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Fatigue life: Age, miles or both.(5 posts)

Fatigue life: Age, miles or both.Juanmoretime
Aug 19, 2003 8:27 AM
Many component manufactures recommend replacing components at a certain age, such as,replace your USE carbon post every two years. I think mileage would be a better gauge for replacement. I doesn't seem right that a person who ride 1,000 miles per year and a person that rides 10,000 per year would replace the same component at the same time, obviously, the peron who rode less, that component would have gone thru many less fatigue cycles. Your thoughts?
All of the aboveFez
Aug 19, 2003 9:04 AM
Age replacement

I think the reason you see age replacement recommendations because manufacturers imply that age goes along with mileage for MOST riders. A set of handlebars that never got any riding time obviously wouldn't need to be replaced every 2 years.

However, you do see age replacement a lot in motor vehicles, for example. Rubber parts get brittle and need replacement after it ages, regardless of mileage. Same with grease or oil or fluid filled components, where age causes breakdown regardless of mileage.


Regarding fatigue, I think constant tinkering, removal and reinstallation may fatigue parts more than gentle use. I'm sure frequent exchanges of stems and bars and front end components fatigues all the parts and bolts more than a set-and-forget scenario with seasonal maintenance. Same goes for constant tinkering and swaps with seatposts and saddles. I've seen many a friend strip binder bolts and seatpost clamp bolts from frequent changeovers.
--- Not quite ---Synchronicity
Aug 19, 2003 6:21 PM
Fatigue means failure through a number of loadings. This type of failure does not include your friend stripping threads after removing/reinstalling parts.

Think of it this way:
It more like a cumulative effect. A part will eventually fail through repeated loading. The time to failure depends, among other things, on the size of the load. There IS a load on, say, a stem or bar. Especially if the road is bumpy, or if you put more weight on those parts.
*However*, some metals have what is called a "fatigue limit"
This means IF the load is small enough, the metal will not fail through fatigue. It may corrode or get totally squashed by a truck, but it will not spontaneously break from fatigue. Steel and titanium have fatigue limits. Aluminium and magnesium do not. But if a steel stem is made from tubes that are too thin, this will effectively act like an increased load (as it is a reduced section) and the part will still eventually break.

So you cannot have your cake and eat it! Either it is light or it is fatigue resistant. There will be a compromise with a well-designed product.

So the point is if a part has just been sitting there and it hasn't corroded, it DOESN'T need replacing after two years. It has been idle. Tightening up the fittings is a different story; that is a different loading cycle to general riding.
Actually, I was talking about something differentFez
Aug 20, 2003 8:53 AM
The part I said about excessive tinkering and installations and reinstallations were about the part failing, but at a different area. For example, the metal in a stem could fail from fatigue after a few years and many miles. OTOH, someone who removed and reinstalled it multiple time might damage the threads or bolts, ruining the stem. The stem would still have to be replaced but the failure would be at a different area from one which was installed once or twice but ridden many miles.

Same with a seatpost. Excessive removals and reinstalls could damage the tube from clampings, as opposed to the tube just fatiguing from staying installed and seeing thousands of miles on the road.
re: Fatigue life: Age, miles or both.zero85ZEN
Aug 19, 2003 7:38 PM
fatigue life, age, load limits etc....

Most non cyclists have the general opinion that I look like a concentration camp survivor being that I'm 5'11" and about 135lbs when I'm in summer riding form. For me it translates into welcoming climbs on group rides and never bothering to even think about contesting a bunch sprint. :-)

It also means that I put a lot less "load fatigue" and stress on my cycle componentry than many of my cycling peers. So many factors figure into "wear and tear" on various component parts that a manufacturer recommendation is just that; a recommendation. Keep in mind that they will always be VERY conservative on these recommendations. Better to tell you to buy a new bar every two years than have a lawsuit against them for a broken handlebar causing an accident and injury.
That being said, the larger the rider, and the more miles you put on a bike, obviously the more wear and tear on the parts. Most serious large riders that I know tend to be very wary of all the lightweight exotic material parts out there. If you have the build of a pro cyclist, but ride about 1/6th the miles per season you probably can confidently ride what the pros ride and ride it for a few seasons longer than manufatures recommendation. On the other hand, if you are significantly larger than the average pro or you ride the miles that the pro's ride (you lucky fool...I want your job that lets you do that!), then you may want to adhere pretty close to what the manufacturers reccommend.

Just my opinionated rant.