My seatpost keeps slipping. Bike is a Marin Pine Mountain --- steel hardtail frame with frame-integrated seatpost binder. Specs call for 27.2mm post. Seatpost is a 27.2mm Thomson Elite setback (though the stock Marin saddle slips as well).
The problem has been, my seatpost slips an inch or two as I ride. It's not loose enough to slide down if I push it with my hands, but my ass pounding against it on the trail causes it to slowly drop, bit by bit, for a couple inches before it stops and holds steady.
I initially switched to a Salsa QR binder bolt, thinking maybe the generic QR binder bolt that came with the bike wasn't pulling its weight. The Salsa QR binder bolt wouldn't clamp tight enough either.
So the other day I bought a regular binder bolt, hoping it would allow me to tighten it enough to hold the post. At moderate tightness, it wouldn't hold. I tightened it further, but the bolt itself began to bend. It did seem to hold for a little while, right up until the stress on the binder bolt caused it to break in half. Yes, the binder bolt snapped in half because I had to tighten it too much to get the seatpost to stay in place.
I'm not sure what to do here. The post fits so snug when I'm inserting it that I can't get a beer can shim in place. I've wiped most of the grease away. The Thomson post is ribbed, so abrading the surface isn't necessary.
My only thought is to possibly try a 27.4mm post. Unfortunately, since that size is harder to come across and I'd want the option of returning it if it doesn't help, I'd probably be forced to pay MSRP or something crazy like that. Would I even be able to fit a 27.4 into the seat tube?
Any other thoughts? I'm at my wit's end, and I'm competing in my first race this weekend. Help.