What I'm not seeing mentioned here is Rider Input. It is very much, a factor, an almost always thing.
Its affect really came to consciousness for me with my R110RT. On that model, it seems BMW had designed the entire bike, also with a specific tire (size) in mind. Before release, a new line of "Sport Touring Class" tires was released by the tire manufacturers, and BMW chose to install them on their new bike.
The new tires were of wider section, and they came close to touching the single-sided swing arm. With both mechanical and physical offset, BMW was able to gain their desire clearance. But now, the front and rear tires were out of line. And this was cause of the infamous Pull To The Right, or PTTR.
The affect varied among bikes because of differing tolerances in bolting the subframe(s) to which the front end is mounted. On my bike, the result was severe.
As the roundness of the front tire was worn away, more and more concerted push on the left bar was required, just to ride in a straight line. But also - and this is the point to my post - more and more pressure was required to "keep the bike down in a turn left turn".
What was going on was Continuous Counter-steering: The tire was being turned oposite the turning direction, and constantly Scrubbing during the turning maneuvers. It is this scrubbing action which produces Cupping.
In talking with Riders about the phenomenon (continuous bar pressure while turning) almost all of them swore they did not do it. I was certain otherwise, so I placed clay on the bars and had them ride a series of corners, doing the counter-steering to initiate a turn by pulling on the "outside" bar rather than pushing on the "inside" bar. All came back with comressed clay after only the first turn.
The need to apply "inside" pressure to a bar to keep a bike on course while turning is called Understeer. The phenomenon, the "need", does not arise with some bike, and some tires on those bikes. It ranges all the way up to X tire on Y bike "needs" it even when the tire is new.
Certainly, as a front tire wears in the center, the "need" is going to increase. The "need" will initially be lower, and "grow" slower with a more round profile tire - Avon Azaros being such. The opposite end of the scale (for me) is the ME-Z4, having a flatter arc section.
The phenomonon will also be more pronounced with lower than higher tire pressures. This is because lower pressure tires will be flatter under straight run dynamics, and much flatter as MV2 loads are applied during cornering.
I don't pay huge attention to other causes of motorcycle tire cupping. I'm certain they can exist. But so far, it's never occurred that I could not prove a rider with a cupping front tire is not using flatter than rounder tires, and/or lower than higher pressures that promote front tire flattening, and that he/she is appling bar pressure to keep the bike down in turns.
Believe or accept what you will. I just wanted you to know about a well proven fact.
Best wishes.
