Thoughts on Fiberglass Rear-Skirt Design
 

Ok, So... I've yet to do anything major, not even as much as a grill block (but if i can get my hands of some black plastic they'll be done this weekend)

But i'm considering rear skirts. The thing is, I can't screw cardboard or plastic on the outside of my car. The car is still new, with 25k on it, and still pretty. What i'm thinking is, Could i make nice looking, smooth wheel skirts out of fibreglass, and mount them in the rear wheel wells? That way I could paint them, (or have a body shop professionally paint them.... i know a guy), and they wouldn't look ghetto. I've never worked with fibreglass, and i have no idea what the costs would be. I'm not sure how you'd make a nice smooth mold either.

Thoughts?

Go for it. You never know until you try. As someone with a newer car, I agree with you...not out for an eyesore on this one. Good luck!

This post was kind of meaningless, but hopefully it was also kind of encouraging!

Edit: Now this is veering off topic! However I notice you are getting better MPG than I am in a car with lower EPA, so I want to learn from you :). What do you do? I saw in your vehicle profile you have a scangauge; what in particular do you pay attention to? Thanks!

stevey_frac,

I've been thinking over rear skirts and partial fronts since last summer.

And, I think the best first attempt to mount a skirt would be the way that MetroMPG did in his Skirt Thread:

http://ecomodder.com/forum/showthrea...html#post37306

Second picture in, shows his attachment method, using screws and flattened copper piping to provide tabs to mount skirts on.

I don't own a Cobalt, so I can't say where the screws are that hold on your plastic inner fender. On my Jetta on the back wheels there are three mounting screws on the top along the outside edge. Likely it's the same sort of setup for your car too.

What I'd suggest is: Make copper pipe "Brackets" that allow you to use your inner fender screws to hold the copper pipe brackets.

A drilled hole in each end of the bracket will allow you to mount on the existing fender screws, and a screw or other means of attachment to your skirt.

If nothing else, these can be your first-generation / prototype brackets ... you could use aluminum or steel bar stock to make something sturdier, if you find you need to.

Paper/Card/Cardboard strips and a pen or marker might be useful to template out where your bends and holes on each bracket need to go.

The nice thing is, even if you need a different size/shape of bracket for each mounting point ... there's an identical one on the opposite wheel well, after you've figured those out. :)


You can only plan so much until you have to dive on in... get to 'er! :)

Oh, and take pictures, if you can! ... MetroMPG was grumbling about his poor pictures in the thread I pointed out. Yet more pictures will get more people to take the plunge after they see how easy it can be.

stevey_frac -

I agree with ChrstphrR, MetroMPG has a really good DIY.

If you don't want to drill any holes, then you need to figure out how to "reuse" whatever existing holes you have. I found some (partial) pictures of Cobalt fender liners :

http://www.griptuning.com/ImgApp/install/0000000130.pdf
(Look on page 10)

From these pictures, it looks like there are a lot of fastening holes to work with, at least towards the bottom.

Can you remove your rear fender liner and take pictures, or count the number and location of fastening holes you have?

Even if you don't use the existing holes, you can make holes in the inner fender liner as needed. This is a *replaceable* piece of the car that doesn't serve an aesthetic purpose. You can always restore the car by covering the holes or getting another set of fender liners.

CarloSW2

A lot of my mileage has to do with the fact that i drive a lot of highway. The rest of it is shifting low and some EOCing. Perhaps the occasional draft of a transport from 100'. My scanguage, i watch my instantaneous and average.

I'll take pictures. Give my wife's digital camera a workout.

I'm not worried about the mounting at all really. What i want is a smooth, professional looking cover. Something i don't have to be ashamed to bolt onto my car.

I'm not worried about attaching the covers, or punching holes in the fender liner. There are actually a LOT of existing screws to stick it on. What I would like to do is make a little bubble that sits around the lip of the fender, and pops out about an inch. I can hinge it at the top, and use rubber feet, and use small bungee cords to hold it to the side. The hard part is making them...

Stevey, I really hope you decide to go through with these the fiberglass way. I think OEM quality skirts would be way cool. Like you just said, making them will be difficult. You will most definitely be going the extra mile but in the end the results could really be worth it. If you have the time and you get it done right, you might be able to make a few bucks selling additional ones off to other cobalt hypermilers.

I, too, have zilch experience with fiberglass but to speculate, you may want to jimmy rig some ghetto skirts first to help you make your mold. I would guess that either wood or a type of styrofoam may work best for a mold. Best of luck.

is the opening of the wheel well close to flat, or does it bend in 2 dimensions? if its close to flat you may better be using aluminium plates, it's cheaper and they are already flat! chances that you can make something of decent quality out of fiberglass from the first try are close to zero. but obviously once you mastered it somehow, then possibilities of making complex shapes become far greater.

I would also think aluminum would be the quickest and lightest solution. I guess I'll find out if I try it on my wagon whenever I finish restoring it.

The aluminum would have to be bent out on my car to accomodate the rubber especially if the tire is allowed to go down. I don't have the ability to form aluminum to be the way i would want it. I agree that it would be fast and easy if you could put a flat panel on it though.

My comments, from having made fiberglass front wheel skirts and Kammback:

- Use epoxy resin, rather than polyester. It's much more expensive, but also stronger. (I didn't. Wish I had.)

- if you want a pro finish, 90% of your time will be spent doing cosmetic work (sanding/fairing body filler) to smooth the fiberglass part once it has been made

Lessons learned:

- In addition to using the weaker polyester resin, I didn't make my front skirt thick enough. So it's gone a bit wavy. When it was initially completed, it was smooth & nice. I need to reinforce it with more layers on the back, and possibly re-fair the front side.

I've been buying my supplies at Crappy Tire. It's not cheap! And I'm not sure if I can get the epoxy resin there (may have to go to a marine supply store.)

Uh ohh... Looky what i found....

Bondo Resin Repair Kit, 1 L | Canadian Tire

Might not be a bad place to start. I just need some foam insulation to make my plug. Might take a run at it this weekend.

Did anyone else think that fibreglass was more expensive?

I'm looking into a local canadian company that sells fibreglass materials. I'm going to see if they'll sell small amounts locally, and what it would cost. ZAt the moment, i'm thinking it'll run around $70 for a set in materials alone, and as MetroMPG indicated... Probably many hours of work to get a profesional product.

-Steve

P.S. The company is called Rayplex.

Why does everybody just try to fill the hole, and put up with whatever flare shapes the stylist did? With fiberglass, you want a convex shape for stiffness, and it will smoothly cover the whole mess around the wheel and as far back as necessary. Mold making is more tedious than mysterious, especially for that shape. If the available holes are insufficient, with the right adhesive, you can probably put mounting blocks around the wheelwell without damaging the paint, if you are worried about resale value.

I'm not quite sure i understand what your saying. You want me to use the wheel wells for... i'm not even sure. I want to put a skirt in place that will cover the back wheels from the bottom of the car body, to the top of the wheel well. It has to bubble out a bit to do this. That's a convex shape...

Please explain, as i'm feeling more then a little lost here.

-Steve

On your car, I believe that the rear tires project beyond the lower edges of the fender, na? Depending on the suspension softness, etc, I'd start to define my new shape by taping blocks about 2cm thick on to the tire, and bending splines smoothy over them to where they meet the body. I'd use a gentle curve, probably going at least 10 cm past the wheel opening in all directions, and much farther near the lower edge. Starting from the bottom, you can work up and make a shape that takes a smooth vertical spline. Where the edge of the skirt meets ridges on the body, you can either use a jagged (coped) edge, or blend the shapes. You want to get a nice-looking edge shape overall by varying the curve in the splines somewhat as you go up.

stevey_frac -

Here's something with a tiny(?) chance of working. Research aftermarket wheel skirts for old classic hot rods. if your wheel-well dimensions are close enough, you may be able to adapt a "classic" wheel skirt.

While Googling for an example, I found this how-to article :

Creating Fender Skirts - Rod And Custom Magazine
http://image.rodandcustommagazine.co...er_skirts+.jpg

CarloSW2

That sounds a LOT more complicated then i want to do. I am thinking i'll build up from around the existing edge (there is about a 1 inch ledge around the wheel that is flat) and build it up enough that it clears everything, probably around 2 inches. That transition can be fairly subtle. I was planning on the main part of it being flat, and parallel to the existing ledge. The 3d sculpting your talking about I think would be too much for my engineer brain to do properly. Keep in mind i'm a computer geek. I have the fine motor control of a spastic 2 year old.


EDIT: Wait.. i re-read that, and i know what your going for. It'd be fairly sexy, and not nearly as much art as i thought it was. :P

That'll be the polyester resin. You want epoxy - it'll say so explicitly on the packaging.

Yes, i caught that. You and I both wrote our posts at the same time, and I wrote mine without knowledge of the contents of your post. :P

That's why i'm going after this Rayplex company. I think they're less expensive (especially in larger volumes) and i also think they have higher quality materials.

If it helps at all, a brand name I know of for marine epoxy fiberglass repair is "West System".

Are you certain that epoxy resin is stronger, and that it's not the fault of a crappy crappy tire product? It just seems to me like polyester plastic is fairly resilient...

EDIT: Nevermind. I looked it up. Epoxy resin >>>> Polyester resin.

West Systems make good products. Used them to repair boat damage caused by the maniac owner of the marina while transporting them from storage to water and back and really beats the Bondo stuff I had used on cars in the past. For any kind of auto body, I recommend marine products as they are designed to withstand a more harsh environment, though the cost often reflects that so it depends on how permanent you want the job to be.

I think my project for this week (read: this saturday) is going to be putting in the synthetic tranny fluid, and fabricating and installing a grill block. That leaves next Saturday wide open for trying to make a mold. I want to think about it a bit more. The way I wanted to do it, i wouldn't be able to get the part out of the mold. Which means i think I have to make the other side of the mold. The female counterpart to the male piece i wanted to make. And that's gonna require more thought.

I've done a lot of fiberglass, and taken a course at Abaris, and I'd say that if you are having trouble with polyester being weak on a simple panel, you are not handling it right. In composite work, we count on the strength of the fibers; the resin is just there to keep them lined up. The advantages of polyester are that it is compatible with Bondo, and MUCH EASIER TO SAND AND POLISH. The more variable cure times are a great convenience. You can get better properties from isophthalic types, or the strength and toughness of average epoxy from vinylester resin, but it has a short shelf life, so you have to find a shop that uses it to buy a gallon. The advantages of epoxy are low shrinkage and a true water seal, even over rust. It also has less odour and a more manageable health hazard. There is one epoxy, SB-112 from System Three that is compatible under bondo and polyester, but you can usually put epoxy over the esters.

Mold Making
 

<grin> Chet Kyle remarked that some of the early entries at the IHPVA looked like they were build by blind men wearing boxing gloves.

Getting from that shape, defined by splines, to a finished product, can be tricky. One way would be to pre-coat one side of thin wooden splines with polyester gel-coat. (It will stick a lot better if you heat the wood first, so the cooling air inside sucks the resin in, while the heat thins it out.) That way, you can build up almost a whole mold surface from splines, and glue them to a backing to hold the shape. Spray foam caulking and plywood might be OK for that, but epoxy putty, blocks, and plywood would be best. Then, you just have to fill the cracks with more gelcoat, sand and polish, and you have a mold. Wax it 3 times before use. Problems over the thickness of the part affecting fit can be sanded out.

Oh, that's a certainty (in my work to date)!

For one, I didn't use enough layers, and for two, the kammback has a cardboard core. :) Can't foresee any problems there, eh?

Fibreglass Questions: Since you guys have worked with it before.

Do i have to wait between applying layers? And how far will one of those kits from Cdn Tire go? Could I make one skirt with one box? or would it take multiple boxes? Do I need to have a core of some sort (i could probably use some thin plywood) or can i just schlep enough fibreglass onto it and it'll work?

Also, i was going to do a kind of raised flat panel, and then i wanted it to have like.. a 1 inch smooth section to rest agains the edge. Can i make the panel large enough to cover the the whole edge, then have a section that angles back to rest against it? It's kinda hard to explain...

Where the wheel well is 'up' from that, and those flat edges rest against it. The problem is.. How would i get it out of the mold. Or am i going about this all wrong?

-Steve

Here is a "simple" method of building fiberglass parts. I think it goes with what bicycle bob had said, but instead of using the wood, you use foam. You can fiberglass over both sides of the foam and have a thick part, or do one side and mechanically (scrape, hack, scratch, sand) remove the foam.

Fiberglass Answers
 

A kit might be good just for the education, but I don't know what is in that one. The most common fiberglass cloth is 6oz per sq. yd "boat cloth" and you would want at least five layers, all mushed together while still wet. Cheaper fabric is a felt mat, sold, confusingly, by weight per sq ft. Two layers of 1.5 oz might be OK. Mat winds up with a lower glass content, but the thickness build-up with cheap resin makes it good for non-structural panels. Plus, it is less likely to show a pattern through the gel coat. I wouldn't bother with a core material on a first project. That is a way to get the lightest possible part, not a successful first project.

To get that edge shape, you'd want a two-part mold, and it isn't always easy to lay 'glass in a tight curve. A larger skirt overlapping the fender could have a much slipperier shape.

Polyester resin won't cure on the surface when exposed to air. This is handy when adding layers to get bonding without sanding. The final layer has wax added to float up and seal the surface. So always check for "waxed" or "unwaxed" labels, or buy the resin and wax separately. You would use unwaxed gel coat against a mold (with release wax - a separate issue) and let it cure before adding the cloth, using wax for at least the last layer. You can usually sand wax off easily as you rough up a cured surface for a secondary bond, but gelcoat might break up.

Can I not use the foam as a mold to make many out of?? Or do i have to make a more complicated mold for that? I was planning on making it out of that foam board insulation, glued together. The difficulty would be in removing the plug after the fiberglass had set.

Sure, you could build a surface of wood or foam splines, and 'glass over the outside to hold the shape, and then do the inside to get a sandwich-core part. That way, you don't have a mold left over. The weight savings from the core will probably disappear into filling and fairing the surface. The wood core would want expensive, hard to sand epoxy, so the urethane foam sounds better, if you can handle it neatly. Using a thick build-up and then removing the foam is also a valid proceedure, but it tends to require more fiddling around the details. Fortunately, it is very easy to add bits to a 'glass part.

You guys are honestly about a 2 leagues above me with this stuff.

Ok. Let me throw this at you for a procedure, and you tell me if it'll work.

1) Build up a foam block by gluing together a bunch of sheets of foam insulation
2) Cut away excess material until the block is the same shape as the fibreglass part i want to make
3) Sand the crap out of it
4) Coat the foam block, with wax / release agent / something to make it not stick to the fibreglass
5) Put on 5 layers of cloth/resin. I think the Canadian tire stuff is all waxed.
6) Separate part of block
7) Sand the crap out of the part
8) Paint, mount, and enjoy increased fuel economy
9) ?????
10) Profit

Only thing i don't have worked out in my head, is how to mount it after it's done that dosen't involve punching holes in it. Somehow it seems wrong to go to all the effort to get a professional looking cover, and then put a screw through it.

The Canadian tire kit comes with 1L of resin, fiberglass mat, and hardener.

You (bob) seem to indicate that if i don't have enough, i can go out, buy another kit, and add layers. Did i get that right?

Hmm, I was chasing a linkback, and found this old unanswered query.

Building up a foam block is popular. The glue lines can be a problem when sanding, but latex-based contact cement, preferably aerosol, is not too bad. To create a big blob at the same price, you can mix Part A and Part B by sloshing them around in a garbage bag which becomes the rough mold.

Snap-off blades make great carving tools but dull very quickly. Sanding is good. You can buy urethane foam, which is impervious to polyester, and use scraps to sand the rest, as blocks that shape to the correct contour.

The foam surface must be filled smooth to release a mold. Candle-wax (Parafin in America) might work, applied with a hair dryer, but I've never heard of it. I have used a mix of microballoons, wall repair compound, and latex paint, which dries to the same sanding-hardness as insulating styrofoam as a filler and top coat. It, in turn, has to be sealed and polished, using either Shellac or Polyvinyl Alcohol (PVA - also handy as a water-soluble mold release)
Then it gets waxed with mold release wax, which is very thin and hard, to preserve details, good or bad. 3 coats are always used on a new mold, along with muttered incantations and/or other safeguards. (OTOH, I knew a guy with a whole shelf full of oddball resins, with the recommended release agent for each. One day, he used Pledge furniture polish on a countertop, and a sample of each resin released just fine.) A "soft" master made of foam and filler will break up during release, so it is probably better to just use regular mat and gel coat for a surface. You can protect styrofoam from a 1st layer of polyester with a layer of masking tape, and use easy-to-sand, quick-to-patch polyester all the way to the 'way-easier-to-polish final surface. It can be good practise with the stuff, too.

Usually, we make a master model of a part, and pull a mold from that, to get the shiny side on the outside of the final part. For a shallow shape like a fender skirt, it might be OK to use a male mold, and finish the outside. With a re-useable mold, you might be able to do the other side by starting with a thin layer, and then turning it inside out. Then add a second layer to give final strength, and tape it on while still "green" to take out any warpage. That won't work for a flanged edge, of course, just a spoon shape.

To create hard points for mounting hardware, you can add layers to make a thicker pad, and then encapsulate a bolt head or nut, visible only from one side. The layers of cloth or mat with the hole have to be well bonded, and spread out to a long tapered edge.

Adding material from a second kit sounds fine. Just remember to thoroughly sand off any wax before making secondary bonds, and leave it rough from 100 grit. While epoxy is quite wonderful, and on average is tougher than polyester, you should not be relying on the resin for strength in any composite part. It is only there to keep the strong fibers lined up.

I noticed that Bicycle Bob is advocating making the skirt from about five layers of fiberglass cloth, and this would work fine....

But there is another way that I have successfully made various shapes over the years.

Make the entire shape of your skirt with polystyrene foam that is bought for home insulation.

As you mentioned, cut/carve/shape and so on to get the shape you are after. When doing this, try to keep the minimum thickness of the foam at least 1/4 inch thick in the thinnest spots, and up to 1 inch thick where you can.

Now when you have the shape exactly as you like it, you can apply the cloth with your choice of resin, but if you use Polyester, then you must coat the foam with something like Latex primer paint before adding the Polyester resin.

For a part this small, I would pay the extra and use West System Epoxy resin, which does not eat the foam in the process, so no painting is necessary before adding the resin.

Add one layer of 6 ounce glass to the outside and inside for strength. No need to add more layers, as this will be plenty strong.

Add wood blocks inside the foam where the skirt attaches to the car, and add several more small patches of glass in these areas for support.

Almost forgot... Making a foam/fiberglass composite will result in lower weight of the overall panel, since there are only two layers of glass, instead of multiple layers when making a fiberglass model (at least five for strength) from a plug (male or female), depending on what surface you want the smooth finish. In the case of the polystyrene filler, the extra thickness of the filler adds hardly any weight, as the density of the foam is low compared to the weight of the fiberglass and resin.

Also keep in mind when doubling the thickness of a panel, the buckling strength goes up four times for this thickness increase, with only a small increase in weight. Formula race cars use this same construction technique, except they substitute aluminum honeycomb for the foam core, and carbon fiber for the fiberglass cloth.

Jim.