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How many spacers safe above stem on carbon steerer

zhangzhang Posts: 222
edited May 2015 in Workshop
I've got a full carbon frameset which has quite a long steerer tube and a very upright position. I'd like to slam the stem but dont want to commit to trimming the steerer tube in case I need to change my position or if I ever sell the frame. How many spacers is it safe to have on top of the stem?

The frame in question is a Planet X model and they don't have any guidance regarding this, but Cannondale say with their forks you should have a single spacer above the stem so the clamping forces of the stem are distributed around the fork and expander bung evenly, whereas Specialized explicitly state that no spacers should be used above the stem.

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  • nicklousenicklouse Posts: 81,520 Lives Here
    Makes no difference.
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  • 964cup964cup Posts: 1,359
    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.
  • drlodgedrlodge Posts: 4,824
    You *should* have a 5mm spacer above the stem, this ensures the stem is not crimping the top of the steerer tube. With a 5mm spacer you probably want 2-3mm of steerer protuding. If you had no spacer, the steerer tube would need to finish 2-3mm before the end of the stem, which isn't ideal.
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  • dabberdabber Posts: 1,555
    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.
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  • drlodgedrlodge Posts: 4,824
    It doesn't matter how many spacers you have above the steerer (other than for aesthetic reasons), there are two rules in my book:
    - the top of the steerer should protrude above the stem, hence why I said at least a 5mm spacer
    - the stack of spacers below the stem should not exceed the width of the steerer, generally recognised to be 25mm-30mm max
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  • ai_1ai_1 Posts: 3,060
    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.
  • rafletcherrafletcher Posts: 1,391
    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.
  • keef66keef66 Posts: 13,286
    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
  • londoncommuterlondoncommuter Posts: 1,518
    Scott say:

    Never use more than 40mm stack height of spacers between headset and stem.

    Do not use more than 5mm stack height of spacer above the stem between the
    topcap of the headset and the stem.

    Do use minimum 5mm stack height of spacer below the stem between the cap of
    the headset and the stem.


    As mentioned by others, I thought the limit on spacers above the stem was to ensure the bung supported the steerer you're otherwise crushing with the stem.
  • veronese68veronese68 Posts: 22,288 Lives Here
    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.
  • ai_1ai_1 Posts: 3,060
    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.
    Your second point is certainly correct. It's purely the impact on bung position and it's support of stem clamping that's a potential issue.
    My Canyon uses the Acros headset which doesn't need pre-tensioning via the bung and steerer cap and therefore there is no bung and the stem is, by design, clamped to the unsupported steerer tube. Canyon will undoubtedly have designed their steerer with this in mind however the question I don't know the answer to is whether other steerers are designed on the assumption that a bung will be correctly located. Presumably there's allowance made for some setup imperfections but they will still increase the risk of an issue, I would think.
  • keef66keef66 Posts: 13,286
    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...
  • jgsijgsi Posts: 5,025
    http://cyclingtips.com.au/2015/01/2015- ... o-peloton/
    above spacer argument...
    yes.. yes... no ... no .. yes.. no scroll down to the C'dale
    naturally no ! :-D
  • MatthewfalleMatthewfalle Posts: 17,571
    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.
    Postby team47b » Sun Jun 28, 2015 11:53 am

    De Sisti wrote:
    This is one of the silliest threads I've come across. :lol:

    Recognition at last Matthew, well done!, a justified honour :D
    smithy21 wrote:

    He's right you know.
  • zhangzhang Posts: 222
    Thanks for the input guys.
  • 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.
  • dric322 said:

    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?

    Depends on make/model of steerer and bung combination. E.g. Giant have a bung that has a rim that sits on top of the cut steerer. This means that they should not have more than a 5mm spacer on to of the stem, as otherwise the stem will potentially crush the steerer.
    Also, one has to consider the shock transmitted up the forks when hitting a pot hole etc, as a pure clamping force on a piece of cut steerer in a vice is not a valid test.
    Cannondale had an issue where riders putting spacers on top of the stem had incidents of steerer sheering off at rather inconvenient moments.
    Some info in this thread:
    https://singletrackworld.com/forum/topic/cannondale-carbon-steerer-spacer-above-stem-or-no/
  • 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.
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