This is mainly a concern in the case of tires for off-road use, where a knobby tire can get a bite of a soft or irregular surface. For pavement riding, smooth tires get as good traction as those with tread, sometimes better. Aggressive knobby tread tires corner very poorly on pavement, because the tall knobs bend sideways under stress. The trail is a function of the head angle , the fork rake , and the tire diameter. Trail has a major effect on the handling of a bicycle.
More trail increases the bicycle's tendency to steer straight ahead.
A bicycle with a smaller trail dimension will be more manuverable and responsive. Joshua Putnam has a good discussion of trail and Bicycle Steering Geometry in general on his Web site. If that link doesn't work, try this one. The horizontal cable linking the two arms of a cantilever or center-pull brake. Traps Short for rat traps .