sets of fibers cross the crack at a 45-degree angle — you’re allowing all of the fibers to develop strength across the crack. As we start our repair work on the Bradley, you’ll see me do this over and over again. BID is good in tension and is great for torsional (twisting) loads. It also takes compound curves very easily but, because of this, it distorts easily. As we go, I’ll show you some special handling techniques that will help you place the cloth without distorting it.
Unidirectional fiberglass (UND or RA 7715) has 95% of the glass fibers woven parallel to the selvage, giving it exceptional strength in that direction and very little at right angles to it. The remaining 5% of the fibers are there just to hold the 95% together in the form of a cloth. UND is great in tension and good in compression. It is sometimes used to accomodate torsional loads but, in these cases, it is usually applied at 30 degrees across the load surface. We probably won’t have need to use UND in this way but the folks who build aircraft use UND across the faces of their wings’ shear webs in just this manner. UND will not bend around a compound curve. You can bend it parallel to the fibers or perpendicular to them, but not both at the same time. UND tends to ravel severely when it is cut. We’ll show you some tricks to minimize this problem when we start the repair work.
Deck Cloth (Aircraft Spruce P/N 1080-50) is not used for aircraft construction very often but it has a useful purpose in our work. The weave of deck cloth is very fine. As such, it can be used in place of BID where subsequent filling and contouring will be required. For example, repairs to the inside of a bumper can be done with BID but repairs to the outside should have one or two layers (plies) of deck cloth outermost. This will make filling, contouring and finishing go a bit easier.
Unidirectional fiberglass tape (Aircraft Spruce P/N 01-06800) is used where very high tensional or compressional loads are expected. The aircraft folks use it to construct the upper and lower spar caps of their wing spars. I’ll probably use it to reinforce the insides of the bumpers in high stress areas such as where the bumper mounts are attached to the bumpers.
The one type of fiberglass that you won’t see me using is glass mat. Most auto body folks use a lot of glass mat and, if you’re used to working with it, by all means do so — mostly as a replacement for BID. However, I prefer to use BID because it is much easier to wet out and control. Hopefully, by the time we’ve repaired all of the fiberglass parts on the Bradley, I will have convinced you of that!
When making a fiberglass repair, start by roughing up the area of to be repaired with either 36- or 40-grit sandpaper. You want to have a really rough surface so that your repair materials will develop a good mechanical bond with the underlying structure. Next, paint a coat of pure epoxy on the area where the fiberglass is to be applied. Be a bit generous with the epoxy. Then lay on the first ply of fiberglass. If you use excess epoxy under the fiberglass, it will wick up thru the glass, driving out any excess air. If the layup is too dry below the fiberglass, you have to force the epoxy down thru the fibers in the cloth by stippling the layup with your brush. This can be done, but it tends to drive a bit more air into the layup, which makes it weaker. Use enough epoxy to fully wet out the cloth, leaving no unwet white areas, but don’t use so much epoxy that it puddles on top of the cloth. As Burt says, “Not wet, not white.”
The number of plies of fiberglass that you need to use is something that you will learn by experience. The designer will specify the required number of plies, if you’re working from plans. However, here is a rule of thumb that you can apply. If a crack goes all the way thru a structure, build up fiberglass to half the thickness of the original structure on both sides of the structure. For a crack that is 4 inches long, place the first ply so that it covers the crack and an area 1″ around the crack. The next ply should extend beyound the first for 1 inch in all directions. The third ply should extend beyond the second for 1 inch in all directions. Continue this procedure until the area over the crack is half as thick as the original material. Then, repeat the layup on the other side of the structure.
If a crack fully penetrates a structure, you must repair both sides of the structure. If you don’t, the repair will be weak at best and will fail at worst. If you repair just the underside of a crack on a car part, the unrepaired outer side of the crack will telegraph thru any gel coat, body filler and paint job in short order.
Always use a hair drier or heat gun to warm the epoxy as you apply it. The kind of heat gun used by model airplane bulders to heat-shrink Monokote over their balsa-wood airplane frames works really well. Mine is made by Top Flight. The use of a heat gun lowers the viscosity of the epoxy, allowing it to wet out the cloth much better. Be careful not to apply too much heat or your epoxy may exotherm. I keep mine about 3-4 inches from the work. Keep the heat gun moving at all times. This is especially important when performing a layup over a core material such as a foam, because excess heat my melt the foam core.
By the way, you don’t need to use a new brush for each layup. When you’re done for the day, use a paper towel to remove as much epoxy from the brush as possible. Then, roll the brush in a zip-lock bag, squeezing out as much of the air as is possible, and store it in the freezer compartment of your refrigerator. The next time you want to use it, give it about 10 minutes to warm up and it will be ready to go. By doing this, you can re-use the brush for dozens of layups. I use 1″ and 2″ chip brushes, bought from Harbor Freight in boxes of 36. That’s the least expensive source of brushes that I’ve found. They tend to shed a bit, but a couple of bristles in a layup isn’t a problem.
You can use standard shears to cut dry fiberglass, but I like to use a rotary cutter (pizza cutter) that quilt-makers favor, along with a nice 4-foot metal drywall straightedge. For cutting wet fiberglass, nothing beats a pair of Dritz rechargeable electric scissors. You can cut thru up to 6 plies at once and easily get right up to the edge of the layup. Clean the jaws with a bit of MEK and a paper towel after use (before the epoxy cures). To protect them from getting all gummed up with epoxy, cover everything except the jaws with a baggie and duct tape.
Enough for now. Soon, you’ll get to see all of this in action. Next time, we’ll look at core materials (foams).
I’m nearing the end of the clean-up phase of the fiberglass pieces and I’m approaching the point at which I’m going to need to start the fiberglass repair work. As such, I thought I’d post some information on the techniques that I’m going to use to do the fiberglass repairs.
As I’ve mentioned before, the method that I’m going to describe is known as “Moldless Composite Construction” — a technique developed by the German sailplane industry but popularized in the U.S. by Burt Rutan. The word “Composite” refers to any structure in which the end result is better than the sum of its parts. By this definition, a peanutbutter and jelly sandwich is a composite structure.
Expanding the idea a bit, composite structures are usually composed of a fabric, an adhesive and perhaps one or more types of core materials. The fabrics are usually fiberglass, carbon fiber or an aramid fiber such as Kevlar. Combinations of these fibers can also be used. The Bradley uses chopped strands of fiberglass for their “fabric.”
The adhesive is usually a polyester resin system, a vinylester resin system or an epoxy system. The Bradley kit car used a polyester resin system. Resin and epoxy systems must not be mixed before they cure. However, once cured, you can use any of the systems to adhere new parts or repair old ones. I’ll be using an epoxy system to repair the damage to my car over the Bradley’s polyester system.
The core materials can be any number of types of foams (urethane, PVC, polystyrene, or proprietary foams such as Clark Foam or Last-A-Foam), or they can be cork, balsa wood, plywood, or corregated cardboards or metals. Most of the Bradley was constructed without core materials. However, plywood is used as a core material in certain portions of the body such as the forward and aft longitudinal ribs.
The main thing to remember about a composite is that we build our structures so as to take advantage of the materials’ beneficial properties while down-playing their detrimental ones. For example, adhesives have both good properties and bad ones. On the positive side, they’re strong and hard when cured, and they flow easily so that we can wet out our fabrics efficiently. On the negative side, they can be brittle and heavy. Foams are lightweight, which is good, but they are weak, which isn’t. Fiberglass is strong in tension, but less so in torsion and shear and it is weak in compression. So, we need to design our structures so that the fiberglass is in tension as much as possible. We need to use the foams as lightweight spacers between the layers (plies) of fiberglass so that the fiberglass takes the loads and the foams do not. We need to use the adhesives to bond the fabrics to the foams so that there is minimal tendency for the fabrics to delaminate from the foam structures and weaken them. That’s a lot to ask from these materials but, by being clever in how how use them, we can maximize the properties of our composite structures in the areas where we need them to perform.
Today, we’ll start our discussion by looking at the adhesives. Adhesives come in three families — vinylester systems, polyester systems and true epoxies. That order is important — its an order of increasng cost and an order of decreasing weight. For an electric car, low weight is good. There is a natural tendency to think of any adhesive that is composed of two parts (which you mix together to activate) as an epoxy, but this is not true. Vinylester and polyester resin systems are catylized whereas true epoxies are mixed. What’s the difference?
Have you ever worked with Bondo? Bondo is a non-structural cosmetic filler that is a part of the polyester resin family. It is non-structural — meaning that it is not intended to take loads — it is intended to fill dips and depressions to produce a pleasing cosmetic appearance. Because it is part of the polyester resin family, it needs to be catylized to be activated. When you mix Bondo, you add a very small amount of a red cream “hardener” to activate the material. The use of the word “hardener” here is unfortunate because, from a chemist’s standpoint, it is not a hardener — it is a catylist called MEKP (methyl ethyl ketone peroxide). In the strict sense of the term, hardeners are components of true epoxy systems — not of catylized resin systems.
In resin systems, all of the chemicals needed to harden the mixture already exist in the mixture. In Bondo, the grey stuff that comes in the can has all of the chemicals that are needed to produce a hard sbstance — but, they need a kick to get the reaction started. That’s what the catylist does. A catylist is a material that promotes a chemical reaction but does not enter into the reaction itself. Technically, a catylist lowers the energy barrier that keeps the chemicals from reacting. When you mix in the catylist, the energy barrier that keeps the chemicals from reacting is lowered by the catylist and the chemicals react, forming a hard substance. The amount of catylist required to kick off the reaction is not critical — you need to use enough to do the job but if you add too much, it’s just wasted. That’s why the mix ratio for Bondo (around 1/2% to 3%) isn’t that critical.
Now, contrast that to a true epoxy system. True epoxies contain a resin and a hardener. Here, the use of the word “hardener” is appropriate. Both components are required to create the chemical reaction, so there is no need for a catylist to promote the reaction. The mix ratios for true epoxies are fairly critical. If you have too much resin or too much hardener, they go unreacted and weaken the physical properties of the hardened material.
The epoxy that I like to use is called MGS 335 and you can learn more about it here…
I use the L335 resin with the H340S slow hardener and mix the material with a 100:45 resin:hardener ratio by volume. This gives me a pot life of over 2 hours before the epoxy starts to gel. The long pot life means that I can only do one lay-up per day on any one given piece. I have to let the material cure overnight in warm weather and for two days if the weather is cold, but that’s a condition that I’m willing to endure. The long pot life means that I can take my time doing each lay-up and do a good job. I don’t feel rushed as I would if my epoxy had a 30-minute pot life.
Now, how do you mix the stuff to get the exact ratio that you need? I use a Sticky Stuff Dispenser by Michael Engineering:
You need to purchase the pump for the particular epoxy system that you plan on using, so that it will mix the materials in the proper ratio. If you change materials, you may need to have the pump modified to accomodate a different mixing ratio, but Michael Engineering will sell you the proper parts so that you can do this yourself. With the pump, you simply put the hardener in the smaller container, the resin in the larger one and then pump the handle. The materials are delivered into your mixing cup via the two aluminum spouts on the pump.
Use unwaxed paper cups to mix the epoxy — the wax from waxed cups will contaminate and weaken the epoxy. Popsicle sticks or tongue depressors work great as mixing sticks. Mix the material for 2 minutes, scraping the sides of the cup every 15 seconds — that’s the standard Rutan protocol. After your lay-up, wipe any excess epoxy off the mixing stick with a paper towel and let any excess epoxy harden in the cup. You can re-use the cup for the next lay-up after the epoxy hardens. Used cups and sticks actually work better than new ones because they tend to soak up less of the hardener (which has a lower viscosity than the resin.
In the photo, you’ll notice that I keep my pump — and a spare gallon of epoxy — and some 5-minute epoxy — and some 30-minute epoxy — we’ll talk about those another time — in a wooden box. We call these things epoxy incubators. There’s a 60-watt lightbulb in the box which is connected to a dimmer. There’s also a thermometer in the box. By futzing with the dimmer, I can heat the box to keep the epoxy warm. I keep mine at about 95F year round, which means that I need to crank up the dimmer in the winter. By keeping the epoxy warm, you lower the viscosity of the two components. This makes it easier to pump and mix the material and it seems to wet out the fiberglass a bit better.
One problem that many of us have had with the Sticky Stuff Dispenser is that some chemical in the hardener embrittles the plastic Tupperware container that holds it. You go down to the shop one morning to work on a lay-up and discover that your pump is leaking hardener into the bottom of your incubator. This results in a lot of colorful language, an abnormally large consumption of paper towels and wasted hardener. As a result, some of us have started to replace the hardener container on these pumps with 1-quart uncoated paint cans from the local big-box hardware store. The down side is that you can’t tell when your hardener container is nearing “empty.” I draw marks on the resin container to give me hints as to when I might need to replenish the hardener.
Another problem with the Sticky-Stuff Dispenser is that the Tupperware containers are apparently porous to carbon dioxide gas. CO2 filters in thru the container and reacts with the hardener, forming a crystalline sludge at the bottom of the container. This only happens if you let the hardener sit unused in the pump for months on end. If you’re a fairly regular user of the epoxy, you don’t need to worry about this. However, if it happens to you, you must throw out the hardener and clean your pump because the hardener is defective at that point. Using a meal paint can to replace the Tupperware container helps with this problem a lot. Some builders, including yours truly, keep an argon blanket over the hardener and this eliminates the problem.
If you don’t feel like ponying up $300 bucks for an epoxy pump, you can mix the epoxy by weight using a postal gram scale. Figure out how much epoxy you want (say, for example, 3 ounces) and then weight out 100 parts of the resin in your mixing cup. Re-zero the postage scale and then add in 38 parts of hardener and mix the material. Note that you have to use different mixing ratios if you’re mixing by weight instead of by volume. Builders who use this technique claim that it’s just as easy to use as a pump once they get into the rhythm of things. Most builders make up a chart that shows how many grams of resin and hardener are required to mix up a batch of a certain size.
By the way, it seems appropriate to mention that, when working with epoxy, get used to the idea of mixing many small batches instead of a few large ones. This extends your working time. Large batches can generate their own internal heat from the curing reaction. The heat accelerates the curing rate and, in extreme cases, can result in something called an exotherm where the epoxy flash-cures. In some cases, exotherms have been known to cause a fire.
The MGS epoxy requires a shop temperature of at least 70F. Anything lower than that and it won’t cure properly. It may look cured, but it won’t have developed the advertised strength. There are ways that you can overcome this by post-curing, but that’s an advanced technique for another day’s discussion. For now, just keep your shop over 70F.
This brings us to the last topic of the day: Safety. The hardeners in true epoxy systems contain a chemical which is an allergic sensitizer. Some people can work up to their elbows in this stuff and never have an allergic reaction. Others only need to walk into a room where the epoxy is curing and they start itching and caughing up green guck. I’m of the latter persuasion. Problem is — you don’t know what your own reaction will be until it happens. Like hay fever, some people never react, some react after a few years and some people get sensitized fairly quickly. Once you become sensitized, you’re sensitized for life. Lucky you. Reactions also vary from one individual to another. Some people itch a bit, some get holes in their fingernails and some wind up in the hospital for a week or more with bad reactions in their lungs. So, if you’re going to work with this material, you MUST protect yourself properly. Here’s my drill…
1.) A Tyvek suit with hood — always. No exceptions.
2.) An MSA-approved, carbon canister organic vapor respirator. No lee-way here either. Just use it.
3.) Butyl rubber gloves. They’re the only ones that have been proven to work. Not latex, not nitrile, not anything else. Butyl! Latex gloves are porous — they aren’t to bacteria and viruses (which are large) but they are to molicules (which are a LOT smaller).
4.) Latex rubber gloves over the butyl rubber gloves. This keeps the butyl gloves clean. The butyl gloves are expensive — about $20 bucks a pair, so we use inexpensive latex gloves over them and just throw out the latex gloves when they get messy.
5.) Some form of eye protection. I use labratory splash goggles.
If you have access to a system that feeds fresh outside air into a respirator, you can use that instead of the organic vapor mask. Just make sure that your fresh air supply doesn’t draw in vapors from the epoxy.
The MGS epoxy is a great material, but is has almost no odor, so you can’t use your nose to determine if you need the organic vapor mask. You also can’t use it to determine when the carbon canisters in the mask are shot, so change the canisters frequently. Once you’ve done your lay-ups for the day, keep the shop well ventilated and don’t re-enter the shop withour some form of lung protection until the epoxy has cured.
I have found that in winter, when the epoxy cures slowly, I need to let the material cure for 48 hours before I can sand it. It’s just too gummy otherwise. Also, you need to use your organic vapor mask when enterng the shop if it contains partially cured epoxy. Make sure to use a dust mask when sanding cured epoxy.
If you’d like to work with this material, I recommend that you purchase it from The Composite Store (CST):
For some reason that we haven’t been able to figure out, both Aircraft Spruce & Specialty and Wicks Aircraft Supply (who also sell MGS epoxy) have outrageous shipping charges for this material. They claim that they can’t ship it by UPS and that it must be shipped by truck. The Composite Store has worked out a system with UPS to ship it and buying it from them will save you mucho dinero. You do have to pay the hazmat charges (as you would with other vendors), but the shipping charges from CST are far more reasonable.
Enough for now. Next time out, we’ll look at the fabric side of the equation.
The Bradley GT II Electric was equipped with a defroster but not a heater. The defroster unit was positioned under the dashboard in a fiberglass plenum that was integral with the firewall and molded into the body of the car. Fresh air was drawn by a fan thru a hole in the forward face of the plenum (the car’s firewall), was warmed by a set of three resistance coils and was ducted to a pair of outlets on the top of the car’s glare shield. One relay controlled power to the blower motor and a second relay controlled power to the heating coils. Switches on the instrument panel controlled the relays.
When I received my car, several modifications had been made to the system. The air inlet hole in the firewall had been blocked off with, of all things, a round amber lens from a school bus’ turn signal. Hey — it was the right size! A smaller inlet hole was drilled into the plenum on the passenger-side of the firewall, reducing the fresh air feed from outside the car. Two trapezoidal holes were cut into the plenum cover. You can see these in the photos above. Adding these holes had the effect of recirculating cabin air thru the defroster. Presumably, the cabin air would have been warmer, making the defroster more effective. There were no valves or dampers in the system, so the relative amounts of recirculated and fresh air were determined simply by the sizes of the holes thru which the air flowed.
The outlet side of the system was also altered. One of the outlets in the plenum chamber cover was ducted to both defroster outlets with hoses and a tee fitting. The other outlet in the plenum chamber cover had a small louvre. This had the effect of directing some of the air from the defroster down towards the driver’s feet. Again, there were no dampers in the system but by closing the defroster vents, all of the air would have been directed towards the floor of the car under the instrument panel. The modifications were really quite clever and probably aided in improving the comfort of the occuppants.
The photos show how the whole thing was assembled. I’m toying with the idea of adding a few further modifications. I’d like to put dampers on the trapezoidal holes. I’d also like to re-open the original hole thru the firewall and add a damper there. Finally, the smaller hole thru the firewall — the one added by a previous builder — would be sealed off. This would have the effect of controlling the relative mix of inside and outside air — similar to what you find in most production cars. I don’t know if I can do this, but I’m going to start thinking about it.
While I’m growing new fingertips, I decided to put a bit of work into figuring out how to mount the seat buckets to the seat sliders today. In the original Bradley design, left and right sliders were bolted directly to the seat bucket. In the car I purchased, a previous owner had modified the design by attaching the sliders to the buckets with a set of custom brackets (see the photo of the seat slider in yesterday’s post).
When I removed the foam and fabric from my seats and discovered how the sliders had been mounted, I knew that I wanted to do something different. The idea of having bolts protruding thru the seat buckets in the vicinity of my posterior didn’t exactly thrill me. Granted, there is roughly 2 inches of foam padding that overlies the bolts, but it just seemed like there ought to be a better way.
In the fourth photo, you can see a pair of metal strips. Each strip has two bolts tack welded to it. The nearer one shows the bolt threads and is just thrown in the bucket; it is positioned in a set of holes that are in the lower right corner of the photo. The far one is properly positioned so that it can be attached to the rear two brackets on the sliders. A bolt was run thru the center hole in each strip and fastened to the seat bucket with a nut. In this way, the strip was held in position and wouldn’t go flopping around (under the upholstery) if the two outside nuts were loosened. Also note the wallowed-out holes for the bolts that secure the rear brackets to the seat sliders. How would you like to sit on one of those? Ouch!
In the third photo, you can see how the bolts welded to the strip protrude thru the seat bucket and attach to the rear-most slider brackets. The second photo shows how the bolts in the second strap are supposed to attach to the front two slider brackets.
I decided to try a different approach — one that would result in no hardware contained inside the seat buckets and under the upholstery. I made a set of 8 hard points out of 1/2-inch mild steel plate. The hard points are 1-1/2-inches square and are drilled and tapped for a 5/16-18 bolt. The parts were cut from steel plate on a band saw, trued up on a small mill-drill, drilled on the mill-drill and then tapped by hand. Four of these will be attached to the bottom of each seat bucket.
If you’re familiar with Moldless Composite Construction, the hard points are just floxed in position and glassed over. If that’s Greek to you, they’re held in position with a material called flox, which is a mixture of pure epoxy and flocked cotton fibers. Flox is a structural adhesive — meaning that it can accomodate loads. When it cures, it has the hardness of concrete. Many of Burt Rutan’s airplanes are held together with flox joints and some of them have been flying for over 30 years now. Once the hard points are floxed into position, I’ll lay 12 plies of bidirectional fiberglass over them and then peel-ply the glass for a smooth transition onto the surrounding seat pans. I’ll have more to say about flox when I do the lay-ups and, by then, we will already be familiar with the different types of fiberglass and epoxies from having done some simple lay-ups. I’ll post pictures when the job is actually undertaken. Until then, this should tweak your curiosity on some of the techniques that we’ll be learning here.
It took about 6 hours to make the hard points but the end result should be worth it — rock-hard mounting points for the seat buckets where all of the hardware can be accessed from the outside of the bucket.
For the last few days, I’ve been sanding all of the old upholstery cemet off the seat buckets, seatback shells, glare shields, door panels, etc. and, at this point, I’ve sanded the fingerprints off my fingers. It may be a few days before I resume that task. The left photo shows a before-and-after comparison on the seat buckets.
The center picture shows how many holes were drilled in the buckets over their 28-year lifespan and that prompts a tale…
The original plans for the GT II show seat sliders that are mounted to the sides of the buckets. I didn’t receive these when I bought the car, but the holes for the stock sliders had been drilled into the buckets. My guess is that the original seat sliders were replaced by a prior owner — and for an understandable reason. The original sliders — there was a left one and a right one on each bucket — were not connected to each other in any way. They were just bolted to the fiberglass buckets. It’s easy for me to see how this could cause a big problem in adjusting the seat positions. There would be a tendency for the seat to twist left-to-right with the two sliders operating independently of each other. One might lock in while the other didn’t. Thankfully, the car I received had a different design.
The picture with the seat slider shows a new slider (bought from Chirco in Tucson) with the old mounting hardware. The black cross-straps ensure that the two sliders move in unison, eliminating the tendency for the seat bucket to twist. This is the setup that came with my car — vastly superior to the original Bradley design.
The sliders that I received were rusted shut — having been in contact with wet carpeting in the car — so I had to buy two new pairs. I was hoping that the original bracketry (shown in the photo) would work with the new sliders but, alas, it was not to be. The rear brackets come pretty close to being OK — if I’m willing to allow those big wallowd-out holes that someone cut into the buckets to allow for bolt clearances — which I’m not (see the center photo). The front brackets are way off with the new sliders. Also, the seat sits a tad too low — resting directly on the spring wire that connects the two locking cams on the sliders.
I’m going to need to devise some new bracketry to hold the seat buckets but, before I do that — or, maybe in conjunction with that — I’m going to need to patch all of the holes in the buckets and reinforce the areas where the buckets will attach to the sliders. This isn’t a big deal — just one more task to undertake.
Then again, a little metalwork might be just the thing to do while I’m growing a new set of fingerprints.
The body came back from Plastic Media Stripping yesterday — pretty amazing service. It took them about 3 hours to bead blast the underside of the vehicle and get it down to raw fiberglass so that I can start the repairs on the main body. I also asked them to remove the gel coat in the area where the left rear fender had been damaged and this came out nicely (see the picture).
By removing all of the paint on the underside of the vehicle, I was able to locate certain defects in the body that weren’t visible before. One of the main concerns has to do with the two longitudinal stiffening ribs located in the front of the car — at either side of the hood. Both of these ribs have delaminated from the body for about the forward-most 8 inches and it’s easy to see why.
The electric version of the GT II has a special set of instructions that have you modify the body of the car in a different way than you would have done had you been building the gasoline-powered version. One of those change-orders has to do with how the hood is cut out and the change has you cut it very close to the stiffening ribs. A portion of the weight of the forward battery box is tranferred into these ribs by a pair of brackets — one each side — that connect the forward battery box to the ribs. These brackets also hold the mounts for the front bumper. With the dynamic loading of the batteries constantly bouncing and pulling on these ribs, it’s a wonder that they didn’t delaminate further. There is very little overlap between the glass on the ribs and the glass on the body — especially on the inboard sides of the ribs, where less than an inch of overlap exists.
Somehow, I’m going to have to grind away the delaminated fiberglass and reinforce the joints. Reinforcing the outboard sides will be easy — there’s lots of surface area there to make a good, strong structural lay-up. Reinforcing the inboard sides is something else again. I may need to wrap fiberglass onto the top (outside) of the body to make these joints strong. That will require a lot of cosmetic work on the outside of the body — on either side of the hood hole — to fix the joints.
The good news is that it’s better to find out now than when the car is on the road.
By the way — take a look at the first photo in yesterday’s entry. It shows the front end of the car with the two longitudinal stiffening ribs. My blog editor has a 4-photo-per-day limit, so I had to place this last photo in yesterday’s entry.
After more weeks than I care to remember, all of the hardware for both the VW donor chassis and the Bradley kit car have been refurbished and inspected. I’ll need to purchase some basic hardware items to replace some of the bolts, nuts, screws and washers that rusted to the point of being unuseable but about 80% of what came with the car was salvageable.
The next job is to restore all of the fiberglass parts. As many readers may not be familiar with the procedures and materials used to do basic fiberglass work, I’m going to add quite a bit of detail to the description. To begin, there are 25 pieces of fiberglass (or ABS plastic) that must be reconditioned:
(1) Main Car Body
(1) T-Top for Main Car Body
(1) T-Top Liner (For Headliner)
(4) Upholstery Shells (For Areas Surrounding Small Rear Windows)
(1) Center Console (ABS)
(1) Glove Box
(1) Defroster Shroud (ABS)
(2) Door Panels (ABS)
(2) Headlight Buckets
(2) Gull-Wing Doors
(2) Seat Buckets
(2) Seat Bucket Back Panels
(1) Glare Shield (Goes On top of Dashboard)
Some of these parts just need to be cleaned up. This usually consists of having the old upholstery stripped and the upholstery cement removed. The Glare Shield and Seat Bucket Back Panels would fall into this category. Other parts may need structural repair or reinforcement (my rear bumper took a bad hit on the left side) and most, if not all, will need cosmetic work (filling, sanding, contouring, priming and painting. There’s an order to doing this work and cleaning up the parts comes first. All of the old upholstery and rubber cement needs to be removed from all of the fiberglass parts — down to the bare fiberglass in some cases or down to the gel-coat in others. Once the parts are cleaned up, structural repairs are next. This may require removal of gel-coat or paint in damaged areas because you cannot perform an adequate structural repair over gel-coat or paint — you have to get down to the raw fiberglass. Once the structural work is done, then the cosmetic repairs can be done.
The method of fiberglass work that I use is called Moldless Composite Construction and, as the name implies, no molds need to be made to repair any of the parts or, for that matter, even to fabricate new parts. We’ll demonstrate this latter concept by scratch-building an overhead center console for the T-Top without building any molds. Moldless Composite Construction was developed by the German sailplane industry but was popularized in the U.S. by Burt Rutan, designer of the Vari-EZ, Long-EZ, Defiant, Voyager, and numerous other experimental and factory-certificated aircraft. The technique was recently used in the construction of Space Ship One and its mother ship — White Knight. The techniques are easy to learn, the materials are readily available and the methods produce strong and lightweight structures. If you’re not familiar with the techniques and would like to get some hands-on experience as I describe them, Aircraft Spruce and Specialty sells a practice kit, a demonstration video and all of the materials that you will need. You can purchase similar items from Wicks Aircraft Supply or The Composite Store (in Tehachapie, CA). Other techniques and materials may be equally applicable here. However, I’m most familiar with the procedures developed and the materials recomended by Burt Rutan — so those are the techniques and materials that I am going to describe.
Back to Step 1: — Cleaning the Parts
You’ll need to strip all of the upholstery and backing foam from all of the fiberglass parts, including the main body of the kit car. Ripping and shredding works well for the bulk of the material but remove the fabric with sufficient care that you can trace patterns on a roll of art paper. Be sure to mark the outline of each fabric piece along with the locations of any folds and seams that have been stitched into the fabric. If the fabric parts aren’t in too bad a condition, you might want to save them for use as models when you are making up the new upholstery. My upholstery was so deteriorated and soaked with pigeon and cat urine that I just made paper patterns and got rid of the original material as fast as I could.
Once the foam and fabric have been removed, you’ll need to remove any excess upholstery cement. A lot of this cement may have partially deteriorated (oxidized or powdered) and trying to fasten new upholstery over the old glue is a waste of your time. Also, if any structural repairs are required for the part, you will need to remove everythng (foam, cement, fabric, bondo, paint, primers, gel-coat…) right down to the bare fiberglass — on both the inside and outside of the damaged areas.
Upholstery cement is basically just a special formulation of rubber cement. As such, Acetone is the best solvent to soften (not remove) the old glue. You can use mineral spirits, but acetone works better. Make sure that you use the proper safety precautions when working with acetone (butyl rubber gloves, respirator with organic vapor cartridge, no open flames nearby, etc… Read the precautions on the can.) I use a paper towel to saturate the old glue with acetone to soften it and then use a single-edged razor blade to scrape the glue off the fiberglass. Once all of the glue has been removed, I sand the entire part with 60-grit aluminum oxide paper. This helps identify any glue that I might have missed and roughens up the part so that the new glue will adhere well. Removing the old glue takes time — it took me about 5 hours to do each of the seatback shells, for example — but it’s a lot easier and more pleasant to repair and upholster a clean part than a messy one.
To date, I’ve got the two seatback shells finished and the glare shield about half-way done. I’ve also got most of the headliner shell cleaned off and I’ve cleaned up the glove box. (In the original Bradley instructions, the glove box is glued to the back face of the dashboard with a structural adhesive. The joint was not glassed over with fiberglass tapes, as it should have been. Bradley left the installation of the glove box as an option, for builders who might prefer to install a radio in this location. I found that my glove box fell off after a couple of whacks with a rubber mallet. The adhesive was very brittle and the glove box popped off easily without any damage. Once on my workbench, I was able to sand off the old structural adhesive in short order. I’m going to use a flocking kit that I obtained from Woodworker’s Supply to flock the inside of the glove box. I’ve done this before and it comes out looking very professional — even for first time users — and it sure beats the carpet that was originally used.)
After you have all of the glue removed from all of the parts, you’ll be ready to start the structural repairs. In the next blog entry, we’ll do a couple of easy ones to introduce you to the materials and procedures.
It’s been a while since my last entry, so I figured I’d better at least make an attempt to update the journal. Most of the last few weeks have been spent painting and cleaning parts. I made a decision a few weeks ago to powdercoat those parts which will be exposed to road grime and the weather, and to paint parts that wouldn’t be exposed to too much grime. The painting job is done except for the front beam assembly. I want to spend a day removing welding spatter from the front beam assembly before I paint it (and from the rear diagonal arms, before they’re powder coated). All of the parts that need to be powdercoated have been sorted out, media-blasted and are just awaiting the funds to do so.
For the past few weeks, I’ve been standing in front of a wire wheel, de-rusting nuts, bolts, washers, screws and all of the small miscellaneous parts that go into making a car. It’s amazing how many there are. If I had a spare $300, I could buy a vibratory tumbler and get the job done in a day — but then I’d have to figure out where to store the darn thing untl the next job came along. So, I’m just doing it the old fashioned way. I made the mistake of backing into the rotating stone on the other side of the grinder’s motor two days ago and discovered that it doesn’t take long for a stone rotating at around 8000 RPM to take a chunk of your elbow’s skin off. Ouch. That’ll leave a mark!
I’ve also set aside a group of parts that will need to be buffed and polished. These include things like door hinges, aluminum moulding strips, key latches, “Bradley” badges, etc. Once all of the hardware has been de-rusted, buffing out parts will be the next job.
In the meantime, I received the new seat sliders a few days ago. The old ones had rusted tight and couldn’t be disassembled because of the way they were constructed. By having the sliders available, I’ll be able to incorporate mounting structures into the sub-chassis instead of just bolting them to the floor pans as the previous builder had done. Mounting the fiberglass seat buckets to the sliders will require the fabrication of some custom mounts and I suspect that that’ll be a job coming up shortly. The seat pans apparently had at least two previous sets of sliders mounted to them and, as a result, they have a lot of unnecessary holes in them. Patching them up will be a job that comes up in the near future.
I took the roll bar (T-Top support) to a racing shop a week or so ago and asked them to weld on a pair of 1/2-13 nuts to act as mounting points for a pair of shoulder harnesses. The original Bradeys didn’t have shoulder harnesses. They suggested a different approach as they were concerned that welded nuts might not hold in an acccident. Instead, they drilled and mounted a pair of 1/2-inch high-strength steel bolts thru the roll bar from the back side (outside) of the roll bar. This is probably a better approach, although I’m not sure that I like the pair of 1/2-inch holes that had to be drilled thru the bar on each side. Seems to me like the welded nuts might have been better.
It hit 108 yesterday here in Las Vegas. That will pretty much kill a landscaping project that I’ve been working on in our back yard until September or October. It’ll also give me the opportunity to work inside on the Bradley a bit more. Oh, the advantages of having an air-conditoned garage…
Work continues, in sort of a hodge-podge fashion. I spent yesterday sorting thru all of the metal parts that came with the Bradley kit car — things like bumper brackets, the accessory battery tray, the T-top post brackets, the roll bar (if you can call it that), seat slider brackets, etc. — and sorting them into piles. One pile is going to the sandblaster (these parts will get painted), one is for the powder coater and one pile needs to be sandblasted, inspected and modified before I cart them off for powder coating. One of the modifications that I want to perform is to weld a pair of 3/8- or 1/2-inch nuts to the insides of the vertical portions of the roll bar — to act as anchor points for shoulder harnesses. There were also a bunch of VW parts that got sorted into these piles — things like the steering column, spring plates, diagonal arms, bearing retainers, brake backing plates, etc… Once the sorting was done, I carted off the first category (75 parts in all) to the sandblaster.
I’ve also been sanding a lot of the smaller fiberglass parts — mostly to get the old upholstery glue off them and to prepare them for making fiberglass repairs. The sanding literally made my fingers raw. 40-grit will do that! To reward myself, I spent two days digging a ditch in the back yard for a new landscaping project — a stone-lined dry wash. Digging with a pick shovel doesn’t seem to bother raw fingertips but it sure is good exercise.
I’ve also been working out a preliminary wiring diagram for the car and a preliminary design for the instrument panel. Some of my background is in designing wiring systems for experimental aircraft. As a result, I’ve decided to wire my car more like an airplane than a car. There won’t be any fuses — instead, I’m going to install a breaker panel with Tyco pull-type circuit breakers that I’ll obtain from Aircraft Spruce & Specialty. The Motor Volts and Speedometer gages will be moved off the gauge panel and will be placed on either side of the steering column. I’m used to having the speedometer in front of the steering wheel and I want to change the design of the instrumentation to permit that. Additional indicator lights will be added to verify that the motor blower is functioning and to indicate an overtemperature condition on the traction motor. The motor has a built-in over-temp sensor — might as well put it to good use. The heater blower switch will be split into a fan (only) switch and a heater switch, with both switches relocated to the main portion of the dashboard. In addition, I’m going to add a battery master switch and battery solenoid for the accessory battery — similar to the one you find in general aviation aircraft. This allows me to disconnect the accessory battery from everything, in case of an electrical problem. A new Accessory Battery Ammeter and Voltmeter will be added to the gauge panel as will a Pak Tracker battery condition monitor.
Once the design is finalized, I’ll fabricate the dashboard and gauge panel out of 1/8-inch aluminum, paint them flat black and then silk-screen them with the appropriate labeling. The original gauge panel was made from some sort of phenolic-like material and it warped badly under the summer heat. I’m also toying with the idea of fabricating a small overhead console out of fiberglass and aluminum. At a minimum, I want to put two reading lights up there. If I need additional space for low-current switches, that’ll be the place where they get added.
Over the next few days, I’m going to start refurbishing all of the nuts, bolts and other hardware that came with the Bradley kit. The VW hardware is finished at this point. I’m shooting for about one more month to finish the chassis and then I can begin the redesign of the sub-chassis and battery boxes.
I’m now two months into the project, and the only unresolved concern is still the rubber parts (gaskets, seals, weatherstripping, etc.) — especialy for the sliding windows in the doors. Virtually none of the rubber that came with this car is salvageable.
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