I took my bike into the Honda dealer for new tires. I know I needed a front tire because it was down to the wear bars. The back was fairly close, maybe 1/16" to 1/32" away, but the dealer had me stand back and see the "shark fins" or "cupping" and said that indicated I definitely needed a new rear tire. I had him go ahead and do the rear as well, I'm just not completely convinced that "cupping" is the end of the world. Isn't "cupping" just another word for "normal tire wear"?