I broke my air dam going over a dip, near my office, but not a route I usually take. I remembered the dip halfway down the road, but decided to proceed. Got to work and decided to take a look, surprise, it's broken! (Surprising because it survived a lot of scraping before breaking.)
You can see the fractures focused on the brace location. At the time I put the braces on I was mindful of how rigid they were, but having more flexibility might not save this dam from fracture on a hard strike anyways, since this dam is angled forwards and as it bends it's just going to go under and get itself in a tight spot.
I walked up the road later and picked up the missing piece, covered in mud and leaves in the gutter.
To start my repairs, I marked, then masked off the blinkers and painted the dam black. I painted the rear surface, since this is clear plastic I can do that and achieve chip proof paint. It seemed like a good time to try brush-on enamel. I've been thinking brush-on should have less VOC smog-contribution than spray, might even be a big difference, but not sure.
Then of course I pop-riveted some aluminum sheets over the fractures. Also I put some foam weather strip along the top. The real purpose is to prevent paint transfer to the bumper.
EDIT: then I enclosed the ducting. It could be better, but I think I'm done, give or take some black paint. The sides are attached to the bottom via L brackets and #12 sheet metal screws. I'm glad I chose to attach the ducting to the braces rather than to the dam. Assembly is easier this way. BTW, the aluminum strip braces bolt onto the car at the engine pan locations (sharing the bolts with the engine pan).
Then I used a white paint pen on a staff and marked a uniform height, and cut by jigsaw. Previously the hard plastic ended at 4.75-5" clearance at the front-corners, and about 5.5" in the middle of the bumper. Now the hard plastic has about 6.75" clearance all around.
Then I added lawn edging using sheet metal screws, final clearance at about 4.75". Coroplast would have been a strong choice too (better wind-stiffness, spring-back, cheaper per area, but requires painting and might be stiff enough to push the whole dam around during a ground strike).
I'm going to do a highway camera test to see how well the lawn edging holds up against the wind. I don't remember anyone else reporting on that subject.
At this point I should note that thin aluminum sheet probably would have been a better choice over the PETG for this project, even considering a little added effort to add clear lenses for the blinkers. Using aircraftspruce.com, Al is a little cheaper than PETG, it is actually more paintable (although doesn't have the magic chip resistance of backside paint), I believe Al is probably easier to bend (heat gun was a pain). I did stretch the PETG a little in this project for more forward angle, not actually sure how much, but I doubt it mattered much aerodynamically and it was a pain to do. Anyways the same thing could be achieved by starting with a wider cut and doing a steeper angle on the sides, or since I cut the bottom back anyways the same thing could be achieved without any tradeoff if the correct height were used initially. Al also has a better failure mode, unless you can't stand the look of beat-up un-bent sheet metal. Either material choice requires adequate clearance and a softer bend-away chin, like Coroplast or lawn
edging.
EDIT: I thought about hiding the rivet and screw heads, and fracture lines, but I think I might just leave them. Maybe I should add some more decorative rivets on the other side. Bullet-hole-decals also crossed my mind for the side of the car. That could be taken wrong though, I suppose.