Old 04-24-08, 10:13 AM
  #11  
imcrushingyerhd
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Join Date: Feb 2008
Location: ATX
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Bikes: '01 Fisher Joshua F3. '89 Master... I don't let go of bikes easily.

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7075 al is used because it is more easily machinable, because it is more brittle. The tensile strength is higher but for aluminum it's still easily machinable compared to steel. This means that the chip breaks quickly so you can hog more material off at a time, especially when boring a hole, which is great for making stuff. Brittleness, however, is not great for the end user. 7075 actually costs more pound for pound than 6061- twice as much, but the cycle times are reduced so much that manufacturing is cheaper in the end.

Because of the severe brittleness of your frame, you won't get much warning when it decides to break all the way. In fact I'm surprised (Godly intervention?) that it cracked like it did and didn't crack all the way through. I can tell by the photo, just from being a machinist and mechanical designer for so many years, that the head tube is WAY to thin for 7075 T6 by design.

It most likely chose to break when it did because while it was sitting you probably got some corrosion on your bearings or races making them ever-so-slightly uneven, which applied an uneven force to the head tube, which is real bad for brittle material. They probably tested the head tube with direct pressure in a lab, since if any indirect pressure from bad bearings would defer liability onto the bike owner.

So here's the easy answer: If you ride it you will die.

BTW Don't try to weld it. It cannot be fixed. The bottom of the head tube needs to be dead-flat or the bearings won't be under even pressure and you'll get another crack. You'll also lose the annealing (T6 most likely) the material was given once you apply the heat from welding, making it completely vulnerable to cracking. Even if you took all of that into consideration, you have to strip the paint to weld it . JB weld wont hold something like that together so don't try.

So you on the Darwin Award's page.....

Last edited by imcrushingyerhd; 04-24-08 at 10:19 AM.
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