Instructables
Picture of DIY Bike Fenders
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Make your own bike fenders for free out of a bucket! 

I haven't a clue what your bike looks like, but if it's anything like mine it has wheels that sit in forks with little holes in miscellaneous locations. Your challenge is to arrive at a shape that fits your bike and keeps liquid substances (icky or otherwise) off of your impeccable white tuxedo. Or am I the only one wearing that on my bike?
 
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Step 1: Design, materials, tools

Design:
Bike fenders can be made from a staggering variety of materials. Just look at this:
http://www.instructables.com/tag/type-id/?q=bike+fenders&sort=none 
In fact, you could probably do a web search for any noun plus the words "bike fender" and you'll find something. Go ahead and do that now, I'll wait.

So, what will it be? All form? All function? So beautiful that it brings a tear into the eye of the crankiest engineer and most jaded artist? So revolting that traffic around you parts like the Red Sea as drivers pull over en masse to vomit?

See doodles. For this first try I settled on a pointy-at-the-ends and narrow-in-the-middle design. I drew quite a few others though, and now that the concept is proven and I discovered how easy it is to cut bucket material, I may crank out a few weird shapes that I can swap out from time to time (think squid, ferret, snake, lightening bolt, cloud). Or maybe something plain and rectangular for formal occasions?

The most restrictive point is where the fender has to fit between the forks so you will probably need to go narrow there, but apart from that there is really no limit to what shape your fender can take! The front fender is usually shorter than the back, but even that's not set in stone. Look at tried-and-true fender designs, then figure out your own!

For mounting, I'm using only two rods instead of the customary four that you usually see in bike fenders on the market. This seems to work well after the testing I've done, and hopefully I won't regret the choice someday as the rod jumps between my spokes and I go careening over my handlebars.

Materials:
I happened to have a partial 5-gallon bucket - the upper half - left over from another project. And also a surplus of bucket handles (because we used a bunch of buckets for chicken nesting boxes, and the chickens didn't need the handles due their lack of opposable thumbs and tiny brains). So because it's hard to argue with a price tag of $0, those are the materials I settled upon.

So, here's what you'll need: 
-Plastic bucket: buy new if you must, but it's way cooler to get one for free!
http://www.instructables.com/id/Get-project-buckets-for-free!,
http://www.theprairiehomestead.com/2011/03/a-frugal-source-for-food-grade-buckets.html
For this project you'll need only one bucket, but four bucket handles (you can use other stiff wire for the support rods, but sticking with bucket parts is more cosmically harmonious).
-Zip ties: they hold the planet together, going where duct tape cannot.
-Fenderless Bike.

Tools:
-Something inky that can draw on a bucket (I used a Sharpie marker).
-Something sharp that can cut a bucket (I used a box cutter).
-A couple of pinchy things to help you bend a bucket handle (I used a pair of pliers and a pair of vice grips and a bench vice and, briefly,  a hammer and some profanity).
-Something pokey or drilly for putting mounting holes in the bucket material (I used a drill).


Cool. My fenders go with coat hangers rear from the seat post supporting dis-guarded drinking cups cut in halves, and a single coat hanger from the rear of my fork [the base of my fork has a through hole just the right size conveniently] supporting a halved Tropicana 50 bottle.

Good luck keeping your feet dry, I haven't figured the practicality of that out yet.
I really like this idea, but I have to say this.

The way you're attaching stays to the fork is inherently dangerous. If it jumps out and gets caught in the spokes of your front wheel when you're going fast it could lock up, making your mudguard into an instant, absolute front brake. Expect to faceplant if this happens.

The reason I know this? The person that taught me bike mechanics was killed by a mudguard stay getting caught in his wheel. He was riding to work, his wheel locked up and he fell in front of a lorry. A sad end for a really nice man.

This is the reason modern full mudguards all use plastic clips that make them jump free if the stay gets caught in something. A pair of SKS ones (SKS are the Mercedes of mudguard manufacturers) will cost no more than the price of a coffee from a bike shop, and might save your life.

Please be careful!
Great point, ChrisTCFS. Looks like these are the clips you're referring to, yes? http://www.rivbike.com/product-p/fe83.htm

Absolutely worth it if it keeps the bar out of the spokes... so is the idea that the SKS ones prevent this because of their width? Like, the big black piece won't slip into the gaps between the spokes at their narrowest points; where they hit the hub?

In my initial tests the weak link was the point that the fenders hit the stay, and I did have the stay swing off once. But where it hit the hub it stayed attached, and the stay hinged off and dragged on the ground, and the fender rubbed on the tire. I was going quite slow at the time so I was able to stop safely, but at higher speed I can clearly see the danger. In that case additinal zip-ties keeping the stays suck in place to the fenders make failure less likely (as long as the zip-ties hold- and the cheap ones can indeed break pretty easily!).

But should the stay come loose from the hub, it would be inclined to either drop straight to the ground which woud be relatively safe, or swing inward, which would be extremely dangerous.

My first inclination is either shore up the attachment point at the hub even further, or add a piece of material that would provide that "widening" effect and encourage deflection if the stay were to come un-stayed.

But for the price, and to leverage the knowlege of actual actual safety researchers rather then hacky guesses by me, I think I may go clip-shopping.

Thanks Chris, and you be careful too out there! The gas-burners are trying to run us over...
Thanks for such awesome bike posts! Keep up the good work.