Trimming a carbon fiber seat post

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2009-09-02 6:18 AM

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Subject: Trimming a carbon fiber seat post
A frined of mine just got a "new" (for him) bike from Craigslist.  It has an aero seat post that is about an inch or so too high for him.  He is too cheap to replace it with another one, and it would offend the bike's aesthetics to get an aluminum post (IMHO).  Any tips for trimming the post? It has a "cap" where the seat clamps are glued, so my thought is we trim off the top inch or so (don't worry, I'll size him up first with a "temporary" seat post to get an exact measure), then pry off the clamp, and re-glue it on the new top with some gorilla glue.  Unless I am missing something vital?

2009-09-02 7:15 AM
in reply to: #2384296

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Subject: RE: Trimming a carbon fiber seat post
Does the post not lower anymore into the frame?  If you want to cut it why would you not cut the bottom of the post?  There is absolutely no reason to remove the top clamp off of the carbon shaft.  That is just asking for trouble.  If you need to cut it, cut the bottom.  Wrap some masking tape around where you want to cut and make sure to use a blade that is very sharp is can cut compsite material.  The tape will help stop any fraying of the carbon.  Lightly sandpaper the cut area afterwards.  And let me repeat....no way should you remove the clamp and reglue it on.
2009-09-02 8:26 AM
in reply to: #2384346

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Subject: RE: Trimming a carbon fiber seat post
The problem is the post is an aero-design, so the bottom is round, but the top half is eliptical.  And the post is already all the way into the frame, so trimming the bottom is not going to add anything. My friend may just have to bite the bullet and get a shorter post, since reglueing the clamp sounds like a potential disaster. Although I wonder if I used epoxy that I used to fiberglass my boat if it would work? 
2009-09-02 8:57 AM
in reply to: #2384490

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Subject: RE: Trimming a carbon fiber seat post

I agree that attempting to re-glue the seat clamps is a bad move.  I personally wouldn't risk it. 

I'd see if you could either buy a shorter aero post or just use a round one.  A round post probably isn't going to have any noticable effect on the aero properties of the bike, not that you could measure outside of a wind tunnel.  It may not look as cool, but last time I checked they didn't award style points at races.

Mark

2009-09-02 10:55 AM
in reply to: #2384296

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Subject: RE: Trimming a carbon fiber seat post
hey, it's not you that's going to get killed (well maybe not killed, but I bet your friend will wish it was death rather than being rectally impaled with a carbon shard) when (not if) it fails.

what model bike is this? I'm having a hard time picturing a situation where cutting the bottom of the post wouldn't allow you to lower it. Maybe the lesson for your friend is a bike that doesn't fit isn't as good a deal as it seems.
2009-09-02 11:22 AM
in reply to: #2384920

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Subject: RE: Trimming a carbon fiber seat post

fausto - 2009-09-02 11:55 AM hey, it's not you that's going to get killed (well maybe not killed, but I bet your friend will wish it was death rather than being rectally impaled with a carbon shard) when (not if) it fails.

what model bike is this? I'm having a hard time picturing a situation where cutting the bottom of the post wouldn't allow you to lower it. Maybe the lesson for your friend is a bike that doesn't fit isn't as good a deal as it seems.


Offhand, I don't recall the model, but the seatpost looks like this:

 

So the rounded bottom fits into the frame, but you can only trim off the top to reduce to overall height.  The LBS which is only half a mile from his house, has a shorter version that would work, but he is the ultimate in cheap b*stards, so I think he will end up going with a cheap aluminum post instead.



Edited by gearboy 2009-09-02 11:23 AM

2009-09-02 1:12 PM
in reply to: #2384987

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Subject: RE: Trimming a carbon fiber seat post
That's just something you don't "F" around with IMO.  Spend the 100 bucks for a new Thompson Elite seatpost or similar...  Or you can even go cheaper and get a carbon fiber post from Nashbar for about 50 bucks...  Do go cutting it.

I would even say don't cut off the bottom end.  Why?  Because there is a certain minimum insertion line inscribed on the seat post.  If you cut the end down, that line will be too close to the end and the minimum insertion length won't be met if you insert to that line.  Maybe not a problem if there is only one rider and they remember that fact.  But sell it or give the seat post away, and they insert to the minimum line?  They could get hurt.
2009-09-02 1:15 PM
in reply to: #2384296

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Subject: RE: Trimming a carbon fiber seat post
yeah, I realized that right after I posted. Giant's first "compact frame" offerings used something similar.


I'd never buy a carbon post (unless it was a frame with a shaped seat tube that couldn't take a round post). I don't think of myself as particularly "cheap," just there just aren't any carbon posts as good as the old standby Thomson.
2009-09-02 1:33 PM
in reply to: #2384296

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Subject: RE: Trimming a carbon fiber seat post

There seems too be alot of fear mongering here.  Cutting cabon fiber isn't brain surgery.  A fine tooth hacksaw and masking tape is all it takes.  THis is no different than how frames with integrated seat masts are resized.

First I'd contact the company that makes the seatpost to see if they sell the seat cap separately.  If you need to shorten the post by more than the distance that the cap is inserted into the post then your good to go.  Cut it, clean out and glue residue drips from the old cap glue the new one on.  You can probably clean up the old cap, depending upon what it's made out of (probably Al) but if you can get a new one I doubt it'll be worth the effort.

But I would not use gorillia glue.  I'd find an epoxy designed specifically for CF to AL bonding.  Not your typical hardware store epoxy.

2009-09-02 11:40 PM
in reply to: #2384296

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Subject: RE: Trimming a carbon fiber seat post
If you do decide to shorten the post only use loctite hysol part #9460 epoxy glue. Do not use anything or you risk almost certain castration. Calfee uses this stuff to bond metal or alloy bits to his frames. I have had experience with this stuff and it works great. I'm a hobby frame builder and needed to shorten a Kinesis carbon aero fork and it works great. When you cut carbon I use a small dremal tool with a small cut off wheel. Make sure you rough up the bonding surfaces with sand paper.
2009-09-03 12:42 AM
in reply to: #2384296

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Subject: RE: Trimming a carbon fiber seat post
Do I understand correctly that you want to cut off the top of the seat post since the bottom is already all the way down in the frame?

Cutting carbon fiber is not complicated. Just be careful, take your time, measure twice (or in my case at least 4 times...), use a fine tooth brand new blade and go ahead cut.

What you want to do is definately something I would NOT recommend. You're basically destroying the bond between the post and the clamping head, then you're going to epoxy it back. Although not impossible, you're taking a major risk here... failure and you will hit the pavement. Just to save $50-100... worth it?! Hardly.

2009-09-03 6:32 AM
in reply to: #2384296

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Subject: RE: Trimming a carbon fiber seat post
Thanks for all the advice.  I passed it on to my friend, who cheap as he is, decided to NOT risk catastrophic failure and will just get an aluminum post. 
2009-09-05 12:09 AM
in reply to: #2386553

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Subject: RE: Trimming a carbon fiber seat post
gearboy - 2009-09-03 6:32 AM Thanks for all the advice.  I passed it on to my friend, who cheap as he is, decided to NOT risk catastrophic failure and will just get an aluminum post. 


Besides, I have read an article stating that wind tunnel tests show that aero seat tubes are actually slower than round ones(!).  I was surprised at that, but the article noted that the interaction between a slim aero seat post and your legs was worse than a round one post and your legs.  A thicker seat mast, on the other hand, did provide aero benefits as it "filled the space" between the biker's legs.

I'd sell the aero seat post and buy a round one.  Just bought a new one myself at Nashbar for $11 (FSA zero setback).
2009-09-05 12:49 AM
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Subject: RE: Trimming a carbon fiber seat post
I think the bike is too big
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