Push

I have been riding my bikes again since 2009, for the first year and a half no issues at all but then I noticed that after a longer ride (which would explain why it didn't appear at first perhaps) that the saddle will be slid back in the seat post at the rails.

It can range from 1/8 inch to 1/2 inch of movement backwards and when I am off of the bike the saddle seems secure meaning that its not loose, wiggly, or anything that would say that there is an issue. I can adjust the seat, tighten the bolt down and ride off without a problem and when riding I cannot feel the seat moving at all but if I do anything further than say 10-12 miles, the saddle is usually slid back a bit after the ride.

I thought that it was the single bolt seat post that is on one of my bikes but I recently bought a Trek Xcaliber and that seat post has 2 bolts to tighten the seat down and I am noticing the same thing happening.

so, anyone else experience this? is it something that us folks that deal with gravity more just have to deal with? or is there a solution out there that I am missing? am I plain old just doing something wrong? I don't see how that last one could be possible but am humble enough to entertain that its a possibility

Nightshade

Mark the correct engagement range on your seat rails then take a file to rough up that range al'round for better bite between the rail & clamp. If that doesn't work try a spot weld.

Bill Kapaun

OR maybe loosen it up thoroughly and slide in a small sliver of emery cloth/sand paper???

contango 

I had a similar issue with my bike although it turned out that what I'd done was partly about aligning the two parts that grip the rails and partly about putting enough torque on it. If the two sections aren't aligned right they will move a little, which will allow the saddle to move. If they aren't torqued up enough the saddle will move.

Once I found the position I wanted on my bike I torqued it up enough I was half expecting my allen key to break in my hands. My saddle hasn't budged at all since.

MileHighMark

Get some carbon assembly paste. It works wonders on slipping parts.

CACycling

Other than a saddle with nonstandard rails or a clamp that is nonstandard, I can't imagine needing anything but adequate torquing of the fastener.

Mr. Beanz

My guess is that somewhere along the line the OP could have overtorqued the clamp thus reshaping the seat rail tubes.

If the rails are the slightest bit undersized, or mis shaped, it may have affected the contact points.

Push

First post I stripped the bolt from constantly having to adjust the seat and getting frustrated so king kong tightening the bolt, bought a new seat post, tightened it good but not over tight at all, same issue with 2 different seats.

On the Xcal, LBS did all the tightening, it slid a tiny bit on a ride, I was a mile away, stopped in and the mechanic adjusted and tightened the seat, I am thinking that it could have something to do with my weight? and is why I posted here instead of over in mechanics.

it is bazaar to me because if I do a 10 mile ride it doesn't move, if I do 4 10 mile rides in a week it doesn't move, the instant that I do a 20 mile section of a trail that I like I get back to the truck and can see that the seat has moved..... 2 different bikes, 3 different seats.

Mr. Beanz

Strange, even with other posts? Maybe you have your saddle positioned wrong, too far forward causing you to sit on the the nose damaging the connection. That would be the only other thing I could think of in your case.

I know other big riders 280 that dont have a problem with seats connections.

Push

Mr Beanz, I position the seat within the markers that are on the rails, the clamp is inside of those marks and not really close to the limit either, the saddle on the Xcal was installed by the LBS when they built the bike and again it sits on the clamp in an acceptable place. Its driving me nuts! (well it was when I was riding more, now that its cold out the Xcal is sitting and the K2 is on the trainer) Its happened on more than one bike/saddle/post and the only common denominator is the rider...

Maybe I am flexing something when I ride? but why then only on the longer rides? see why it drives me nuts I like the idea of assembly paste and had seen that as a possible fix when searching on google.

so far looks like I am the only fellow with this problem... any other ideas are appreciated!

fietsbob

consider something with a better rail clamp..

Race Face , has a different bolt for holding the rail, and another for tilt.
but some are made ratyher light, thinwall tube..

Traditional Campag 2 bolt on the top seatposts.. perfect combination with Brooks' saddles.

Cane creek Thudbuster ST might be a good upgrade also..

LT takes a taller post extension, they color code the weight range on the elastomers
black and purple are the 200# range.
the ST are all the same color but the density is different, for the rider weight too.

a Vibrating engraver cross hatching the rail's clamp area is a texture that may help.


maybe not the seat rail, as they are often a solid steel rod,
but the saddle clamp parts of the post itself.