Metal Fatigue over Time??

[CarreraS2]

Does metal tend to fatigue over time, even if not used?

[island911]

No, steel does not tend to fatigue over time, when not used.

by def'n, Fatigue happens when the metal is loaded, then unloaded, repeatedly. With steels, the load/unload difference needs to be (most always) much more than what it will see "just sitting around."
The point where the load/unload difference is a problem is called the "endurance limit" . . .if the loads are below this, it will never fatigue.

Corrosion problems are more likely to give problems (for steels). . . but you can usually see that.

(re Fuchs) Aluminum doesn't have this "endurance limit" . . so PANIC!

heheh. . just kidding (about panicing) Even when used (read: lots of load/unload difference) they should last a good long time. Sitting in storage, multiply "a good long time" by 7,227,911

[Randy Webb]

That's the std. engineering line. I'm always suspicious unless an actual study has been done, but I can't imagine any problems based on your facts.

20 year old bike for heavy off-road use?! Thats pretty commendable. Hope you have the appropriate riding gear... the vintage would be tie-dyed bandana, heavy denim jeans, mesh roadie gloves & a full beard I think.

[911pcars]

It depends on the storage conditions. In the case of steel, excessive oxidation (rust) will change the strength of the part. However, strength should not be affected if it's only surface rust. With bike frames, rust often forms on the inside where the metal tubes are often bare and unpainted. You can remove the seat post and take a look inside. The bottom bracket area (the lowest point on the frame) will collect the most moisture.

If the Fuchs wheels look okay on the outside, they should be fine. If oxidized, you can have them refinished. No structural issues. Your closet sounds like a pretty good storage environment. I wish I had NOS Fuchs in mine.

Sherwood Lee
(who has stored numerous car parts during the restoration years)
http://members.rennlist.org/911pcars

[island911]

Originally posted by Randy Webb
That's the std. engineering line. I'm always suspicious unless an actual study has been done, . .

I believe most every mechanical engineer student does do fatigue "studies" .
. . .and I don't mean mental fatigue studies . .although, those were done too.

For me, it was samples of various Steels & Aluminums; with various surface conditions. . .machined, shot-peened, polished.
They get spun by a lathe, with a load pulling the end of the sample off to the side. . . .yada yada . . a few million cycles later, SNAP . .change the load, do it all over again, ad-nausium.

Anyway, if you don't trust "the std. engineering line" I suggest you don't exceed 10mph. . .and certainly, don't drive over any bridges. Those were designed w/ all-kinds of "std. engineering lines"

[BoogieOnSkis]

For all practical purposes, Islands answer is the best your gonna get!


Just wait till the philosophers join this thread

[Chuck Moreland]

Typically for steel the Endurance Limit is about 50% of the yeild strength. So if the stress stays below that level it can be cycled indefinitely without degrading. Even when exceeding the Endurance limit, steel strength tends to stabilize as the cycles climb.

As dr. isle says aluminum has no Endurance limit and degrades with every cycle, no matter how small the stress. But the lower the stress, the less the degradation.

Here is a table that compares the performance of steel and aluminum:



Notice the scale, we are talking 10's and 100's of millions of cycles!

So don't panic. In the absence of corrosion, your bike is fine.

[Drago]

Saw the title to this one and just knew Island would quickly respond, and look at that, first even....

He's right, you know.....

[Bobboloo]

Ouch, if aluminum degrades like that I'd hate to see what magnesium does. Makes rebuilding a mag case motor a worse proposition every mile/year that goes by.

There's a guy in Redondo that restores old RSK's and 550's and such. I guess restore is a loose term in his case. He basically builds a new car from the blueprint of the old car. He even builds the space frame from scratch for these reasons.

[cstreit]

I'm wondering the same thing about my Mtn Bike. I used to race for Pedros in college... I have an early Carbon Fiber frame with Aluminum lugs... I heard that the two tend to react negatively towards each other over time...

I wonder how stable it is now?

[911pcars]

Chris,
The early carbon frames were assembled with lugs. Was it because they didn't have an efficient (cheap) method of joining tubes or maybe not the know-how back then? Don't know. They used some sort of glue or expoxy to assemble. I don't think there's any galvanic reaction (or equiv) that causes separation but instead the age of the epoxy and/or constant cycling (flexing I mean) between tube joints that fatigues the bond.

I'm partial to ti. I wonder how long these frames are going to last? Maybe one of my potential grandkids can use it when they get tired of flying around in anti-gravity toys.

Sherwood

[johnnyo200]

Would it not more likely fatigue quicker due to the additional weight we have gained by getting older? Just following the engineering theory stated above.

[cstreit]

I *gasp* still wear the same *erp* jeans I wore in *oooof* HighSchool *zip* Phew!

[Superman]

I've been told by engineering dudes that heat cycling and vibration also plays a role. Also, metal usually gives some warning before it fails completely. You'd see cracked paint at the joints prior to a failure, if you were looking for it.

Sudden unexpected structural failure of high quality metal framework is not what keeps me up at night. As I've said before, when it comes to finding stuff to worry about, there is so much to choose from that you have to be careful. I choose to ignore the threat of suffering sudden catastrophic failure of structural steel framework. I'm probably more likely to be struck by lightning. I don't worry about the lightning thing either.

I do worry about our nation's foreign policy, and domestic policy as well, but that's probably OT.

[jpachard]

I used to work for Fat City Cycles and Merlin Metalworks back in the day and I can safely say that your bike should be fine. The worst killer of old steel Mtn. bikes is rust. Before you take the bike out shine a flashlight down the seat tube as well as any other open tubes if possible and check for rust and or flakes of metal coming out. If the bike was stored corectly you should be fine. Also check for any hairline cracks under the headtube on the downtube( this is where the most failures occur). Best of luck!! Just keep in mind that a bike of this age should not be ridden to the absolute limit. Also, if the frame was originally shipped with a rigid fork, don't install a suspended fork. These frames were not designed to withstand the abuse a sprung fork can deliver.

Cheers, James

[Randy Webb]

"Anyway, if you don't trust "the std. engineering line" I suggest you don't exceed 10mph. . .and certainly, don't drive over any bridges. Those were designed w/ all-kinds of "std. engineering lines""

- I'm a scientist. Here's a quote: "the first time, it's science, after that it's engineering."

[island911]

Originally posted by Randy Webb
- I'm a scientist. Here's a quote: "the first time, it's science, after that it's engineering."

OR;

science is discovery arising out of analysis,
engineering is application arising out of synthesis.

. . .oh, that's good (patting myself on the back)

[dean]

Hey James

I have one of those Fat Chance bikes, actually the wife has one also.

[jpachard]

Blown, You folks keep coming out of the woodwork!! I met a couple at a DE at Watkins Glen last summer who had Fat Chance Bikes. I'm always amazed what people are drawn to and how it brings folks together.

Cheers, James