Seat Post Mounted Racks
 

I'm considering buying a seat post mounted rack for my XC.
The manufacturers rate them with a capacity of 20/25lbs.
I tend to think this is a bit overrated.
Am I correct?
Would this mounting clamp permit the rack to swing side to side as there are no brackets whatsoever?
Any comments?

Does your seat post rotate from side to side or does the seat-post clamp hold it tight?

These racks use a similar type of binder so if you fasten them properly they are fairly stable and don't move around much. However, if you bump the back end of the rack it will rotate due to the leverage.

My seat post is a suspension type (Post Moderne).
It does rotate slightly from side to side which seems to be the case with this seat post.
I was concerned also with the weight rating of 20lbs. Seems like a lot for this kind of rack.

Check out some of the Carradice saddlebags which can clamp onto the saddle or post. These hold the load much closer to the post than seatpost racks.
Seatpost racks are very heavy for their specified load. Saddlebags are a much more efficient solution.

I tried a seat-post mounted rack once - and after a 30km ride home with a broken seat-post (and consequently no seat), I vowed never to do it again. If you want to carry 20/25lbs, I'd suggest getting a frame mounted rack instead.

My seat post mounted rack doesn't move around at all because I have it clamped tight. The back section will bounce if you hit a bump but no sideways movement.
Once because of the bounce a lens fell of one of my blinkies.
http://home.mindspring.com/~rhorne/Raleigh-rack.jpg

I'm not sure if I've ever carried 25 pounds on it. My panniers weight about 4 pounds (per manufacturer) and I usually carry work clothes (trousers, shirt, socks, underwear, belt), rain gear (just in case), toiletries and towel, shorts and jersey for the ride home, lunch, and a magazine to read at lunch. Is that 25 pounds?

I have carried around 20 pds in mine before. Took my usual load of gear + my laptop one day. My problem was not my seatpost but spokes. I hit a big bump and after that started losing a spoke every 50 miles. I have had no problems with my seatpost mounted rack.

Ok, I have a question about the seat post mounted rack too - usually when I see them they have a trunk pannier on them. I would think that the side mounted panniers would have a tendency to swing into the spokes. Is this true, or do side mounted panniers work just fine?

The mechanic at your bike shop ought to be able to fix your seatpost so it does not rotate. My Post Moderne (on a Trek hybrid, same as yours, I bet), used to rotate a little, but it does not now.

I have never had a problem with a pannier hitting the spokes and I normally use just one so I would think that would tend to twist more. The racks are pretty secure. I have used it with both bags and not had a problem there either.

The idea of using a Seat post rack on a suspension seat post doesn't sit right with me. I'd be concerned about side loading on the post causing problems.

I have a TopPeak that I use for quick trips to the store etc. It is great for small loads. Easy on easy off. As for the suspension seat post, I've had no problem using it with a ThudBuster.