I'd bought a set of the extra-long, 36mm width Honjos from Velo Orange for the Mercian, and I loved the extra coverage that the fenders had. I probably don't even need to worry about a mudflap with these fenders, the coverage is so good. We'll see.
The only problem was that the front-most portion of the front fender rattled and vibrated whenever I rode on rough surfaces like a bumpy or chip-sealed road. And riding with one hand off the bars (whether drinking water or signaling, or whatever else) made the problem waaay worse. The Mercian has a fender boss under the fork crown, and I'd tried all sort of mounting setups: rubber washers, metal washers, different bolts, etc to no avail.
Since the read fender has 3 mounting points (chainstay bridge, seatstay bridge, and fender stay), it was rock solid. The front fender only has 2 mounting points (fender stay and fork crown boss), and it obviously wasn't enough to keep the entire thing steady.
I spent a bunch of time googling around, but I didn't really find a whole lot of useful information about this problem. I wasn't ready to commit to having a Nitto-style rack on the front, just to get the extra fender-mount that those racks provide. Finally, after seeing this post on the wonderful Cycling Spokane blog, I decided I would just add an extra stay to the front fender. I also found a few images of some sweet bikes that had extra fender stays on the front, and they looked good:
(image courtesy of Vintage Bicycle Press)
Since I only had one set of eyelets on the fork, it was a little challenging, but I was able to mount both stays on the one eyelet. I put an r-clip on the outside of the fork as usual, and another r-clip on the inside, with a Nylock nut to hold it on to the long hex bolt that I used.
Here's a crappy picture of what I'm talking about:
Turned out pretty good I think:
As soon as I took the first ride after getting it all set up, I knew I'd done the right thing. Vibration gone, rattling gone. Hopefully by putting this online, it'll be found by the next person who encounters the same issue. I know not everyone that buys these extra-long fenders has a front rack with a fender mount to keep the front fender steady.