Chimneys: Safety Issue?

So I just watched a video by Cam Nicholls that got me to thinking. Besides being less aesthetic, excessive steerer tube above the stem seems hazardous.

Case in point: one morning commute, I was jra in the drops, a bit too relaxed, when I checked over my shoulder for traffic, and just as I turned my head back around, I hit something—pothole, debris, whatever—and both hands slipped off the bars, leading to chestplanting on the stem and handlebars, followed by bodyplanting on the asphalt. Had there been a large amount of steerer tube sticking up, that woulda hurt a lot more.

So my q to you: is excessive steerer above the stem a safety issue, or was my (dumbass) crash a one-off?

(PS: the vid mentioned every chimney issue but safety, which is why I’m asking.)

I think Leuscher Teknik talked about this in a video…in addition to the hazard you unfortunately experience consider that the compression of the steerer tube against the stem clamp is structurally very important. If your steerer tube extends past the stem so far that your compression plug is above the stem clamp that could compromise the steerer tube. Something I hadn’t considered but it makes sense.


Good call. @Brennus . The vid I watched didn’t mention that either.

I think a 3 or 5mm spacer on top is the way to go. It provides just enough extra support. People with 3-4-5cm of steerer tube sticking out are going out of their way to make their bikes look stupid for the sake of resale value.

As far as getting hurt in a crash, I don’t think you are likely to get more hurt from a top cap that sticks up 5mm versus one that is flush. You’d still be hitting the same thing with your body.

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In fact both my wife’s trek domane and the enve road disc fork I have require at least 1 mm of steerer above the stem with a 5mm spacer between the stem and cap. It seems to be often required by manufacturers for carbon steerer tubes. So that’s a little bit of chimney for safety right there.

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Take out your steerer plug and see how long it is.

If it’s 40mm and you also have 40mm chimney then I would say it’s asking for trouble.
Not sure about anyone else but I’d want steerer plug behind most of the material that the stem is clamped to.

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Yes! #1 danger of too much spacer stack above the stem - especially with Carbon steerer tubes - is the steerer tube breaking while you’re riding, probably at a high impact moment of your ride, probably sheering off at the stem… and you can imagine then that with your bars and hands no longer attached to the fork the crash won’t be pretty.

The plug piece of the compression plug in carbon steerers (same is true for star nuts with aluminum steerers actually) is meant to sit fully inside the stem clamp area - not above or below it. By being there it provides structure inside the Carbon steerer to prevent it crushing under the stem clamp. So as said above, check the length of your compression plug to make sure it’s placed deep enough. If not, then your steerer’s likely too long for safety.

Another factor: some compression plugs can get fairly long and allow you to pass the test above. But then, if you have say a few CMs of stack above the stem, the top cap has that much more leverage to apply torque at unintended angles to the steerer.

How much stack is good above stem? I’ve seen a lot of fork specs call for 5-10mm. There’s often both a minimum and a maximum. For minimum many manufacturers say not to have 0 because then to get good top cap compression on the headset you’ll necessarily have to have the top of the steerer a few mm below the top of stem clamping area - and that’s not recommended either :slight_smile:

For maximum, depending on the compression plug length check, you can probably exceed 10mm if you want to preserve some fit flexibility. But personally I wouldn’t go beyond 15mm for long term.

Why doesn’t a carbon seatpost require a compression plug? There’s a heck of a lot more leverage on a seatpost than a steerer.


Not only does it look awful but it is dangerous. Just cut the fork tube down to the correct height for yourself, once you know your optimal riding position.

Gonna guess just the level of death that you incur when your steerer breaks as opposed to a Seatpost

Once the stem clamp is tightened, the cap and the compression it generates have no structural importance. The cap and star nut are only there to pre-load the bearings while you install the stem - same mechanical concept as the pre-load nut on non-drive-side Hollowtech cranks.

You could have a mile of spacers stacked above the stem, it would make no structural difference. The load is from your hands to the stem to a bending moment on the steerer tube below the stem.

If the steerer tube relies on the star nut not to crush, you have a major design issue on your hands. If the steerer tube can handle all the loads imposed on it - heavy braking on a bumpy road with a heavy rider - the stem clamp is a minor design requirement, and the star nut would be an extremely flimsy way to get there.


Great post @rocourteau, much appreciated and accurate, IMO.

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Once a MechEng, always a MechEng.

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Just an FYI for interested readers out there…check your manufacturers recommendations. It might not make a structural difference (an assertion which I don’t necessarily agree with! :smiley: ) but it may certainly make a warranty difference. If you have >20mm spacers above the stem you should double check you aren’t violating manufacturer’s recommendations.

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I have to put the safety benefits of this right up there with leaving the reflectors on your deep carbon wheels or ensuring you don’t remove the dork disc behind your cassette. :man_shrugging:

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You appear to be using ‘Star nut’ and ‘compression plug’ interchangeably. But they are different: Star nuts are used in aluminum steerers, while compression plugs are used in carbon fiber steerer tubes. They’re shaped very differently too.

We know (right?) that carbon fiber can be laid up in endless different ways, and often has strength in some dimensions but not in others? E.g. people crushing their carbon frame tubes by clamping them in a work stand.

So what I’ve heard and believe from several pro mechanics is the carbon fiber steerer tube needs the solid block of the compression plug situated inside it to prevent getting crushed by the horizontal clamping force of the stem clamp (not top cap). And this logic lines up with manufacturers instructions and warranties (as @Brennus noted).

For me, the end result of a steerer tube failure while riding is likely too catastrophic for me to ignore the manufacturer and industry guidance.

Agreed - yes, potentially, the expansible plug may help in protecting the steerer tube (particularly the top). This said, there still is no load on the cap once the stem is installed, and the length of exposed tube above the stem has no mechanical impact - assuming a longer tube does not bring the plug out of its manufacturer-specified location if there is such a specification.

TL;DR: You can undo the cap and ride. It’s not smart, but nothing will break.

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And that’s the point. Too much spacer stack above the stem, or “chimney” length, and it’s very likely that most carbon fiber compression plugs (the insert part, not the top cap) are situated too high in the steerer tube to give that protection.

Pics of a Star nut for an aluminum steerer tube vs compression plug for use in a carbon one. Carbon steerers are designed assuming that hunk of metal is inside the area being clamped. Without it is possible for a ham fisted mechanic to damage the steerer.



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