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  1. #1
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    alan seatpost dilema

    hey guys, I have a used carbon frame alu luggen alan cx frame which i picked up second hand from belgium. I am having a seatpost problem. I picked up a 25mm seatpost which is what I have read was the correct size. Now the post fits in fine, but once done up tight, the two bolts at the back are touching and you can still twist the post from side to side. it does apear to be a little loose compared to other bikes/posts. So i tried a 25.2mm, which fits in the lug nice and snug butonce into the carbon tube itself its too big. i was wondering if anyine had any ideas? there does seem to be a weird grease inside the seat tube already. (should i have some sort of carbon paste?).
    cheers, austy
    Last edited by austy37; 12-08-10 at 06:04 AM.

  2. #2
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    Seat posts have to be sized according to the seat tube, not the seat lug, where there's often a bit of distortion. In a perfect world, you'd have access to a hole mike, or bore gauge like on of these that would allow measurement of the seat tube below the lug.

    The fact that the ears touch seems to indicate that the post is probably too small, but you can't force a larger post into the tube. I suspect the lug is a bit distorted, or the ears bent bent from over tightening. Fit the 25.0 post and gently tighten it. When the ears touch, does the top of the slot touch also, or only the ears? If the latter, the ears are bent and the post is the right size.

    The fix is to file the inside of the ears so they can close more, but you'll be limited in how much stress they can take, so you'll want to reduce the clamping force needed by using coarse lapping compound on the clamping area of the seat lug and post. When clamped under pressure the grit in the compound bites into both the post and lug making a mechanical engagement, not dependent on friction alone.
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  3. #3
    Charles Ramsey
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    I sanded down a post one once it was a lot of work. What you want is a post that is 25. mm in the carbon and 25.2 mm at the clamp. This won't hurt the post however don't force the post into the carbon you will crack it. Take any hard tool say a nail mark groves in the part you want sand down then sand until the grooves are gone. This will ensure you don't take too much off one side. you will need to do this perhaps 100 times your frame is worth the trouble.

  4. #4
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    Quote Originally Posted by FBinNY View Post
    Fit the 25.0 post and gently tighten it. When the ears touch, does the top of the slot touch also, or only the ears? If the latter, the ears are bent and the post is the right size.
    If the ears touch but the slot still has some clearance there is another possible fix and that's to file off the ears completely and fit a seat tube clamp. That "cure" requires two things:
    1. The outside diameter of the seattube lug has to match a standard seat tube collar size
    2. The seattube lug has to project above the toptube enough to allow the collar to fit.

  5. #5
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    Quote Originally Posted by HillRider View Post
    If the ears touch but the slot still has some clearance there is another possible fix .....

    .....2. The seattube lug has to project above the toptube enough to allow the collar to fit.
    I would have suggested that but it's not an option based on this condition, look at the photo.
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  6. #6
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    Quote Originally Posted by FBinNY View Post
    I would have suggested that but it's not an option based on this condition, look at the photo.
    I did look at the photo but really couldn't tell how far up the lug projected. You may be quite correct that there isn't enough room for the collar.

  7. #7
    coprolite fietsbob's Avatar
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    I use to own an AlAn, 25.0 is right, previous owner may have overtightened the ears
    Its a cross frame, so flying remounts may have pushed the post down,
    causing the owner to crank down on the bolt, compressing that housing hanger. It's in the slot that is where the seat post clamp squeezes.

    I have a run the cable thru the seatpost method I used , so as to not need that cable hanger inserted between the frame seat post clamp.

    It's a Campag Super Record single bolt , will sell ..
    hand machined roller inside the seat tube,
    to make nearly no friction..

    If you get a collar machined to go just around the seatpost It would sit on top of the frame.
    And stop pushing the seatpost in , if you are going barrier leaping.
    Last edited by fietsbob; 12-08-10 at 01:14 PM.

  8. #8
    Senior Member Chombi's Avatar
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    I'm wondering if the seat clamp ears and lugs might have stretched from overtigntening as previously mentioned. Once the Al material is stretched, there's no bending back that will cure the problem as might be done on steel frames, but you might be able to file off some material between the ears and the slot the get back the tightness needed to hold the seatpost properly, just watch out for any cracks that might be developing at the clamp/lug area as the Al might have been stretched and stressed to its limit already. If you see any, drop everything and check out the possibilty of using and external, add-on calmp instead, as already recommended.
    If the lug and clamp area seems to be sound, you might also be able to use some sort of split steel or Al shimming tube too, instead of filing material off.

    Chombi

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    coprolite fietsbob's Avatar
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    Check with AlAn in Italy, they may be able to restore it, to new specs,
    after all being screwed and glued , they do come apart , to replace things like cracked lugs and such .

    At least You can use surface freight, air freight across the Atlantic is a deterrent.

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    in the 3rd photo I wonder if putting solder on the lugs would work, to build it up a bit?? the last pic is the nut done up as you can see the edges are allmost touching. the 1st pic also shows the post looking pretty loose. thanks for all the replys.
    p.s what is the part you would like me to file??

  12. #12
    coprolite fietsbob's Avatar
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    You Could widen the slot with 2 hacksaw blades stacked. till you can get a thin file in between
    to polish the roughness created by the sawing
    I would be careful though ..
    by mentioning solder I suspect you are in over your head.
    lacking experience with working with light metals and carbon Composites.

  13. #13
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    Nothing seriously wrong, clean it up, improve the clearance between the ears to get a bit of extra pinch, and use lapping compound or carbon assembly past to improve the bite, so you don't have to over tighten it.
    FB
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    An ounce of diagnosis is worth a pound of cure.

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    WARNING, I'm from New York. Thin skinned people should maintain safe distance.

  14. #14
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    Quote Originally Posted by fietsbob View Post
    by mentioning solder I suspect you are in over your head.
    lacking experience with working with light metals and carbon Composites.
    yeah mate, i am on an online bike forum

  15. #15
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    sorry, what are the "ears", are they the round bits that the bolts go through? do you mean if that slit between the bolts is wider, then it will grab the post better?

  16. #16
    bike whisperer Kimmo's Avatar
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    We're guessing that the reason the clamp won't hold the post is that those ears the clamp bolt goes through have bent in and meet before the clamp is tight. Enlarging the gap so this doesn't happen should fix it, except none of your photos show the ears actually touching.

    But see how the sides of the slot aren't parallel? The holes through the ears are on the same angle...

    If the ears don't touch, the clamp might have been prevented from clamping further by the angle of the holes changing so much that the clamp bolt prevents the ears coming any closer, because the angle increases as the clamp tightens. You should be able to check this by trying a thinner bolt that won't interfere with the holes, with some washers to transfer the force to the ears.

    You might need to run a rat-tail file through the ears to allow room for the bolt in the clamped position... and then you may find that the ears now touch and you also need to enlarge the slot, filing the inside of the ears to make the slot parallel again.
    Last edited by Kimmo; 12-08-10 at 09:55 PM.

  17. #17
    Senior Member Chombi's Avatar
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    BTW, looks like the PO had already been sticking some tool between the seatclamp ears to spread them out, judging from the scars on the edges, maybe to get the clamp and lug slot edges parallel again and gain back some clamping force. So it looks like the frame has had this loose seatpost problem for a while already. Again, be careful when you work on this problem so you don't end up causing an overtight condition that could suddenly cause cracks at the clamping area of the lug. Aluminum is not that forgiving a material. The crud that's built up between the slot and clamp does not help either, I'd start with cleaning that off first so you have a good baseline to work with, just in case it's getting in the way of the clamp from functioning correctly to grip the post.
    Vitus frames with similar aluminum seat lug seat post clamps sometimes suffer from the same problems with stretching and cracking of the aluminum, specially the CF/Al frames they made for Peugeot during the mid to late 80's. That's why I think Vitus went to a jamb screw type of seat post anchor, instead of a clamp on their later Al and CF frames. They still had problems with the jamb screw sometimes stripping the threads on the seat lug on some owners after a while, but I think the damage is easier to fix if it happens.

    Chombi

  18. #18
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    Maybe a Dremel with a cutoff wheel to it ? The reinforced ones last longer,and less likely to fly apart. Its an otherwise nice looking frame.Chris

  19. #19
    coprolite fietsbob's Avatar
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    a stiff piece of material with a section of Carborundum cloth, wrapped over it,
    [sand paper for metal, on a cloth backing]

    Can be used to remove metal , with a back and forth motion
    by hand.

    a file only cuts in one direction, like a saw.

    abrasives are able to cut back and forth.

    Like sandpapers, by increasing the grit number the grit is finer
    and so makes surface smoother.

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