Don't do it, John. The heat treating recipe for large diameter,
thin-walled aluminum tubes is not simple, was worked out over a lot of
trials. Cannondale produced many warped frames to learn how to do it.
If you don't heat treat it, it will probably quickly break, resulting in
expensive dental work or worse.
Cannandale is pretty good at standing behind their products, but good
luck if you are not the original owner. Rather than repair, they give
you a new frame, and you have to swap the components. Some
manufacturers want you to cut out the bottom bracket of the frame on
which you are making a warranty claim.
It is possible, but the frame will not be as strong as original.
Not sure on the alloys they use, but they are heat-treated frames, so
you would have to re-heat-treat the frame to restore the factory
When you weld heat treated alloys, like 6061-T6, the welded area will be
at around a T2 after the weld.
Over time 6061 will precipitation harden back to around a T5, after
about 3-4 weeks, but it can't get back to a T6 without a complex series
of heatings and coolings in a computer controlled furnace.
So if you choose to weld the frame I would recommend not riding it for a
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