The Improbable Bulk
Join Date: Jul 2005
Location: Wilkes-Barre, PA
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Depending on the bike it could be an easy fix.
I had this issue with my Giant Sedona, and tried a variety of fixes before I finally found the fix, and when I found it, I did a slap of the palm to my forehead...
I tried everything, including eliminating a QR collar and going to a bolt... I assumed the problem was my 350 pounds, and I may have done better at a lower weight, but the issue was corrected without having to lose a lot of weight (although the weight loss is still an objective).
For some reason, certain bikes (even from the factory) come with a seat post shim... and if the slot in the shim doesn't align with the slot in the seat tube it is virtually impossible to tighten the clamp enough to keep the seatpost from sliding into the seat tube during a ride. You can slow it down, but you can't stop it.
Once this was pointed out to me, I aligned the shim, and my problem went away.
If you have a shim... Try it, you'll like it.