This winter, I decided that I needed to do whatever I could to protect my chain, downtube, and bottom bracket from all the salt and road spray.
So, I rigged up a DIY fender extender and have been riding with it for months now.
After every ride, I do tend to wipe my chain and any parts that got sprayed, but I'm blown away by how clean the bike stays simply by having that front fender extender.
The photo shows the fender extender, held in place using duct tape (didn't want to drill holes in the fender, although, some fenders are designed so fender extenders can be put on easily). It's held like this through numerous puddles and all kinds of weather, so "it works". Make it out of a plastic file folder, which I got from Walmart for a few dollars. I can probably make another 3 out of the single file folder.
The only downside I see, which can probably be tweaked by trimming the extender to mitigate it, is that it causes more drag as it acts like a mini sail that's always working against you. LOL I'm not racing with this bike, so it's a small sacrifice for keeping it clean! I'm sure someone more crafty could come up with a design that could be easily removed, although, my setup only requires you to pull the tape off. Easy enough.
It needs to be shaped like a scoop, and not a flap. A slightly more rigid material (i.e. thin plastic sheet) would be better than something softer.
Most fender extenders you can buy are rubber, but they aren't nearly as long as the one I made, and thus, not nearly as effective.