Molding acrylic plastic is not hard to do (I have made a couple of items this way before, but not a windshield), and the material is readily available, so I decided to try making one. This one has a slight compound curve (bend in two diections) but most windshields are just a simple curve.
The bike in the picture is a 2001 Honda Magna 750, with my homemade windshield installed. This type of windshield would probably work for most any bike. It is fairly small, and I did not see any need for metal bracing on the windshield. It is mounted with four struts of 1/4" all-thread, bolted through the plastic. It has held steady at up to 75 MPH (legal, here in Texas), with no vibration or movement. It has killed allot of bugs that would otherwise have been splatttered all over me!
Step 1: Materials & Tools
These are the materials that I used for the mounting hardware. This will vary from bike to bike, depending on what is available to attach it to. I attached to the forks only, but a taller windshield might be braced to the handlebars.
4 Electrical Conduit Clamps - plated (Select size to fit on the forks and/or handlebars)
2 feet 1/4"-20 All-thread Rod - plated (struts)
4 1/4"-20 Acorn Nuts - stainless steel or chrome plated
8 1/4" washers - stainless steel
12 1/4"-20 Nuts - stainless steel
4 Rubber Grommets - 1/4" ID, for 1/4" thick material.
2 feet 3/8" dia. Black Shrink Tubing (to cover the all-thread rod)
1 Inner Tube (to cut into pads to isolate the clamps from your chrome)
1 yard Felt Cloth - synthetic or wool
Consumables which you will need:
Sandpaper: Various grits from 200 to 800 or 1000, depending on how shiny you want the edges.
Cardboard. The exact size and type will depend on the shape and size of your windshield.
Razor blades, Dremel cut-off disks, saw blades, etc.
Contact or rubber cement. This is to glue the rubber to the clamps
Full size baking oven. I used the one in my kitchen. We are only going to soften the plastic, and you should not be ble to even smell the plastic when it is heated. Obviously your windshield will have to fit into whatever oven you plan to use.
Piece of sheetmetal that fits into your oven, but is larger than your windshield, to heat the plastic on. This should be clean and smooth, no paint, preferably galvanized.
A sabre (reciprocating) saw, jig saw, or coping saw, to cut the plastic. I used a cheap hand-held jig saw with a fresh metal-cutting blade and it worked well.
Drill; manual or electric. A stepped drill is very good for drilling plastic. If not available you will need at least a 1/8", a 1/4", a 3/8", a 1/2" and a 9/16". See the section on drilling the holes in the windshield for details.
Files, course and fine.
Wrenches to fit nuts.
Vice: for bending the struts.
Step 2: Design
I did the layout in a CAD program. Alternatly, you could cut out cardboard and adjust it until it looks the way you want it too. Fold it in half and trim the shape so that it is symetrical.
The CAD program I used is available for download from Dassault Systems. It is called DraftSight. The 2D version is free for personal use. Look here to get it:
The drawing that I used is attached below, along with a PDF file for printing. I took this file to FedEx/Kinkos and had it plotted full size. This produces an outline which can be cut out and used to trace the shape onto the plastic with a permanent marker.
Step 3: Trace the Pattern and Cut the Plastic
Cut just outside the line with your saw. Use a course file, working lengthwise along the edge, to cut down to the line and smooth out any inconsistancies. Use a fine file to remove the course file marks, then sand with progressivly finer sandpaper on a sanding block, to make a smooth edge. Use a hard sanding block for this, or you will tend to round off the edge.
I do not recommend using a flame to polish the edge of the windshield. This is quick and makes a nice edge, but the plastic will craze and crack if it ever comes into contact with a solvent. Acrylic adhesive and alcohol are two common solvents which can cause crazing. Check YouTube for videos showing this effect.
Step 4: Make a Mold
I used a piece of heavy corrigated cardboard for a mold base. A table top or plywood would work as well. My windshield was to have a rise at the edges of about 3", so I started by making two 3" tall struts out of corrigated cardboard, doubled over for strength. These were taped down to the base, parallel to each other, and a couple inches further apart than the width of the windshield. See the first picture.
A piece of card stock was then laid across these two struts, and taped down to the board in the center and to the struts at the edges. This defines the major crosswise curve of the windshield.
You could stop at this point and have a functional mold with a simple curve. I wanted the bottom of the windshield to curve back, so I added two pieces of card stock, tight to the mold surface on one edge, and 2" above the surface at the corners. See the end view picture.
It is most important that the mold be symetrical. If one side is higher than the other the curve will be off, and it will look odd on the bike.
When a satisfactory curve has been achieved the mold is lined with two layers of felt cloth. This prevents the hot plastic from picking up the patern of the tape and seams in the mold. To ensure that the plastic is correctly positioned the paper pattern is laid on the felt in the mold, and the outline is traced onto the felt with a marker.
Step 5: Mold the Plastic
Place the piece of sheetmetal in the oven on a single rack at about the center of the oven. The oven I used was gas fired. Preheat the oven to about 325 F degrees. Remove the protective paper or plastic film from your windshield plastic, and wipe it down carefully. You do not want bits of plastic stuck to the surface.
Make sure your mold is ready, and you can easily move the hot plastic from the oven to the mold. Wear long sleeves and long oven gloves to protect your hands from the hot plastic. Be very careful. You can easily burn yourself.
Place the plastic flat in the center of the sheetmetal in the oven, close it up, and wait about 20 minutes. You should NOT smell plastic. If you do, it is getting too hot.
When the time is up, open the oven and lift the plastic gently off the sheetmetal and lay it directly in the mold with a minimum of handling. Slide the plastic around in the mold to align with the outline of the windshield, and leave it to cool for about 30 minutes. If all goes well, you have a molded windshield!
Step 6: Mount the Windshield
Locate the holes to allow for a straight line from the bend to the clamp. Drill very carefully. The stepped drill shown will make a clean hole without risk of cracking the plastic. Otherwise drill successively larger holes to minimize chipping.
The struts run from the holes in the ends of the clamps to grommets installed in drilled holes in the windshield. It took a great deal of trial and error to determine where the holes needed to be, and how to bend the all-thread rod. The idea is to orient the bend in the rod so that it penetrates the windshield perpendicular to the surface of the plastic at that point.
The strut assembly consists of a bent piece of all-thread, with a nut on each side of the hole in the clamp, a nut after the bend, where the rod penetrates the grommet in the windshield, and an acorn nut on the outside of the windshield. I put a washer on both sides of the grommet to keep the nuts from pulling through.
All-thread rod bends fairly easily using a vice. If you thread a nut onto the rod down to the point where you want the bend, and put another nut right at the end of the rod, you can clamp both nuts in the vice. This lets you clamp the rod without ruining the threads. If you put a piece of tubing over the other end of the rod, down to the point of the bend, you can get a clean bend without curving the rest of the rod.
When everything was fitted correctly I took the struts out one at a time and slid heat-shrink tubing over them. A lit match pulls the plastic tight, and covers the threads. This looks better, and it keeps the plating from rubbing off. It also protects any cables (throttle, brake, wiring, and speedometer cables) that might touch the strut.