gkamieneski

A few years back when I was setting up the front end on a new CF road bike, there were very clear warnings from almost every manufacturer that carbon fiber steerer tubes required 5mm spacers above and below the stem. Have there been any recent changes in interpretation of this practice?

Mine is an endurance frame with a rather large head tube. It's replacing a pure racing bicycle and for some time I have wanted to remove the 5mm spacer under the stem, or at least replace it with a 3mm alloy spacer that the bicycle came with originally. Somewhere along the line, I replaced the minimalist FSA compression plug the manufacturer provided with a more robust 50mm Origin 8 compression plug, so I feel the stem tightening area of the steerer tube will find full support from the compression plug with the removal of the 5mm spacer as well as how things are currently set up (I have a 10mm spacer on top of the stem). Also, the FSA Orbit headset features a substantial 15mm top bearing cap of a conical variety.

FBinNY 

I've never endorsed the spacer requirement which wouldn't be necessary if stems fitted properly ---- zero clearance (barely) running fit. I understand the need for the spacer above, so a sloppy fitting stem cannot cam against or ovalize the end of the tube. However, I don't see how the spacer below would change anything, and IMO bracing would be better if the stem were as low as possible.

So, in your shoes, I'd eliminate the lower spacer entirely, put the bars where I want them and enjoy life, free of worrying about stuff that doesn't matter.

FWIW - what we all need to remember that CF structures lack the ductility of metals, and therefore are much less forgiving of things like sloppy fits or any distortion. Doesn't mean they're unsound or unsafe, just that we have to respect those considerations.

Kontact

I am not aware of this ever being a common manufacturer restriction - and I was working in a shop and building major brand bikes every day. The exception was Trek, who always seems to have some issue with steerers.

5mm above is a good idea for lots of reasons. Below? I don't see the point.

gkamieneski

Yes, mine is not a Trek but it was Trek's warning that has caused me pause.

"Below the stem on all steerers
Always place at least one 5mm spacer below the stem. This spacer is in addition to the bearing cover (Figure 1), which must be at least 5mm. If the stem rests against the bearing cover, a point load is created on the steerer that could eventually cause the steerer to break."

FBinNY 

I get their point, but as a practical matter you get the same bending moment at the base of the lowest spacer.

It's possible that some flex might happen between the stem and spacer, taking some of the strain, but bending loads tend to be greatest nearest the fulcrum, so any improvement would be minimal.

IMO if the designers are working so close to the edge, that the spacer is important, the right answer is to add more safety margin to the steerer. (which they probably did).

One consideration in all this is how squarely the bottom of the stem is faced. If its a forged stem and not faced, it might not make 360 degree contact, and any gap could be an issue.

Kontact

A lack of 360 contact could effect how the headset bearing is supported, but shouldn't have anything to do with how the stem clamps the steerer.


I think TREK has had several very public steerer tube failures, and the 20-something "engineers" they have on staff used computer models to come up with these recommendations as a panacea to the public. They are not industry recommendations.

Enve, for instance, specializes in forks with all carbon steerer tubes, and makes absolutely no recommendations about spacers. In fact, they make a stem that can't be used with a top spacer.

FBinNY 

I think we agree that the spacer requirements are nonsensense, possibly resulting from early experience with poorly designed and built CF forks.

The problems were compounded by stems that were fine with steel steerers but problematic with CF. Comparable issues involved early CF seatposts, that couldn't tolerate the stresses of seat tube distortion.

But I want to be clear about my reference to unfaced stems. The problem isn't about clamping, but is about the kick of 360 degree support. If the stem is supported on one side, that would allow a bending moment toward the unsupported side. Not a serious issue most of the time, but can be if the steerer is built too close to the edge.

gkamieneski

Mine is a 2014 Bianchi and I know Bianchi Italy produced a similar warning document to Trek’s. I use a FSA SL/K stem and never noticed a gap where stem meets spacer. I do notice that the conical headset cap spins in unison whth the stem and spacer when turning.

FBinNY 

Of course it does. How could it not?