This details a simple way to add a virtually free fender to your bike to keep mud/rain/blood off of your duds. This design uses recycled materials and the only cost is your time. The presented design uses a rear rack as the support, but you could easily use some other found items such as wire from a tomato cage or similar to provide structure for the fender. Face it though, your bike needs a rack so just get one, or view my instructable (coming some day) on making your own bike rack out of used automotive brake and fuel line and get to it.
Step 1: Materials
The materials for this are easy to find. You need a plastic gallon jug. I used a windshield washer fluid jug since it had vertical sides and was pretty thick. The circular indents and thin plastic on a normal milk jug make it less desirable, so keep an eye out for a thicker jug like those used for vinegar, windshield fluid, etc. Pretty much anything will work. You will also need a pair of strong scissors or a utility knife to cut the plastic.
Additionally you will need electrical tape or some zip ties or other method to secure the fender to the rear bike rack.
The figure below shows the kind of jug I used, along with a pattern that I used for the fender.
Step 2: Cutting the Fender
Now that you have a jug, some scissors, and some tape let's get to it. First, take the scissors and cut around the jug to get the vertical portion of the plastic. The idea is to cut the top and bottom off, and then cut this hoop so that you have a nice long flat rectangle of plastic to work with.
Next, draw your fender pattern on the plastic. The keys to sizing the fender is to make sure it is wide enough to cover your whole tire and long enough to keep spray from whipping off the rear of the tire up onto your back. My fender is 4 inches wide and about 24 inches long. This is pretty long as you can see in the pics. You can trim the fender afterwards if desired once the spray pattern of your specific tires is characterized. Spray comes off of your tires tangentially, so if you look at the layout of your bike you can see how long your fender needs to be to keep spray off of you in the saddle.
My fender (below) includes some notches on the sides for clearance of the side bars on my bike rack. My pattern also includes a pair of slots in the middle and a slot at the front to allow the fender to be taped and zip tied to the rack. Your pattern may differ and could include provision for mounting to a different type of rear rack, a support structure of your own design, or might include fancy scallops and designs for no good reason other than aesthetics.
Use your preferred cutting implement to cut the fender out of the plastic.
Step 3: Attach to Your Bike
The final step is to attach the plastic fender to your bike. I used some electrical tape and a zip tie to attach mine. You can do the same or use whatever is handy. See the picture below for a better idea of how this is done.
There you have it! For a front fender you can use the same type of plastic taped to the downtube on your frame which will shield you when you are going straight (most of the time) or get creative. I have found that in most cases a front fender is not needed unless it is actively raining, at which point you are wet anyway. Going through puddles and other wetness can be handled by a rear fender only for the most part. As long as you aren't wearing a three piece suit or equivalent evening attire. Different tires pick up and spray water differently so as always, try it out and tweak it. Otherwise, you could try and build a front fender out of old tires, which is awesome.
With rainy spring on the way and melty snow in the road, your fenders will keep you from being a bike-skunk so get building.