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@gbj_tester
The geometric factors are weight distribution and trail, but since these are unlikely to be out of whack on a Surly, unless you've put a very wrong fork in it, it's actually more likely that mechanical factors which are not strictly geometric are the problem. Torsional stiffness of the frame, inertia of the steering system about the steering axis, wheel axial stiffness, tyre compliance, etc. etc. Basically, you have a whole bunch of inertia-compliance systems coupled together, and they will have a set of resonant frequencies. The high frequency ones are what causes "shimmy", but there are low frequency ones which, if excited by pedalling frequency, can cause a weave which is usually damped by your holding the bars, and only manifests as a problem when you let go.