Quote:
All of the resistance to twist is in the outer surface area of the bar... The metal in the center offers little resistance & extra weight..
To be more precise, if the interior space on the bar is say 7/8", one could deduct the rating of a 7/8" solid bar from rating of a larger diameter solid bar to reach the new rating of a tubular bar. It is not as maybe implied above that all the resistance to twist is on the outer surface, it is that the outer surface on any object is more resistant/effective to resist twist then the interior. 
Reality check, that half the population is smarter then 50% of the people and it's a constantly contested fact.
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