[...]I don't think of myself as an aggressive rider. Last week-end I was riding with a bunch of twenty-something sport bike riders. There were five of us and we were on some twisty roads in Wisconsin. When I was riding at a more spirited pace, the bike was handling great and I kept up and even moved up in the order to third. I started thinking "you better cool it". I fell back to sweep and immediately noticed the bike wasn't handling right. Something felt loose in back. The feeling was like making a right or left turn over crosswalk paint. The only difference was this was happening on smooth dry roads. There was no anticipating it like "here comes the paint". The rear would just shift slightly.
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I have experienced this exact problem with a set of pr2s. I put brand new pr2s on my FJR one weekend then started on a 3000 mile interstate ride the next weekend. I noticed less than stellar grip during the first week while riding my normal commute. After returning from the trip with the flat-spotted tires I frequently felt the rear tire just go away when leaning. That condition has generally improved as the flat-spot wears off. I have put an additional 2500 miles of twisty wear on these pr2s since returning from the interstate trip, so the flat-spot is not as noticeable as it was and the handling has become progressively more consistent though still not pr2-like. Still, I experience extreme drifting of the rear tire if I lean low and corner sharply at low speed--turning the corner on city streets for example. I expect to get another 500 miles out of this set. The contour should be back to the original profile about the time the threads come through!
My take is this, when the tire squares-off, leaning the bike is sort of like balancing a coin on edge. You can bump it a little and it will come back to vertical, but beyond a certain critical point the coin cannot recover.
Still, this set of pr2s just doesn't seem like my first two. The stellar grip was absent this set from the beginning but I feel guilty for complaining because I've already got 6k miles on this set and that is absolutely unheard of for me. Typical mileage for me is 3500 to 4500 per set of any good tire.