terrymorse 

I never need to cite my professional credentials on rides, but here it's a quick way to shut down the unscientific myth repeaters.

terrymorse 

None of what you wrote above explains how a metal's stiffness might change over time.

Nothing you write can, because it doesn't change.

One more time: no amount of stress cycling will soften a metal.

Please stop, this is embarrassing.

StanSeven

Enough of the personal insults here. No names mentioned for now. I’ve cleaned up the thread so let’s let it continue the way it should.

Eric F 

Didn't take me long to find this...

Quote:

Originally Posted by Look

There is no limitation because carbon has a natural flexibility. It can be used a hundred years while maintaining the same stiffness.

Please stop spouting false information.

terrymorse 

The "metal softening" myth does pop up periodically.

Jobst Brandt contributed some comments about Frames Going Soft it in rec.bicycles.tech in 1998. His "technically correct but colorful" comments hold up pretty well.

Trakhak

Continuing the theme of materials engineering, here are the results of fatigue tests of 12 high-end frames reported in the German Tour magazine back in 1997. Note that two aluminum frames and one carbon fiber frame passed the tests with flying colors while all of the steel and titanium frames failed. (The testers observed that the failures were mostly the consequence of errors in design and fabrication.)

Chandne

Like I said, the carbon fiber does not seem to break down but the resins do...the resins that are the glue, essentially. Now, I do not work with carbon but am close to the composites made for the aerospace industry (by hexcel, for example). In the bike industry, they use similar composites but often much more resin. The resin can break down over time (and with heat). Bikes may take way too long to notice and maybe you never will in most cases. Ti is probably still the king if welded very well. Now the alloy thing is also from personal experience with some softer alloy rims that were great at first but eventually after a couple of years, started to feel a bit softer and needed more truing. I did not examine the alloy at a molecular level, obviously. When I called Notubes, they said that the lighter rims would break down and soften up over time and to stick with the Flows or stiff carbon rims. My wheelbuilder (who may have read the same "soft alloy" discussions) said basically the same think. What happens from a crystalline structure level is obvious in metals when you heat and bend them but in rims over time/beating...does the crystalline structure slowly change too? Good debate and discussion. Fine...let's keep the alma mater out. It made me no smarter, though my dad's lessons probably was more of a nourishing mother.

terrymorse 

Resins used in carbon composites remain stable for a very long time. They do soften at high temperatures, but the temperatures that bicycle components experience are nowhere near high enough to induce this softening.

The one exception: carbon rims with rim brakes. Extended braking downhill can generate enough heat to raise the rim temperature above the resin softening point, and the rim will fail. To avoid this, manufacturers now use higher temperature resin in rims.

Notubes was mistaken. The metal in those rims rims wasn't softening up. The rim was probably failing. Lightweight aluminum rims usually fail from cracks near the spoke holes.





Your wheelbuilder was also mistaken.

No. The crystalline structure of aluminum alloy does not materially change from stress cycles. And even if it did, the stiffness of the metal wouldn't change.

But cracks do form in aluminum rims, and grow, leading to failure.

Chandne

Except in my case, there were zero cracks or fissures I could see when laced up or unlaced...not in the Crest and not in the Arch. We examined them closely and did not find a single crack. If there were not visible to the naked eye, then that is certainly odd. I moved to carbon Knights and Nox rims and have had no issues. I'm not a particularly aggressive mountain biker (no drops over two feet and no drops to flat) but carbon seems to serve me better on the mountain bike and feels the same for the three years I have had them, unlike the alloy rims which started to feel softer and less precise on hard corners in under two. They started out great and were quite comfy too, though the carbon rims are just sharp handling.

BillyD

aka the New Tappan Zee Bridge. #pushback

btw the bridge does not connect to NJ, it connects Westchester county to Rockland county.

genejockey 

I dunno. "The Last Bike You'll Ever Buy" sounds sorta like a threat.

PeteHski

Your alloy rims probably had worn spoke nipple seats. Stress concentration around the spoke holes where the nipples are seated eventually leads to local deformation and then the spoke nipples only contact on one side of the hole. Eventually you get micro cracking around the holes and later on much more visible cracks propagating out from the holes. The rim material itself doesn’t soften up. But once the spoke holes start to wear and deform it becomes more difficult to maintain consistent spoke tension.

The only time I’ve seen carbon “soften” is due to excessive heat (exhaust gases blowing over carbon suspension wishbones on F1 cars). I’ve been involved in testing stiffness of many carbon race chassis and IME they don’t lose stiffness with normal use. Only delamination from severe impact damage causes problems with longevity. I would expect a well designed carbon bike frame to last pretty much indefinitely unless heavily crashed.

indyfabz

For old times' sake.

SoCal_Cyclist

I've tried titanium. Carbon bikes are lighter and more comfortable for me. More shock absorption and vibration dampening, plus lighter.

The only real advantage of titanium is durability if you're going to travel with it and bang it up.

SoCal_Cyclist

What is FEA?

palesaint

Federal Everythingbutcarbonbikes Association

Litespud

the steel hairspring in my watch coils and uncoils 8 times/sec, day in day out. That’s ~250 million times/year, and will do this for decades. This is in a mechanism where the most infinitesimal change in the behavior of the material will manifest itself in measurable speed variation. Since my watch has run 24/7 for the last 5 years, we’re looking at ~1.25 Billion flexes without change. Tell me again how metal inevitably fatigues or “softens” 🤔

Chandne

Your watch spring is not under near as much load and is built to basically be an extremely tensile leaf spring, I assume. Since we are talking about stiffer alloy rims, they do fatigue like any high-strength alloy will. The frame is probably not under that type of deflection stress. In the transportation industry (for example) over 70% of failures are due to fatigue caused by alternating stress. It is sort of if you keep bending a thin fork or an alloy clip- they don't have tensile strength. They will break. This is called microplasticity and you can easily find this issue in several industries where they keep trying to strengthen alloys for this reason. It causes a permanent weakness in the weak points of the microstructure and starts to weaken or soften it, and then eventually leading to a crack or fissures that permanently weakens or destroys the part. Alloys have developed very good tensile strength so they can bend early in this potential fatigue cycle to avoid this but it is not infinite and very high tensile strength cannot be applied in many parts due to rigidity etc needed. It isn't like the bike or rim will simply become putty-soft. Your watch probably has a pretty cool tiny leaf spring instead of a coil spring. Leaf springs are springy and built to avoid the type of weakness/softening and eventual fissure or crack because of the design. It is a totally different material than an alloy rim which is probably designed to deflect at its contact points with the ground over and over but not forever and certainly not designed to take too many impacts while doing that. I hope this has helped, since your watch spring's incredible fatigue life appears to have perplexed you.

terrymorse 

FEA: Finite Element Analysis. It's a numerical method for determining the stresses in a structure. Or heat transfer.

terrymorse 

<sigh>

For the last time: METAL DOESN'T SOFTEN FROM STRESS CYCLING!

A stressed component maintains its stiffness, right up to the point when it fails.

Chandne

Are we still talking about watch springs or titanium frames? Hopefully we have the resin situation sorted out.

Sorcerer

This became a mildly contentious thread didn't it.

All in good sport.

A 6'4" friend of mine stopped riding his Ti mountain bike because of a crack which was slowly spreading near the top of seat tube below the seat post clamp area across the middle of the seat tube and not across the weld(s).

There are threads about this sort of titanium frame failure with photos and some frames have been repaired. It might not be worth it.

I'd I had an unlimited budget I would have disc brake bosses welded on the rear ends of my old frames, but I can't, won't, and will ride a more appropriate bike when the conditions are slimy if I can.

After reading through this thready conclusion is that titanium bike frames are not forever or the last one you will ever own or need. Titanium isn't perfect or infallible.

One thing is certain to me is that riders get used to what they ride and riders, not all but most, will compensate for the shortcomings with their riding style and technique and expectations.

I ride a a variety of bikes like most here do and I like them all. I know that my two old titanium mountain bikes which are single speeds are very capable especially with good tubeless wheels on them, but they aren't any near as good as it gets.

A late model carbon hardtail will blow my old bikes away - no contest.

There is a certain aesthetic about titanium.

As I write I'm catching up on the TDF and I see so many wrecks on the first stage! Tragic stuff.

Now I ask you and me, which crashed bike would you choose to be more likely worthy to continue to ride given an inspection for visual damage? I'd pick Ti. Carbon may have concealed damage.

​​​​​​Mind you that's just an opinion.

Back in the day I drove a Giant Cadex on a roof rack into an overhang and it pulled the bike rack off the car and the bike showed no damage at all! I kept rising it for a few more years before selling it to someone at work who rode it in Washington on trails for a dozen more years.

Anecdotes abound. Anything can fail. Nothing is Forever.
​​

Trakhak

True. The term of art is work hardening, not work softening.

tempocyclist

Carbon can and does last. It's not as fragile as many people make it out to be. I've got a carbon fibre road bike that is 20 years old and still going strong.

pgjackson

Man, I disappear for 5+ years and people are still arguing steel vs aluminum vs CF vs titanium. Some things never change.