Is this safe ??

[Chris James]

If you are interested then google 'fracture mechanics'.

This didn't sound quite as I meant it. You may already be an expert on fracture mechanics for all I know!

 

[Wobblers]

Yes, fatigue is progressive, unfortunately once the crack reaches a critical length it will fail immediately and catastophically. It is many years ago but I did my B Eng dissertation on the subject.

If you are interested then google 'fracture mechanics'.

Steel is pretty ductile, even the high strength ones so will tend to bend prior to snapping, which is a useful property in a safety critical component. As far as I can see the only real drawback to steel is the weight...

Isn't the idea to spot the crack and replace the part before the crack becomes free running?

Sounds like you want titanium there: even 6/4 Ti has more elongation than chromoly (4130) steel and is a wee bit lighter!

Edit: don't worry, your post never came across as condescending or anything like that (and I'm not an expert in fracture mechanics either!)

 

[Wobblers]

Let me clarify...the failure is due to fatigue, but that's the symptom and not the cause - which is spoke mistensioning, either due to poor manufacture or inept user repair.


Not routine, not proven. Following the composite tailfin failure of American Airlines flight 587 in 2001, James H. Williams, Jr., Professor of Applied Mechanics in the Mechanical Engineering Department at the Massachusetts Institute of Technology, wrote:
"There are several unvoiced issues that remain; two of which are the aging and the repair of nonmetallic fiber reinforced composites. By aging, I mean the deterioration that occurs during long-term in-service mechanical and environmental loading, in the absence of an overloading or catastrophic event. When subjected to the loading histories of some aircraft, composites will lose both strength and stiffness. Furthermore, studies of the long-term effects of exposure to aircraft environments of moisture, pressure and temperature, as well as fuels, hydraulic fluids, lubricants and deicers remain to be conducted for many composite materials."

Funny you should mention Flight 587: I had edited out a comment on how the rudder failed when it was subjected to almost three times its expected failure load. Carbon fibre is very much stronger and stiffer than any metal: this allows the designer the luxury of designing in large safety factors without having to compromise too much on weight. It also has to be added that carbon bikes as a rule are not subjected to the same environment that aircraft are! Also note that the report does not mention whether the composites used were tested for aircraft environments - but given the regulatory environment and liability issues I would be extremely surprised if they weren't. Neither Boeing or Airbus could afford the lawsuit if they hadn't!!!

Even perfectly tensioned spokes can and do fail from fatigue: there are reports of this over on yacf.

You raise other valid points - I'll try and get round to them tomorrow (though this thread's already degenerating into something from a engineering PhD thesis!).

 

[dellzeqq]

Yes, fatigue is progressive, unfortunately once the crack reaches a critical length it will fail immediately and catastophically. It is many years ago but I did my B Eng dissertation on the subject.

If you are interested then google 'fracture mechanics'.

Steel is pretty ductile, even the high strength ones so will tend to bend prior to snapping, which is a useful property in a safety critical component. As far as I can see the only real drawback to steel is the weight...

eh? And the vibration. There is no way that I'd go back to a steel bike, I rode 531 and 531C frames for twenty years and I'd rather walk than get on one of those horrors!

 

[Chris James]

eh? And the vibration. There is no way that I'd go back to a steel bike, I rode 531 and 531C frames for twenty years and I'd rather walk than get on one of those horrors!

Blasphemy! Don't you know steel is real and all that?

You have mistaken vibration for 'road feel'!

Funnily enough my old 531c frame was a bit whippy for me, so I guess it shows that the design of the frame is more important than the material it is made from in terms of feel.

(NB I have ridden aluminium and carbon fibre frames and forks too so am not a steel evangelist)

 

[snailracer]

...
You raise other valid points - I'll try and get round to them tomorrow (though this thread's already degenerating into something from a engineering PhD thesis!).

Well that's a relief- my fingers were starting to ache

 

[Wobblers]

eh? And the vibration. There is no way that I'd go back to a steel bike, I rode 531 and 531C frames for twenty years and I'd rather walk than get on one of those horrors!

<sharp indrawn breath>

You take those foul words back, else I'll fit mudguards to your Colnago!

Oddly enough, I find I get more vibration on my Ti Yukon than the 531 Galaxy. That's probably down to the 32 mm tyres on the Galaxy compared to the 25's on the Yukon. I think that frame material is very much overrated when it comes to ride quality: any frame ought to flex a lot less than the tyres (if it doesn't you're probably in trouble!). Geometry and how well it fits you seems to be the most important thing when it comes to comfort.

 

[Wobblers]

There are lots of CF forks that come factory-fitted on new bikes. But they are pretty chunky items, not the lightweight full-carbon units that kicked off this thread. I agree that they have proven reliable - so far. While we're arguing about why metal will fail because of fatigue, why don't we start another thread of why CF snaps because it isn't ductile?

I thought that was something to do with dislocation mobility?

Seriously, though, that's carbon's Achille's heel. I certainly wouldn't trust a carbon fork after a heavy impact or crash. And I would also have sent back the fork that the OP had (you still remember that, don't you? We have gone slightly off topic ). But it also has to be said that I wouldn't trust an alloy fork in the same circumstances. Steel, maybe, if there's no deformation.

Even lightweight forks are reliable - the nature of the material (reliable service up to the point that it's subjected to forces exceed it's ultimate strength) mean that failures in normal use would be common, if there were major issues in design or construction. As you rightly point out, that doesn't mean anyone should be complacent though! The experience of carbon fibre composites in aerospace indicate that long term, degradation is not a major issue. Remember that aluminium is very susceptible to salt corrosion, and corrosion pitting greatly reduces fatigue life.

Personally, I'm more worried about the join between the carbon forks and the aluminium steerer tube in my bikes - that is a natural site for stress concentration. I'd actually prefer an all carbon design which avoids the issue: I can always take care never to overtighten the stem bolts.

Well that's a relief- my fingers were starting to ache

Damn! If only I'd known!

 

[Berlinbybike]

It is odd that there is so much publicity about carbon frame failures. Either there are a lot of them or it's a popular subject. But we get so much disinformation from the manufacturers that we can't make much of an informed decision. There was a comment above about poor quality or poor design resulting in commercial pressure. Trek have had a very negative press about their frame and fork offerings and their response has been, evidently, to apportion culpability elsewhere and to issue ridiculous use guidelines. Then they change models and up the price, every single year. So they can get away with it, and it is us who pay.

 

[bad boy]

Personally, I'm more worried about the join between the carbon forks and the aluminium steerer tube in my bikes - that is a natural site for stress concentration. I'd actually prefer an all carbon design which avoids the issue: I can always take care never to overtighten the stem bolts.

+1

New forks are on the bike now courtesy of my LBS. They also stuck on my new stem which I done a DIY job on sanding it down, they said it was smooth enough so hopefully now ive covered all basses.

I told then to Torque the stem clamp bolts to 4Nm aswell as opposed to 5-8 as PX recommends

went for a test ride only very short no noise all feels solid headset adjustment seemed fine.

Im so tempted to take it apart and check !!?? my OCD is running away with me over this but im resisting the urge to take a glance and will just check it once a month from now on by removing stem and spacers etc. Every ride ill check headset pre-load and to see if I can turn the bottom spacer by hand before setting off.

Thanks for all the hep very interesting thread, as someone with OCD perhaps carbon wasnt the best choice for a new bike