I've experienced a shimmy and been experiencing instability at high speed on this bike. Are these welds normal. It seems the down tube radius is too big and the welding has buckled the tube edge in. Would this contribute to a wobble?

Can you post a better photo?

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it looks like the down tube may curve in at the edge to reach the head tube. whatever it is, it is extremely unlikely that a cosmetic weld "flaw" has anything to do with high speed shimmy. as a search will discover, shimmy is a harmonics issue, and changing one or more parts of the system (which includes you, the frame, fork, wheel, tire, air pressure, stem, rider positioning...) can get rid of it.