Slipping seat post

on-yer-bike
on-yer-bike Posts: 2,974
edited July 2013 in Workshop
The carbon seat post on my steel frame very slowly slips down when riding. The fit when inserting the post is very tight, much tighter than on my other bikes which dont slip.
I have used carbon assembly paste;
I have changed the clamp to a Thomson with the the bolt positioned at the front as per their instructions. Thomson only specify 2.2 Nm but I have gone to 3.5 Nm and it still slips;
A LBS mechanic cracked the last seatpost by doing up the original clamp too tight.
Any ideas?
Pegoretti
Colnago
Cervelo
Campagnolo

Comments

  • sungod
    sungod Posts: 16,891
    did you thoroughly degrease the seat tube and post before putting the paste on?

    fwiw my cf post in steel frame used to slip: i tried tacx assembly paste, it slipped; i tried finish line paste, it slipped; i tried mixing the two pastes together, that worked!
    my bike - faster than god's and twice as shiny
  • turnerjohn
    turnerjohn Posts: 1,069
    3.5Nm is bugger all really....I torque mine up to 6Nm to stop it slipping. Have a F-Force and part of the issue is the gloss lacquer coating.
  • Monty Dog
    Monty Dog Posts: 20,614
    None of this assembly paste nonsense - degrease and clean thoroughly and apply hairspray to the post. It's mildly tacky.
    Make mine an Italian, with Campagnolo on the side..
  • on-yer-bike
    on-yer-bike Posts: 2,974
    The frame was treated with Framesaver when it was new but I imagine that has worn off by now around the seat clamp area. Im slightly worried about using too much torque because I can only get the seatposts that fit (29.4) from Italy and they cost a fortune for a pretty average post. Not sure why Thomsons require such low torque compared to other clamps.
    Pegoretti
    Colnago
    Cervelo
    Campagnolo
  • Monty Dog
    Monty Dog Posts: 20,614
    If the frame is steel and the seatpost alloy, then 5-6Nm should help - 2.2Nm is naff-all. If the frame is slightly oversize and because the material is stiffer, then needing a bit more torque isn't surprising.
    Make mine an Italian, with Campagnolo on the side..
  • mr_eddy
    mr_eddy Posts: 830
    One trick I have used in the past (although not on a carbon post) is to spray the post with hair spray, when its tacky after about 15 seconds put it back in and tighten to the suggested torque. I have used on Alu and titanium posts and it works a treat.

    I reckon it would be fine for a carbon post, at the end of the day its hairspray so if it was corrosive then I doubt it would be allowed to be used on someone's head.

    Another trick is Teflon tape around the post, this has also worked for me in the past.

    A more permanent solution maybe a new seat clamp, often the clamp's that come with bikes are pretty naff as this is an area where manufacturers save a few pence. Onza / Bontrager / Chris King are all good bits of kit and may resolve the issue.