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I'm looking at expander plugs as I work on a modification for my new build.

I note that the M6 bolt that holds most of them - like the Enve on my desk - threads into a larger, internally threaded, sleeve which in turn threads into the bottom cone.

I can't see why though.

Is it just to allow you to "stretch" the bolt to accommodate different numbers of spacers above the stem? It would be much simpler to have the M6 run straight into the bottom cone and be long enough to handle different heights with the excess just sticking out of the bottom of the plug.

What am I missing here? The threads are both cut the same way so there's no reversal going on. Deda do this on their expanders too.

I'm sure it's going to be obvious enough to make me feel stupid.

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    There is one screw for the expanding action (to keep the expander plug in place) and one to set the bearing preload. Usually you need much more torque and force for the expanding. So it’s not a good idea to combine both (but some cheap plugs do so).
    – Michael
    Sep 19, 2021 at 6:52

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In designs like you describe (most of them out there), the plug is held in place from expanding wedges. When it's installed with good lubrication and torque it will sit there doing its thing indefinitely. It's solid and unmoving when the headset is adjusted or when the stem is changed.

In a design like you propose, the same bolt that adjusts preload on the headset also tightens the wedge in place at the same time. To get it started, you have the plug and topcap assembled outside of the fork completely (they have to stay together like this because the lower parts would just fall through if the bolt and topcap weren't there), and you engage the bolt enough that it fits snugly in the steerer as you install it. There needs to be a spring so that the wedge pieces are pressed down and engage when they're supposed to. When it's engaged just enough to catch, you push it down until it starts and begin tightening the bolt to wedge it in place while also drawing slack out of the headset.

Designs like this have existed. For a user or mechanic that's good at adjusting headsets and understands what the design is doing, it's not really a problem. But, a lot of people have difficulties with those things and don't need another analog or subjective feel-type element to master when adjusting their headset. I don't believe you can get away from needing the spring in these designs either, which implies steel, and while I don't have weights to compare, it probably makes it hard to save weight doing it that way.

One imagines that over-torquing the expander wouldn't be an issue, but a design like this in the wild in carbon steerers could prove otherwise. I could easily see a user switch stems and accidentally be left with insufficient stem gap. The plug you propose in that situation will no longer be splitting input force between preload and expansion. It will all go into expansion and the user won't know it, they just know that tightening the bolt takes out looseness. That could be a recipe for a destroyed steerer.

rockwerx plug

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  • Nathan - thank you. Makes perfect sense now. The plug design is doing two things, staying in place and setting preload. With vastly different amounts of force needed to achieve each task it makes sense to have two screws which can be set independently. My modification just got more complicated!
    – Mr73
    Sep 19, 2021 at 18:47
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    One way I could see doing what you propose for the hose pass through is use the plug part of a glue-in plug for the reinforcement under the stem, then use an expanding headset spacer to set preload with the stem clamped on, then make a simple friction fit topcap for the hose to pass through. Sep 19, 2021 at 20:18
  • Nathan - do you mean expanding plug? I don't know what an expanding spacer is. Problem with plugs is they don't have space for the hose. I think I'm going to just machine a part with a hollow bolt sized for a 5mm hose. I can scavenge some parts from an existing plug and machine the cones and bolt. I don't really want to use glue and the top of the steerer will be neater than a simple exit. HANG ON - THIS! bikerumor.com/2015/09/10/…
    – Mr73
    Sep 20, 2021 at 0:06
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    The thing about the product you link to is that it's a real thing that many manufacturers call out the need for a plug that provides internal support. I realize opinions about this are mixed but I'm pretty cautious about it. Sep 20, 2021 at 2:52
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    For anyone wondering I am going to try this. intend-bc.com/products/smarty It's a perfect solution given the design, with ample cable room. Really quite elegant and plenty of support for the stem. eeWings also have something but the design is a little aggressive/weight-weenie for my liking. Making a custom top cap is much simpler and cheaper than machining a complete replacement part.
    – Mr73
    Sep 20, 2021 at 17:27

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