| As Pete points out the seat load is supported in front of the crack so its weight from behind, adding a set of panniers at 10 - 20 kg would load it quite a lot. From the ones i've seen its not just load related, its a flexed stress area and it looks like crack comes from the end of the weld, thats a fairly common one on mountain bike frames so we try and avoid welds ending like that. alloy tubing is thin, 1.2mm - 2mm being the norm. | 
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