I have an American Classic titanium seat post (linky) with a Pivit seat mounted to it. Better image of post is second item on this page.
Its a two bolt post: one bolt clamps, the other adjusts. Yesterday, both were tight at the beginning of my ride. 35km in, the seat moved backwards as I was riding, so I stopped and checked. The clamp bolt was VERY loose, so much so the seat could easily be slid back and forth along the whole rail length without any real arm force. I tightened it, checked the adjusting bolt was tight and started again.
I found the first time I used it that if you adjusted the angle without retensioning the clamp or vice versa, it would always shake loose. But now, even with them both tight, its come loose 3 times in 5 months. I do have a Trek saddle rail mounted bag (small) on this seat as well.
Is this a possible case of the clamp bolt having exceeded its tension and stretching? (The threads LOOK fine, but there's no way to tell if its been overtensioned. I have other bolts that may fit, but havent had a chance to replace as of yet.)
Is it a peculiar symtpom of two bolt posts that they require constant checking?
If its stretched, is there a chance its done the same to the body of the clamp, so replacing a bolt would have the same result in time either way?
PS: Before the invariable 'do a search' reply, I'll just point out that ITS NOT WORKING. Three computers, logged in, with IE7, IE6, FireFox, Opera all yield the same blank window with no result.