Originally Posted by HTupolev

It's representing the case where the tire width is exactly the same as the width of the anchoring point (i.e. "the bead"), not narrower than it. Or, rather, it's representing the narrowest case where that's true. It's not really possible in the model for the tire to be "narrower" than the anchoring point, since the tire extends to its own anchoring point.

But yes, it's true that the red arc doesn't resemble real-world road tire setups. I was just using it to explain geometric reasoning, not describe a realistic use case.