Structural?
So, do plugs actually support the carbon tube walls? How can a stem clamp crush the walls when the force is all the way around? I'm assuming the plug does some kind of support internally, but a badly torqued stem (too loose or way too tight) seems to be the main cause of failures. And I wonder about the sharp edge of the machining at the stem clamp circle on some stems.
A loosened stem is going to mangle the carbon even with a plug correctly installed. That's what the photos below imply. Not a crushing split, but a slow mangling of the steerer fibers as the stem gets loading from rider and road bumps.
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It's interesting that some manufacturers
require a spacer, while others, like Felt,
prohibit it.
Back in 2017, Bianchi said: no more than 5mm above the stem. My Bianchi came with a minimalist FSA plug, not nearly as tall as the stem. I assume
the 5mm limit is to avoid ignorant mechanics placing the plug too high in the steerer. My plug sits near the bottom stem bolt.
Some plugs have a lip on the top, to sit on the cut end of the steerer. Spacers above would move those upwards. See this
google image search -- plug styles are hugely varied.
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From an
old thread on another site:
...a safety warning on Bianchi's global site. This has to do with possible catastrophic failure of full carbon forks with carbon steerer tubes when assembled incorrectly. I checked Bianchi USA's site and there is no such recall/warning and no mention of the issue. Additionally, I had registered my Bianchi on BianchiUSA.com (and this information was actually logged on the global site in Italy), yet I did not receive notification of the possible problem.
The main theme of the warning is that they are claiming that stems and spacers were installed incorrectly on Sempre Pros and Intensos, but not limited to these models. They state that assembling with more than a 5mm spacer above the stem can cause damage to the steerer tube.
Interestingly, when I went into the full details and instructions, they also said that a 5mm spacer was required under the stem (and not more than 35mm) and they also said that carbon assembly paste was not to be used on the steerer tube for stem installation.
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I had linked the "damaged" and "not damaged" steerer photos from Bianchi, they no longer link. But an image search showed them on Bianchi Japan, so I downloaded them.
Not damaged. There's a shiny outline of the stem, just on the surface of the steerer. Looks like a 5mm spacer was above the stem, I think.
A damaged steerer. All their damage photos show the bottom, fork end of the stem crushed the steerer.
More extreme damage! A loose stem clamp? And a huge amount of spacers above, so the plug was likely located too high? I wouldn't be surprised if they had more than 40mm of spacers below the stem, too.
