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  1. #1
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    Expansion Plug, Carbon Steerer

    In the Bianchi section, I recently wrote of a safety recall that I found on the Bianchi international website (notably, nothing said on the Bianchi USA site). It had to do with maximum/minimum spacers above and below the stem and the position of the expansion plug.

    This led me to consider replacing the minimalist FSA expansion plug and to replace it with an Origin8 plug that is 50mm in length. I used the same total amount of spacers as I didn't want to have to cut the carbon steerer again.

    What I didn't take into consideration correctly is that this new expansion plug, unlike the minimalist FSA plug that it replaces, is of the design that has a 2mm lip so that it doesn't slide down further into the steerer tube. So, guess what? That's just enough extra addition to the steerer tube that I can no longer get the preload with the top cap bolt. It looked like it might work, but what was once 2mm or 3mm gap for the spacers above the steerer is now more like 1mm or maybe even flush. When I torqued things down, I noticed that I can "spin" the spacer below the top cap somewhat and this could only be if I wasn't getting the appropriate preload. I would have to cut the steerer and additional 2mm - 3mm.

    Does this make sense? The ability to spin the spacers, means the preload is not enough correct?

  2. #2
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    To overcome this you'll need a temporary spacer at least.

    I always use a temporary spacer, just to be sure, when setting preload. Maybe a 20mm spacer, at least 10mm. This temporary spacer is placed above the stem and under the top cap when setting preload. After the stem bolts are tight, I can then remove the temporary spacer and just put the top cap on without it. You very well now might need a 5mm or 3mm spacer above the stem if the top cap won't tighten without one.

    To test to see if the preload is tight enough or not, do the front brake test. Hold the front brake tight and try to rock the bike back and forth. If you feel or see any movement in the headset or where the fork meets the headtube, it's too loose.
    use a torque wrench

  3. #3
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    Thanks for those ideas.

    I hate the thought of not having a predetermined, constant downward force on the assembly though. I do the front brake test and the front end drop and the assembly seems tight enough yet with slight twisting force I can spin the spacers slightly by hand. Can't help but feel that if I can spin them, there cannot be enough downward force being applied.

  4. #4
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    Once u tighten the stem on the steer tube, you can take the top cap off and leave it on the work bench, u don't need it!
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  5. #5
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    I hear you, but I will contend the downward "backup" force means something. Should a stem bolt fail catastrophically (yes unlikely), to top cap bolt could get you safely to the side of the road.

    I agree with MMS for using a larger spacer to get the preload then reinstalling the smaller spacer as long as you can still get the top cap tight. If you cannot, then all you can do is resize the steerer or use a larger spacer. In my situation, it looks like the top spacer I planned on using is only going to be flush with the top cap.

    I just poorly planned that this more substantial expansion plug had a 2mm lip built in while my original "minimalist" FSA expansion plug had no such lip.

  6. #6
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    I was on a bike tour, my top cap fell off at day 2, I rode the rest of the week without it. There is always a chance for failure, usually there are 2 stem bolts, and those bolts are no where near the tension that they would fail along the axis, that would take about 500 lbs tension. At that point your steer tube would fail first and you don't have a back up for that!
    I would be more worried about where u're going.
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  7. #7
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    Quote Originally Posted by GKSki View Post
    In my situation, it looks like the top spacer I planned on using is only going to be flush with the top cap.
    Not all top caps are the same. Some are heavily conical and some are dead flat. You can leave things as they are and get one of those dead flat ones like Carbon Ti makes, or you can trim the steerer a little more, or you can put a slightly larger spacer above the stem. Just as long as the spacer above the stem isn't over 5mm.
    use a torque wrench

  8. #8
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    Quote Originally Posted by GKSki View Post
    I hear you, but I will contend the downward "backup" force means something. Should a stem bolt fail catastrophically (yes unlikely), to top cap bolt could get you safely to the side of the road.

    I agree with MMS for using a larger spacer to get the preload then reinstalling the smaller spacer as long as you can still get the top cap tight. If you cannot, then all you can do is resize the steerer or use a larger spacer. In my situation, it looks like the top spacer I planned on using is only going to be flush with the top cap.

    I just poorly planned that this more substantial expansion plug had a 2mm lip built in while my original "minimalist" FSA expansion plug had no such lip.
    Maybe I'm missing something but why don't you just get a spacer that works?
    Last edited by Jay Strongbow; 02-22-2017 at 10:07 AM.

  9. #9
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    MMS, I tried the larger spacer and was able to set the preload, then tighten the stem bolts, remove the top cap and replace with the 5mm spacer. Remember I am trying to limit top and bottom spacers to 5mm (obviously subject to discussion) to comply with Bianchi's safety warning. Previously I had 0mm under the stem and 13mm above the stem but with the minimalist FSA expansion plug (although I did have it centered inside the stem with no signs of injury to the carbon steerer).

    Now, because the top is really flush, I have that top 5mm spacer loose and I think I even hear a rattle from it.

    Best thing I should be doing is cut the steerer again to the correct size but this would mean only taking 2mm-3mm off and that sounds like an iffy cut. Alternatively, I know I can go back to the old "minimalist" plug making sure it is positioned correctly. I kind of liked the Origin8 plug because it was more like those used by Colnago and Pinarello, but that additional 2mm lip has caused my issue.

  10. #10
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    You could probably cut a full 5mm off, leaving the steerer slightly below the stem. With this plug it will then be just about flush with the top of the stem.

    Or a flat bottom top cap.

    https://fairwheelbikes.com/carbon-ti-x-cap-top-cap/




    All of the top end Bianchi's, by the way, use these Carbon Ti top caps.
    use a torque wrench

  11. #11
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    Quote Originally Posted by MMsRepBike View Post
    You could probably cut a full 5mm off, leaving the steerer slightly below the stem. With this plug it will then be just about flush with the top of the stem.
    That's exactly what I would do, give it a try OP.

  12. #12
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    Thanks, all. I am inclined right now to run 5mm spacers top and bottom of the stem and go back to the supplied "minimalist" FSA expansion plug to be in compliance with Bianchi's safety warning. If later I am to cut the steerer I think I would only take 2mm or 3mm off and then I would be able to utilize the more substantial expansion plug with the 2mm lip on it.

    Question for MMS; is cutting 2mm or 3mm difficult when dealing with carbon fiber? Any mitre box or clamp cutting guide that I would have, would have trouble dealing with such a shallow cut and I sure don't want to disturb the fibers on the end cut.

  13. #13
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    Quote Originally Posted by GKSki View Post
    Question for MMS; is cutting 2mm or 3mm difficult when dealing with carbon fiber? Any mitre box or clamp cutting guide that I would have, would have trouble dealing with such a shallow cut and I sure don't want to disturb the fibers on the end cut.
    I would only use the Park Tool specific carbon cutting clamp jig and carbon specific blade. I've cut 2-3mm off lots of times using one, it's a very common thing around here, haven't had a problem yet.
    use a torque wrench

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