Lowering stem, carbon steerer, bung length.

johngti
johngti Posts: 2,508
in Workshop
So, already chopped 10mm off the carbon steerer and lowered the stem. Initially had a 5mm spacer above and 20mm below, now have 10mm above and 15mm below. I’d like to drop it by another 5mm as I seem to be getting a bit more flexible but the expander bung basically is as long as can be seen in the picture below (between the arrows).

My question is, given that I don’t want to cut the steerer any shorter, should I fit a longer expander bung before lowering it any more? My inclination is yes, it would be a good idea but keen to hear from others.



Thanks!

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Comments

  • imposter2.0
    imposter2.0 Posts: 12,028
    Controversial comment - but the purpose of the bung is purely to provide a secure anchor point to allow the top cap to be tightened sufficiently. It serves no structural purpose, so its length should not be an issue.
  • MattFalle
    MattFalle Posts: 11,644
    imposter2.0 said:

    Controversial comment - but the purpose of the bung is purely to provide a secure anchor point to allow the top cap to be tightened sufficiently. It serves no structural purpose, so its length should not be an issue.

    agree with this.

    some may say that it provides further strength inside the steerer where you clamp the stem but hey, have you ried to compress and snap a carbon tube?

    if you can be bothered lower it down, if you can't i really wouldn't loose any sleep over it.
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • johngti
    johngti Posts: 2,508
    Well I guess Trek must know what they’re doing when they supply a shorter bung. They must realise that there will be many looking to lower it…
  • masjer
    masjer Posts: 2,653
    If bungs aren't used for structural support, why were they invented? Everyone would be using a star-nut. If you truly believe they're not structural (with a weight obsession in cycling) they would only be used as a tool for installing the headset, then removed.
    My guess is to keep using them for the intended purpose, resisting the crushing force from the stem pinch bolts.
  • MattFalle
    MattFalle Posts: 11,644
    Blimey, how tight are you doing up the stem bolts? Its 5Nm not FT with a 1/2" long bar.

    Seriously dude....
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • MattFalle
    MattFalle Posts: 11,644
    And yup, you can use a star nut if you want to.

    #controversialcomment
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • imposter2.0
    imposter2.0 Posts: 12,028
    masjer said:

    If bungs aren't used for structural support, why were they invented? Everyone would be using a star-nut. If you truly believe they're not structural (with a weight obsession in cycling) they would only be used as a tool for installing the headset, then removed.
    My guess is to keep using them for the intended purpose, resisting the crushing force from the stem pinch bolts.

    A star nut will score the inside of a carbon steerer, creating potential stress risers and failure points, which is why non-damaging bungs were 'invented'. This is less critical on an alu or steel steerer, which is why they are fine to use on those.

    If you take the time to study the composition and function of a typical expander bung, you'll see that they offer no effective barrier against 'crushing' forces. The clue is in the name 'expander'. Something which expands under tension is also going to collapse under load.

  • Ben6899
    Ben6899 Posts: 9,686
    masjer said:

    If bungs aren't used for structural support, why were they invented? Everyone would be using a star-nut. If you truly believe they're not structural (with a weight obsession in cycling) they would only be used as a tool for installing the headset, then removed.
    My guess is to keep using them for the intended purpose, resisting the crushing force from the stem pinch bolts.


    The main reason we don't use star nuts in carbon steerers, is the method of installation. You'll basically wreck the steerer before you've used it.
    Ben

    Bikes: Donhou DSS4 Custom | Condor Italia RC | Gios Megalite | Dolan Preffisio | Giant Bowery '76
    Instagram: https://www.instagram.com/ben_h_ppcc/
    Flickr: https://www.flickr.com/photos/143173475@N05/
  • Ncovidius
    Ncovidius Posts: 229
    Once you’ve got the preload you need and torqued up the stem bolts, you can chuck the top cap and bung in the bin, if you want to. It’s probably wise to keep hold of them in case you ever need to do anything requiring a removal of stem, but once they’ve done their job ( top cap and bung ) you could take them off / out. Carbon fibre is ridiculously strong stuff, but it doesn’t like shock. So banging a star nut into a carbon steerer wouldn’t be clever.
  • MattFalle
    MattFalle Posts: 11,644
    Ben6899 said:

    masjer said:

    If bungs aren't used for structural support, why were they invented? Everyone would be using a star-nut. If you truly believe they're not structural (with a weight obsession in cycling) they would only be used as a tool for installing the headset, then removed.
    My guess is to keep using them for the intended purpose, resisting the crushing force from the stem pinch bolts.


    The main reason we don't use star nuts in carbon steerers, is the method of installation. You'll basically wreck the steerer before you've used it.
    best tell that to my EC90 SLXs that have had a star nut in there for 13 years.

    oh.
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • MattFalle
    MattFalle Posts: 11,644
    edited August 2021
    Someone else had better tell Cannondale as well because they used to use them in carbon steerers as well - Systen Sixes for one had them fitted from the factory as standard.
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • masjer
    masjer Posts: 2,653
    Anyone actually not use a bung? Thought not. Only the designers of the forks know if it needs a bung or not. The installation manuals indicate they do.
    It's obvious a tube with an internal support is much stronger than one without hence the need for a bung.
    It's a bit like saying a car wheel designed to be held on by 5 studs only needs 3, it might hold, but I wouldn't try.
    Ncovidius said:

    Once you’ve got the preload you need and torqued up the stem bolts, you can chuck the top cap and bung in the bin, if you want to. It’s probably wise to keep hold of them in case you ever need to do anything requiring a removal of stem, but once they’ve done their job ( top cap and bung ) you could take them off / out. Carbon fibre is ridiculously strong stuff, but it doesn’t like shock. So banging a star nut into a carbon steerer wouldn’t be clever.

    Carbon fibre is strong like you state, but only in the directions it's designed to be. A top-tube for example is strong but can be crushed very easily. As for CF not liking a shock, you must ride on some very smooth roads and MTBs seem to do alright.
  • imposter2.0
    imposter2.0 Posts: 12,028
    masjer said:

    Anyone actually not use a bung? Thought not. Only the designers of the forks know if it needs a bung or not. The installation manuals indicate they do.
    It's obvious a tube with an internal support is much stronger than one without hence the need for a bung.
    It's a bit like saying a car wheel designed to be held on by 5 studs only needs 3, it might hold, but I wouldn't try.

    It sounds like you haven't read - or understood - any of the answers here.

    The purpose of the bung has been explained clearly enough and as already stated, the way the bungs are typically constructed means they can offer no comprehensive defence against compressive forces anyway.

    I think it was Canyon, a few years back, who offered a carbon steerer setup with an open top cap and no bung. They eventually returned to a more conventional setup but it is evidence - if any more is needed - that the bung serves no structural purpose.



  • Ben6899
    Ben6899 Posts: 9,686
    And there’s a reason that underground tunnels are tubular in section…
    Ben

    Bikes: Donhou DSS4 Custom | Condor Italia RC | Gios Megalite | Dolan Preffisio | Giant Bowery '76
    Instagram: https://www.instagram.com/ben_h_ppcc/
    Flickr: https://www.flickr.com/photos/143173475@N05/
  • imposter2.0
    imposter2.0 Posts: 12,028
    Ben6899 said:

    And there’s a reason that underground tunnels are tubular in section…

    Yep - the tube train itself acts like a bung. If the tunnel collapses, the train will hold it up. Or maybe not... ;)
  • MattFalle
    MattFalle Posts: 11,644
    imposter2.0 said:

    masjer said:

    Anyone actually not use a bung? Thought not. Only the designers of the forks know if it needs a bung or not. The installation manuals indicate they do.
    It's obvious a tube with an internal support is much stronger than one without hence the need for a bung.
    It's a bit like saying a car wheel designed to be held on by 5 studs only needs 3, it might hold, but I wouldn't try.

    It sounds like you haven't read - or understood - any of the answers here.


    its what he/she does best

    .
    The camera down the willy isn't anything like as bad as it sounds.
  • masjer
    masjer Posts: 2,653
    Imposter, Canyon DESIGNED the forks not to use a bung, big, big difference.
    So every manufacturer uses one, but you state they're superfluous. You stated you have a controversial opinion about not using bungs, now you state it's fact.
    I'll stay with a mainstream opinion and just keep using one.
  • MattFalle
    MattFalle Posts: 11,644
    starnuts will only score carbon if you try and rip them upwards.

    they slide in and sit there and the minuscule amount on Nm you use to preliad bearings has no hope of moving them.

    think of it as a fish hook.

    unless you're Masjer and his/her 1/2" breaker bar that is....
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • masjer
    masjer Posts: 2,653
    So, everyone stating you don't need a bung, uses a bung. The manufacturers instruct you to use a bung. I rest my case. Use a damn bung.

    MF, you don't need a bung as your bikes are shed trinkets that never go on the road but I can sure think of a use for your bung.
  • imposter2.0
    imposter2.0 Posts: 12,028
    masjer said:

    Imposter, Canyon DESIGNED the forks not to use a bung, big, big difference.
    So every manufacturer uses one, but you state they're superfluous. You stated you have a controversial opinion about not using bungs, now you state it's fact.
    I'll stay with a mainstream opinion and just keep using one.

    The entire Canyon assembly was designed not to use a bung. I don't believe there was anything particularly unusual about the steerer tube design, unless you can demonstrate otherwise.

    I don't think you've understood what I said. I didn't say bungs were superfluous, as they serve a purpose. That purpose being to provide a secure anchor point for the top cap bolt. Go back and re-read what I said.

    The opinion is self-evidently controversial, as you are disagreeing with it. The trouble is you are disagreeing without actually presenting any compelling argument to the contrary.
  • imposter2.0
    imposter2.0 Posts: 12,028
    masjer said:

    So, everyone stating you don't need a bung, uses a bung. The manufacturers instruct you to use a bung. I rest my case. Use a damn bung.

    Once again, nobody has ever said 'you don't need a bung'. Please read what people have written.
  • masjer
    masjer Posts: 2,653
    imposter2.0 said:

    masjer said:

    So, everyone stating you don't need a bung, uses a bung. The manufacturers instruct you to use a bung. I rest my case. Use a damn bung.

    Once again, nobody has ever said 'you don't need a bung'. Please read what people have written.
    So, now we agree we do need bungs. You think they're not structural, but give zero evidence. I strongly disagree. Don't forget, it was you who started this "controversial statement"
    Since people spend a fortune on lightweight forks, why would they leave a heavy metal bung fitted if after installing they aren't needed? Clue they are needed- structurally.
  • lesfirth
    lesfirth Posts: 1,382
    Ben6899 said:

    And there’s a reason that underground tunnels are tubular in section…

    The reason is that it is the easiest shape to make them.
  • pblakeney
    pblakeney Posts: 27,314
    lesfirth said:

    Ben6899 said:

    And there’s a reason that underground tunnels are tubular in section…

    The reason is that it is the easiest shape to make them.
    Only it isn't.
    On either front.
    The above may be fact, or fiction, I may be serious, I may be jesting.
    I am not sure. You have no chance.
    Veronese68 wrote:
    PB is the most sensible person on here.
  • imposter2.0
    imposter2.0 Posts: 12,028
    masjer said:



    So, now we agree we do need bungs. You think they're not structural, but give zero evidence. I strongly disagree. Don't forget, it was you who started this "controversial statement"
    Since people spend a fortune on lightweight forks, why would they leave a heavy metal bung fitted if after installing they aren't needed? Clue they are needed- structurally.

    I have already explained several posts ago to you why the bung is needed. Bungs are not exactly 'heavy' - not sure if you've ever handled one. They are generally made of aluminium, with a couple of spring wires or rubber bands to keep the alu split sleeve intact. If you can explain what level of structural reinforcement a thin alu split sleeve provides - especially in the gap between the two expander wedges, then I'm listening.

  • masjer
    masjer Posts: 2,653
    I'll try and explain in a way you'll understand. The bung is fitted with about 8nm. This imparts a considerable outward force on the internal walls of the tube. The stem uses pinch bolts, causing an inward force on the tube walls. Both inward and outward pressures cancel each other making for a stronger tube. Simple.
    An analagy- think of an unopened coke can. You can't crush it because the walls are being pushed outward. Open can, pressure is released, you'll probably be able to crush the can.
    In terms of weight saving a bung is very heavy since people spend vast sums to save just a few grams.Simple.
    Anyway think of Gorilla Greipel sprinting, he'll be wanting a strong stem/steerer combo ie with a bung in it.
  • MattFalle
    MattFalle Posts: 11,644
    masjer said:

    So, everyone stating you don't need a bung, uses a bung. The manufacturers instruct you to use a bung. I rest my case. Use a damn bung.

    MF, you don't need a bung as your bikes are shed trinkets that never go on the road but I can sure think of a use for your bung.

    Cannondale don't use a bung.
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • MattFalle
    MattFalle Posts: 11,644
    masjer said:

    I'll try and explain in a way you'll understand. The bung is fitted with about 8nm. This imparts a considerable outward force on the internal walls of the tube. The stem uses pinch bolts, causing an inward force on the tube walls. Both inward and outward pressures cancel each other making for a stronger tube. Simple.
    An analagy- think of an unopened coke can. You can't crush it because the walls are being pushed outward. Open can, pressure is released, you'll probably be able to crush the can.
    In terms of weight saving a bung is very heavy since people spend vast sums to save just a few grams.Simple.
    Anyway think of Gorilla Greipel sprinting, he'll be wanting a strong stem/steerer combo ie with a bung in it.

    eh?

    what you on about Willis?
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • MattFalle
    MattFalle Posts: 11,644
    imposter2.0 said:

    masjer said:

    So, everyone stating you don't need a bung, uses a bung. The manufacturers instruct you to use a bung. I rest my case. Use a damn bung.

    Once again, nobody has ever said 'you don't need a bung'. Please read what people have written.
    bloke above and Canyon both say you don't.
    .
    The camera down the willy isn't anything like as bad as it sounds.
  • MattFalle
    MattFalle Posts: 11,644
    edited August 2021
    masjer said:

    imposter2.0 said:

    masjer said:

    So, everyone stating you don't need a bung, uses a bung. The manufacturers instruct you to use a bung. I rest my case. Use a damn bung.

    Once again, nobody has ever said 'you don't need a bung'. Please read what people have written.
    So, now we agree we do need bungs. You think they're not structural, but give zero evidence. I strongly disagree. Don't forget, it was you who started this "controversial statement"
    Since people spend a fortune on lightweight forks, why would they leave a heavy metal bung fitted if after installing they aren't needed? Clue they are needed- structurally.
    no, we don't need bungs.

    and no, they aren't needed structurally.

    .
    The camera down the willy isn't anything like as bad as it sounds.

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