When doing pullups or chinups coming out of the bottom part (with arms straight) is by far the most difficult thing for me. I can do at least 15 chinups in a row when I go only 50-60% down, but if I go down all the way I can do only 1 or 2 strict chinup/pullup at once (with 5 sec break between each rep I can do 20 of it, but not in a row). 

So are there any specific drills or exercises to improve the bottom part of my chin ups and pullups?