I see a lot of images on various sub-forums here that show panniers mounted such that they are a fair distance behind the rear wheel axle center. Back in the 1980s I had really be shimmy on a bicycle with the panniers a bit behind the rear axle and moving the rack forward so that the panniers were basically centered over the rear axle eliminated that shimmy.

How is it that these bicycles with the panniers so far behind the rear axle don't cause shimmy?

Cheers