It has taken half a day but at last we are on the same page.
Maybe the problem is that one of the rods is underloaded.
Hi Aitor
Underloaded? thought the big spring theory was exploded
I'd go with the notion that in the OH cast, the majority of the line is accelerated in one direction, in the Roll, only a part of it is, and some of the energy will be wasted making the rest of the line move in the direction we intend. Since I'm crap in physics, I'm probably wrong, but I really like your explanations, so I'm looking forward to it