That`s the second thing I mentionned. Drop between the top of the saddle and top of armrest. Typically a road configuration will limit the drop and limit the nice and low aero position you would be able to get with your TT or Tri. My road bike is able to give me about 9 cm .. with a Cinelli angel bar. set to the lowest position (ie stuck on the headset). a pure TT would have a smaller headset (10-11 cm instead of 13-14 cm) ..