How do you interpret "minimum insertion" of a quill stem?

Question

I have a 1" threaded fork here, longer than usual. The headset nuts have been separated by a significant number of spacers. So many, that if the quill stem is inserted to the minimum, almost none of it will be within the frame's headtube and therefore will be clamped in the threaded section.

I am not convinced that this setup is a good idea, but leaving that aside for a second, is there any official/technical advice/material that states this is incorrect and the quill must have some length within the headtube(i.e. below the top bearing).

Answer 1

It's a surprisingly controversial topic.

Many technical sources (Barnett's Manual is where I first learned it) say the expander wedge should be below the threaded section. If this is true, there's not much point in using a spacer stack on threaded headsets, because the lower end of the threaded section is your limiting factor.

In order for this rule to work, there would either need to be a standard for thread depth below the locknut, or the thread depth would need to be checked every time the height is adjusted, or the depth chosen to mark the minimum insertion would need to be conservative enough to cover almost all scenarios without either of the above being true. It's my impression that the third option is more the world we're living in, though this means some exceptions will slip through even if users and mechanics do things "right."

Other sources (Rivendell is one I know of) seem unconcerned about the whole thing and imply you can add spacers with impunity.

Before threadless, bikes tended to come with a different steerer for every size, with the threaded section starting higher up for each larger size bike. The framebuilding world still has such a range of threaded steerers available.

Contemporary threaded repair forks usually come in only one, very long thread length, which allows one part to be applicable to a wide range of bikes. These forks will thus have the expander stressing the threaded section on all but the smallest bikes. The availability of these forks in a liability-sensitive world (they're the only option in many cases) can seem to imply the design is safe.

I've seen several heavily used forks break their steerers when the expander is clamped under the threads, including repair forks as above. Many more are in regular use without issue. I suspect but don't know that rider weight/power and overtightening the wedge are large factors, but even so the risk of a steerer breakage has to be taken seriously. I've come to view clamping with the wedge under the threads as a bad idea, and am reluctant to use replacement forks that force one to.

Answer 2

Here are some of the mechanical/physical considerations at play here:

• The purpose of the minimum insertion marker is to ensure that the two pieces (stem and fork) are connected via a sufficiently long lever. If the lever were too short, the shear forces at the ends of the connection may lead to failure. So, that seems fine.

• The quill clamping mechanism puts nonuniform forces on the steerer tube. This will turn its circular cross section into a slightly elliptic shape. And this may interfere with screwing stuff on its threads. Not so good, but probably not a real issue.

• A thread is always one hell of a notch by definition. And any notches may become the starting point of fatigue cracks. As such, applying high bending forces on a threaded tube may lead to abrupt failure of that tube. And if your stem does not reach beyond the end of the thread, such high bending forces are exactly what you are subjecting your threaded steerer tube to. Now, this would be really bad.

All in all, I think that it's probably a good idea to ensure that the stem does reach down below the threaded part of the fork. Especially because the last failure mode would definitely strike out of the blue and force an immediate, dangerous accident.