Tell me about steerer tube expander plugs

So taking my bike into the LBS the other day and the mechanic voiced his displeasure with the expander plug used by my bike’s manufacturer.

This is the type of plug

He said they shouldn’t use that plug and should be using one like this

He also pointed out that for the FSA style plug and top cap there should not be any spacer above the stem.

I didn’t know what to say other than that’s what the bike manufacturer ships and that’s the expander plug they sell on their website for their replacement parts. He seemed pretty adamant about it.

I just assumed the bike manufacturer uses a properly tested part. Is he wrong or is it a preference thing?

longer generally better and the fsa one is about as short as they come.
also can be a PITA when you have to fully knock the bottom tapered bit down into the fork to get the expander out.

What exact bike? Specifically, the fork & steer tube material & design are items of most interest.
Aluminum is pretty much ‘anything goes’. Carbon is more variable due to construction variation and some even use Alum/Carbon overlays. These may lead to many options while others are more a single ‘safe’ option.

There are general guidelines about material and expander use that he may be referencing. Otherwise it could he his personal experience and preference. The best way to know would be to dig more into the ‘why’ behind his comments.

Those things can be a real pain. If you loosen it too much the bottom falls into the frame. Sometimes when torqued to proper setting the headset will still be loose. Need to use plenty assembly compound. There is also something about the gap on the steerer tube. Probably what you mechanic said.

Generally speaking I don’t like to put compression stress on the steerer tube unless there is an offsetting non-carbon/resin laminate on the other side. So if you put spacers above the stem and you have a short plug that doesn’t extend to overlap the compression of the stem…the carbon laminate is not strong enough to withstand the compression of the stem during use.

Eventually it will break.

You want a plug that is long enough to overlap the point where your stem compresses the steerer tube.


I think this is his view

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is anyone (bike or fork company) recommending expanders for aluminum steerer tubes?
my only experience with alu steerer is on mtb forks and they are all star nut, so wondering if it is diff for alu steerer road forks etc

the inside expander parts (metal coated with some sort of gritty compound) on the FSA plug are shorter in height than the stack height of most normal stems to begin with IIRC

To be honest, I don’t have a ton of info. I just know I’ve seen expanders in aluminum forks on several of the bikes I’ve fitted or worked on over the years. Star nut is most common, but not apparently exclusive from the ones I’ve touched.

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Lauf true grit.

The why is because the steerer tube cracked part of his view is that expander plug is not long enough to overlap both stem bolts. But of course that is the plug lauf has always shipped with their bikes.

I’ve always used a torque wrench when tightening the stem and bars. And lauf’s instructions state to leave at least a 5mm spacer above the stem.

This bike is 4 years old. I’ve been working on my fit so swapped bars recently but torque things as normal and left spacers as they were. I noticed the crack not while riding but when I was swapping stems. I can only guess when it occurred. On the last ride I had a drop off a big rock that was enough to practically give me whiplash. I’ve gone over this rock a bunch but normally there’s is some clay on the other ride so I roll over the rock. But rains apparently washed it away and turned it into a drop. So I wonder all my weight going forward like that put a ton of force on it a cracked the steerer tube.

It wasn’t cracked through and I never heard a sound but that’s the only thing I can think of

Just another data point here, my CAAD12 came with the FSA plug and they also recommended a 5 mm spacer, but not more, above the stem.

It’s definitely not long enough to overlap both stem bolts no matter how you place it

caad less likely to be used for off-road cycling with drops etc, different level of load/force on the steerer.

Yeah. It’s not like it’s some unusual plug.

That is an excellent point. The main reason I chimed in was about the 5 mm spacer above stem.

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I agree. It’s not like lauf is doing something that NOBODY else is doing.

Ive had the FSA type on my TT bike since 2017 and my road bike since 2018 (with carbon steerers), FSA also but longer bungs, and they have been into the mechanics a few times and they’ve never said anything and I’ve never had a problem. AFAIK there’s only really carbon and aluminium type steerer plugs.

Yeah. I don’t know if it’s just this mechanics preference. It’s not that I’ve had it in changing the stem a bunch so it’s not something he would have noticed until now.

This gave me flashbacks to the headset failing on my first mountain bike. Could either tighten it to the point that my shoulders got very tired steering or a normal amount and hope that when I turned the bars the wheel would also turn (which, it would, until it didn’t).

My wife has a Lauf Anywhere (True Grit but rigid fork) and I absolutely hate this plug.

We travel with our bikes a fair bit and each time we come to build the bikes back up, this always causes problems.

It’s fallen into the fork several times. I tighten it just right and still play in the headset. I put a fair bit of carbon blue paste into the steerer tube and that helped a lot, but it was still a complete PITA.

I think I’ll get one of those Cane Creek expander plugs and replace it.

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