The All-Weather Helmet is also completely waterproof, so it's great to have when it's raining, sleeting, or snowing.
Virtually any helmet can be modified into an all-weather helmet, the only requirement is that the plastic decorative shell can be removed.
One bike helmet in your size. Make sure the plastic part comes off.
About one square yard (or one square meter) of nylon fabric
Contact cement or some other glue with a long open time, that won't melt foam
A plastic face shield (optional)
A sharp knife
Step 1: Remove the Plastic Shell
If your helmet has a visor, pop it off and set it aside.
Start by cutting the tape around the perimeter, or just peeling it off. Lift the plastic cover at one end, and feel for the first spots of glue. Gently break the glue connection, being careful not to dent the plastic or pull pieces of foam off the actual helmet. Use a metal shim, such as a ruler, to make this easier. Do this for all the glue spots, until the plastic cover comes loose.
Step 2: Cut the Nylon Fabric to Shape
Dry-fit the nylon onto the foam of the helmet. Obviously, something is going to have to be done to fit the flat fabric over the severely curved helmet! We don't want to have to cut the fabric or else the weatherproof-iness may be compromised. So, it will need to be folded and overlapped instead.
Place the plastic cover on top of the nylon. Center the fabric so that at least an inch of material sticks out on all sides. Tug the material so that it's pulled taut, then trim around the edge with scissors. Leave at least an inch all the way around.
Now, experiment with the fit of the fabric by tugging on it in various places. Try to set it so that the fabric is pulled flat across the vent holes, with no folds or wrinkles visible.
Step 3: Glue the Nylon in Place
Run glue along all the ridges, and around the perimeter where the edge of the plastic cover stops. Lay the fabric in place on the helmet - again, make sure that there is an inch of material all the way around! Fold over any major folds in the fabric to get rid of some of the wrinkles, then put the plastic cover back on.
With one hand holding the cover in place, tug on the fabric with the other hand so that all of the vent holes are covered by a relatively smooth and tight nylon layer. You'll need to shift, fold and tuck bits of fabric up underneath the ridges of the cover to achieve this effect. It doesn't have to be perfect, and there will be some "waves" visible in the vent holes here and there. Don't fuss with it too long either, or the glue won't set properly.
When you're satisfied, hold the plastic cover in place for 15 minutes or so (if using contact cement). Once the glue is dry you should be able to lift off the cover, and the fabric will stay in place.
Step 4: Gluing Down the Plastic Cover
With scissors cut the remaining bits of nylon as close to the edge of the plastic cover as you can manage. Then, finish trimming it back right to the edge with a sharp knife. Try not to cut too deeply into the foam when you do this.
To finish the job, use electrician's tape or even reflective tape to hide the edge of the plastic cover. Don't stretch the tape or it will eventually pull away and leave a sticky residue. Press it down firmly so there are no bubbles or folds for water to enter.
Step 5: Add a Face Shield
Since I'm used the purchased face shield I don't have build pics to help you make your own, but you should be able to figure it out. You could hack a face shield designed for industrial use, cut up a piece of packaging, or maybe even use a plastic soda bottle. The face shield wraps all the way around the front of the helmet from ear to ear. It also extends downwards to protect part of the rider's cheeks. It attaches via a strip of velcro that runs along the top of the shield. The velcro allows you to remove the shield when it's not needed.
With the face shield in place, you may reattach the visor. The All-Weather Helmet is finished! You will, of course, need to protect the lower half of your face with a scarf or half-mask. I use the #2705 Neofleece Combo Scarf from Seirus, which I find to be fantastically effective on even the coldest days.
Step 6: Testing
I would really like to know how effective this helmet is for other people, especially in temperatures lower than I've experienced. If you build your own all-weather helmet, be sure to let me know how it works out for you!