What makes a carbon seatpost stick?

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  • What makes a carbon seatpost stick?
  • Premier Icon nedrapier
    Subscriber

    What's the frame made from?

    There's apparently galvanic corrosion beteeen ally and carbon, due to difference in the ionic(?) charge between the 2 materials.

    Some people say that's bobbins, because all of the carbon's encased in plastic resin which doesn't conduct.

    All I know is, the one on my mate's bike is stuck, and plenty of others have been too. Think I might treat him to a Thomson after I've destroyed the one that's in there…

    YETIboyJAY
    Member

    Its actually down to what grease you use. Carbon is porous so swells with certain substances. Petrolium derived products are one of them. It swells the post so its stuck solid in your frame.

    You should only use Silicon based greases on carbon to avoid this.

    Hope that helps!

    billybob
    Member

    no grease ever on carbon posts I've always been told…

    druidh
    Member

    Carbon assembly paste

    don simon
    Member

    Just read the thread below with the stuck carbon post and decided to have a look at mine. A good couple of years on the bike with use, another 18 months with no use (broken frame) and out it popped, no problem. I have more problems adjusting the height with a Ti post.

    What causes carbon seat posts to weld themselves to frames?

    trail_rat
    Member

    Carbon assembly paste….silicon grease with beads in it …. Lowers torque needed and stops swelling !

    Love Tubs
    Member

    Ionic interaction LMAO, I'll put my money on a 'size mis-match – delaminating' combo. Yes, some metals will react with each other but, more often than not, they have undergone 'some' 'type' of force to bring them into close proximity to initiate corrosive cascade. I would also expect the oxidation from the frame (dust like stuff, usually white on alloy) to clog up the carbon, even becoming 'glued' into due to the frictional (heat) of the seat post moving in the tube.

    For carbon to bond (ironically) with a metal it would have to loose some of its electrons which would require more energy than someone's fat arse sitting on it for 6hrs, or a hairdryer FFS!

    How amusing the lay person can be 🙂

    Stands well back for onslaught 🙂

    jd-boy
    Member

    They do that just before they snap and go up your jacksee.LOL

    don simon
    Member

    What's the frame made from?

    7005 Aluminium, and I do't think it was greased, if it was, then I would have used std grease.

    I'll put my money on a 'size mis-match – delaminating' combo.

    My immediate thought was a size mismatch, but only through manufacturing tolerances rather than wrong size.

    Would climate have an effect?

    There doesn't appear to be any white powder deposits either.

    Macavity
    Member

    Not all plastics are the same, even if they are reinforced with carbon fibres. CFRP.

    neninja
    Member

    A mates carbon post stuck in his Ridley road bike the other day – it took 3 of us about 40 minutes to maul it out.

    Bizarrely it wouldn't go back in that afternoon but the following day it went in easily and has behaved since so it must have absorbed something that made it swell and being left out in the air evaporated it off.

    Premier Icon nedrapier
    Subscriber

    So the penetrating oil that I've liberally applied could actually be swelling the post even more?

    Macavity
    Member

    Could BP throw a lot of Broken Plastic or Bloated Posts into the Gulf of Mexico to soak up the oil?

    z1ppy
    Member

    nedrapier, I used GT85 & got mine out, though it took a week, but it was at the right height to ride at so undid the seatclamp & rode it. Eventually came out by twisting the saddle.

    trail_rat
    Member

    Maccavity that sorta technology exists…..we were trying to figure out if they explored swellable packer tech….stick it in – swells ….job done….although you might end up with a rocket once the pressure built up !

    Premier Icon nedrapier
    Subscriber

    z1ppy, so you rode it for a week and the movement from bumps and pedaling worked the oil round and loosened it over the course of the week?

    Or did you just leave it for a week and then have a proper go at it?

    Ta

    Premier Icon P20
    Subscriber

    I don't know about modern carbon posts, but the old ones weren't perfectly round and had lots of flat edges on them. This would allow dirt into the gap and cause it to stick.

    sq225917
    Member

    If you doubt it's galvanic corrosion I suggest you take up windsurfing and see just how long your carbon bits last with ally plugs in them in salt water.

    Don't use Lithium grease any scratch in the outer coat on the post and it's toast, aluminium oxide all over the place from the battery between carbon and aluminium. It sticks because aluminium oxide takes up more space than the alloy in the seat tube wall.

    Don't use petroleum based greases as they penetrate the epoxy resin and cause it to swell.

    Sodium hydroxide down the bottom bracket will help dissolve the aluminium oxide and remove a jammed carbon post from an ally frame, if you used Petroleum based grease, yer f-ed.

    Premier Icon nedrapier
    Subscriber

    Right, I got the bleeder out. 3 kettles of boiling water, left foot in the downtube/seattube junction, right hand on the back of the saddle twisting, left hand on the nose, pulling. I won about a millimeter of seatpost per quarter turn, then I had to return it to center for another twist n pull. slosh of boiling water every 3 or 4 goes. Did that for about 3 1/2 inches before it freed itself, when the clouds parted and choirs of angels sang beautiful harmonies.

    Runs out it's an ally post with a carbon skin. The carbon skin is less than a quarter of a mm.

    My Thomson wouldn't go in the frame without some sanding away of hard white stuff. Now the Thomson and the carbon skin post fit with exactly the same snugness.

    So in this case, I don't buy the swelling idea as the main cause. Galvanic or not, there was definitely some corrosion down there.

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