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Spacers above stems (engineering not cosmetic) question

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  • April 3, 2017 at 11:54 am #26966
    velochris

    Bianchi are stating no more than 5mm of spacers should be above a stem.

    http://www.bianchi.com/global/news/news_detail.aspx?newsidmaster=360100&newstitle=important-and-urgent-safety-warning

    In the past I always left 10-15mm, did a few rides to ensure I was fully happy with my position and then cut down to leave 5mm above.

    Can anyone provide an engineering explanation why more than 5mm could be dangerous?

    Ps I use a torque wrench and set just below recommend torque to prevent tube damage.

    I try to make the expansion part of the expander bung sit in the middle of the stem. The one Bianchi provide is so small I doubt this makes any difference.

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Viewing 15 replies - 1 through 15 (of 15 total)
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  • April 4, 2017 at 1:09 pm #890427
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    trohos

    Safety i thing. In many cases

    Safety i thing. In many cases when you leave that space, the topcup comes above the handlebar height, sticking out.

    April 4, 2017 at 10:51 am #890425
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    davel
    gonedownhill wrote:
    beezus fufoon wrote:
    Woldsman wrote:
    Erm, hoping this a manufacturer-specific thing…

     

    is there a reason you’ve done that – like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

     

    It is also a good idea to not cut the steerer short in case of resale. I’m about as flexible as a breadstick and would never buy a bike where the steerer was cut such that I had to ride slammed.

    This is the excuse I use to explain why I’ve not cut my steerer and have got a longer and uncut ‘woldsman’ on 2 of my bikes.

    But it’s mainly because I just can’t be arsed… 

    April 4, 2017 at 10:04 am #890423
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    gonedownhill
    beezus fufoon wrote:
    Woldsman wrote:
    Erm, hoping this a manufacturer-specific thing…

     

    is there a reason you’ve done that – like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

     

    It is also a good idea to not cut the steerer short in case of resale. I’m about as flexible as a breadstick and would never buy a bike where the steerer was cut such that I had to ride slammed.

    April 4, 2017 at 4:15 am #890421
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    CXR94Di2
    dottigirl wrote:
    I managed to bollocks up a carbon steerer tube by overdoing the expander nut thing. Was almost impossible to get spacers on and off. Stem tension was probably the only thing keeping it together. Luckily, the same fork was subject to a product recall a short time later (for a different matter), and the replacement fork had an alloy  steerer. đŸ˜€

     

    You were lucky.  You only need a small clamping force for the top cap, then using the stem bolts to hold that position.  If, after a few rides the fork has a little play just reseat it again, literally a 2 minute job for 3 bolts

    April 3, 2017 at 10:30 pm #890419
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    Woldsman
    beezus fufoon wrote:
    Woldsman wrote:
    Erm, hoping this a manufacturer-specific thing…

     

    is there a reason you’ve done that – like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

    Exactly.  I felt that my back was uncomfortably rounded and I was also sticking up in the air (my main bike is a 2010 Kinesis Racelight T2 with a 10.5cm head tube). 

    So I fitted a longer stem and put the spacers on top for a similar position to that on my ‘winter bike’.

    April 3, 2017 at 10:15 pm #890417
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    beezus fufoon
    Woldsman wrote:
    Erm, hoping this a manufacturer-specific thing…

     

    is there a reason you’ve done that – like having a bit of a backache occasionally and switching the spacers and stem for a more relaxed position?

    April 3, 2017 at 10:13 pm #890415
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    dottigirl

    I managed to bollocks up a

    I managed to bollocks up a carbon steerer tube by overdoing the expander nut thing. Was almost impossible to get spacers on and off. Stem tension was probably the only thing keeping it together. Luckily, the same fork was subject to a product recall a short time later (for a different matter), and the replacement fork had an alloy  steerer. đŸ˜€

    April 3, 2017 at 9:52 pm #890413
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    Woldsman

    Erm, hoping this a

    Erm, hoping this a manufacturer-specific thing…

     

    April 3, 2017 at 9:21 pm #890411
    0
    wycombewheeler

    guyrwood wrote:

    guyrwood wrote:
    Not really, no. Keeping the spacers to 5mm over the stem keeps the star fangled nut within the stem clamp area which may help prevent the steerer tube being crushed if the stem’s over-tightened but that’s grasping at straws. Once the headsets adjusted and the stem clamp is tightened, any steerer tube above the stem is largely redundant from an engineering point of view.

    star nut position within the sterner tube is the same whether the spacers are above or below the stem.

    Bianchi are probably against it because it looks so bad and they like their bikes to be beautiful.

    April 3, 2017 at 7:49 pm #890409
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    Rich_cb

    Could be a safety thing.
    Could be a safety thing.

    Geraint Thomas ruptured his spleen on his spacer stack!

    April 3, 2017 at 7:09 pm #890407
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    Nixster

    Those are the engineering

    Those are the engineering reasons but why the 5mm limit I don’t know, although I believe Cannondale say the same.  The expander plug on my Supersix acts over about 10-12mm length of steerer and the stem clamp length must be close to 40mm so it’s entirely possible to position it within the clamp with more than 5mm of spacers over the stem.  So I file it under BS personally although there are obviously limits. 

    April 3, 2017 at 3:50 pm #890405
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    StraelGuy

    Sorry Dave, I’m not as posh

    Sorry Dave, I’m not as posh as you, my steerer tube is aluminium and has a SFN .

     

    Beezus, thats a good pont, having the expander wedge thing within the stem area would prevent accidental crushing of the steerer tube.

    April 3, 2017 at 1:32 pm #890403
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    beezus fufoon

    if the steerer is carbon then

    if the steerer is carbon then it’s preferable to have the expander bolt in line with the stem clamp

    April 3, 2017 at 12:27 pm #890401
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    DaveE128

    The only thing I can think of

    The only thing I can think of is that a steerer top cap retaining wedge (a star fangled nut in a carbon steerer probably isnt’ a great idea) exerts a significant outward force on the inside of the steerer that could perhaps crack it if not counteracted by the stem clamp. In my experience steerer spacers don’t sit tightly around the steerer (so you can slide them on and off easily and without causing scratches. Some even only make contact at three small points to save weight.) so they can’t provide this reinforcement. If there are too many spacers above the stem, the steerer top cap retaining wedge will be partially or completely above the level of the stem clamp.

    That’s my best guess as an engineer.

    Alternatively, they’re just trying to enforce “the rules” or something! đŸ˜‰

    April 3, 2017 at 12:16 pm #890399
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    StraelGuy

    Not really, no. Keeping the
    Not really, no. Keeping the spacers to 5mm over the stem keeps the star fangled nut within the stem clamp area which may help prevent the steerer tube being crushed if the stem’s over-tightened but that’s grasping at straws. Once the headsets adjusted and the stem clamp is tightened, any steerer tube above the stem is largely redundant from an engineering point of view.

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