Originally Posted by Jumpa
That's not a crack that's a complete fracture through and through. take it back to Wally world and complain, that is a fabrication error not from vibration
Actually it's not its an engineering error. Aluminum is alloyed with magnesium and silicon to make it strong. That is what the 6061 numbers mean. It work hardens and get's brittle over time. Vibration and bumps all speed up the process. And it always seems to fail at the edge of a weld. A cantilever frame should be a spring from the axle all the way to the head but where it welds to the seat post it's not a spring anymore its just like putting your knee in the middle of a stick and pulling on both ends. It's gonna fail at the end of a weld everytime. It's even worse on those Panama jacks where they welded a tube in the middle of the seat stay to to rear rack. You load the rack with some weight and hit a bump guess where its going to break? Right at the edge of the weld on that rack support. I've repaired this stuff thousands of times on headache racks for trucks, irrigation equipment, all kinds of aluminum stuff the weld won't break the aluminum will. Damping the shock might help like on a full suspension bike frame but eventually that's where it will fail. If it fails. I hate aluminum imo its total garbage. You don't see aluminum motorcycle frames around do you? There is a reason for that.
But in fairness I don't think engineers thought the bicycles they designed were gonna get motors stuck on them either so really when we jump curbs, and hit potholes at thirty miles an hour with a case of suds on the back rack its sort of our fault for pushing the envelope too far. A heavier steel frame is really needed for this stuff or else you get what you get. Just pay attention and look it over often and take it easy.