A mirror helps a cyclist avoid automobiles and ride more safely. You can make a mirror for your helmet. You will need a mechanic's inspection mirror ($2 at Harbor Freight Tools), some heavy wire (a coat hanger will work very well), an aluminum soft drink can, some screws, and some aluminum bar stock (not shown).
Step 1: Cut and Bend the Wire
The wire will be attached to the helmet's visor when finished. Cut 6 inches of wire. Bend it in the pattern you see. Bend at 2.5 inches from one end and 1.25 inches from the other end.
Step 2: Make Tabs for Attaching the Mirror Frame
Cut two pieces of aluminum from a soft drink can. Make them 0.75 inch by 1.5 inch. Fold them over the wire and crease around the wire. Fold them so the printed side is on the inside of the fold.
Step 3: Drill Mounting Holes
Drill holes in the aluminum tabs and in the visor.
Step 4: Attach the Frame to the Visor
Use small screws to mount the tabs and wire frame to the visor. Use a flat washer and a toothed washer behind the nut. The flat washer adds rigidity to the aluminum tabs. The toothed washer keeps the nuts tight. Use a scissors to trim sharp corners from the aluminum tabs. The screws may also be shortened with a grinder or any other suitable tool. The "L" shaped bend in this part of the wire frame keeps the mirror from rotating up or down. This general design also eliminates any need to weld or braze anything.
Step 5: The Mirror to Frame Connection
Cut two squares from a piece of 1/8 by 3/4 inch aluminum bar stock. Dress corners and edges with a file.
Step 6: Make a Groove for the Wire Frame
This part is a little tricky. Make a groove in each aluminum square. The aluminum squares will grasp the wire frame with these grooves. Cutting them by hand with a file is one way to do the job. Clamping the squares together and drilling between them is another way to do the job. It is faster, but the bit tends to wander and go into one square more than the other. If the bit does wander, some correction can be made afterward with a file. The width and depth of the grooves should be a little less than the diameter of the wire so the squares grasp the wire firmly.
Step 7: What You Are After
Pictured is how the squares should fit onto the wire frame.
Step 8: The Aluminum Squares With Drill Holes
The aluminum squares need two sets of holes. One set forms a socket for the swivel ball on the mirror. (The rest of the mirror as it came from the store is not used.) Drill a hole a bit less in diameter than the swivel ball. Dress and fit it with a countersink bit. Drill another hole for the clamping screw.
Step 9: Finished Assembly and Use
Sandwich the aluminum squares around the wire frame and the mirror's swivel ball. Clamp with the screw. Use a toothed washer to keep the nut from working loose. Tighten the screw and nut enough that the mirror stays where you want it. Leave it also loose enough that you can adjust it while riding your bike. I find the mirror works well for me if I can just see a bit of my ear in it. That gives me a reference point for what I see in the mirror. It is not a problem if you adjust the mirror so that you need to incline your head just slightly to see the traffic lane fully. If necessary, you can tweak the bends in the wire frame to get the mirror roughly where you need it and then do the fine adjustments with the swivel features of the mirror and the aluminum squares.
I have been using one of these for quite a while and really like it.