Carbon Steerer Tube plus Ignorance; DAMN!
Discussion
I recently had my (carbon lower, aluminium steerer) CAADX fork replaced under the model-wide recall. Once home I wanted to adjust my spacer arrangement to achieve the correct position. Made the adjustments and tightened everything up (compression bung rather than star nut on the replacement, top cap bolt, stem bolts) with an allen key. All fairly tight.
Failed to appreciate that the steerer on the new fork is carbon until everything was back together. Just taken another look out of curiosity and the steerer tube (internal and external) is visibly deformed into the profile of the overtightened stem. The compression bung now no longer seems to 'bite down' and there is major play in the headset.
Knackered then? Only consolation is that I didn't find out when the steerer failed.
An expensive lesson learned I think.
Failed to appreciate that the steerer on the new fork is carbon until everything was back together. Just taken another look out of curiosity and the steerer tube (internal and external) is visibly deformed into the profile of the overtightened stem. The compression bung now no longer seems to 'bite down' and there is major play in the headset.
Knackered then? Only consolation is that I didn't find out when the steerer failed.
An expensive lesson learned I think.
Edited by AdviceHunter on Monday 5th October 15:06
I would be wary sadly of any damage to the steerer . It depends on what damaged has been caused, but must be aware of leverage on the bolts to not damage it. A simple torque driver for under £50 or so should be used on whatever bolts are on a bike. But hindsight is a wonderful thing.
As to what to do ? I would get it assessed by someone who deals in carbon and see if the deformation is a structural issue . You could bond in a new bung , if it is deemed useable , as when the stem is tight , the bung does nothing. The bung can actually stiffen the steerer, internally with regards to clamping the stem , which a star fangle nut wont. The other alternative is to remove spacers ( if possible) and try and gain the height back, if thats what you are trying to achieve with a stem with a rise.
As to what to do ? I would get it assessed by someone who deals in carbon and see if the deformation is a structural issue . You could bond in a new bung , if it is deemed useable , as when the stem is tight , the bung does nothing. The bung can actually stiffen the steerer, internally with regards to clamping the stem , which a star fangle nut wont. The other alternative is to remove spacers ( if possible) and try and gain the height back, if thats what you are trying to achieve with a stem with a rise.
AdviceHunter said:
I recently had my (carbon lower, aluminium steerer) CAADX fork replaced under the model-wide recall. Once home I wanted to adjust my spacer arrangement to achieve the correct position. Made the adjustments and tightened everything up (compression bung rather than star nut on the replacement, top cap bolt, stem bolts) with an allen key. All fairly tight.
Failed to appreciate that the steerer on the new fork is carbon until everything was back together. Just taken another look out of curiosity and the steerer tube (internal and external) is visibly deformed into the profile of the overtightened stem. The compression bung now no longer seems to 'bite down' and there is major play in the headset.
Knackered then? Only consolation is that I didn't find out when the steerer failed.
An expensive lesson learned I think.
It may be okay. C.F. isn’t as weak as some people think. It may just need a gentle heating with a hot air gun, with the bung in place. You will need a torque wrench to gauge 5Nm of torque on the stem bolts though.Failed to appreciate that the steerer on the new fork is carbon until everything was back together. Just taken another look out of curiosity and the steerer tube (internal and external) is visibly deformed into the profile of the overtightened stem. The compression bung now no longer seems to 'bite down' and there is major play in the headset.
Knackered then? Only consolation is that I didn't find out when the steerer failed.
An expensive lesson learned I think.
Edited by AdviceHunter on Monday 5th October 15:06
CF is nowhere near as weak as people seem to think, and a steerer particularly so - given the load it has to resist when you smack a pothole and try to drive the bearings through the tube, I'd happily bet that you'll strip the bolts on the stem before crushing the steerer - or at the very least need something more than the average 4mm allen wrench to achieve enough torque. While torque wrenches are nice, generally all that is needed is a bit of common sense. I.e. I'd be very dubious that you've crushed it.
THAT said, if the tube is deformed, I strongly suspect you're done. Repairs to frames are entirely achievable, I'm rather more doubtful that a steerer tube can be reasonably or cost effectively achieved
The only metal tube I've seen bonded into a steerer is a sleeve to allow a star nut to be used. Always thought it was an odd solution, but it's not really there to stop the carbon being crushed.. logically you'd just use more carbon, not introduce an inferior material and the potential for electrolytic corrosion
THAT said, if the tube is deformed, I strongly suspect you're done. Repairs to frames are entirely achievable, I'm rather more doubtful that a steerer tube can be reasonably or cost effectively achieved
The only metal tube I've seen bonded into a steerer is a sleeve to allow a star nut to be used. Always thought it was an odd solution, but it's not really there to stop the carbon being crushed.. logically you'd just use more carbon, not introduce an inferior material and the potential for electrolytic corrosion
Edited by upsidedownmark on Monday 5th October 23:16
The two posters above are correct that CF is stronger than a lot of people think - you can build F1 cars and jet fighters from it - it is more than capable of resisting loading from normal use in a bike.
However (and its a big however) - this does not mean that it is capable of withstanding a load the designers have not allowed for (see stories of crushed steerers, frames ruined by bike rack clamps etc.). It also does not mean that once it has deformed, it will still behave how you want it to.
There are some very important things to bear in mind:
CF is not equally strong in all directions.
CF frames and components by design are strong where they need to be and weak elsewhere - thick, chunky carbon around your BB, thin, weedy carbon for your seatstays. Downtubes that are thin in the middle, but thick at the ends... It is part of the beauty of CF - its why designers can make light frames that are super stiff when pedaling, but do not remove your fillings. it also means that if you add in a load which should not be there, you may break something.
Damage to CF can be much harder to detect than damage to alloy/steel - you can see a crack in metal, but potentially not see anything in carbon. If you dent alloy, you can see it, if you dent carbon it may look fine, but not be structurally sound.
Finally - do you feel lucky? I can't think of many bike bits I would like to fail less than a steerer tube. For me, I would always err on the side of caution. If you can see it is visibly deformed, are you going to be happy riding it?
I don't think I really need to tell you this, but do not take a heat gun to it.
However (and its a big however) - this does not mean that it is capable of withstanding a load the designers have not allowed for (see stories of crushed steerers, frames ruined by bike rack clamps etc.). It also does not mean that once it has deformed, it will still behave how you want it to.
There are some very important things to bear in mind:
CF is not equally strong in all directions.
CF frames and components by design are strong where they need to be and weak elsewhere - thick, chunky carbon around your BB, thin, weedy carbon for your seatstays. Downtubes that are thin in the middle, but thick at the ends... It is part of the beauty of CF - its why designers can make light frames that are super stiff when pedaling, but do not remove your fillings. it also means that if you add in a load which should not be there, you may break something.
Damage to CF can be much harder to detect than damage to alloy/steel - you can see a crack in metal, but potentially not see anything in carbon. If you dent alloy, you can see it, if you dent carbon it may look fine, but not be structurally sound.
Finally - do you feel lucky? I can't think of many bike bits I would like to fail less than a steerer tube. For me, I would always err on the side of caution. If you can see it is visibly deformed, are you going to be happy riding it?
I don't think I really need to tell you this, but do not take a heat gun to it.
Parsnip said:
I don't think I really need to tell you this, but do not take a heat gun to it.
Why not? From what you’ve just posted above I deduce you clearly don’t have a lot of experience of / knowledge about the sorts of CF materials used in bicycle components, or how they work / behave. A heat gun really won’t harm it. A blow torch would, but a hot air gun is not a problem.Edited by BrundanBianchi on Tuesday 6th October 16:59
BrundanBianchi said:
Why not? From what you’ve just posted above I deduce you clearly don’t have a lot of experience of / knowledge about the sorts of CF materials used in bicycle components, or how they work / behave. A heat gun really won’t harm it. A blow torch would, but a hot air gun is not a problem.
And therein lies the joy of internet forums - the person posting could be anyone.Edited by BrundanBianchi on Tuesday 6th October 16:59
For all I know you are head of steerer tube development at Cannondale and a heatgun is a certified repair method for a deformed carbon steerer. Enlighten me - why would heating the damaged CF help? Is the composite used not a thermoset? What will this applied heat actually achieve?
Personally, I'm not sure I'd be trusting "bloke from an internet forum" when thinking about the consequences of a bodged repair going wrong. I would hope the engineers at Cannondale have done a bit of detailed analysis on their design - maybe "deformed steerer repaired with heat gun" was a load case they ran and the OP will be fine?
Indeed.. and Parsnip is absolutely on the money.
I'm not a design engineer, but: Crushed frames, yes.. steerers, my argument would be that they're mostly designed for radial loads - steering load (torque) is negligible really.. and that unless it is defective, you'd need a pretty deliberate effort/abuse to crush. If I look at the steerer behind me, it is a 3mm wall thickness, and largely composed of fibres wound round the circumference - about as good as you're going to get in crush/radial loading. A frame on the other hand - much thinner, and likely to have most of the fibres aligned lengthways.. not so hot on the crushing..
Hot air isn't going to do much to help. Yes, there are thermoplastic resins used for things like moldable shoes, but the frame is almost certainly a thermoset, and a one way deal. Mild heat won't damage, severe heat will degrade the resin, but I can't see a case where it's going to do anything useful.
FWIW, my experience is breaking and fixing a bunch of carbon on boats, and I've repaired a broken carbon frame, once.. Nothing formal.
P.S. just to be picky, aluminum can (and does) fail unannounced due to fatigue/crystaline fracture.. but as I say, picky.. but not convinced I like it too much for steerers either.
I'm not a design engineer, but: Crushed frames, yes.. steerers, my argument would be that they're mostly designed for radial loads - steering load (torque) is negligible really.. and that unless it is defective, you'd need a pretty deliberate effort/abuse to crush. If I look at the steerer behind me, it is a 3mm wall thickness, and largely composed of fibres wound round the circumference - about as good as you're going to get in crush/radial loading. A frame on the other hand - much thinner, and likely to have most of the fibres aligned lengthways.. not so hot on the crushing..
Hot air isn't going to do much to help. Yes, there are thermoplastic resins used for things like moldable shoes, but the frame is almost certainly a thermoset, and a one way deal. Mild heat won't damage, severe heat will degrade the resin, but I can't see a case where it's going to do anything useful.
FWIW, my experience is breaking and fixing a bunch of carbon on boats, and I've repaired a broken carbon frame, once..
Nothing formal.P.S. just to be picky, aluminum can (and does) fail unannounced due to fatigue/crystaline fracture.. but as I say, picky.. but not convinced I like it too much for steerers either.
Thanks for all of the responses and advice.
Given that I’m so technically incompetent as to get myself into this mess in the first place, I think heat guns and ‘reforming the steerer’ are out of the question!
I’ll take it into my local LBS for a diagnosis. Realistically, I think it will have to be a new fork. As somebody above said, I know what is potentially there and would wince over every bump. Slightly concerned that it might not be commercial viable (few years old, average spec, not cosmetically great etc.) which is a shame as I like the bike.
Given that I’m so technically incompetent as to get myself into this mess in the first place, I think heat guns and ‘reforming the steerer’ are out of the question!
I’ll take it into my local LBS for a diagnosis. Realistically, I think it will have to be a new fork. As somebody above said, I know what is potentially there and would wince over every bump. Slightly concerned that it might not be commercial viable (few years old, average spec, not cosmetically great etc.) which is a shame as I like the bike.
Tech info for 2020 carbon synapse - worth a read especially the warnings.... It appears the stem bolt(s) compress the carbon tube against the expander bolt wedges; effectively providing a metal reinforcing component. Quite clever really. Any misalignment risks crushing the steerer tube. Especially if over-torqued.
If you want to drop the stem it means cutting the tube down to align the stem bolts properly. I'm sure it's possible to ride in a temporary configuration but for me, as an engineer, the permanent trimming of the tube is the correct solution.
Edited by ian in lancs on Thursday 8th October 08:11
Had a high speed crash while racing cyclocross, knocked myself out, split my new helmet in two, and cracked the steerer tube halfway through. The handlebars were flexing on the remaining bit of carbon that hadn’t snapped. I epoxied a bit of old aluminium handlebar in, and cut down a star nut to suit, been like that four or five years now.
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