Mine only had one diaphragm and it has never touched another one when I put a passenger car behind it. Perhaps someone before you put a larger one on, or yours is not on correctly. I ran mine on O36 with things behind it all the time.
EDIT: Another thought, I think this had the long coupler on the back, so maybe someone replaced it with the short one, which would also cause this issue. Perhaps you can post a photo or two. I'll try to remember to take some of mine tonight to post.
