"Trail can be found by supporting the bike on a flat surface in an upright position for measuring purposes. A centerline is run down through the head tube until it hits the flat surface. A vertical line is then dropped from the front axle until it hits the ground. The distance between these two points on the ground is the trail. The comfort range of trail is 50 to 65 millimeters. Beyond these limits in either direction, it would be considered less desirable."
Dynamic Stability Of Bicycle Design : Part 2
Dynamic Stability Of Bicycle Design : Part 3
Dynamic Stability Of Bicycle Design : Part 4
A Bicycle Model To End All Models
"Linearized Dynamics Equations For The Balance & Steer Of a Bicycle" - J. P. Meijaard, Jim M. Papadopoulos, Andy Ruina, A. L. Schwab, 2007
History Of Bicycle Dynamics