Intro: Aerodynamic Bike Helmet!
Make your helmet more aerodynamic while revealing your inner artist! This instructable will introduce you to composite manufacturing and get you closer to being Lance Armstrong.
Step 1: Sketch the Shape. Dream Big!
Drawing inspiration from both the fish and a nearby airfoil, I put a piece of butcher paper underneath the helmet so I could sketch a few ideas and see how they looked. Here's a parts list for this project:
-Fiberglass cloth, epoxy resin, slow cure catalyst (can buy at Home Depot; ask for fiberglass boat supplies)
-Foam sheets you can cut with a hotwire (plus a hotwire cutter; or make your own)
-String, masking tape, hot glue, 5 minute epoxy, string, masonite scraps (see step 2)
-spackle (same kind used for drywall is fine) and spackling/putty knife
-metal bonding compound (should be sandable)
-Sandpaper (80-100 grit), razor blade/scissors for cutting fiberglass cloth
-latex gloves, safety glasses, respirator with organic vapor/HEPA filters
-a BIKE HELMET you're ready to convert into a MASTERPIECE!!!
Step 2: Cut Out the Desired Cross-section.
Scissor out your desired cross-section from the butcher paper and trace it onto a piece of 3/16"-thick masonite. I used a bandsaw to cut the masonite (with safety glasses, of course). Use a file to smooth out the edges of your masonite cutout.
Step 3: Fit the Cutout to the Back of the Helmet.
I prototyped the shape using paper again.
Step 4: Add a Sheet of Foam on Either Side of the Cutout.
Sandwich the masonite in between two sheets of foam. Using a hotwire, shape the foam so that it matches the contours of the helmet. Gaps of 1-2 cm are ok.
Step 5: Make Two More Cross-sections.
Dust off your imagination and use it to envision a cross-section that's one foam sheet away from the original one. I chose to cut this piece to match one of the vents in the back of my helmet (see photo) so that I would have an easier time with assembly. After you've decided on the shape, cut out a second identical version to go on the outside of the other sheet (e.g., in the picture we can see the cutout that goes on the right side of the helmet; a copy of this cutout will be attached to the opposite foam face).
Step 6: Preserve the Alignment of the Cross Sections (drill Alignment Holes)
In the previous step, I was just holding the pieces together with my hand (although in retrospect a clamp might have been useful). Still holding them tightly, I placed the foam-masonite-foam-masonite stack onto the table and carefully removed the upper layer of foam so that I could maintain the orientation of the smaller masonite piece with respect to the larger, central piece. I then traced the small piece's outline onto the big piece. What we're going for here is a way to preserve the alignment of the pieces so that the overall piece will be symmetrical (in other words, I want the whole helmet to be symmetric about the plane that the two bike wheels are in, if the helmeted rider is facing forward). After tracing the little piece's profile onto the big piece, I drilled two holes in the little piece, lined it up with its outline again and then traced the holes onto the big piece. What's pictured here is all three pieces (two small identical, one big) stacked up; each piece has holes in the right place. There's a *related instructable* about how to drill a hole called "Drill a Hole" where you can read more about the details I left out: Drill A Hole
Step 7: Sandwich Foam Sheets Between the Masonite.
The masonite pieces will serve as a guide for the hotwire you will use to sculpt the foam. In this step, you need to attach foam in between the sections. I used regular masking tape folded over on itself (an approximation to double sided tape). Actually, there's an interesting point here: since my design was a "fishtail", as you've probably guessed by now there wasn't going to be a simple way to get the final fiberglass version to slip nicely off of the pointy mold (which is to say, the points were in the way -- like trying to slip an ice cream cone wrapper off the wider end of an ice cream cone). So, in this first picture you can see that I actually cut the middle masonite section into two pieces in case I needed to destroy the mold and remove it from the inside of the fiberglass shell. Fortunately, I didn't have to; but depending on your design, this would be the appropriate time to start thinking about how you'll separate the final piece from the mold.
After taping on one cross section, I used a pencil to puncture the two holes through the foam. Then, with two pencils sticking out the other side, I aligned the other two cutouts to get the final 'sandwich.'
Step 8: Attach the Foam/masonite Combination to the Helmet.
In a stroke of wild luck, since my alignment holes went through all the layers I was able to just run string through the entire assembly and just tie it onto the helmet (those ventilation holes really come in handy). This is another reason why drilling at least two holes in the masonite is a good idea. I added two or three globs of hot glue at the joints where the masonite met the helmet for redundancy. I was pretty paranoid that the assembly would fall off while hotwiring the foam, so as a result it was very sturdy.
Step 9: Add Two More Pieces of Foam on the Sides.
Again, using the hotwire was useful to get these pieces to match the curved sides of the helmet. I didn't bother with any holes for these two pieces; instead, more sticky tape did the trick (however you choose to attach them, just make sure the force from the hotwire won't cause them to pop off while you're trying to shape them).
Step 10: Using a Hotwire, Sculpt the Foam Wackily. This Will Be a Rough Cut of the Final Shape.
Good hotwire technique is an art I'm still trying to perfect. In general, I can say that the cut surface is much smoother if you don't let the wire snag on anything and if you try to make sure you move the wire with relatively consistent speed. If the wire snags, often this can result in a deep groove where the wire burns into the surrounding foam as it sags. if you can keep it moving that keeps tension on the wire and makes the cut more predictable. Doing a few practice cuts is a great idea; after you cut 4 or 5 random little pieces you start to get the hang of it and will certainly be able to do this project. If you want to make your own hotwire you should check out the instructable Hot Wire Foam Cutter.
Step 11: Cover the Foam With a Thin Layer of Spackle. Let Dry Overnight.
If you put the spackle on too thickly like I did small cracks can develop. I would suggest a layer that is less than 0.5cm thick if possible. Mine was 0.5cm thick in the thickest regions -- I could live with the cracks, but much thicker and you probably would have to fill them in at this step instead of moving on.
Step 12: Sand the Spackle Until the Surface Is Smooth.
It's ok if you start to see the foam in some areas as long as these patches are small. The goal is just to make the surface uniformly smooth. 80 or 100 grit sandpaper should do the trick.
Step 13: Cover the Rest of the Helmet With Plastic to Protect It.
This is to protect it from epoxy drips in future steps. I used masking tape to attach the sheet of plastic (I cut a ziploc bag along the seams).
Step 14: Cut Pieces of Fiberglass Cloth -- Enough for 3 Layers.
If you've done a lot of fiberglass work, skip the next few steps. Otherwise, here was my sequence: before mixing any epoxy, I took a sheet of fiberglass cloth and draped it over the spackled area I had just sanded. While pressing it onto the surface so that it conformed to the edges of the piece, I drew lines on it so that I would know where to cut. The goal is to cut pieces that won't crease or bunch up when you epoxy them on later. When this happens, it's easy for bubbles to form between adjacent fiberglass layers. For an aerohelmet this may not be a big deal, but such bubbles (called "delaminations") are the subject of much thought and test design in manufacturing scenarios. So, your objective in cutting the cloth pieces is to cut shapes that lie flat against the surface you are working on. Cut as many pieces as you want. I ultimately had three layers of fiberglass -- the underlayer was 20 oz. variety and the next two layers were 7 oz. (these refer to the weight of the cloth per square yard). If you're using the stock fiberglass cloth you can buy at Home Depot, I'd say two layers with reinforcement in the weaker areas (such as the sides) would be a good bet. Or three layers. Both should still be very light and yet flexible enough so that it's easy to remove the final piece from the plug.
Step 15: Cover the Entire Plug With a Layer of Epoxy.
I recommend use of an epoxy with a 90-110 minute working time so that you can relax and put the pieces on carefully. I used West Systems 105 epoxy resin and West Systems 206 curing agent. This has the specified work time and cures overnight. Spreading a thin layer of epoxy over the mold helps saturate the cloth you are about to put on with epoxy (which is your goal).
Step 16: Place Layers of Cloth and Cover With Epoxy.
While cutting the fiberglass cloth in the previous step, I made dotted lines along ridges / edges in the shape so that I could remember the orientation of each piece (I did this so that I could get even coverage of the plug and maintain the same number of layers across the entire surface). In these pictures, the dotted lines are more apparent. The process is to lay down a piece of cloth and then smear epoxy over it (thinly) being careful not to cause bubbles or stretch the weave. Since you're wearing gloves, your fingers are an excellent tool to use in this process. Another instructable that descriibes fiberglassing is called Homemade Rudder for Surfski in case you'd like to see another example.
Step 17: Sand or Cut Off Any Fibery Protrusions. Be Aware of Sharp Corners.
NOTE! Fiberglass shards can cut you! I used 80 grit sandpaper to smooth off sharp corners; a Dremel with an abrasive bit would also be useful. Safety first!
Step 18: Apply a Thin Layer of Metal Bonding Compound. Let Dry (20 Min).
Use _much less_ catalyst than the manufacturer specifies (you can do a test piece to make sure it hardens). It makes your life easiest if this compound is pretty liquidy and goes on like paint. Too much catalyst makes it more like bondo and just means it will take longer to sand it smooth. After it dries, you should sand it in the same way you sanded the spackle earlier. Perfectionists may choose to wetsand the surface.
Step 19: Wax and Buffer the Surface.
Regular car wax may be used. To be safe, you should apply at least three layers (letting them dry in between). By "buffering" all I mean is using a cloth to smooth out the last layer of wax (the goal is to leave most of the wax on the surface, just making sure there are no ridges in the wax).
Step 20: Apply Mold Release Compound.
Step 21: Layup Two or Three Layers of Fiberglass. Allow to Cure Overnight.
Use the exact same process you used for applying the layer of fiberglass over the spackle. It is not necessary to allow each layer to dry individually; you can apply all three immediately one after the other. (sorry I don't have a new picture for this step)
Step 22: Cut a Seam in the Fiberglass If Necessary and Pull It Off the Mold.
(first sanding off any fibery protrusions that might cut you) Because my design had two points on it, it wasn't possible to slip the fiberglass off the mold in a single elegant motion. Instead, using a Dremel I cut along the entire lower edge and part of the trailing edge. I'm a safety nut, so I wore some stylish safety goggles. 8)
Step 23: Remove the Final Piece From the Plug!
First, run a screwdriver along the seams you just cut. The screwdriver should be squeezed between the fiberglass and the surface of the mold (see picture). You should see the fiberglass starting to separate from the mold. Next, I repeated this process along the edges where the fiberglass meets the helmet itself. After a few rounds of loosening, I was able to pull the two edges apart and remove the entire thing. Voila!
Step 24: Repair the Seam.
Using blue masking tape on the outside of the piece to hold the seam closed, I layed down a single layer of glass on the inside of the seam (same as before -- a little epoxy, a small piece of fiberglass, a little more epoxy). The top corner was easy to glass over, but for some reason the other corner hadn't come out right. Since there was a hole in the lower corner, I just dripped some epoxy down into it from the inside. NOTE: for this step I used 5-minute cure epoxy. We're getting close to the end! No time to wait!
Step 25: Do a _crazy_ Paint Job.
I used Liquitex acrylic paints.
Step 26: Hit the Road With Your New Creation!
You'll be the coolest kid on the block.