Or is it just down to the flexibility of an old steel frame?
It's probably this. The weight you're carrying probably makes the frame flex. Add into that the weight distribution and a slightly uneven centre of gravity and it probably hits a sweet spot, causing the frame to oscillate.
It's probably this. The weight you're carrying probably makes the frame flex. Add into that the weight distribution and a slightly uneven centre of gravity and it probably hits a sweet spot, causing the frame to oscillate.