I would imagine it's something to do with the overall length. A longer shaft can produce more deflection/oscillation.
1950 Chevy Advance Design 3100 in Scotland In the Stovebolt Gallery More pix on Flickr. I've definately got this truck thing in my blood ... my DNA sequence has torque settings "Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.
