Evolution of the Humble Steering Tube Plug
 

What is up with the ever growing steering tube compression plugs?

This thing started out life as a simple little device to provide preload of headset bearings, and nothing more:

https://cimg4.ibsrv.net/gimg/bikefor...0c06d0215d.png
It was a practical alternative to the existing star nut, which left gouge marks on the inside of the tube. Nobody seemed to care about scratching fatigue-prone aluminum tubes, but many pearls were clutched over the thought of scratching carbon.

Here's a star nut, just look at those mean edges:

https://cimg5.ibsrv.net/gimg/bikefor...ec29d4b606.png
So that was that, and compression plugs became the standard for carbon steering tubes.

But at some point, someone looked at the compression plug and thought aloud, "It's obviously there to protect the fragile carbon tube from being crushed by those mean old stems." Never mind that no manufacturer had made those claims. Oh well, that idea spread like wildfire through the bike rumor mill, until today we are offered this monstrosity:



https://cimg0.ibsrv.net/gimg/bikefor...af9cc6ba53.png
"Extra long 90mm height reinforces carbon steerer tube beneath the stem"


90 millimeters, could this be the ultimate length? Or will these things keep growing? Only time will tell.

As far as I know, the function of the expander plug is to (1) counteract the clamping force by the stem on the CF steerer, and (2) enable the top cap to pull the steerer up into the head tube of the frame. To accomplish the first, the expander plug should extend past the bottom of the stem. The height of the expander plug needed depends on how far down the steerer the stem is clamped. Some expander plugs have an adjustable lower section which can be set at the height of the lower stem bolt. To me, if the foregoing is done correctly, but the lower, unclamped section of the steerer still needs further reinforcement, the fork manufacturer might as well have molded a metal tube within the CF steerer or just use an alloy steerer.

Why don't carbon seatposts come with compression plugs? They are:

1. about the same thickness as a steering tube,
2. clamped more tightly with the seatpost clamp,
3. are subjected to much more stress during use.

I guess the question I'd ask is - has anyone ever ruined a carbon steerer by crushing it with a stem?

If improper installation is the criteria, one should ask the same question about aluminum steerers.

Only if somebody's crushed a carbon steerer. If not, there's no need to ask about the aluminum ones, because nobody's proposed anti-crush plugs for them.

Crushing the tube potential is not so much of the problem as slippage of the compression plug is. I don't know if if is the smooth interior of carbon steerer tubes or if it is a shortage of knurling or grippy surface area of the plug itself but I will say that once you lose confidence in your fork bearings during braking - well that is a problem. The Reynolds stock steerer tube plug with my Reynolds carbon fork would always eventually slip even with grip paste and with torquing my stem bolts to higher torque than specified.

Well, if the big scare is improperly over-compressing a steering tube by a stem clamp, one ought to more scared about over-tensioning the steering tube by the compression plug.

Cylinders are substantially stronger against external compressive stress than they are against internal tensile stress.

https://cimg5.ibsrv.net/gimg/bikefor...eb814f2cac.png
https://cimg9.ibsrv.net/gimg/bikefor...0549387b8.jpeg

Specialized had some headtube failures on the Tarmac a couple years ago, and went to longer compression plugs. But I suspect that was a hedge against damaged steerers that slipped through the recall. The recall came from a poorly shaped internal stem part that damaged the steerer.

Trek, years ago, had some aluminum steerers break off. They went to mandating a 5mm spacer above the stem.

Stems can and do damage carbon or aluminum steerers when overtightened. It's why stems now have torque limits and carbon paste, and why wedge type stems have gone away. But stems don't cinch down like hoseclamps, crushing the steerer all around. They pinch the steerer at the gap where the bolts are, digging into the carbon at the spot.


As Enve still sells the same old short top cap, I don't think the industry has shifted to 90mm plugs. But given the ham fisted things bike owners sometimes do, I wouldn't be shocked if they did go to crush resistant plugs.

Why are these a thing? They should be removed after tightening the stem bolts.
They serve no purpose when riding.

The obvious difference is that (hopefully) your seatpost isn't clamped right on the end.

If MTB bar-ends were still a thing, would folks with carbon bars want a little reinforcement in there?

In my experience the cap itself helps prevent creep of the stem that is tightened to only 5nm. It probably shouldn't work that way, but they do.

Crud, hoping for an explanation of the wood plug hammered into the bottom of the Pug steerer.

True: it seems at least possible that the minimal force exerted on the minimal top (and bottom) surface of a stem contributes to holding the stem straight. Never thought of that.

Interesting that the first Aheadsets came with a plastic top cap, a brilliant engineering choice that nevertheless didn't account for the average low-wage bike mechanic misunderstanding the design and consequently cranking down on the tensioning bolt as if it were a conventional quill stem. I imagine that the following then happened.

Mechanics, blaming the plastic top cap for the resulting damage rather than their own ignorance, complained to shop owners, who complained to sales reps, who filtered the complaints upward through channels until the manufacturers said, fine, we'll make it an aluminum top cap.

This. Unfortunately the rumor persists that it helps protect the fragile steering tube from damage.

One of several "carbon is fragile and poorly understood" myths (apparently).

I haven't seen this stem creep/loosening of bearing preload, and I rode without a preload tensioner for several years.

​​​​​​​5 N-m stem bolt torque doesn't sound like much, but it's more than enough to keep the stem from twisting under heavy torque, so it seems that it ought to prevent the stem from creeping up the steering tube. Might something else in the headset stack be moving/deforming? Bearing race on the fork crown, maybe?

If the expander plug is removed, what would the top cap bolt thread into?

I saw this working in shops. Frequently the bikes that had loose headsets had poorly designed expanders that allowed the cap to creep up with the stem. This would happen a few times, then we'd replace the expander with something grippier and the headset/stem/cap wouldn't go anywhere.


My experience recently with plastic caps is that they are not strong enough to deal with internally routed cables and brake lines. They can crack just trying to compress the lines into their correct paths on new bike builds. So we used a metal cap for assembly and then the plastic one for looks. But the owner is screwed if they needed to make a change to the headset adjustment.

Threads? We don't need no stinkin' threads.

https://cimg9.ibsrv.net/gimg/bikefor...7b6776a688.jpg

BF will get very quiet without them.

Thats the whole point. The star nut is part of a calibration tool that is typically used a few times in the life of a bike.
BUT YOU CARRY IT UP EVERY STUPID HILL.

None of my bikes have a compression plug anymore. The entire area from the top of the steering tube to the bottom of the stem is filled with solid epoxy, with a 6mm thread insert at the very bottom. That makes it impossible to crush the steering tube and nothing will ever slip. I also don't need a 5mm spacer above the stem. I've only got a 10mm headset top cover and a 10mm spacer under the stem. If someone wanted to cut the steerer 10mm shorter, they could. If you've got 40mm of spacer, you'd need to place the thread insert lower, so a future owner could cut the steerer shorter. The top of the steerer is the only place where crushing by over tightening the stem is thought to be a problem.

I use star nuts as the thread insert and grind down the OD, so they slide easily into the steerer and self-center. I use a foam rubber plug to set the depth of the star nut and keep the epoxy from running further down the steerer.

Removing a compression plug after adjusting the headset bearing preload isn't the smartest thing to do. You might find a need to make an adjustment out on the road.
​

If I understand you correctly, you have filled the entire internal volume at the top of your steerer down to the bottom of the stem clamp with epoxy and embedded a star nut (with its OD ground down) at the bottom of this epoxy cylinder, which is supported by a rubber plug. Do you then drill a hole through the axis of this epoxy cylinder so that you can thread a top cover bolt through the embedded star nut? How to set preload?

I do something similar, if less massive. I made fabric/epoxy plugs. Slightly lighter

Pics or it didn’t happen.

I have a picture from several years ago in my album, but I can't figure out how to add it to a post.