When I cycle I can feel wobbling in the direction. I feel it even more and can even see it if I ride no handed. It gets really bad when I put luggage on my low rider.
The bike is a 62cm touring steel bike with 700 wheels, the head tube is really long, about 9 inches without the crowns. The wheel oscillate by about 2 degrees with luggage on at about 35 km/h (22 mph). The handle bar being kept about straight. So all the wobbling is torsion in the steering tube.