I am installing Planet Bike Cascadia Fenders on my Trek 520 disc. I had researched and these fenders are supposed to be disc brake compatible. I think the rear will not be too difficult but I am having difficulty solving the riddle of the front and am seeking advice. I am not a terrific mechanic but figured this would be something I could do.
The struts need to avoid the housing for the front disc brake. I took this picture of one method. I had searched on ye olde web and found a handful of pictures. I saw one this way and another with the struts bent around the housing. I decided to try this last night and broke the small plastic piece where the struts, which are really one piece, meet. I think this is not critical as the shape of the meeting point is held and a washer seems to hold the struts when I affix them closer to the hub. However, I wonder about the bending and whether they will reach. I was getting minimal rubbing with the pictured method and could have played with it. I think I may be compressing the brake and have no movement with the latter, more hub oriented method.
I've mounted fenders(Honjo) but never on a disk brake bike. Saying that I would recommend permanently bending the fender stays, no matter where you mount them, so they will stay in place when not fastened down.
If the stays are just bowed and fastened the fenders and stays will always be in stress and eventually fail.
Two box end wrenches can be used at the point where the bend will be made, one to hold the stay and the other to apply the leverage to bend the stay. They need to be near enough that there is only a gap between them for the bend.
If the stays are V shaped with an eye where they attach to the fork I think that I would mount them to the fender and then bend one of the stays to touch the fender, in a relaxed state, where it will be attached, and then do the same at the other.
The picture isn't really clear, so I am going to assume that you have fender eyelets further down the fork by the ends. With the quick release tabs for the fenders I am not sure it is a good idea to mount the fenders to the mid-fork eyelets as shown - you might find the stays releasing over hard bumps.
Go to the hardware store and pick up a long(er) M5 screw (socket-head cap screw or philips head, doesn't matter, and make sure the screw is stainless, or else it will corrode in place!) and a spacer sleeve of sufficient length that will allow you to run the long screw through the fender quick release tabs, the spacer and then the eyelet fully, but not so far as to interfere with the hub. You can either try measuring the length needed or buy a few different sets of increasing lengths to get the right size (the hardware is cheap); you probably want to start with an M5 x 20mm or M5 x 25mm for the screw and use a spacer sleeve 5mm shorter than the screw length.
This is the set up I have on my Raleigh Tamland to get my fenders to fit without interfering with the brake caliper; a drop of blue Loctite at the end of the screws will keep them from backing out until you want to remove them. You really only need to get the longer hardware for the brake side stays, but if you are OCD about the fit and spacing like I am then get a second set to install the opposite side stays with.
I've had a couple bikes with mid-fork eyelets or where the lower eyelets wouldn't allow mounting around disc calipers without a lot of bending, stand-offs, and other rigamarole, so I've always mounted to the mid-fork eyelet if it was available. If there was no mid-fork eyelet and routing the stay around the caliper was too much b.s. I've just used rubber cushioned P-clamps to make a mid-fork mounting point. The quick release on the front fork fender stays will work if a stick gets jammed in there whether the stay is mounted down at the tip of the fork or up in the middle - doesn't matter. Your mounting location is fine, but you want to make sharp bends to those stays using a pair of channel lock pliers and an adjustable wrench. Using those two you should be able to get nice even bends on one side, then take the stay from the other side and use your first one as a template to bend the second one the same way.
I came home shortly after that picture was taken and decided I didn't like the way it looked and have attempted to bend around the lower eyelets. I have not been able to get the fender to avoid hitting something and knocked the rubber piece off the end of the fender. I may have botched the job but will still work with it.