Catch up on all of our Project 1.2 posts here!

With its simple two-bolt head and unidirectional carbon fiber finish, FSA’s entry-level SL-K carbon seatpost is an attractive piece of kit that coordinates well with many of FSA’s cranks, bars, and stems.  The company bonds an aluminum head to the full carbon shaft for both cost-effectiveness and peace of mind.  Is this $110 seatpost a good way to get more fiber in your riding diet?  Hit the jump to find out!

Strong enough for mountain bike use, the FSA’s SL-K posts remain light enough to be seen regularly  on road tri bikes.  Available aftermarket with either red or white accents (shown), the seatpost is available in 27.2x350mm, 30.9x400mm, and 31.6x350mm sizes.  Both 20mm and zero offset heads are available to accommodate different riders and fits.

While FSA advertise a 220g weight for a 27.2x350mm post (Thomson’s benchmark Elite post is about 10% heavier but 20mm shorter), our 31.6x350mm sample actually came in lighter at 211g.  Sure, there are lighter aluminum and carbon posts out there, but FSA’s wide OEM spec mean that their posts need to be able to handle the wide range of uses (and abuses) seen by widely-spec’d hardware.

The SL-K’s only demerit an easily-addressed one.  In an effort to shave weight, FSA have carved a hole in the saddle cradle- providing direct access to Narnia a route for all manner of crap to get into the frame.  A piece of electrical tape easily sealed off the passage- but not until after our Project 1.2 Lurcher sounded like trailgoing maraca.  As anyone who’s tried to tease fine gravel out of a frame will attest, the task is a painful one.

The SL-K’s only real fault is easily fixed- but should really be addressed on future versions by FSA.   FSA have announced a fix: read about it here Though it doesn’t provide the same compliance as a flex-tuned seatpost (like Syntace’s HiFlex or Ritchey’s FlexLogic models), the SL-K will damp more trail vibration than aluminum models and appeal to anyone looking for a solid, attractive perch at a reasonable weight.  Just be sure to seal up that hole before your first ride.


Note:  Post’s shaft does not use FSA’s CSI aluminum core as initially stated.  Thanks to our commenters for the correction and our apologies for any confusion!


  1. Honest question (since I’m medical and not an engineer): aesthetics aside, what is the advantage to having a “carbon-wrapped” seatpost a/k/a a “carbon” post with an aluminum core?

  2. The SL-K has an aluminum head, but the shaft is 100% carbon (unless they’ve changed it for 2013). I know because I own several, and I have sawed off a few inches on them. Both the 27.2 and the 30.9 are full carbon tubes.

  3. A carbon wrap seatpost gives you vibration damping from the carbon, combined with protection against catastrophic failure since the aluminum core will usually bend before breaking, unlike a full carbon post which will just snap. This is why my CX still has its original equipment FSA Carbon Pro aluminum cored seatpost installed instead of the FSA K-Lite full carbon post in my parts box.
    As an aside, carbon wrapped parts are easier to make than laid up or molded full carbon parts since you form the CF around the aluminum core rather than a removable mandrel or bladder.

  4. Agreed with Matt, the SL-K posts are carbon except for the head. The FSA Team Issue post is their CSI (carbon wrap) post for the road. The don’t do a mountain specific CSI post. Afterburner is the step down from SL-K, and its all AL.

  5. I can never dig the aesthetics of these carbon posts with the simple-looking tube/head junction. It looks…cheap, like lower end posts such as Cannondale’s C2; a bit half-assed looking to throw carbon in the mix to appeal to the consumer. Won’t deny the SL-K’s actual functional performance, though.

    Just saying, if one really wanted a nice carbon post why not just hold out for a full monocoque (K-Force,etc) or at least a more blended finish? There’s a lot out there not way off the SL-K’s price.

  6. Matt & Mateo,
    You’re right- thanks! I hadn’t trimmed ours so it wasn’t obvious. It’s the SL-K bar that uses a carbon wrap over an aluminum core. The post has been updated.

    While aesthetics no doubt play a part, a big reason to go to carbon is for the vibration damping it can provide when compared to aluminum posts. This is especially the case on hardtails- though the Lurcher’s large 31.6mm ID seat tube dictates a relatively stiff seatpost. For a given weight, fatigue strength should also be better in carbon.


  7. Like Nick said, everything breaks, including Thomsons. I’ve cracked two seatposts and a stem (the older design, with the wedge style), and an X4 faceplate (with a torque wrench). I’m 135 lbs a week after Xmas, so I’m not exactly a clyde, either. Still love their stuff, but don’t delude yourself into believing that Thomson stuff is infallible.

What do you think?