Looking at the metallurgical data collected by Alan Williams, and comparing it to the performance of heavy English bows and thickness of surviving harness, it seems there was quite a broad range of armor effectiveness. With a top quality harness of medium carbon steel hardened by effective heat treatment, 1.25 mm plates would need about 120 Joules to penetrate, and would defeat a 140 lb bow generating 100 J hitting dead on at close range. 1.25 mm is the low end of the range for helmet sides and cuisses measured by Hardy from this period.
On the other hand, low quality iron armor of the same thickness might be defeated by about 40 Joules from an arrow striking perpendicular to the surface, and a 70 lb bow could generate over 50 J at close range. Even at 180 yards, with an arrow descending at about 30 degrees, a 100 lb bow could probably deliver enough energy to defeat such plates if it hit squarely in the horizontal plane.
The thinnest value measured by Hardy for the top front of a bascinet was 2.47 mm, which if made of iron would require about 110 J to defeat with an arrow. A 1.5 mm iron breastplate worn over iron mail and padding would probably require at least 130 J to defeat with an arrow, so even an inferior harness would offer excellent protection for the vital organs from a frontal attack.