Seat post will not stay put

[GeorgeJettson]

I have a new (about 3mo) old Front suspension hardtail aluminum frame - I wont mention brands at this time.

I like the bike. I've been using it a lot. However, I have one really annoying/painful problem.
I can't get the $*($%$ seat post to stay where I put it, it slowly sinks into the bike, unless I literally use 2 hands to close the QR.

Having the QR this tight obviously makes moving the seat up or down when I want too, less then a quick process.
I have tried putting a small shim into the seat post tube - this has not helped.

The seat post tube does not have 'ears', rather the seat post is held in place by a clamp which slips over the top of the tube.
On my older bikes, which have 'ears' on the seat post tube, this has never been a problem.

Does anyone have any suggestions on how to remedy this situation??
Thanks

[sjc115]

I bought a Richtey Pro seat post and had the same problem. I switched back to my Race Face Ride XC post and the problem went away. I don't know if it was because the Ritchey post had a thinner tube wall, or it was just a tolerance stack-up problem between my particular frame and that particular seat post. I chalked it up to a tolerance issue because I am sure many, many people have run Ritchey posts without issue.

[Spangaru]

You mention that it's a new bike to you, so if you've already ruled out a mismatched frame/clamp/seatpost size, you might try cleaning the surfaces well and reapplying a friction paste (especially if you've got CF components). Not sure if it's an option, but if you're not dropping and raising the post regularly, I'd replace the QR clamp with a bolt on one. This solved a similar problem on my wife's Gary Fischer.

[Jason]

X2 on friction paste, such as Tacx.

Is this something that you built up or did the post, frame and clamp all come together from the factory?

If you built it, measure the post and the inside diameter of the frame. 0.1of a MM is enough to cause problems. Possibly the frame is reamed too large or the post is too small.

If it all came from the factory like that, try and warranty it, that is not right.

[GeorgeJettson]

Thanks for the suggestions -

note: the bike is not just new to me - it is new.

The post *is* a RItchey post. The original/stock post was not long enough, maybe I get the retailer to find me another manufacturer's 400mm post.
The post is supposed to be 30.9. I did look into a slightly larger diameter post, but apparently 31 or 31.1 is non-standard/doesn't exist.

The post is not factory, but the clamp is.

I just looked up the Tacx stuff. I 'll see if the retailer will supply some to try. The last time I was there and complained, they got a 300+lbs person to just crank the QR lever tighter....

what are "CF components"?

Thanks again.

[sjc115]

"CF" means 'Carbon Fiber'. I would try a different brand seat post and see what results you get.

[GeorgeJettson]

ahh CF --
I have one of those frames...

a Graftek )