is a slightly-scratched carbon seatpost a problem?

[Nathan Dushman]

In the process of adjusting the seat height (so my sister could ride my bike), I scratched my carbon seatpost a bit. It had been in the same position for years, and was quite stiff and difficult to move. I think they're just light scratches on the surface coat. Questions:
- Should I worry about this, or are a few light scratches harmless? It looks a ugly (some zig-zags) but the surface still feels smooth so I don't think the scratches are deep.
- Since I may need to do this again, what can I do to prevent it, other than the obvious replace it with a metal seatpost? The frame is aluminum.
- Is there any reason to fix the surface coat (with clear nail polish or whatever), or is that purely cosmetic?

Nathan

[Ben Kochie]

That surface scratching seems unavoidable with carbon posts.  I wouldn't worry about it.

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[Jason Thorpe]

I think you’re probably OK.  But to prevent permanent chemical bonding (carbon<->aluminum), get proper carbon friction paste slathered on that thing where the post makes contact with the frame.

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[Patrick Kitto]

I thought it was aluminium<->titanium connection you had to worry about?

Ive only ever used the friction paste for carbon<->carbon interfaces to prevent slipping without over-torquing.

[djconnel]

The electronegativities of Ti and Al are extremely similar.  Sorry, but my level of chemistry knowledge doesn't go that much further :(shall we discuss again why CO₂ leaks from butyl inner tubes? :))

Pardon the following for any errors: Scratches in the epoxy layer are essentially nonstructural.  Broken fibers in the carbon reduce strength.  But the layer inside the seat tube is under relatively low stress.  The highest stress, by far, is the material immediately above the insertion point.  If it fractures, that's most likely where it would occur, and deep scratches act as stress risers which increase the stress in addition to thinning the material and reducing the mechanical strength.  It's similar with handlebars which are most likely to break near the stem, or at transition points in thickness (stress risers).  But basically scratches within the seat tube aren't much of a concern.

Specialized did a cool paint job on a Venge to match the stress pattern during pedaling.  Stresses are much higher above the seat clamp than below:

http://theradavist.com/2013/06/recent-roll-garrett-chow-and-his-fea-specialized-venge

Now why does the stress increase moving up from the exposed tube toward the seat rather than be highest nearer the seat clamp?  I honestly don't know, but I'd want to know what forces are being applied for this particular stress pattern.  In any case, not much happening below the clamp here.

Dan

[Patrick Lea]

It has to due with the loading and it being a dynamic system. Although I am guessing they used a static analysis for that paint job. I think it is basically a bending mode out of the plane of the bike. Your body essentially being very heavy and stiff compared to a bike frame. Your butt basically holds the saddle fixed and you apply a moment to the BB giving but don't really let it move left and right so you get higher stresses at the two moment restrain locations (bb and saddle). There are some other things going on there too but that is part of the explanation. Note I am guessing that is some combination of forces probably Von Misses.

Also that analysis means nothing about your seat post breaking. I don't think anyway here is putting out the power to break their bike. Hitting a pot hole is must more likes the break any part of your bike and in that case your going to have the highest stress at the clamp, although that is a truly dynamic event making it the answer less obvious/easy.

[Ben Blizard]

Setback seatpost? The ass-weight is cantilevered off the back of the
seatpost. The post needs to be applying torque to your butt to keep
it in place.

I didn't get why the head tube didn't have any red (high stress?)
paint. I thought it took a lot of stress, given all the to-do about
increasing head tube bearing diameters for "high performance" bikes.

Ben Blizard
Voice: (650) 450-0037

[Daniel Connelly]

I never understood the beefy head tube thing....

Here's a Guru FEA model during some sort of bending mode:

http://steam.ca/dev/guru/images/tech-focus/holistic/pt/animation-1.gif

				View on steam.ca	Preview by Yahoo

No seatpost shown here, but the cluster has a lot of stress, so there's clearly a constraint of some sort applied there.

In any case, everyone agrees the bottom bracket lights up.

Dan

---

From: Ben Blizard <ben...@gmail.com>
To: Patrick Lea <patri...@gmail.com>
Cc: Dan Connelly <djco...@gmail.com>; sf2g <sf...@googlegroups.com>; Jason Thorpe <tho...@me.com>; Nathan Dushman <n...@abtech.org>
Sent: Thursday, January 8, 2015 1:53 PM
Subject: Re: is a slightly-scratched carbon seatpost a problem?

[Viet-Trung Luu]

On Wed, Jan 7, 2015 at 4:33 PM, Patrick Kitto <patric...@gmail.com> wrote:

I thought it was aluminium<->titanium connection you had to worry about?

Ive only ever used the friction paste for carbon<->carbon interfaces to prevent slipping without over-torquing.

+1

Also, it's possible that friction paste may in fact cause scratches. (I sometimes use it between metal parts, in particular between the stem and the bars; the last time time I did, the paste decidedly scratched the paint.)

On another topic: Nathan, is this the stock seatpost? Are you sure it's actually all carbon, as opposed to "carbon-wrapped" (i.e., carbon around an aluminum tube)? (I'm not sure how this would affect things, since I don't know the degree to which the carbon wrap is structural versus decorative.)