Originally Posted by Tourist in MSN

I had a cheap Nashbar rack that I occasionally clamped onto my seatpost. It had a rubber spacer, thus there was rubber between the seatpost and the clamp. It worked ok with a rack top bag, but not so good with panniers.

If it was not balanced, meaning the same amount of weight on each side of the rack, it would turn on the seatpost so that the heavier side moved towards the center. I have a small pannier I often use as a gym bag, once I put that on it and I think I went less than 100 yards when the pannier support started rubbing on the tire because it rotated on the seapost.

Actually I had two of those racks, one with the lower pannier supports like on your rack, one without the pannier supports. The one without pannier supports could only be used for a racktop bag , but that worked ok because if it rotated on the seatpost, nothing rubbed. Photo below shows the one without the pannier supports on my road bike.