Ride an 2003 RSV with a sidecar. Have a front end vibration at 55 up to 60. Only there, no other speed. Occurs in 4th or 5th. Straight roads or curves. Bike has 19,750 on it, going in for the 20K in the next couple of weeks. I doubt it's cupping, not willing to believe that-seems every shop says 'cupping' so they can sell a tire. So, any suggestions? I think it's a balancing issue. Front tire is a Dunlop D404F. Thanks.