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  1. #1
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    Aluminium frame fatigue

    Hey. I'm looking at an older mtb from around 95 or 96. Now I've heard that aluminium does fatigue over time and I'm quite a heavy person, around 280 pounds and I'm worried I'll just end damaging the frame. Anyone know anything? I only really drive on road, forest paths with some larger stones and roots and gravel roads.

  2. #2
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    true fatigue on alu bikes is super rare. typically when they fail it's because of a defect, even if it took awhile for it to manifest.

    One major concern with an aluminum mtb from the mid 90s has nothing to do with fatigue, though, is the fact that that was the weight weenie drillium era. LOTS of substandard bikes and parts were manufactured then (often drilled full of holes to make them lighter), and LOTS of stuff broke because it was simply too light and not durable enough. If it was a high quality lightweight bike for that era, I'd stay away.

    If it was a cheap, heavy aluminum bike, it'd probably be less risky from a durability standpoint, but the other issue with something of that age is its need for maintenance. If it had suspension, that's probably just trash at this point, and you're unlikely to find anything that will fit the bike now. So you'd have to install a rigid fork. It probably has other maintenance needs, too, and the costs will mount quickly.

  3. #3
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    A bike 10 years newer would be questionable. Thats a lot of years of someone doing god knows what to it.

    Definitely find a newer bike, thats far, far too old.

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    Quote Originally Posted by Harold View Post
    true fatigue on alu bikes is super rare. typically when they fail it's because of a defect, even if it took awhile for it to manifest.

    One major concern with an aluminum mtb from the mid 90s has nothing to do with fatigue, though, is the fact that that was the weight weenie drillium era. LOTS of substandard bikes and parts were manufactured then (often drilled full of holes to make them lighter), and LOTS of stuff broke because it was simply too light and not durable enough. If it was a high quality lightweight bike for that era, I'd stay away.

    If it was a cheap, heavy aluminum bike, it'd probably be less risky from a durability standpoint, but the other issue with something of that age is its need for maintenance. If it had suspension, that's probably just trash at this point, and you're unlikely to find anything that will fit the bike now. So you'd have to install a rigid fork. It probably has other maintenance needs, too, and the costs will mount quickly.
    It's a gt avalanche 96

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    Aluminum doesn't fatigue with time, but rather riding or the stress cycles that come from riding (loading and unloading). And Harold is right that aluminum is more susceptible to defect fractures induced by fatigue than steel.

    So, an old bike doesn't pose a problem in and of itself (other than the bad designs Harold also noted), but rather the amount of riding that occurred in it's long life. If you had any reliable way of knowing that it wasn't ridden much, it would be ok. Of course, you can't really know that.

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    Just stay away from the cannondales from that era.

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    Thanks everyone. I think I might have to reconsider. I just love the look of that old bike.

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    Quote Originally Posted by thesmokingman View Post
    Just stay away from the cannondales from that era.
    Was that the early-mid 90s? Besides being horribly stiff, did those break a lot? Assuming we're talking the first aluminum frames that Cannondale was mostly pushing.

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    Quote Originally Posted by TwiceHorn View Post
    Was that the early-mid 90s? Besides being horribly stiff, did those break a lot? Assuming we're talking the first aluminum frames that Cannondale was mostly pushing.
    Cannondale was notoriously bad early on with aluminum, hence all the joke names about them.

  10. #10
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    Quote Originally Posted by TwiceHorn View Post
    Was that the early-mid 90s? Besides being horribly stiff, did those break a lot? Assuming we're talking the first aluminum frames that Cannondale was mostly pushing.
    Their big thing was grinding down the welds for a "smooth look", it turned out to be counter productive and actually decreased the strength, probably because welds are not perfectly consistent, so if you grind them down to a consistent looking finish, the actual thickness would differ drastically in places.
    "It's only when you stand over it, you know, when you physically stand over the bike, that then you say 'hey, I don't have much stand over height', you know"-T. Ellsworth

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  11. #11
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    I've seen quite a few cracked al rear triangles, down tubes, and seat tubes on mtbs ridden by fit experienced riders. These all happened after quite a bit of use.
    Do the math.

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    I agree with everyone else here about a bike that old for you considering your stated weight. I would also say that you should look for a bike that will give you more of an upright riding position, bars and saddle height being about the same height. The bikes of that era, in addition to the aforementioned, will have a more aggressive geometry with the bars lower than the saddle that may be uncomfortable for you. The end result being that you may not enjoy the hobby if you are uncomfortable on the bike. I would suggest looking for something newer. Have you considered any plus/ fat bikes?
    I don't know why,... it's just MUSS easier to pedal than the other ones.

  13. #13
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    I'd look at something from the last 5 years. An aluminum 27.5+ bike would probably work well for you.

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    Quote Originally Posted by thesmokingman View Post
    Cannondale was notoriously bad early on with aluminum, hence all the joke names about them.
    Yeah, I remember that pretty well in the context of road bikes. I just couldn't place the timeframe exactly, would have guessed late 80s, maybe earlier. Maybe it took them that long to get right with Al.

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    Quote Originally Posted by ChrisHoffmann View Post
    It's a gt avalanche 96
    GT from that era made very good bikes. I am still riding one. Just saying.

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    Quote Originally Posted by Jayem View Post
    Their big thing was grinding down the welds for a "smooth look", it turned out to be counter productive and actually decreased the strength, probably because welds are not perfectly consistent, so if you grind them down to a consistent looking finish, the actual thickness would differ drastically in places.
    I'm surprised at that because my engineering text says that a better surface finish improves fatigue life, the weld reinforcement does nothing to improve weld strength, and the step edge at the toe of the weld is a stress riser.

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    Quote Originally Posted by MikeDee View Post
    I'm surprised at that because my engineering text says that a better surface finish improves fatigue life, the weld reinforcement does nothing to improve weld strength, and the step edge at the toe of the weld is a stress riser.
    A good weld shouldnít have a step edge at all; thatís a heat problem. Fillet welds should join the base material smoothly. I donít ever grind my welds...

    But Iím also not an engineer.
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  18. #18
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    Quote Originally Posted by MikeDee View Post
    I'm surprised at that because my engineering text says that a better surface finish improves fatigue life, the weld reinforcement does nothing to improve weld strength, and the step edge at the toe of the weld is a stress riser.
    A good weld shouldnít have a step edge at all; thatís a heat problem. Fillet welds should join the base material smoothly. I donít ever grind my welds...

    But Iím also not an engineer.

    Aluminium frame fatigue-f5df92e9-913e-4597-a323-f9be35ed14ac.jpg
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  19. #19
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    Quote Originally Posted by Sage of the Sage View Post
    A good weld shouldnít have a step edge at all; thatís a heat problem. Fillet welds should join the base material smoothly. I donít ever grind my welds...

    But Iím also not an engineer.

    Click image for larger version. 

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    Do your fillets have a concave profile?

  20. #20
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    Quote Originally Posted by MikeDee View Post
    I'm surprised at that because my engineering text says that a better surface finish improves fatigue life, the weld reinforcement does nothing to improve weld strength, and the step edge at the toe of the weld is a stress riser.
    But it's not fatigue life that is in question, it's the limit, lower thickness or drastically differing thickness (because you ground it down) will create localized areas where the limit is significantly less. I'm guessing that maintaining constant weld thickness is very difficult to impossible when ground down like C-dale used to do. They may be able to handle stress up to that limit all day long, but if the limit is lower, it'll start a crack and that will progress with each subsequent cycle.

    Not only that, but grinding the weld to smooth it will introduce heat back into the metal and may change the properties of the weld, even if it "looks" all smooth. Control over that process would be critical.

    Assuming everything else is equal, I would agree that a smooth consistent surface finish when the wall thickness is constant and the heat and weld material are distributed evenly is the best outcome, but that's probably extremely hard to control given how thin the aluminum that is being welded is, which is also likely why a lot of manufacturers moved on to hydroforming to come up with more radical tube-shapes. I can't think of many applications where the welds are ground down and if one was doing so, I'm not sure how you would adequately control the thickness of the resulting material that is left.

    Bikes generally don't break from fatigue cycles, they break due to a flaw, under-designed part, or massive over-load. In the case of an under-designed part, it's a local over-load, but at or below the designed load limit for the entire bike. Again, that'll start a crack forming and eventually it'll snap one day.

    In the case of fatigue due to cycles, that'll usually take a long time, although a heavy guy on a 20+ old bike may be pushing it or at least needs to proceed with more caution.
    "It's only when you stand over it, you know, when you physically stand over the bike, that then you say 'hey, I don't have much stand over height', you know"-T. Ellsworth

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  21. #21
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    Aluminium frame fatigue

    The grinding does not produce any significant heat. Cannondale used a small belt sander to grind down their welds.

    I agree on stress risers causing cracks, although I had two Gary Fisher frames that cracked and failed. I could see no reason for the cracks. The last one made by Trek is still going strong after a number of years. Go figure.

    I think the welding process is critical to preventing future cracking. What may look like a nice weld may not be. Too much heat damaging the tubing in the HAZ, subsurface defects, lack of fusion or penetration, etc.

  22. #22
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    Quote Originally Posted by MikeDee View Post
    The grinding does not produce any significant heat. Cannondale used a small belt sander to grind down their welds.

    I agree on stress risers causing cracks, although I had two Gary Fisher frames that cracked and failed. I could see no reason for the cracks. The last one made by Trek is still going strong after a number of years. Go figure.

    I think the welding process is critical to preventing future cracking. What may look like a nice weld may not be. Too much heat damaging the tubing in the HAZ, subsurface defects, lack of fusion or penetration, etc.
    Yeah, there's too much going on in a weld to apply the rule of thumb that "better surface finish increases fatigue life." That applies to a wrought or milled structure, not to a weld. Although, I suppose between two otherwise identical welds (ha), the one with better surface finish might have a longer fatigue life.

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    Quote Originally Posted by MikeDee View Post
    Do your fillets have a concave profile?

    Either slightly concave or flat.

    Aluminium frame fatigue-3ea84567-9781-4308-8ba6-fed37a1df949.jpg
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  24. #24
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    Here's a timely article on the subject.

    https://www.velonews.com/2018/11/bik...fatigue_481150
    Do the math.

  25. #25
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    Quote Originally Posted by Lone Rager View Post
    Here's a timely article on the subject.

    https://www.velonews.com/2018/11/bik...fatigue_481150
    Yep, but IMO way too many people bought into the titanium thing back in the 90s, and the frames cracked as much as, or more than, anything else. It was because of the same basic idea above, frames don't generally fail from fatigue, it's very very rare, it's an under-designed part or flaw, which is overloaded and starts a crack, which many Ti frames succumbed to. The other failure is an overload above the designed limit, which starts the same process. The "theoretical" part of the material properties never really materialized IME. Not that there is no reason to get these, just that people made them out to be something they were not and lightspeed, merlin, lnysky and everyone else dealt with lots of cracked Ti frames.
    "It's only when you stand over it, you know, when you physically stand over the bike, that then you say 'hey, I don't have much stand over height', you know"-T. Ellsworth

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  26. #26
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    So much facepalm in this thread. I don't even know where to start with some of you people.

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