Best explanation of fork trail that I have read is from Tom Kellog;
http://www.spectrum-cycles.com/geometry.php

Too often, I think new builders try to tune a desired fast or slow steering feel with trail instead of with HTA.


For the two examples bikes having same trail with 74° and 70° HTA, keep in mind that the combine effect of shallower HTA+increased rake also will both contribute to making the front-center measurement (Front wheel hub to bottom bracket) longer, so increase the wheelbase of the bike and change the weight distribution (more rearward). These later two side-effects probably have at least as much enfluence on the overall handeling of the bike (if not more), than the direct effect of HTA.

Low trail touring bikes need the low trail because every time you turn the bars you are also lifting all the dead weight of the gear being carried on the front end, increases the effort it takes to turn bars. The low-trail minimizes the amount of lift produced during turning so get the steering effort back to an easier, more familiar level. A high trail bike with heavy front-end load would turn like a freight train.