ljsense

The typical expansion plug weighs around 30 grams, and doesn't serve a whole lot of purpose once the stem is tightened. This has always bothered me.

It bothered me to the point that I tried the Tune Gum Gum expansion plug, which is a rubber spacer, a silicone-ish expansion disc, and two alum alloy discs, one threaded. The idea is that the two alum alloy discs squeeze the silicone to get a grip on the inner steer tube walls. I've never found them all that effective -- they can keep a top cap on ok, but the hold isn't all that great for putting tension on the headset bearings when installing a stem.

My idea is to make a lightweight disc of carbon fiber sandwiched around something light in the middle, such as balsa wood, drill a hole in the middle and press in (or epoxy in) an aluminum alloy riv-nut, as frame builders use for water bottles (or used to) with ti or other non-steel frame types. I then would epoxy that into the steer tube.

Riv-nuts are generally for M5 bolts, and steertube top caps are designed around M6, but this can be solved with an M5 bolt with the right head profile.

This plug would be semi-permanent, sort of like star-fangled nuts are, but that doesn't concern me -- my forks are cut to the length I know I want them, and I feel comfortable removing an epoxied piece if I needed to.

Anyone tried something like this? It would save about 20-25 grams, and I think be a more elegant solution than what exists. I know I should be posting this to the weight weenies forum, not bike forums, but who needs another login credential.

fietsbob

Re: Riv Nuts.. go outside bike biz sources yet?

an aluminum pop rivet, once set , can be threaded the ball on the end of the pin, can fall out after forcing the expansion ,


If really weight weenieing it get a custom machined plug of magnesium .. Mg lighter than Al aluminum.. and further cut away to minimize the amount of that..

then use a bolt of Titanium.. and a carbon fiber stem cap..


Epoxy the magnesium plug in what I assume is a carbon fork steerer..

My late father had a home machine shop I did not get in the estate..


FWIW,
Hexcel is a bee's hive like aerospace material of more air than carbon finer..





You: go...



...

AnkleWork

Why not just use any old plug and cap, then remove them after adjusting the preload? Zero weight.

ljsense

Right, magnesium alloy would be great -- I'm the same, I don't have a lathe. Carbon fiber and balsa I can do pretty easy with sandpaper, etc. But I agree, the right Mg alloy, you could just drill and tap -- that would be nice

ljsense

Yes, true -- I've done that from time to time. I like having a top cap, though. It looks right to me, but I also think it does add some amount of clamping security in addition to the stem, which isn't usually needed, but on one bike I had a tune gum gum, it felt like the headset would come loose a bit over rough terrain when relying only on a stem to keep it in place.

fietsbob

I plugged a steerer tube (that did not need a threadless headset pre-load,) with a Cooks Champagne cork .. to keep the rain out..

what do you mean "Only a threadless stem keeping it in place" , once the stem is tight on the steerer the stem cap bolt is making No Contribution..


wtf is "a tune gum gum" you had ??







/..

AnkleWork

No doubt, the ultra-minimum balsa plug you propose will provide lots of extra security.

ljsense

That's what I'd assumed, too. But with a stem clamped to torque spec on a carbon steerer, I actually have evidence to suggest that the plug and top cap contribute to holding the load.

Some bs weight weenie part from the fine people at Tune, in Germany. I think they stopped making it, though.

ljsense

The balsa would just be there to give dimensional thickness desired for the laminate -- the load carrying layers would be carbon.

nfmisso

Just shave all your hair off; that would be more weight savings without impacting a well engineered bicycle.

After you do that, skip socks and underwear for even more savings.

fietsbob

show it.. references need backing data..

AnkleWork

Safety First, or weight weenie -- pick one; you can't have both.

ljsense

Yeah, I figured I'd get a fair amount of that kind of response. Fair enough. I take joy in tinkering, I love cycling, and I've always worked at making myself and my bike as light as possible while being durable and reliable.

A glue-in insert to take the top cap bolt seems like a good way to shave a bit of weight and achieve as much, or more, reliability than the traditional expansion plugs. Once you guys see the photos, I think a few souls will be with me.

ljsense

If we're debating by adage, I think it's: strong, light, cheap -- pick two.

In this case, I bet I will be able to build a glue-in plug that has all the holding strength of a compression plug at 80 percent less weight.

ljsense

When I'm ready to publish the numbers from the lab, I will, but in the mean time, I suggest removing the top cap from a mountain bike or a fat bike with a carbon steerer, and riding it for a few days over rough terrain and medium sized drops. Make sure the stem bolts aren't over torqued. You may be surprised to find that your headset has loosed. The stem cap is actually producing pretty significant downforce if the bolt holding it is torqued, as most specify, to around 4 Nm.

ljsense

Also, I glanced over this part -- "skip underwear"? Come on man, know your audience. You think I ride in underwear?

79pmooney

Have you considered using a 1/4" bolt? Those rivnuts would be easy to find. (1/4" = 6.35mm. The topcap is the only place where the bolt has to fit and it will be no less than 6mm. Common bolts are considerably under the nominal dimensions so I'd be surprised if a Home Depot 25 cent bolt doesn't just slide through. Maybe go an Ace Hardware and look though their selection to find a bolt with a head that will look good on the bike. That will probably drive the price up to 75 cents

To take this a step further into weight-weiniehood - make an aluminum bolt. I haven't figured out uet how you fabricate/attach the head, but getting there is half the fun. THreading 1/4" aluminum rod wouldn't be too hard and the end result would be light. (I'd set the headset and stem with a steel bolt, then replace it with the aluminum one to spare the threads.)

Ben.

DaveSSS 

A 1 inch star fangled nut, with a little filing off the edges, will slip into a 1-1/8" steering tube. Stuff a little foam rubber into the tube first, add some epoxy glue, then slip the star fangled nut in place and add more glue to fill. Can't weigh too much. I've done this and even extended the steering tube length by 1cm, using a 1" copper tube coupling as a mold. The ID is 1-1/8", so it slips right over the steering tube. JB Weld epoxy recommended.

You can buy threads inserts in 6mm, no proble.

dsaul

I've had similar real world experiences with carbon steerers and the stem slipping. One was a road bike and the other a mountain bike with a rigid fork. In both cases, the expansion plug didn't have enough surface area to prevent the stem(and expansion plug) from slipping and causing a loose headset. On my road bike, I cured the problem by switching to a better expansion plug. My friend fixed his MTB by epoxying the expansion plug into the steerer. The old adage of not needing the top cap after tightening the stem does not necessarily apply to carbon steerers.

ljsense

Thanks to both DaveSSS and dsaul, as well as everyone else -- you've helped me think through this. Here are my thoughts so far:

1. I should have stated the problem a bit differently -- I think the top cap and its ability to screw down into something up to its recommended torque setting are important.
2. A chemically bonded insert seems like a good solution that would offer as much (or more) hold as an insert at a fraction of the weight
3. Better tools, like a lathe, would open up more options, or at least make it easier, but I have files, a tap set, a punch that centers a starfangled nut and other stuff.

My plan is to make a little disc out of lightweight alloy, glue carbon fiber to its top and bottom, glue up a second layer of carbon in the shape of a starfangled nut, glue this to the first layer of carbon and the sidewalls of the steer tube, then drill and tap through it to establish the M6 thread hole. All patents pending.

fietsbob

Alternatives include, as I said .. Filling the ID of the Carbon steerer solid and epoxying it in. and perhaps use that grit filled grease called carbon assembly paste...
to bite into the epoxy matrix on the outside.. (Or Red Loc Tite.)




but I simply avoided the' I want a 9 pound bike' obsession..

for my much heavier bike I needed a taller stem and found a NL company BBB that made a shaft quill lining the Steel steerer, stem raiser
( the BBB BHP 20 7/8" & 21 1", is aluminum with a steel wedge bolt)

of course with such a light bike, its odd you don't want to do any safety checks before each, I assume, Race..







.....

woodcraft

How about a piece of dowel, drilled in the center,

recess & glue a nut in the bottom, and glue the whole business into the steerer tube?

Should be about 10g.

ljsense

Yeah, that'd work -- I think that would be an elegant solution for the simplicity of executing it. I want something that honors all the engineering that went into the rest of the bike, and, if I ever showed it to anyone -- as I hope to show you all -- they might say, "huh, that's kind of neat. How long did that take you? Aren't you afraid it's going to crack? Wouldn't it have been easier just to take off your underpants?"

Or something along those lines.

woodcraft

LOL.

"Psst- hey, wanna' see my steerer tube?

TimothyH

The expansion plug isn't a place to save weight.

It isn't about just holding the stem from slipping and preload on the headset bearings. The plug supports the steerer tube so that the steerer is not crushed by the stem.

Steerer tubes have fibers runing lengthwise which are resistant to bending loads. There are little to no fibers running around the circumference (hoop fibers) and so steerers are not very resistant to crushing loads. It only takes 2 Nm extra to start crushing a steerer and breaking fibers and causing delamination.



People are free to do what they want.


-Tim-