Did I bust my fork ??

For discussions about bikes and equipment.
AndyA
Posts: 428
Joined: 21 Mar 2009, 9:16pm
Location: Edinburgh

Re: Did I bust my fork ??

Postby AndyA » 30 Jan 2020, 9:02pm

thatsnotmyname wrote:
AndyA wrote:The bung definitely provides resistance to crushing the steerer. It probably also reduces the stress on the steerer if it is long enough to finish beneath the bottom of the stem


Most steerer bungs consist of a couple of expander wedges (one at the top, one at the bottom), surrounded by a thin aluminium split sleeve. If you can explain to me how a device like that adds to the structural integrity of the steerer, then I’m listening...


It's self-evident that it's harder to crush a tube if there is something inside the tube. Think about the way the top wedge and middle sleeve fit together, they make a part that's very thick and pushed very tightly against the inside of the steerer at the top where the risk of crushing is greatest.

The second point about reducing stress on the steerer is pretty marginal, but real all the same. If the bung extends below the stem, some of the loads and stresses will be shared between the steerer and bung. The stress riser caused by the end of the stem is reduced because the change in effective thickness is smaller with a bung that extends below the bottom of the stem.

bungle73
Posts: 43
Joined: 26 Feb 2016, 10:19pm

Re: Did I bust my fork ??

Postby bungle73 » 30 Jan 2020, 9:37pm

Marcus Aurelius wrote:When you say Teflon grease the “headset interfaces” I hope you don’t mean the stem against the steerer. Do not put any grease there.


Obviously

If you’re referring to the bottom of the lower bearing and the crown race ( or fork lip if it’s the integrated / raceless type ) and the top of the upper bearing and the top cover, then the Teflon grease will be just fine.


OK, thanks. I did message Speciliized about it, and they told me "So long as the grease you wish to use is safe for use with carbon components, it should be just fine to use in areas where it might contact the steerer tube". They could not have given me a more useless answer if they had said "It will be ok as long as it is ok".

How am I supposed to know if its "safe" for use with carbon components? The tub doesn't say - probably because when I bought it carbon bikes and parts weren't really a thing.

thatsnotmyname
Posts: 386
Joined: 23 Jan 2020, 10:23am

Re: Did I bust my fork ??

Postby thatsnotmyname » 30 Jan 2020, 11:20pm

AndyA wrote:It's self-evident that it's harder to crush a tube if there is something inside the tube. Think about the way the top wedge and middle sleeve fit together, they make a part that's very thick and pushed very tightly against the inside of the steerer at the top where the risk of crushing is greatest.


Whatever is inside the tube will only 'reinforce' it if it has any structural integrity itself. A split aluminium sleeve which has been expanded to fit the cavity inside does not meet that criteria. Come on guys, this is basic stuff.

AndyA wrote:The second point about reducing stress on the steerer is pretty marginal, but real all the same. If the bung extends below the stem, some of the loads and stresses will be shared between the steerer and bung. The stress riser caused by the end of the stem is reduced because the change in effective thickness is smaller with a bung that extends below the bottom of the stem.


The bung does not share any load under normal tightening forces. If you think it does, then please explain how much of the load it shares and what the 'crushing load' of a typical steerer bung is. You won't find any data on it. Canyon used to offer stem/carbon steerer top cap (Acros) with no bung at all - if a bung was so critical, why would they do that?

AndyA
Posts: 428
Joined: 21 Mar 2009, 9:16pm
Location: Edinburgh

Re: Did I bust my fork ??

Postby AndyA » 30 Jan 2020, 11:52pm

thatsnotmyname wrote:
AndyA wrote:It's self-evident that it's harder to crush a tube if there is something inside the tube. Think about the way the top wedge and middle sleeve fit together, they make a part that's very thick and pushed very tightly against the inside of the steerer at the top where the risk of crushing is greatest.


Whatever is inside the tube will only 'reinforce' it if it has any structural integrity itself. A split aluminium sleeve which has been expanded to fit the cavity inside does not meet that criteria. Come on guys, this is basic stuff.


The top and bottom part of the bung assembly results in a part that's 5-7mm thick almunium. The middle part will vary between 1-3mm thick depending on design, but that's plenty to stop someone from damaging the carbon steerer by overtightening the clamp.

thatsnotmyname wrote:
AndyA wrote:The second point about reducing stress on the steerer is pretty marginal, but real all the same. If the bung extends below the stem, some of the loads and stresses will be shared between the steerer and bung. The stress riser caused by the end of the stem is reduced because the change in effective thickness is smaller with a bung that extends below the bottom of the stem.


The bung does not share any load under normal tightening forces. If you think it does, then please explain how much of the load it shares and what the 'crushing load' of a typical steerer bung is. You won't find any data on it. Canyon used to offer stem/carbon steerer top cap (Acros) with no bung at all - if a bung was so critical, why would they do that?


As mentioned upthread, people have damaged the steerer on bikes with no bung. Possibly through cackhandedness, definitely exacerbated by the lack of bung. Do you disagree that having a bung there reduces the risk of crushing damage to the tube? Why?

thatsnotmyname
Posts: 386
Joined: 23 Jan 2020, 10:23am

Re: Did I bust my fork ??

Postby thatsnotmyname » 31 Jan 2020, 8:25am

AndyA wrote:The top and bottom part of the bung assembly results in a part that's 5-7mm thick almunium. The middle part will vary between 1-3mm thick depending on design, but that's plenty to stop someone from damaging the carbon steerer by overtightening the clamp.


So the top and bottom parts of the bung offer reinforcement, but not the middle? :lol:

AndyA wrote:As mentioned upthread, people have damaged the steerer on bikes with no bung. Possibly through cackhandedness, definitely exacerbated by the lack of bung. Do you disagree that having a bung there reduces the risk of crushing damage to the tube? Why?


There you go again, making claims you can’t support. If you claim that it is ‘definitely exacerbated by the lack of a bung’ then you should be able to explain what these forces are and how much resistance is contributed by the bung - but you can’t. At the same time, you could also explain why other parts of a bike where carbon might be clamped do not require the same levels of reinforcement - eg, carbon seatpost/clamp interface, carbon bar/stem interface and brake lever/carbon bar interface. None of those other parts require a 'bung' of any sort to be used for reinforcement, despite similar - and often greater - clamping forces.

Finally, I already said that bungs do not offer structural benefit and that no structural benefit is needed, and if you can find any manufacturer descriptions of bungs which claim to offer anything other than secure headset fastening, then I’d be happy to see them.

colin54
Posts: 1176
Joined: 24 Sep 2013, 4:34pm

Re: Did I bust my fork ??

Postby colin54 » 31 Jan 2020, 10:19am

I watched this video a while back it's by an Australian carbon repairer and tester, he shows various different designs of fork steerer compression plugs and what he thinks of them. Worth watching all the way through ( about 11minutes long). He illustrates the need for a well designed plug towards the end of the video .
A lot of interesting stuff on this channel about carbon bicycles.

https://www.youtube.com/watch?v=8bSnbjHiFXc
A carbon repair and test company in the UK from a google search, I think they use ultra sound to test
suspect components, it might be worth asking their or a similar company's opinion if you are still at all unsure.
https://carbonbikerepair.co.uk/#services

slowster
Posts: 1198
Joined: 7 Jul 2017, 10:37am

Re: Did I bust my fork ??

Postby slowster » 31 Jan 2020, 11:01am

thatsnotmyname wrote:Whatever is inside the tube will only 'reinforce' it if it has any structural integrity itself. A split aluminium sleeve which has been expanded to fit the cavity inside does not meet that criteria. Come on guys, this is basic stuff.

If it's basic stuff to you, then you should be able to provide technical references and links to support your assertions. Instead, dismissive comments like "Come on guys, this is basic stuff" and "There you go again, making claims you can’t support" sound like bluff and bluster.

thatsnotmyname wrote: Canyon used to offer stem/carbon steerer top cap (Acros) with no bung at all - if a bung was so critical, why would they do that?

You cherrypick your authorities. You infer that because Canyon used to do that, it must be OK, and dismiss the vast majority of the other manufacturers who do and say other otherwise because they are only doing it "So they have their ar5es covered in the event of litigation".

thatsnotmyname wrote:Whatever is inside the tube will only 'reinforce' it if it has any structural integrity itself. A split aluminium sleeve which has been expanded to fit the cavity inside does not meet that criteria.

That statement says to me that you are not an engineer.

thatsnotmyname
Posts: 386
Joined: 23 Jan 2020, 10:23am

Re: Did I bust my fork ??

Postby thatsnotmyname » 31 Jan 2020, 11:16am

slowster wrote:If it's basic stuff to you, then you should be able to provide technical references and links to support your assertions. Instead, dismissive comments like "Come on guys, this is basic stuff" and "There you go again, making claims you can’t support" sound like bluff and bluster.


You're asking me to prove that something which wasn't designed for a particular task, cannot indeed perform that task??? My assertion is that they are not necessary for structural support - only for holding on the top compression cap. I can't find a steerer bung on the market which claims to offer such support. Manufacturers themselves do not seem to claim any such ability for these devices. You'd think if it was such a critical issue, they might mention it. Anyone who disagrees with that needs to be able to explain why. I can't prove a negative.


slowster wrote:You cherrypick your authorities. You infer that because Canyon used to do that, it must be OK, and dismiss the vast majority of the other manufacturers who do and say other otherwise because they are only doing it "So they have their ar5es covered in the event of litigation".


I offered an example of a manufacturer who did not use a steerer bung on a carbon steerer tube in order to underline the point I'm making. Canyon found another way of keeping the headset in compression, so dispensed with the steerer bung. It's not an argument against the wider or legitimate use of steerer bungs, it's simply evidence that one is not necessary to reinforce the steerer tube against clamping forces.

thatsnotmyname wrote:Whatever is inside the tube will only 'reinforce' it if it has any structural integrity itself. A split aluminium sleeve which has been expanded to fit the cavity inside does not meet that criteria.

slowster wrote:That statement says to me that you are not an engineer.


Well, I never claimed to be an engineer, but feel free to discredit my contention with facts and supporting data, rather than a dismissive ad hominem. Ironically, you accused me of being 'dismissive' earlier.

AndyA
Posts: 428
Joined: 21 Mar 2009, 9:16pm
Location: Edinburgh

Re: Did I bust my fork ??

Postby AndyA » 31 Jan 2020, 9:24pm

thatsnotmyname wrote:
AndyA wrote:The top and bottom part of the bung assembly results in a part that's 5-7mm thick almunium. The middle part will vary between 1-3mm thick depending on design, but that's plenty to stop someone from damaging the carbon steerer by overtightening the clamp.


So the top and bottom parts of the bung offer reinforcement, but not the middle? :lol:


No the top and the bottom offer more reinforcement in the usual design, but there is still reinforcement. Apples to oranges comparison here, but a aluminium downtube is often less than 1mm thick in the middle butted section.

thatsnotmyname wrote:
AndyA wrote:As mentioned upthread, people have damaged the steerer on bikes with no bung. Possibly through cackhandedness, definitely exacerbated by the lack of bung. Do you disagree that having a bung there reduces the risk of crushing damage to the tube? Why?


There you go again, making claims you can’t support. If you claim that it is ‘definitely exacerbated by the lack of a bung’ then you should be able to explain what these forces are and how much resistance is contributed by the bung - but you can’t. At the same time, you could also explain why other parts of a bike where carbon might be clamped do not require the same levels of reinforcement - eg, carbon seatpost/clamp interface, carbon bar/stem interface and brake lever/carbon bar interface. None of those other parts require a 'bung' of any sort to be used for reinforcement, despite similar - and often greater - clamping forces.

Finally, I already said that bungs do not offer structural benefit and that no structural benefit is needed, and if you can find any manufacturer descriptions of bungs which claim to offer anything other than secure headset fastening, then I’d be happy to see them.


You're being so annoying about this that I might actually do some experiments! I've got a whole bunch of dead carbon forks that I can experiment on.
Meanwhile, see page 20 and 21 for Deda's instruction on how they think carbon steerers, stems and bungs should be fitted together.
http://www.dedaelementi.com/wp-content/ ... web-EN.pdf

Deda Elamenti wrote: Make sure that the expander (not included) has enough length to ensure the handlebar stem rear bolts seat on the expander portion of the fork steerer. This will reduce the risk of fork steerer notching while tightening the rear bolts
dedastem.jpg


thatsnotmyname
Posts: 386
Joined: 23 Jan 2020, 10:23am

Re: Did I bust my fork ??

Postby thatsnotmyname » 31 Jan 2020, 9:43pm

AndyA wrote:No the top and the bottom offer more reinforcement in the usual design, but there is still reinforcement. Apples to oranges comparison here, but a aluminium downtube is often less than 1mm thick in the middle butted section.


That comparison is certainly 'apples & oranges' to the point where it's not even relevant. The thinner the wall thickness of a downtube (in this case aluminium), the greater the overall diamater needs to be. A split sleeve which is expanded into the gap within the steerer tube will offer no reinforcement. Despite being asked twice now (or is it three times) you have still not explained how it might do that.

AndyA wrote:You're being so annoying about this that I might actually do some experiments! I've got a whole bunch of dead carbon forks that I can experiment on.
Meanwhile, see page 20 and 21 for Deda's instruction on how they think carbon steerers, stems and bungs should be fitted together.
http://www.dedaelementi.com/wp-content/ ... web-EN.pdf

Deda Elamenti wrote: Make sure that the expander (not included) has enough length to ensure the handlebar stem rear bolts seat on the expander portion of the fork steerer. This will reduce the risk of fork steerer notching while tightening the rear bolts
dedastem.jpg


You really think I'm being 'annoying'? Or is it just that you can't put forward a convincing counter argument? Anyway, in terms of the Deda stuff - lots of manufacturers have 'belt & braces' guidelines like that. Some of them seem to have no basis in fact (much like the Specialized documentation discussed earlier) - but the 'notching' issue seems to relate to the outside of the steerer tube and not to anything being discussed here.

AndyA
Posts: 428
Joined: 21 Mar 2009, 9:16pm
Location: Edinburgh

Re: Did I bust my fork ??

Postby AndyA » 1 Feb 2020, 1:05am

thatsnotmyname wrote:You really think I'm being 'annoying'? Or is it just that you can't put forward a convincing counter argument? Anyway, in terms of the Deda stuff - lots of manufacturers have 'belt & braces' guidelines like that. Some of them seem to have no basis in fact (much like the Specialized documentation discussed earlier) - but the 'notching' issue seems to relate to the outside of the steerer tube and not to anything being discussed here.


Deda Elamenti wrote:The lower lip of the stem exerts pressure on an area not supported by the expander which will cause fork steerer ovalization and notching and serious risks for your safety: the steerer could suddenly break under stress!

slowster
Posts: 1198
Joined: 7 Jul 2017, 10:37am

Re: Did I bust my fork ??

Postby slowster » 1 Feb 2020, 10:11am

thatsnotmyname wrote:
slowster wrote:If it's basic stuff to you, then you should be able to provide technical references and links to support your assertions. Instead, dismissive comments like "Come on guys, this is basic stuff" and "There you go again, making claims you can’t support" sound like bluff and bluster.

You're asking me to prove that something which wasn't designed for a particular task, cannot indeed perform that task??? My assertion is that they are not necessary for structural support - only for holding on the top compression cap. I can't find a steerer bung on the market which claims to offer such support. Manufacturers themselves do not seem to claim any such ability for these devices. You'd think if it was such a critical issue, they might mention it. Anyone who disagrees with that needs to be able to explain why. I can't prove a negative.

The steerer bungs themselves are not the component that is at risk of failure or causing damage, and a lot of them are generic items, so it's not surprising that the manufacturers of the bungs themselves do not bother to make such claims: they don't need to. It's more likely that such claims would be made for a branded aftermarket bung, e.g. like this one ("Extra long 90mm height reinforces carbon steerer tube beneath the stem").

Since the structural function of the bung is to resist distortion and crushing of the carbon fibre as a result of (most likely careless/excessive) tightening of the stem bolts, it's more likely to be the manufacturers of stems, like Deda, and more especially large manufacturers like Trek and Specialized who make complete bikes with their own brand components throughout, who specify how the steerer bung should be installed and stipulate that it should be in the same plane as the stem.

thatsnotmyname wrote:I offered an example of a manufacturer who did not use a steerer bung on a carbon steerer tube in order to underline the point I'm making. Canyon found another way of keeping the headset in compression, so dispensed with the steerer bung. It's not an argument against the wider or legitimate use of steerer bungs, it's simply evidence that one is not necessary to reinforce the steerer tube against clamping forces.

As I said, you cherrypick your sources to support your argument: according to you Canyon doing differently is 'evidence', but guidance from Trek, Specialiazed, Deda and anyone else is just the manufacturer covering their backside against litigation or a 'belt and braces' approach.

Canyon doing differently is not evidence. The history of bicycles is littered with examples of manufacturers who came up with designs which were flawed or dangerous.

thatsnotmyname wrote:
thatsnotmyname wrote:Whatever is inside the tube will only 'reinforce' it if it has any structural integrity itself. A split aluminium sleeve which has been expanded to fit the cavity inside does not meet that criteria. Come on guys, this is basic stuff.
slowster wrote:That statement says to me that you are not an engineer.

Well, I never claimed to be an engineer, but feel free to discredit my contention with facts and supporting data, rather than a dismissive ad hominem. Ironically, you accused me of being 'dismissive' earlier.

It's not an ad hominem. With remarks like "Come on guys, this is basic stuff" you've implied that you have significant knowledge and expertise. However, when challenged, you expect others to prove their point, rather than provide evidence why you are right. It's normal for manufacturers to issue instructions on how their components must be used without providing the technical knowledge and test data they have which has led to those guidelines. If you tell people that those guidelines can be ignored, then the onus is on you to provide evidence, not the manufacturers.

I too am not an engineer, but I know that it's not difficult to find examples of a material or component which has little or no structural integrity or strength itself, but which may be very strong when combined with another material or component.

As I understand it the ability of the bung to perform a structural function boils down to whether it exerts an outward (expanding) force on the steerer tube, which I believe it does*, and the extent to which that outward force does or does not make a difference to the steerer tube being crushed, deformed or stressed by (careless/excessive) tightening of the stem. It's probably not going to be possible to give a definitive answer to the latter point without having some very sensitive measuring equiment or testing to destruction. And that's unlikely to happen and the results be put in the public domain unless and until there are some indicators that the current widely recommended practices regarding stem fitting and bungs are inadequate (whereas the reports I've seen so far of failures have probably been due to manufacturers' instructions not being followed). That sort of testing is probably already undertaken by some manufacturers, but they would consider the results commercially confidential, because knowing what and where the failure thresholds are gives them an advantage in knowing how to design cheaper/lighter/stronger parts.

* I've seen at least one report where someone was able to measure a slight increase in the external diameter of a carbon steerer when the bung was tightened.

To be clear, I expect that >95% of the time this issue is probably not critical, because of the margins of safety involved. However, the laws of large numbers probably mean that over time there will be a small number of people who come to grief because they have eroded one, or likely more, of those margins of safety, such as positioning the stem in line with the steerer bung.