I have no idea why the better older bikes did not shimmy, or at least the 85 Schwinn Le Tour luxe, Voyageur, and Voyageur SP didn't shimmy, not sure about other manufactures. I did have a friend who years ago toured on the regular Schwinn Le Tour and he had shimmy issues, but I don't know what year his Le Tour was; but my 85 Luxe did not. The 85 Le Tour Luxe and the Voyageur both used Columbus Tenax Chrome Moly double butted main tubes but the Voyageur used a custom blend, not sure what that blend was, there was a difference in the fork between those two as well; and of course, the SP was the top of the line and had a better tube set and fork. 1985 was the best year for Schwinn bikes of any mid-level and up model.