I used to break these bumpers on a regular basis. The web part of the bumper does absorbe the hits quite well but the Delrin material is too brittle that it tends to crack the base of the bumper.
I have found that if you trip the bumper down (remove the webbing part) so you are left with a smaller portion of it thats solid delrin. You will break less of this. a Foam bumper is highly suggested to take the initial impact.
Cut the webbing off the bumper it will last much longer