On my touring bike, with front and rear panniers, I hit 47 mph on a downhill in Montana and started feeling the frame flex, so I back off. Years later, on a Santana tandem in Colorado, I hit 58 mph and it was a solid ride; I would have taken more, but that was all the descent gave me. Another time in Colorado, I again hit 58 mph on my long-wheelbase recumbent.