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  1. #1
    surly old man jgedwa's Avatar
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    how forgiving are aluminum rear dropouts?

    I know, I know.

    Aluminum does not bend, it breaks.

    Can't be cold set.

    But, I have an aluminum frame MTB (bonded and screwed, by the way), with rear spacing of 135mm, and I noticed that the wheel I just put in was a couple of mm shy of that. No less than 133. I could simply put in a spacer, but the axle is already just barely long enough as it is.

    I know the proper fix would be to put a longer axle in with another spacer to bring it to 135.

    But, if I was lazy or cheap, would it be crazy to just press in the dropouts 2mm to make it work? Surely that amount would only change the angle of the stayed by a smidge.

    go ahead and pounce on my stupid idea.

    jim

  2. #2
    me have long head tube TallRider's Avatar
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    This will probably be fine. People pretty frequently use 130mm hubs in AL frames spaced for 126. Just tighten the Q/R down and clamp it shut and squeeze the frame the extra 1mm shut on either side. But if it breaks and you lose your teeth, don't sue me.

  3. #3
    we are 138 Philatio's Avatar
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    I don't see it being a problem either. that's just 1mm bending for each stay.

    And for the record, aluminum does bend elastically before it "breaks" just not nearly as much as steel.

  4. #4
    You know you want to. Eatadonut's Avatar
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    I've bent plenty of aluminum - I don't know why people insist it doesn't bend other than hearsay. Do it.

    It IS more brittle than most steel, maybe that's what they're trying to say.

    EDIT: oh snap. beaten to the punch.
    Weather today: Hot. Humid. Potholes.

  5. #5
    we are 138 Philatio's Avatar
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    Quote Originally Posted by Eatadonut
    I've bent plenty of aluminum - I don't know why people insist it doesn't bend other than hearsay.

    It IS more brittle than most steel, maybe that's what they're trying to say.
    I beat you by 5 seconds!

  6. #6
    biked well well biked's Avatar
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    If you're really just talking about a couple of millimeters, it should be fine. Heck, you'd occasionally find that much variation on brand new bikes I would think........Nashbar sells an aluminum touring frame that's spaced at 132.5mm, with the idea being that the frame can be squeezed in or out 2.5mm as necessary to accept either 130mm or 135mm hubs-
    Last edited by well biked; 02-01-07 at 05:25 PM.

  7. #7
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    I wouldn't worry much about it. The stays probably flex more when you hammer down on the pedals out of the saddle.

  8. #8
    Your mom
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    I have the same issue on the front wheel of my Cannondale - I have to squeeze the fork to tighten the QR. I never thought about it, but of course, now I worry.

  9. #9
    Sir Fallalot wroomwroomoops's Avatar
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    Quote Originally Posted by Philatio
    And for the record, aluminum does bend elastically before it "breaks" just not nearly as much as steel.
    Actually, that's incorrect. Aluminum has no fatigue limit, which means, any deformation is plastic at least partially. Any deformation to an aluminum object will cause permanent changes in the crystal structure.

    In titanium or steel, a small enough deformation can cause no permanent changes in the crystal structure. Of course, if you go over a certain limit, permanent changes will happen. That limit is the fatigue limit.

  10. #10
    Senior Member TimJ's Avatar
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    Quote Originally Posted by wroomwroomoops
    Actually, that's incorrect. Aluminum has no fatigue limit, which means, any deformation is plastic at least partially. Any deformation to an aluminum object will cause permanent changes in the crystal structure.
    Aluminum can flex plenty. If aluminum officially has no fatigue limit then I don't think it means what you think it means. Flexing isn't the same thing as bending and aluminum can flex plenty with no permanent change. Airplane wings and the fango bike attest to that.

    Edit - Oh I see, I suppose you were specifically addressing the guy you quoted who used the word "bend".
    fun facts: Psychopaths have trouble understanding abstract concepts.
    "Incompetent individuals, compared with their more competent peers, will dramatically overestimate their ability and performance relative to objective criteria."

  11. #11
    biked well well biked's Avatar
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    Aluminum will definitely bend before it breaks, i.e. bent rims, bent frames, bent handlebars, etc. It has no fatigue limit, correct, so any stress at all brings it closer to failure, but that's not to say it can't bend before it breaks........The reason you can't expect to cold set an aluminum frame successfully is that in order to get the rear triangle to permanently "set" at the desired spread, you've got to bend the stays way beyond the targeted distance, otherwise the frame just springs back to the original dimension. Aluminum frames aren't suitable for that, they'd crack first-
    Last edited by well biked; 02-02-07 at 10:34 AM.

  12. #12
    Sir Fallalot wroomwroomoops's Avatar
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    Quote Originally Posted by TimJ
    Aluminum can flex plenty. If aluminum officially has no fatigue limit then I don't think it means what you think it means. Flexing isn't the same thing as bending and aluminum can flex plenty with no permanent change. Airplane wings and the fango bike attest to that.

    Edit - Oh I see, I suppose you were specifically addressing the guy you quoted who used the word "bend".
    I think my comment was clear, but that's because I'm into metallurgy and am pedantic about the terminology and the meaning behind it. This was the sentence:


    Quote Originally Posted by Philatio
    And for the record, aluminum does bend elastically before it "breaks" just not nearly as much as steel.
    "Elastic" means the material will return to the same exact position after the stress. That is not true for aluminum, for any amount of stress. I did not say you can't bend aluminum to a certain extent without breaking. Sure you can, but you will accumulate a certain permanent deformation in the crystal structure, no matter how little you bend it. Any deformation with aluminum is plastic, at least in part.

  13. #13
    Senior Member TimJ's Avatar
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    Quote Originally Posted by wroomwroomoops
    I think my comment was clear, but that's because I'm into metallurgy and am pedantic about the terminology and the meaning behind it. This was the sentence:



    "Elastic" means the material will return to the same exact position after the stress. That is not true for aluminum, for any amount of stress. I did not say you can't bend aluminum to a certain extent without breaking. Sure you can, but you will accumulate a certain permanent deformation in the crystal structure, no matter how little you bend it. Any deformation with aluminum is plastic, at least in part.
    Wait a second. What do you mean by stress? If aluminum changes its shape every time it moves then how could an airplane fly? The wings can flex several feet, it sounds like what you're saying is every time they flex they return to a different position? How could a plane fly without having every panel on its wings replaced every year?
    fun facts: Psychopaths have trouble understanding abstract concepts.
    "Incompetent individuals, compared with their more competent peers, will dramatically overestimate their ability and performance relative to objective criteria."

  14. #14
    Sir Fallalot wroomwroomoops's Avatar
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    Quote Originally Posted by TimJ
    Wait a second. What do you mean by stress? If aluminum changes its shape every time it moves then how could an airplane fly? The wings can flex several feet, it sounds like what you're saying is every time they flex they return to a different position? How could a plane fly without having every panel on its wings replaced every year?
    The plastic deformations are very small in correctly dimensioned applications (like planes hopefully are). Hey, it's the same deal as with aluminum frames: there you have flexing all the time, and still aluminum frames last for many years. Same thing: the plastic deformations are very small in well-designed frames, hence the frame lasts very long.

  15. #15
    we are 138 Philatio's Avatar
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    Quote Originally Posted by wroomwroomoops
    Actually, that's incorrect. Aluminum has no fatigue limit, which means, any deformation is plastic at least partially. Any deformation to an aluminum object will cause permanent changes in the crystal structure.

    In titanium or steel, a small enough deformation can cause no permanent changes in the crystal structure. Of course, if you go over a certain limit, permanent changes will happen. That limit is the fatigue limit.
    Didn't know that, thanks I guess everything I've done has been steels and I assumed aluminum would behave similarly.

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