Originally Posted by
SoSmellyAir
I wonder, since most (new) tires have a thicker tread than its sidewalls (for wear and puncture protection), would that not cause the tread to be slightly less flexible than the sidewalls, such that the circle is flatter or less curved at the top?
Yes, and no. Consider that the air is exerting a purely radial force on the tire, which produces tension in the wall fabric. So, while a thicker tread might be slightly resistant to flexing, it's also molded into a curve, so we're talking about tiny flex changes in the tread vs the high pressure within.
In any case, we always allow for differences between theory and reality, and only factor them if they're material. As a practical matter, the acceptance of a circular profile serves the purpose of allowing me to get a decent estimate of changes to width without needing CAD or an internet search.
It's also not purely about self reliance. I'm from the slide rule generation and still prefer to have an estimate of the answer as a reality check against the results of a "precise" calculation.