MinnMan

This is kind of a complicated story, but I'll try to leave out the parts that aren't directly related to my question.

The carbon steering tube of my 7 year old Felt cracked. The crack was underneath the stem.

Felt is supplying a new fork and it is my understanding from the LBS that they won't charge for it. That's very nice.

The LBS and Felt had an extended discussion about the cause, including sharing photos, and some of this information was relayed to me second hand by the LBS mechanic.

Felt noted that there was a spacer above the stem, and, I am told "disapproved", indicating that this over the long term can place undue stress on the steering tube.

(It is not clear that this was the cause of the failure in my case. There are other possibilities but they are besides the point of my question.)

When the replacement for arrives, the LBS intends to cut the steering tube so that the stem is in the correct position without a spacer above it.

This means that I or some subsequent owner could not later raise the bars without replacing the fork.

What does the BF/BM hive mind think? Is having spacers above the stem a potential cause of steering tube failure?

fietsbob

very few spacers .. max 3cm.. because its not metal , under it

But a spacer above.. maybe 5mm is so stem is fully in contact with steerer, being carbon..
the extra 5~10 mm above is for headset pre-load adjustment , underside of the cap clearance..

dsbrantjr

If having spacers above the stem cau cause failure I would be very interested to understand the mechanism. A quick online search shows many recommendations that one needs spacers above the stem.

Since it is presumably allowable to put the stem all of the way at the top of the steerer, I do not see how putting it lower, with spacers above, could be anything but equally or, more likely less, stressful.

Perhaps one or more of our mechanical engineer members will ring in.

You may be in the middle of a finger-pointing/a**-covering exercise.

jideta

Interesting.

Only thing I can think of is the perhaps the depth of the starnut/anchor in the tube in relation to the stem.

Lets exaggerate the issue: what if you had say, 1 foot of tube above the stem? Not seeing how it would affect stress on the tube below the stem. Well, unless your stem was 1 ft above the headset. Sounds like Felt doing some CYA.

I'd have the LBS install the race on the fork and install myself.

I'm with you, I always leave a cm or so on top for the next guy even though there never seems to be a next guy.

fietsbob

Carbon, no star nut >>> an expansion sleeve is put in , & spread out
So, making no scratches in the carbon composite..


and then inside it is a nut like fitting .. several makers ..


a number of bad revues on buying 2nd hand carbon stuff... not a universal good.









....

dsaul

It depends on how high the spacer extends above the stem. Carbon steerer tubes need to be fully supported by the expander, in the area where the stem clamps the steerer. If the steerer extends far enough above the stem, the expander will not be supporting the clamped area of the steerer, leading to cracking and failure at the lower edge of the stem clamp. 5 - 10mm of spacer above the stem should be fine.

Sapperc

You’ll find a lot of folks trying to get away with doing things that are contrary to manufacturer recommendations. Doesn’t make it a good idea.

Specialized also states in their carbon steerer instructions that spacers are not to be placed above the stem except for initial fitting. For permanent installation the steerer tube is to be cut to the correct length. The reason has to go with how the expansion plug is installed. Doing otherwise can lead to failure and voids the warranty. Basically there is no going back.

What I have done to drop the stem without cutting the steerer tube is to install the stem with a reverse angle. Reverse angled stems are perfectly sound and more common than you might think.

Good luck!

Litespud

You're the consumer - if you want to leave some steerer above the stem for future flexibility (or lack of flexibility!), that's your prerogative, and if the shop done agree, then take your fork (which came from Felt, not the LBS) and go elsewhere. Having the steerer extend above the stem is irrelevant as there is no torque exerted on that portion of the steerer. The important thing is to have the expander plug support the portion of the steerer clamped by the stem, and the length of the extension above the stem is limited only by the length of the screw connecting the top cap to the expander plug, necessary to preload the headset bearings. Extend the steerer as long as you like, as long as you can find a sufficiently long screw.
All that being said though, I had a 10mm extension above my stem for years, for "future flexibility", until I came to the conclusion that I wasn't going to be raising the stem any time soon, nor was I planning on selling the bike ever, so I cut off the extension, and the bike looks much nicer. Sometimes it's OK to prioritize the actual now over some nebulous future - and screw "the next guy" - that'll be his issue.

Dave Mayer

Having a spacer above the stem could not have caused the failure. Extra steerer tube above the stem places zero stress on the fork or the stem. Whoever 'disapproves' is simply making a poor attempt at deflecting blame.


The only downsides of having surplus steerer tube sticking out above the stem is a little extra weight, it is somewhat of a hazard to the (male) anatomy, and it looks dorky. I leave about 1cm of extra steerer above the stem, for the future flexibility of raising my stem.


In fact, it is good practice to put a 5mm spacer above the stem, held on by the top cap. Why? because it indicates to mechanics that the full width of the stem is securely clamped to the steerer tube.


I volunteer at a high volume city bike Co-op, and we see many horror shows of stems being half-way clamped onto steerer tubes. In fact every time we see a threadless system without a spacer on top, we remove the top cap to see if the stem is adequately secured to the steerer tube, or whether it is: 'hanging on by a thread'.


Carbon steerer tubes... you CANNOT install a star-nut in these. You need a carbon-specific expander system. Once the headset bearings are properly loaded, and the stem bolts secured, the expander can in fact be removed. It does NOT provide structural integrity benefit to the system. The expander and top cap ONLY exists to allow the headset bearings to be properly loaded.

Kimmo 

This is highly debatable. If you've seen enough old carbon parts, you'll see deformation sooner or later. I've seen carbon steerers with reduced diameter wherever the stem was clamped, with a bulge where the hole in the stem at the back of its extension was. Seen the same on bars. It's a pretty illustrative rationale for the 5mm above precaution.

A well-designed expander plug should support the steerer properly against crushing force, but many are sub-par. I make sure to use good ones so I can cut the steerer level with the top of the stem. Then I countersink them and modify the top cap if necessary to sit flush.

HillRider

I have two bikes with all-carbon Easton forks and have cut both steerers to allow a 10 mm spacer above the stems. One bike has 49,000 miles and the other 43,000 miles with no signs of distress so I have to conclude it hasn't done any harm. I use Profile Designs expansion plugs and these support the stem quite well.

Another important thing is to tighten the stem clamp bolts to the recommended or less torque. I use a torque wrench and usually tighten them to 4 N-m which is a bit less than the recommended maximum. I expect a lot of steerer failures come from insufficient support of the stem and excessive clamp bolt torque.

MinnMan

This has all been very informative. This is the set up that the bike had. It had just one spacer above the stem. Too much? Really?




And btw, this was the setup from the fitting done when I bought the bike from an LBS that was/is an authorized Felt dealer. If Felt did not recommend spacers above the stem, then either the LBS didn't know or was ignoring the guidance.

Marcus_Ti

Look up a manual for the fork and expander plug. All of them are a bit different in recommended best practice. If your LBS didn't follow it and they're an authorised dealer--wag of the finger to them

For example, my Niner plug for my Niner fork advises :

A) No more than 40mm stack height
B) NO spacers above stem
C) Cut the steer 5mm below the top of the stem when installed with spacers
D) Expander plug torques to 7Nm.

katyjcrow

For Trek to warranty your carbon fork, you must have a spacer above and below the stem. I have three Treks with carbon forks and they all have a minimum of 5 mm above the stem. The Boone has four seasons of cyclocross with no problems.

MinnMan

Well, it's a Felt OEM fork. Where exactly would I find the manual?

HerrKaLeun

i have nothing to contribute that will help OP.

but I wonder how much weight is saved vs. a CF fork with AL-steerer? I have an Al steerer and first wished I'd rather have full carbon to save weight, but reading all this I'm glad i have an AL steerer.

I fiddled with the bar height many times (and ended up higher now than after the first year of riding). Had I followed the recommendation to not have any spacers above, I would have had a too short steerer now. Keep in mind finding the correct fit can take forever since the body also adapts and changes. Having future flexibility is important, not only for resale.

Kimmo 

People talk as if only one angle of stem is available, and they can't be flipped.

Sapperc

How about contacting Felt Support?

While it’s possible that authorized dealers may have tech sheets and installation updates not available to the public (Specialized does this by design), Felt structural design engineers have the definitive answer to your question about your specific fork which they are likely to share with you if you ask.

It really doesn't matter how some other carbon fork/ expansion plug combination may or may not be installed. Experience counts for nothing, and what you may or may not be able to get away with counts for nothing until you have a straight answer about proper installation from the company.

You did mention that you did crack the steerer tube while it was installed the same way you propose to install it again. I’d want to be sure that spacer positioning wasn’t part of the problem.

hope it all works out.

Dave Mayer

Kimmo: I have immense respect for your mechanical chops, but I think you have it wrong here.

I am looking at a row of uncut forks with carbon steerers. The wall thickness of these is such that it would take immense compressive forces to crush these. Far far beyond what is reasonably necessary to hold a stem onto a fork steerer.

A well designed carbon steerer should NOT need internal reinforcement to counteract the clamping force of the stem. That would be a very poorly designed system, as there are all different forms of stems and expander plugs on the market. The chance for mechanics swapping parts, or installing the expanders in different ways over the life of a fork is basically 100%.

So back to my original point: there should be no limit on how much extra steerer is left above the stem, except for the extra weight, the dork factor, and the safety risk.

HerrKaLeun

My 2 bikes came with the stem already "up" and only a 5 mm spacer on top. And there is a limit on the stem rise, typically 35° or so unless you go to adjustable stems. It would be good manufacturers just gave us uncut steerers, even if they had to add 2 cm of spacers on top.

Many modern bikes are aggressively low, while many actual riders require comfort over speed.

Just another reason to build bikes from frame up. .......

MinnMan

Another shot of the current setup. Really guys? That spacer above the stem caused a problem?

rm -rf

Structural?
So, do plugs actually support the carbon tube walls? How can a stem clamp crush the walls when the force is all the way around? I'm assuming the plug does some kind of support internally, but a badly torqued stem (too loose or way too tight) seems to be the main cause of failures. And I wonder about the sharp edge of the machining at the stem clamp circle on some stems.

A loosened stem is going to mangle the carbon even with a plug correctly installed. That's what the photos below imply. Not a crushing split, but a slow mangling of the steerer fibers as the stem gets loading from rider and road bumps.

~~~

It's interesting that some manufacturers require a spacer, while others, like Felt, prohibit it.

Back in 2017, Bianchi said: no more than 5mm above the stem. My Bianchi came with a minimalist FSA plug, not nearly as tall as the stem. I assume the 5mm limit is to avoid ignorant mechanics placing the plug too high in the steerer. My plug sits near the bottom stem bolt.

Some plugs have a lip on the top, to sit on the cut end of the steerer. Spacers above would move those upwards. See this google image search -- plug styles are hugely varied.

~~~~~~~~~~~~~~~~~~~~~~

From an old thread on another site:
~~~

I had linked the "damaged" and "not damaged" steerer photos from Bianchi, they no longer link. But an image search showed them on Bianchi Japan, so I downloaded them.


Not damaged. There's a shiny outline of the stem, just on the surface of the steerer. Looks like a 5mm spacer was above the stem, I think.



A damaged steerer. All their damage photos show the bottom, fork end of the stem crushed the steerer.



More extreme damage! A loose stem clamp? And a huge amount of spacers above, so the plug was likely located too high? I wouldn't be surprised if they had more than 40mm of spacers below the stem, too.

fxbike

Yes, thank you your answer

base2 

Would there be anything wrong with getting a longer bolt to preload bearings but still allow for proper placement of the plug in the clamp area of the stem?

I inserted a star nut about a centimeter & a half lower than intended once & the preload bolt wouldn't thread in. A longer bolt took care of the issue & I haven't thought about it since. But now I wonder.

I would think that proper placement of the plug to offset the stem compression would be the primary concern...

That being said, there are an awful lot of carbon seatposts out there with no plug. So in my mind, that calls into question the whole premise.

Kimmo 

Haven't you seen deformed carbon parts, including steerers? I wouldn't say it's quite normal, but it's definitely fairly common, IME. It's just plastic, after all. Without a robust expander plug, the steerer can end up effectively tapered if there isn't a bit poking out of the top, neatly explaining the 5mm above precaution. That's why when I cut my steerers flush, not only do I use a good, well-positioned plug, I also tighten the bottom clamp bolt a tad more.

My theory about the deformation is that it isn't necessarily caused by overtorque, but can happen as a result of normal clamping pressure and riding loads, plus enough time. Heat cycles, too, maybe?