Trek actually replied to me with the following when I asked about the Alpha gold:
Hello,
Thanks for writing in. Molding and shaping of the tube-sets are what will differentiate the various levels, the material being utilized for it is a 6000 series 6061 tube. Please let us know if you have any further questions.
Best,
Andrew Pajak | Trek Bicycle Corporation | Technical Service Representative
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Ive been looking at an aluminum 29'er but I am kinda on the fence about 6061 vs 7075.