Failed Cog

[Chief]

I was climbing a steep hill in my 39-14 when my chain started skipping. I stopped and looked at my rear cog and observed that I sheared off one tooth on my 14 tooth cog. The fracture surface indicates it was fatigue crack. There is some stamping on the side of the cog and this may be were the fatigue crack initiated, but I can't be sure. Fortunately, I had 8 other cogs so I was able to get home, but I did skip some of the hills so as not to press my luck. This was a SRAM cog with about 17,000 miles on it and I often use the 14T cog. Has anyone else had a similar experience with SRAM cogs.

[Ralph]

17,000 miles?

[Cantdog]

Holy bajesus, how do you keep your stuff looking that nice after 17k miles? Will you come wash my bike?

[Dave]

I think they went overboard with the weight reduction. Far too many holes.

How steep could a hill be if you're using a 39/14?

[OtayBW]

I was climbing a steep hill in my 39-14 when my chain started skipping. I stopped and looked at my rear cog and observed that I sheared off one tooth on my 14 tooth cog. The fracture surface indicates it was fatigue crack. There is some stamping on the side of the cog and this may be were the fatigue crack initiated, but I can't be sure. Fortunately, I had 8 other cogs so I was able to get home, but I did skip some of the hills so as not to press my luck. This was a SRAM cog with about 17,000 miles on it and I often use the 14T cog. Has anyone else had a similar experience with SRAM cogs.
Another victim of too much torque???

Seriously though, that seems like a pretty high gear to be running in your small ring. Could the chain line have had anything to do with it?

[Chief]

Another victim of too much torque???

Seriously though, that seems like a pretty high gear to be running in your small ring. Could the chain line have had anything to do with it?

I don't think chain line is an issue. I run a 9-spd 11-21 cog and the 14 is about one cog below the center. I believe the stamping on the cog produces a stress riser that causes the initiation of a fatigue crack. I have seen chain links fail at a letter that had been stamped on it.

[Pete Serotta]

I don't think chain line is an issue. I run a 9-spd 11-21 cog and the 14 is about one cog below the center. I believe the stamping on the cog produces a stress riser that causes the initiation of a fatigue crack. I have seen chain links fail at a letter that had been stamped on it.


In my low tech view, I think it is a cog set issue only. I base this on how cogs look.

[flydhest]

17,000 is many, many miles from my perspective.

My view: Be happy you didn't lose teeth and be happy you got more than your money's worth out of the cassette.

[Louis]

I was climbing a steep hill in my 39-14

I wish I could climb steep hills in my 14T cog :)

[Ralph]

I think the cog was just wore out.

[Pete Serotta]

17,000 is many, many miles from my perspective.

My view: Be happy you didn't lose teeth and be happy you got more than your money's worth out of the cassette.
:beer: :beer: Seth I will be coming for RED and some of your great cooking in the fall!!!! :banana: PETE

[firerescuefin]

Sounds like when I threw a rod on my datsun station wagon with 360K miles. Clearly a quality control issue with datsun :rolleyes:

[auto_rock]

17 thousand miles on 23mm tires means about 13 million load-unload cycles distributed over your cassette. Even if you didn't favor your 14T, that's still 1.4 million cycles per cog.

I mean, holy cow, man. Can you blame it for getting fatigued? I'd be freaking exhausted. Especially if 39-14 is your hill-climbing gear.

:beer:

[firerescuefin]

Previous thread by Chief....Titled, "They don't make em like they used to"

After 58,000 miles and just over 7 years I had to replace my 7703 Dura Ace bottom bracket. :crap:
Can you believe it? :confused:
Since it was sealed, I never even considered servicing it. They just don't....


Chief, it would break your heart if you knew how often I switched out chains.

I really hope I can put miles on like you do as I get older. :beer:

[flydhest]

:beer: :beer: Seth I will be coming for RED and some of your great cooking in the fall!!!! :banana: PETE

We are waiting for you. If you get a chance, drop me a ring. Got some news for you.

[William]

Another victim of too much torque???

...Could the chain line have had anything to do with it?


I showed Andy the photo....he says "no!" :no:





William ;)

[soulspinner]

17,000 is many, many miles from my perspective.

My view: Be happy you didn't lose teeth and be happy you got more than your money's worth out of the cassette.


+1-look how badly some of the other teeth are worn...........

[AngryScientist]

thats a lot of miles on a cassette, how often do you switch chains??

regardless, i would think the teeth would wear out before a cog off and breaks like that. glad to hear you were able to keep the rubber side down.

[Chief]

thats a lot of miles on a cassette, how often do you switch chains??

regardless, i would think the teeth would wear out before a cog off and breaks like that. glad to hear you were able to keep the rubber side down.

I replace chains about every 6500 miles. I keep the chain clean and well lubricated, and in San Antonio one need not worry about riding in the rain which helps with regard to chain life.

[firerescuefin]

Chief,

I was combing through some of your old posts and know that you forget more about this kind of stuff (wear cycles, etc) with your professional background than most of us know. What was your expectation with regards to mileage you would get out of this cassette?...and is your main issue essentially that it failed catastrophically?

[Chief]

Chief,

I was combing through some of your old posts and know that you forget more about this kind of stuff (wear cycles, etc) with your professional background than most of us know. What was your expectation with regards to mileage you would get out of this cassette?...and is your main issue essentially that it failed catastrophically?

I had no preconceived expectations with regards to the life of the cassette. I figured that the cassette would wear out to point where skipping would occur with a new chain, my typical experience with cassettes, before a cog would catastrophically fail. Therefore the reason for putting forth this thread was recording the catastrophic failure. Moreover, my LBS had never seen the failure that I experienced and that is why I posted the photo here to see if anyone else had experienced such a failure. I examined the fracture surface of the failed cog and it is clear that fatigue crack growth took place. I'm guessing that the cogs are forged because of the fine grain structure of the fracture surfaces. Regardless, the fine grain structure will have a reduced fatigue crack growth resistance and fracture toughness of the cog material. Unfortunately, I was unable to recovery the portion of the cog that broke out to see if there was some surface imperfection that was the initiation site of the fatigue crack growth. My LBS tells me that SRAM no longer machines into its cog the reduced section to facilitate smooth shifting and that this reduced section may be responsible for the failure. Having said that, no one else has reported such a failure so without further evidence I treat the failure as fluke or an outlier, or the fact that others have not put as much mileage and stress on their 14T cog, a cog that I use a lot, as I have. I plan to continue using SRAM cassette.

[flydhest]

Unrelated to the thread, but directly related to Chief's epigram, Einstein should have learned how to track stand.

:banana:

[dlflemingos]

Not a good idea to stamp parts. You suspected the failure started at a stress riser (stamped letter or a machined grooves) Can you add an arrow where the crack initiated and describe the striations with an arrow?

I also calculated ~1.4E6 revolutions. I'm wondering if it was in plane (pure shear or tooth bending) or if a side load (you disregarded this) was induced somehow (warped tooth?) that would have caused out of plane bending. The striation pattern will give a clue.

Also, any idea how long the crack was before it went critical? How much do you weigh and what was the grade/speed at the time of failure?

Anyway, I think you should send that part to a lab (or Louis) for further analysis.