Thanks to 7 responses. I have a much clearer idea of where low trail is for 27/700 & 650B:
for 27/700, low trail appears to be 45 to 50 and less. Cpt Bligh reports no problems at 38 mm on 27x1-1/8 tires
for 650B, low trail appears to be 30 to 35 mm or less.
Shimmy appears to be the problem when trail is low. Corrected by knees on top tube.
John Hawrylak
Woodstown NJ