1973 Dodge Dart sport speed related shimmy

Wheels with other than OEM offsets,specifically neg.offset,can initiate a shimmy due to the magnification of the scrub-radius. Worn control-arm bushings can allow it to continue.I have also seen it happen with structurally unsound front unibody weldments and control arm mounting points, or mismatched parts. Non-factory alignments may be able to compensate for the wheels but not the other problems.My experience also shows that A-bodies like as much caster as you can find.At 3 deg. they start to drive nice. At 4 to 4.5 they become very stable at speed. If you go for the caster, you will also have to fix the bump-steer.I have also cured shimmies with a change of tires.Tires worn on the edges actually making them cones are sure to cause problems,esp. on cars with worn parts. Hope this helps.