Originally Posted by Dan
Thats good advice, Cti.
But the main problem with fenders are not that the fasteners loosen, but that the brackets themselves fail and let go. Often with the nuts, bolts, rivets still secured to them.
Very often it is just an L bracket that is just bent. That causes a weak spot at the outside of the bend. So with the vibrations, it wiggles it self to death.
Well there's something very wrong with the bending job on those "L" brackets. At least with aftermarket style, plastic or plastic-aluminum sandwiched, bike shop quality fenders, like the Esge's, Mt Zephal's and Bike Planet's, the L brackets are made of a thick bit of stainless steel with proper radiused bends so as to not cause a stress riser in the sharpest part of the "L" bend.
They also often times have more than one pair of stays, this tends to keep from allowing bad cyclic vibration from certain ranges of the RPM band. This is something I've fought with my present friction drive bike. Sometimes adding a bit of rubber isolation at the fork or chainstay crosspiece damps this out, or, makes it worse.
They can still have issues with the of the stay bits and and "L" bracket riveting to the fender and sometimes the plastic can crack, but they tend to stay attached at the stays and fork attachment points even with cracked plastic.
I'd be more inclined to replace these suspect "L" brackets with custom made 12 gauge or thicker stainless steel L brackets cut from old rack mounting hardware, that you've taken the time to bend with at least an 1/8" or larger radiused bend. These brackets can be really tough to bend sometimes and will require a good vise and cutting tools to shape.