How many spacers safe above stem on carbon steerer

nicklouse wrote:

Makes no difference.

I wonder. Does the expander bung help the steerer resist the clamping force of the stem? Clearly many spacers above the stem could mean the expander was partly or entirely above the stem also. Notably Cervelo expect you (in theory) to epoxy a reinforcing alloy tube into the steerer once you've set the length, and then use a star nut inside that tube, with the tube not only occupying the whole clamping are of the stem but also extending past the upper headset bearing, which implies a need to reinforce the steerer. On the other hand, a) I said "in theory" because I've always used an expander instead, with no issues, and b) Canyon's system, for example, uses an external collar to apply preload, so there's nothing resisting the stem clamping forces except the steerer.

I'm presuming (hoping) up until now that the number above doesn't matter. I've a new bike and have been gradually moving spacers from underneath the stem to above it and testing the comfort. At the moment I've got 2 underneath and 4 above. This is comfortable for me but I plan to move one more above and test that before I cut the steering tube.

For a conventional steerer and top cap, my understanding is that it's not intended that you have spacers above the stem since the bung will then be too high up to provide support for the steerer where the stem is clamped. It DOES make a difference, the real question is how critical is it. It seems many people don't follow the guidance given.

rafletcher wrote:

964Cup wrote:

nicklouse wrote:

Makes no difference.

I wonder. Does the expander bung help the steerer resist the clamping force of the stem? Clearly many spacers above the stem could mean the expander was partly or entirely above the stem also.

Yes, the bung is necessary to resist the crushing effect of the stem clamping.

A longer bolt could be readily substituted if there was, say, 50mm of spacers above the top of the stem.

A longer bolt wouldn't work on my CR1 because the supplied bung sits just proud of the top of the steerer. As a consequence I have no spacers above the stem.

If I wanted to have spacers above I'd have to find an alternative bung that would sit further down in the steerer, and then the long bolt you mention

I stuck the offcut of my carbon steerer in a vice out of curiosity. It took quite a lot to break it, I'd think you'd have to be going some to crush one with a stem. I tend to agree with Dr Lodge and Nick here, spacers above make no difference as they aren't causing any change in leverage the bars could apply to the steerer unlike those below the stem.

Veronese68 wrote:

I stuck the offcut of my carbon steerer in a vice out of curiosity. It took quite a lot to break it, I'd think you'd have to be going some to crush one with a stem. I tend to agree with Dr Lodge and Nick here, spacers above make no difference as they aren't causing any change in leverage the bars could apply to the steerer unlike those below the stem.

Must be a bloke thing. I did exactly the same with my offcut, but with added ineptitude. It deformed slightly before shooting out of the jaws of the vice and pinged off the garage ceiling. Still perfectly round when I eventually found it.

I suspect the manufacturers are @rse covering at the insistence of their lawyers...

So you're going to ride around with a million spacers above the stem (with the daft look and the safety issue reference ripping your testicles off should you ever go over the bars) in case one day you may sell the frame? Note the use of "one day you may sell the frame". May.

Best not ride it at all then - getting dirt on it will devalue it so best keep it in the box forever. And the cable stretch. And brake block wear. Man, there's a whole world of devaluation going on before your eyes.

If you're that worried about it, cut these down and buy some more forks from Planet X and keep these in a box should the potential sale of a P/X bicycle ever nearly fall through due to cut forks.

I thought the only reason these expander bungs exist for carbon steerers is because they do not (and should not) have a star nut fitted, so the bung simply replaces the star nut, nothing to do with stems crushing the steerer?

Those expander bungs are flimsy and aren't made beefy enough to resist two 8Nm bolts on a stem anyway from what I can tell looking at these bungs and using them.

Other places where bands are clamped onto carbon tubes don't have bungs and metal tubes reinforcing anything, for example 5Nm on brake levers with 5Nm on shifters within an inch of each other, 5Nm on a front derailleur clamp, 4x 5Nm on the center of a carbon handlebar where the 4 stem bolts clamp.

Well I tried 40mm of spacers below the stem once on a 1" steeper and it sheared in the uci amateurs worlds.

10mm of spacers maybe 20mm. More is not safe from experience.

I work on the guideline of <80mm of carbon steerer above the headtube, with at least a 5mm spacer above the stem so the bung is supporting the steerer securely for the two stem bolts and enabling the topcap to do its job of preloading the headset before tightening those stem bolts.