Originally Posted by Bklyn
Does a chain tensioner effectively act as a rear derailleur? Will it pick up slack? If the front derailleur moves the chain to the small chainring, aren't you going to have an unusable, saggy chain?
Most chain tensioners pick up slack, but if you want to handle a 14T difference in your front chainrings [52 --> 38], you can use any mid or long-cage derailleur. You might be able to use a short cage road derailleur - 105 or Ultegra might do it, but I don't think Dura-ace [especially older stuff] will. The main benefits of a rear der. are cheapness and easy setup.