Can you cut a carbon seatpost?

Discussion in 'Cycling Equipment' started by Skyward, Apr 25, 2004.

  1. Skyward

    Skyward New Member

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    My carbon frame manufacturer recommends inserting the seatpost no more than 100mm. Most carbon seatpost are too long for what I need. So, can you cut off the extra?
     
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  2. boudreaux

    boudreaux New Member

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    Usually, except for a few that have a flat area down the back of them that is designed to reduce stess from the clamp. Use a fine hacksaw blade and some masking tape to prevent fraying,and a light touch.
     
  3. mjw_byrne

    mjw_byrne New Member

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    Don't take this as gospel, but I'd have thought that if it's OK to cut a carbon steerer tube (which it is), it can't be disastrous to cut a carbon seatpost. I'd guess that if there is any risk, it's due to the possibility that cracks could grow from the cut surface - but this isn't too likely because the post isn't under the kind of stresses that would cause this. Also, it's held in compression by the frame, which will inhibit cracks growing up the post. But to minimise the possibility of cracks growing, after cutting the tube, give the cut surface a good going over with progressively finer grades of sandpaper, to make it nice and smooth.

    Out of curiosity, why exactly shouldn't you insert the seatpost more than 100mm?
     
  4. boudreaux

    boudreaux New Member

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    Well, steerers get cracked by being clamped too tight by the stem. Same for cf bars cracked by too tight stems. Record CF posts get lots of bad press,cuz they get craacked by the seatposr clamp.
     
  5. mjw_byrne

    mjw_byrne New Member

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    Yup, all true - although I was talking more about cracks propagating from the cut surface at the bottom of the post, i.e., if pressure from the clamp isn't already causing damage to the post, it should at least be hindering the growth of cracks up it. But like I said, these aren't exactly hardcore scientific data, I'm only speculating.
     
  6. Skyward

    Skyward New Member

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    This is from the Kestrel web site:

    Seatpost. Seatpost size is 27.2mm. Maximum seat post insertion is 100mm (3.9 inches). The metal seat tube insert extends only this far into the frame. Inserting the seatpost beyond this depth may put pressure on the composite walls of the frame, potentially damaging it. seatposts may easily be cut down if a lower seat height is desired. As with any frame, grease seat tube and seat post before insertion and regularly thereafter.
     
  7. mjw_byrne

    mjw_byrne New Member

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    Ahhhhh, I see. One lives and learns!

    I must say, it's very refreshing for a manufacturer to actually give some explanation as to why they impose a limit like that, instead of just saying "100mm max insertion - exceed it and you'll burn in Hell".
     
  8. boudreaux

    boudreaux New Member

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    No grease on a CF post.
     
  9. Skyward

    Skyward New Member

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    That was my next question.
     
  10. gclark8

    gclark8 New Member

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    On cutting, an Exacto Razor (balsa) Saw from a model shop may be the go.
     
  11. dennis dee

    dennis dee New Member

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    Aside from the good advice given my mjw_byrne, you may also put a thin coat of epoxy glue on the cut and sand-papered surface. Only apply the epoxy on the cross-section and inside surface of the cut.
     
  12. dorian

    dorian New Member

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    The only thing I've heard read about cutting a carbon post is not to cut all the way through from one side. This could cause some delamination of the outer layers of the post, but if you use a very fine toothed blade and smooth strokes I doubt there is much risk of this. I think there's something about this in a velonews tech article.
     
  13. mjw_byrne

    mjw_byrne New Member

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    Oooh, and another thing - obvious, but worth saying: don't just clamp the post in a vice when you're gonna cut it! Find some gentler method of holding it. Maybe make a couple of hollow wooden "jaws" and wrap the post in a towel and then clamp it between the jaws, using a vice.
     
  14. marlon1

    marlon1 New Member

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    NEVER grease a carbon seatpost!
     
  15. boudreaux

    boudreaux New Member

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    Geeze, just lay a towel on a step,put it on the towel,grab it with one hand, use a hose clamp if you think you need a guide,masking tape to prevent fraying,and have at it with a fine hacksaw in the other hand.It Ain't a moon shot folks!
     
  16. fwcyclist

    fwcyclist New Member

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    To sum things up and to throw in a couple of new ideas:

    Greasing a SP (or stem) helps prevent dissimilar metal corrosion (salt + vibration + small electrical currents = stuck posts and stems). CF material doesn't require grease, maybe a little talcum powder to make insertion/extraction easier.

    The "hose clamp" is a great idea for a saw guide. Over masking tape.

    A good choice for a saw is a Japanese "Pull" saw that woodworkers use. VERY sharp, cuts on the pull stroke, no offset (teeth angled outwards). Nice long comfortable handle for control. (and useful around the house)

    DON'T BREATH THE DUST. CF materials are very hazardous.

    The 100mm rule may only apply to the Kestrel or other carbon frames. But having EXTRA post down the seat tube, creates extra corrosion area and is just extra weight.

    Good idea to fine sand the fresh cut, a slight taper to the outside edge will make insertion easier and "Skin coating" with expoy glue over the cut/sanding. Also, a rounded surface is less prone for cracking than a sharply angled one. (short metal cracks can be stopped from running by drilling a small hole at the end of the crack)

    ANY CF tube (Seat Post, handlebars, FRAMES) will crushed. So no vises or clamps appling flat pressure to small areas. If cutting a seat post or fork steerer tube. Wrap a old inner tube around it and then secure the tube with just enough force to prevent movement and take your time.

    Dean
     
  17. crystal_tears_

    crystal_tears_ New Member

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    Always remember children ! never grease a carbon fiber seatpost .
     
  18. Chris Bryson

    Chris Bryson New Member

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    If you have a local archery shop ask them to cut it. Carbon Arrows are always cut with a very high speed circular saw so the carbon does not delaminate
     
  19. BIANCHI_EURO

    BIANCHI_EURO New Member

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    That's a good one, Chris!
     
  20. RC2

    RC2 New Member

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    So do you let the glue dry before re-inserting the seat post into the frame?
     
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