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Old 05-22-14, 10:46 PM   #51
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Old 05-23-14, 08:37 AM   #52
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Are you saying he bought the seat post?
no saying they assumed , without specific ..situational confirmation, by asking directly..


Next time , replace a seat post or Quill stem that is not inserted far enough .. before it causes a failure of the frame .
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Old 05-23-14, 09:39 AM   #53
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The fatigue life for carbon fiber isn't infinite but roughly just higher. (depends on which one though)




(snip)
Note the curve trend and the fact that the horizontal axis is logarithmic. At 50% stress and a hundred thousand cycles, all the metal frames have failed. The trend of the carbon fiber at 50% stress is probably failure at 10^20 or so cycles. That's an effectively infinite lifetime.
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Old 05-23-14, 05:04 PM   #54
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There is the concept of cumulative fatigue damage. Also, crack initiation. A well used carbon frame may have some ready made crack initiation sites that I suspect would significantly shorten its remaining fatigue life. That begs the question, is the carbon bike market, both on the supply and the demand side, really seeking "infinite life"?
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Old 05-23-14, 05:34 PM   #55
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My Raleigh Venture is steel, and that is one of the reasons I chose it.

The biggest problem with Aluminum is that its yield strength and ultimate strength are so close, this is why the failures seem so sudden. The aluminum used for bike frames is always 6061-T6 from what I can tell, it isn't the strongest, but it is the strongest that is easily welded
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Old 05-23-14, 08:18 PM   #56
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I've owned 13 bikes over the years. 4 frames failed when the down tube weld cracked and 1 had a cracked head tube. 3 were alu and 2 were steel. I've owned 4 crabon bikes and so far have not had a single crabon frame failure. In fact, I am certain that I will never have a carbon fiber frame fail due to normal fatigue.
I've owned 27 bikes over the years. All were crabon. Every one broke catastrophically at the down tube, head tube and seat tube, all at the same time.

Fortunately, I was wearing a helmet.
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Old 05-23-14, 10:26 PM   #57
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I've owned 27 bikes over the years. All were crabon. Every one broke catastrophically at the down tube, head tube and seat tube, all at the same time.

Fortunately, I was wearing a helmet.
ironically my second carbon frame was a warranty replacement from trek after they replaced my second broken alu frame. cheap factory welds simply do not stand up to an aggressive rider. that being said my custom steel bike is a glorious frame that will stand the test of time. my triple butted tange prestige race frame from the early 90s is also holding up gloriously.
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Old 05-24-14, 06:58 PM   #58
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Note the curve trend and the fact that the horizontal axis is logarithmic. At 50% stress and a hundred thousand cycles, all the metal frames have failed. The trend of the carbon fiber at 50% stress is probably failure at 10^20 or so cycles. That's an effectively infinite lifetime.
Assuming that the process used to make carbon composites don't generate any major defects that could eventually weaken the carbon nanotube structure like what is happening with graphene.
Defects and disorder in carbon nanotubes
http://www.physics.uci.edu/~collinsp...38.Collins.pdf
Though there is no info on how this graph has been made but we can assume it has been made from actual experiments.
Also the graph doesn't say what kind of test has been used. Carbon nanotubes are not as strong when under compressive, torsional stress and tend to undergo buckling.
https://en.wikipedia.org/wiki/Carbon_nanotube

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Old 05-24-14, 07:23 PM   #59
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A question for all those who feel that aluminum bikes are inherently more prone to failure than steel bikes.

Do you Fly? I don't mean by flapping your arms, but in ALUMINUM airplanes.

There is nothing wrong with aluminum bicycles, if they're built right, and set up correctly.

Usually when I see seat tube failures, they're not because of the material, but because the post wasn't inserted deep enough (1" below the bottom of the top tube. This kind of failure is relatively new, and the result of a design change where the tube extends above the top tube, rather than ending at the seat cluster.

There's nothing wrong with extended seat tubes, but they require extra attention to seatpost depth.
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Old 05-25-14, 06:27 AM   #60
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A question for all those who feel that aluminum bikes are inherently more prone to failure than steel bikes.

Do you Fly? I don't mean by flapping your arms, but in ALUMINUM airplanes.

There is nothing wrong with aluminum bicycles, if they're built right, and set up correctly.

Usually when I see seat tube failures, they're not because of the material, but because the post wasn't inserted deep enough (1" below the bottom of the top tube. This kind of failure is relatively new, and the result of a design change where the tube extends above the top tube, rather than ending at the seat cluster.

There's nothing wrong with extended seat tubes, but they require extra attention to seatpost depth.
As an engineer that works in the aerospace field I would like to point out that aircraft have inspections every so many flight hours, and I don't mean someone just looking at them. They go through non destructive inspection (NDI), some of which involve an ultrasound machine used around known stress points. In addition limits are placed on aircraft and whenever these limits are exceeded they also go through NDI.
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Old 05-25-14, 07:28 AM   #61
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My Raleigh Venture is steel, and that is one of the reasons I chose it.

The biggest problem with Aluminum is that its yield strength and ultimate strength are so close, this is why the failures seem so sudden. The aluminum used for bike frames is always 6061-T6 from what I can tell, it isn't the strongest, but it is the strongest that is easily welded
I have two aluminum frames, neither of which is 6061-T6. One is 7005-T6 and the other is EA60-T6.
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Old 05-25-14, 08:50 AM   #62
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I have two aluminum frames, neither of which is 6061-T6. One is 7005-T6 and the other is EA60-T6.
I wasn't familiar with 7005 so I looked it up and found that it is used almost solely on bike frames and it only has slightly higher properties than 6061. I was unaware that any alloys in the 7000 series were weldable by standard means.

EA60-T6 however doesn't follow standard naming convention so I suspect it is just a standard alloy with some marketing thrown in. My guess would be something in the 6000 series of aluminum. Is it welded at the seams?
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Old 05-25-14, 08:57 AM   #63
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My bike uses 6011a aluminum as near as I can tell. Not that that means anything to me

has anyone posted that study of fatigue life on steel vs aluminum vs cf where steel performed the worst and the good cf frames showed no signs of ever fatiguing to death?
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Old 05-25-14, 08:59 AM   #64
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I wasn't familiar with 7005 so I looked it up and found that it is used almost solely on bike frames and it only has slightly higher properties than 6061. I was unaware that any alloys in the 7000 series were weldable by standard means.

EA60-T6 however doesn't follow standard naming convention so I suspect it is just a standard alloy with some marketing thrown in. My guess would be something in the 6000 series of aluminum. Is it welded at the seams?
The EA60 is on my 1995 Trek 8000. According to the 1995 Trek specifications manual it is stronger than 6061 and 7005. It is not weldable. The frame is bonded.
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Old 05-25-14, 09:10 AM   #65
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Early aluminum frames interest me a lot. Love my mid nineties specialized "metal matrix" ceramic composite frame. The rear end stiffness and responsiveness seem to out preform my modern aluminum frame with grossly oversized bb joints, modern bb, etc... Granted the front end is quite noodly, giving it a strange yin yang thing of stiffness and flexiness going on which doesn't give me confidence in the twisties.
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Old 05-25-14, 09:17 AM   #66
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The EA60 is on my 1995 Trek 8000. According to the 1995 Trek specifications manual it is stronger than 6061 and 7005. It is not weldable. The frame is bonded.
Interesting, I am guessing that was a very expensive bike.
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Old 05-25-14, 09:50 AM   #67
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As an engineer that works in the aerospace field I would like to point out that aircraft have inspections every so many flight hours, and I don't mean someone just looking at them.
Airplanes are also designed with a safety factor of 3.0 or less - by necessity or they wouldn't fly. Airplanes also routinely operate for long periods at loads very near the design load. Neither of these factors apply to bicycle design and operation.
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Old 05-25-14, 10:09 AM   #68
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Airplanes are also designed with a safety factor of 3.0 or less - by necessity or they wouldn't fly. Airplanes also routinely operate for long periods at loads very near the design load. Neither of these factors apply to bicycle design and operation.
And bikes are designed to a safety factor of? My point is the loads on aircraft are much more predictable.

For single loads this safety factor that aircraft are designed two is not for standard flying maneuvers, it is "hard landings", depending on the aircraft that can be 40Gs or more. Couple that with the fact that aluminum has no lower fatigue limit you have to factor in expected life, anything will fly with enough power strapped to it. Navy aircraft are built like tanks because they basically crash every time they land on the deck of an aircraft carrier, not unlike the stress that mountain bikes can undergo.
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Old 05-25-14, 11:20 AM   #69
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And bikes are designed to a safety factor of? My point is the loads on aircraft are much more predictable.
Sorry, I wasn't arguing with you, I was adding on. Bicycle design is hardly at all like airplane design. I don't think most bike manufacturers do any real engineering at all - they just use what's available and what's standard with occasional destructive testing for quality control; and for aluminum that means they're generally over-built. The more boutique and custom makers who do design closer to the edge have to spend more time with engineers.

Now... knowing what you do about how fatigue happens and with a general understanding of loads on a bike frame; how would you evaluate the OP's claim that his previously undamaged seat tube failed from fatigue, not near a weld?
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Old 05-25-14, 11:40 AM   #70
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Sorry, I wasn't arguing with you, I was adding on. Bicycle design is hardly at all like airplane design. I don't think most bike manufacturers do any real engineering at all - they just use what's available and what's standard with occasional destructive testing for quality control; and for aluminum that means they're generally over-built. The more boutique and custom makers who do design closer to the edge have to spend more time with engineers.

Now... knowing what you do about how fatigue happens and with a general understanding of loads on a bike frame; how would you evaluate the OP's claim that his previously undamaged seat tube failed from fatigue, not near a weld?
Oh I didn't take it as that at all, I am a huge geek and love to talk about metal, structures etc. Being new to biking I really appreciate the discussion.

I would love to see a picture, closeup, of the failure. Considering fatigue failures are caused over millions of cycles I do not think it is a fatigue failure simply because I do not think the human butt has the endurance that would be required to cause a true fatigue failure.

I would like to add that I chose a steel bike because steel is more forgiving to damage and is much easier to fix for anyone with with even poor welding skills (me).

Last edited by WVU_Engineer; 05-25-14 at 11:42 AM. Reason: Minor addition
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Old 05-25-14, 11:46 AM   #71
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And bikes are designed to a safety factor of? My point is the loads on aircraft are much more predictable.
.
My reference to aluminum aircraft wasn't intended to involve a direct comparison, but to counter the myth that aluminum in inherently inferior.

Any material can be used well, or used badly, so questions of safety that focus only on the material miss the point.

In the matter of seat cluster failure. This happens on steel, carbon and aluminum frames, and is a design, not a material issue. It was unheard of until they started Tig welding frames, and extending the tube more than 1" past the joint. It's this cantilever of the tube beyond the last point of support that is the issue, and not what the tube is made of.

The problem is compounded by reliance on the minimum insertion mark on posts which is usually positioned without regard to the implications of the extend seat tube.

For those who don't know yet, the minimum insertion for seat posts must meet 2 requirements.

1- minimum insertion equals 2.5 diameters of the post
2- minimum depth in the frame equals 1 diameter below the bottom of the intersecting top tube, or other seat tube brace.

Then there's the issue of flex modulus. Posts flex under heavy load. If the cantilevered section of the seat tube is more rigid than the inserted post, it takes more of the load, and will break near the weld eventually (regardless of material).

In any case, if the post is inserted deep enough, a seat tube fracture will be telegraphed well ahead of separation by the appearance of cracks, and the post itself will prevent a catastrophic failure, though it might mean a walk home.
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Old 05-25-14, 12:41 PM   #72
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My reference to aluminum aircraft wasn't intended to involve a direct comparison, but to counter the myth that aluminum in inherently inferior.
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Considering fatigue failures are caused over millions of cycles I do not think it is a fatigue failure simply because I do not think the human butt has the endurance that would be required to cause a true fatigue failure.

So it is agreed - the OP's quest for a cheap steel bike to replace his cheap aluminum bike because FATIGUE! is misguided.
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Old 05-25-14, 01:17 PM   #73
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My reference to aluminum aircraft wasn't intended to involve a direct comparison, but to counter the myth that aluminum in inherently inferior.

Any material can be used well, or used badly, so questions of safety that focus only on the material miss the point.

In the matter of seat cluster failure. This happens on steel, carbon and aluminum frames, and is a design, not a material issue. It was unheard of until they started Tig welding frames, and extending the tube more than 1" past the joint. It's this cantilever of the tube beyond the last point of support that is the issue, and not what the tube is made of.

The problem is compounded by reliance on the minimum insertion mark on posts which is usually positioned without regard to the implications of the extend seat tube.

For those who don't know yet, the minimum insertion for seat posts must meet 2 requirements.

1- minimum insertion equals 2.5 diameters of the post
2- minimum depth in the frame equals 1 diameter below the bottom of the intersecting top tube, or other seat tube brace.

Then there's the issue of flex modulus. Posts flex under heavy load. If the cantilevered section of the seat tube is more rigid than the inserted post, it takes more of the load, and will break near the weld eventually (regardless of material).

In any case, if the post is inserted deep enough, a seat tube fracture will be telegraphed well ahead of separation by the appearance of cracks, and the post itself will prevent a catastrophic failure, though it might mean a walk home.
Not too familiar with carbon fiber frames but what would TIG welding have to do with it in that case?

Maybe I am not familiar enough with the terminology but wouldn't the 2nd criteria to prevent this failure not be possible on a womens bike?

Not sure I would trust a seat post inserted only two diameters depth at my size.
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Old 05-25-14, 01:19 PM   #74
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So it is agreed - the OP's quest for a cheap steel bike to replace his cheap aluminum bike because FATIGUE! is misguided.
I'm thinking there was damage, possibly even a defect. A failure starting out as just a crack doesn't necessary mean fatigue.
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Old 05-25-14, 01:30 PM   #75
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Not too familiar with carbon fiber frames but what would TIG welding have to do with it in that case?

Maybe I am not familiar enough with the terminology but wouldn't the 2nd criteria to prevent this failure not be possible on a womens bike?

Not sure I would trust a seat post inserted only two diameters depth at my size.
Tig welding per se has nothing to do with anything. It's the design change (extended seat tube) that was implemented when seat lugs were eliminated. There's also the issues of the heat affected zone, and edge of weld stress risers of welded frames vs. lugged or bonded structures.

As for women's frames, Yes they lack a top tube, but I allowed for "other seat tube brace" which would include the seat stays.

You can do a stress analysis for the difference between a cantilevered extension of a seat tube above the last joint, and one braced at the end, and the import of the difference becomes obvious.

But as I said a fw times already, it's not the material or construction, but the totality of the design and how it's executed.

However, I do agree, that given the very short life (LT 6 months), fatigue in the classic sense is unlikely. Plus the OP's reference to a nasty surprise leads me to believe that this is a case inadequate seat post insertion depth, and not the frame's fault. If that's the case, he'll suffer a similar failure regardless of the frame material.

I have no real issue with the extended seat tube design, but users aren't made aware of the implications regard seat post insertion depth. Maybe the smart approach would be for makers of these frames to apply a decal on the frame saying "seat post must extend below this mark".

For any product to serve well in the field it has to be designed right, built right, and used (or maintained) right. If any of these three elements aren't met, product life will suffer.
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