I have a feeling that the slightly exaggerated angle of our forks sets off a strange harmonic, that becomes readily apparent when any imperfections are present in the tread.
A note from days of yore: I used to haul tankers and flatbeds. When loaded just so, the 40 ton GVW flatbeds and bulk tankers would jump up & down a half or quarter inch at around 2 or 3 cycles per second on a perfectly flat road at certain speeds, usually around 40 to 45 MPH. Weird harmonics. Suspension bridges do this too. It's pretty 'exciting' to stop in traffic on the George Washington Bridge, and the entire 3,500' span is jumping up & down an inch or so.
