On my Giant OCR 1 I began thinking the seat was sinking. After putting a piece of tape on the post then going for a long ride it had sunk quite a bit. Anyone else have this problem? How do I correct it? I have noticed I had to keep bumping up my seat for a while but I thought it was my imagination.

It is a new bike? Or one with an extra-long seatpost? There are a few possibilities and solutions:

1. The seatpost is loose. Plausible if it's a new bike. All 27.2-mm seatposts are not created equal and all clamps are not created equal. If it's a new bike, see your LBS who might try a slightly larger seatpost.

2. The binder bolt is bent, broken, etc. and not doing its job. THat sometimes happen, especially if you like a tight fit. I just replaced the binder collar and bolt on my 2.5-year-old bike.

3. Bent seatpost or one with a "variable" diametre. If you sqeeze very hard the seatpost at the 10-cm mark, keep it there a good time, then raise it to 10.2 cm, is it possible to have a post sliding down to the place it was previously squeezed? Something might be measurable with a caliper.

4. Too much grease on the post. Yes, I know everyone suggests to grease the post to prevent it from sticking in place. I actually prefer non-greasy ones.

Regards,

The bike is a 2001. I dont think the seat post is extra long. What are all those parts you described? Can I just pick them up at a LBS?

The most likely culprit is a stripped seat binder unless you have a carbon post and have greased it. They, carbon, should be grease free.
With alloy posts, I prefer greased as I have had to remove several stuck posts by slotting them with a hack saw blade, tedious.

If you have a qr on your seatpost collar, sometimes a 1/4 spin on that will help.. not sure if that's what you meant mgagnonlv, but that works for me all the time. I also just throw a round of electrical tape on my seat post right where it should be, that way I can see really easy if it has been slipping...

Hope it works out for you!

Joe

I never had a quick-release seatpost collar, just the one with an M5 bolt.

On my 2.5-year-old bike, I have succeded to snap the bolt once and to break the collar once. It definitely showed a crack near the bolt hole.


BTW, I never had a stuck seatpost. Maybe I'm too finnicky with height adjustment, because I tend to readjust it twice a year (for Winter boots and Summer shoes respectively), and it then takes me a full month each time to fine tune the adjustment. I guess it prevents it from rusting in place.

Regards,

Does copper anti-sieze work better than a std lithium grease for this purpose. ie is it less slippery ?

Antiseize is less slippery, its purpose is to keep stuff from bonding so it kind of sticky to stay on every thing. Unless you like finger prints in your wonderbread don't eat a sandwitch after using antiseize.
Mgagnonlv, I usually have to do this when someone has purchased a used bike and needs it fitted to them.

I know that I have had to get a file going and rough up the seat post a bit according to where you have it for your height?After roughing things up with sandpaper and a file it worked good

I would try one thing. Loosen up the seat clamp. When the seatpost is in the seattube, is there any freeplay? If so, it is possible that either the seatpost is the wrong size, the seatpost is so old that it has actually grinded to a smaller size (somewhat unlikely), and/or your frame has a manufacturing defect and the inner seattube diameter is too large. It doesn't take much.

If the seatpost is snug and there is no freeplay, then I would assume that the seatclamp is not strong enough. I also like the suggestion of roughing up the seattube with a file or sandpaper.

If you do have freeplay, one suggestion is to make a shim out of an aluminum can. They are easy to cut with a pair of scissors. I suggest rounding off the corners. I have an aluminum can shim on my seattube and my seatpost no longer slips.

Also, if there is grease in your seattube, make sure that there is just enough grease to let the seattube slide in.

For some time I pestered the fame manufaturer about my slipping seatpost. It was not the post. I could hardly believe what I found. One of the rails was broken on the saddle. I never even thougt to look for that