Thank you ld. Now it makes more sense
EDIT: Might vinyl deformation play some part in where typical elliptical shape could offer advantages pushing the deformed vinyl away differently? Or is the deformation even that big considering modern, rather low stylus pressures?
You're welcome, Hepokatti.
As to vinyl deformation, my opinion is that it does not happen. At least to any significant extent as to affect contact mechanics and radii. But there's a plentiful body of opinion that it does.
If many ellipticals typically actually have the same curvature as sphericals, indentation would be the same irrespective of whether it's zero or some actual number. So it's sort of academic to the main point of this thread, of course.
One of the contradictions to indentation happening, IMO, follows on from consideration of what happens in styli with long contact lengths (major radius), or reduced minor radius. Both drag and indentation would increase, versus sphericals, if typical claimed contact region profiles are true. But friction/drag does not increase.