Changing wheel base has no effect on steering trail. Trail is a function of head tube angle, rake offset, and wheel size. Changing fork length does change head tube angle. Increasing the length decreases HTA and increases trail. Decreasing rake offset increases trail.
http://yojimg.net/bike/web_tools/trailcalc.php
In bicycle terminology "rake" is fork steering offset. In motorcycle terminology "rake" is the steering axis angle from vertical.
http://en.wikipedia.org/wiki/Bicycle...cycle_geometry