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frame fatigue(15 posts)

frame fatiguetemplecat
Nov 6, 2001 5:35 AM
Seen all sorts of tests showing that materials fatigue and eventually break, but how does this apply to the cycles we ride? Is 10 years @ 4k per year even come close to the stresses that the testers give frames? What should you look for in terms of flex?

thanks

J
re: frame fatiguemorey
Nov 6, 2001 5:52 AM
I have had two steel CIOCCs from 1984, One Durham-Moore steel from 1987. I did not notice any appreciable fatigue at this time on the steel bikes. My alum Cannondaler4000si is a 2001, I worry about the fatigue on this metal. I think each metal has a fatigue factor, but I think it is a long time.
I thought that's why we took recovery rides- nmfiltersweep
Nov 6, 2001 6:14 AM
engineers will havescottfree
Nov 6, 2001 6:00 AM
answers for you based in science. All I can tell you is, in the real world, I and my elderly friends (45-55)have been riding the hell out of old steel bikes (the oldest is 31 year old, the 'youngest' 12 years)for decades, on rough roads and in bad weathers, and not one of us has ever suffered a frame failure or anything close. We've had plenty of other failures, but the frame, in my experience, is a non-issue. Can't speak to aluminum, ti, carbon.
re: frame fatigueJohn-d
Nov 6, 2001 6:07 AM
I bought a steel frame in 1957, In those days I was riding 18 - 20,000 miles per year. When I got married the bike propped up the wall of the garage for 20 years. I gave the frame to my niece who still, occasionally, rides it. It is still going strong.

I bought a Cannondale with alu CAAD 4 frame, 6 weeks ago. So far it seems fine.
20,000 miles per yearHA
Nov 6, 2001 1:05 PM
GooD OnE!!!
Yes, in those distant days..John-d
Nov 7, 2001 1:54 AM
us club cyclists who raced, used our bikes as the principle means of transport, we didn't have cars then. The week consisted of riding to work, weekday training rides of up to 50 miles a session, weekend riding, training and racing, holidays, shopping and in addition the bike was the centre of our social life. Easter weekend was a training holiday to Wales that covered over 500 miles alone. I kept a log for three years, The lowest was just over 14,000 miles and the highest, that was the year we toured the Alps and Pyranees, the log went just over 20,000.
yes it doescyclopathic
Nov 6, 2001 6:53 AM
if you ride with randonneurs you hear occasionally a story about frame failure.

Is 10/4k enough to break frame? Prob not but it might. There're a lot of things to factor in like your weight, riding style, roads etc. Touring with loaded panniers seems to speed up this process. I know someone who has ~120K on steel lugged frame and it still going strong.

I would just watch it. Frames usually brake in stress areas like BB, head tube, chain/seat stay. It is not likely to be a catastrophic failure. You would see a small crack which if ignored would progress fast.

If you worry about it happening on "serious" ride either get another frame or buy "race" bike and use your beater for commute/training good luck
Yeah, it goes I'm tired of this frame and want a new one! nmdzrider
Nov 6, 2001 7:12 AM
Basicsvanzutas
Nov 6, 2001 1:00 PM
yes metals have a fatigue life. In general Steel has the best fatigue life then titanium then Aluminmum. Fatigue happens when something is loaded and unloaded a large number of times. low cycle fatigue happens between 0 and 1000 cycles. for any given load there is a coresponding ammount of times that a material can be loaded and unloaded. there is a limit though. there is a certain load that can be applied an infinite number of times and the material will not break. any load under this limit is safe to be loaded infinitely.

So on your bicycle you are continually turning your cranks. this loads the bottom bracket and the chainstays the torque on the bottom bracket will fluctuate throughout the stroke. if you do not load the frame above its infinite limit you will not break the frame. you can load it above that limit occasionally just not repeatedly.

Fatigue occurs due to crack propogation. it starts with a small crack and every subsequent load will make the crack bigger until the metal breaks. So most frams will be built so the average rider will not excede the fatigue limit of the frame however you run into problems if there are any nicks, dings, or (god forbid) rust on the frame at a critical point. these imperfections act as great spots for fatigue cracks to start.

The metal itself does not get softer or more flexy as fatigue occurs. the only difference will occur when the crack starts. once that happens it is goodbye to that frame.

I would hope that a frame builder is buiding frames that will last infinitely for most riders. but sometimes I think frames are built with a certain lifespan I just hope the builder makes it quite clear that the span exists.

Adam
Just inspect regularly. Unless the frame is poorly builtMB1
Nov 6, 2001 2:37 PM
you shouldn't have any problems from normal riding. If you have been riding it for 10 years without a problem, unless you crash you've got a keeper there.
Like a Litespeed? (more)js5280
Nov 6, 2001 3:28 PM
Okay, I've been waiting for this opportunity to pass on a classic moment at the 24 hours of Moab this year. It was about 8pm, just after dark and I'm waiting for my teammate so I can start my lap along w/ probably a hundred other people. This guy comes running in, bike on shoulder. It's very odd though cause the wheel isn't in front of his body. Soon everyone is staring. The guy lifts it above his head and the bars, fork, and wheel dangling from the cables. The headtube was completely sheared off! The crowd ERUPTED in cheers and whispers went through the crowd, "Oh sh!t, it's a Litespeed." After he clocked out so his teammate could start, several people came up to take pictures of him and his broken frame. Classic moment! Now granted I don't know the details behind how the frame broke. However, I got to bust Litespeed owner's chops cause some like to unfairly dis the welding on Airbornes ;-)
mine cracked along head tube toocyclopathic
Nov 7, 2001 5:38 AM
and I am only 140lbs!
I think they ought to reinforce head tube welds
frame fatigue definedDaveG
Nov 6, 2001 4:05 PM
Frame fatigue is simple: you get tired of your old frame; you get a new one! Unless, you are riding the new crop of maximum lightness, limited-life frames, frame fatigue is not likely a real issue for most riders. If you are interested there used to be some fatique testing data on Damon Rinard's website
re: frame fatiguegrzy
Nov 6, 2001 5:38 PM
Ultimately it comes down to the design. Fatigue is measured in cycles (millions, hopefully), but it's for a given stress level. So you can have two pieces of metal from the same batch and expose them to the same number of cycles, but one can fail beofre the other if it's stress is higher (i.e. thinner, more load etc.). You're really asking a variation of the old "How long is a piece of rope?" question. It depends. If something is properly designed, well understood and the proper factor of safety is used, and proper workmanship applied to quality materials you should have no problems. Obviously this is a LOT of "ifs." In the bike industry not that many outfits can afford to do detailed FEA (finite element analysis) and extensive testing, so they wing-it and use their judgement. Couple this with everyone focused on weight and you have a real conflict of interest. It all comes down to: light, cheap, durable - pick any two. If you're big and strong then you shouldn't be on a wimpy little noodle - if you're a light weight hill climbing wonder boy then the noodle might be just the trick.

You should focus your attention on to reputable bike manufacturers who make the bikes themselves, offer a decent warranty, maybe sponsor a race team or two, and has an engineering department with a solid R & D budget. This is pretty ideal and it's still no gaurantee that you won't get a failure. However I'd be leaning more towards an outfit like C'dale over Specialized, but that's not to say you can't get a perfectly good product out of Specialized or that Cannondale never has any failures. You're also advised to stick to established and more traditional designs over something new, exotic, and untested.

One way to use the numbers you threw out would be along the following:

4,000 miles @ 20 mph --> 200 hours riding time. Estimate a cadence of 100 rpm (I know these numbers are high, but they're easy to do in my head - use others if it makes more sense), so 200 hours x 60 min/hr x 100 rpm = 1,200,000 pedal revolutions. So we're talking about roughly one million cycles in a year or 10 million in 10 years. So this is 10^7 which is significant, but not an unreasonable number of cycles that a frame should be able to take. Now bear in mind that not all of these strokes are at the design limit. This is critical since once you back off from the desing limit for the fatigue you can increase the number of cycles by several orders of magnitude (powers of 10). So really it comes down to how hard you work the frame - if you take a bike for a test ride and notice frame flex when you really get on it, and this is your riding style, then maybe you shouldn't get that bike. It's more likely that you'll get hit/hit something and wreck the frame before it's time. Also, realize that aluminum doesn't fatigue well, while steel is a lot more resilient.

So I guess it depends, but I wouldn't lose any sleep over it.