Your question reminds me of an old joke that would get me banned, or at least punted down to the basement. But just lookin' at it suggests that the edge against the rim ought to be straight to keep the rounded nose from sliding toward the tread when the lever gets depressed. The tread side of the "penetrator" probably needs to be angled more sharply than the sidewall is gonna' be pushed in at full stroke. Maybe I should take this project over to the University of Kentucky's school of engineering to see if I can shake down a research grant. At the very least I can discuss it w/ LexWife - she was a mechanical engineer in a prior life. I'm sure that she'd be VITALLY interested in the question.