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oval at the BB shell, which makes sense
It makes some kind of sense, although I wonder whether the real reason for it isn't just to make the welding easier rather than the "laterally stiff vertically compliant" marketing justification usually applied to that design feature. After all these years it's the signature style of the PreCursa, so they're probably not going to change it now
Wouldn't it make more sense to have the fat part of the downtube at the welds instead of in the middle?