I have a fiberglass kit and it'll flex to a certain extent, much less than urethane of course but it. I hit shit all the time with my from bumper and it never cracked, only scraped it up a bit. Except for when I didn't see that long concrete block that they put on the end of parking spaces and i drove right into it.... then the fiberglass gave. It wasn't a total separation in the crack though, it was just that, a crack. So I took the bumper off, layed a couple of layer of fiberglass and resin on the back, sanded down the front and bondoed it and sanded. Then I took it to get painted. cost me less than $75 to repair instead of $250 for a new bumper.