The 'end-caps' are the hard plastic that bolt to the aluminum master channel.
The soft PU bumpers are screwed to through the end cap to the master channel to protect frames from the brush.
If you are tearing them, check your technique as you may be applying unnecessary force against the frames.
When replacing, be careful not to over-tighten the screws for the bumpers.
If you are replacing BRISTLES, please note that you choose this option at your own risk. There is no warranty or guarantee as it is possible for these bristles to be distorted and difficult to load into the brush. No returns, no refunds. Caveat Emptor.