For a given head angle, wheel diameter and axle-crown dimension, the more offset the fork has (the distance that the wheel spindle is ahead of a line through the centre of the steering head) the less trail the bicycle will end up with, trail being the distance measured on the ground between a line projected though the centre of the steering head and a line, perpendicular to the ground, dropped from the wheel spindle.
So, as all else being equal trail increases with wheel size, forks designed for 29" wheels often have more offset than for a 26" wheel - say 45 to 55mm as compared to 38mm for 26" wheels.
This also compensates for the slightly "slower" response of a larger wheel and permits a slightly slacker head angle (which has the added advantage of providing more toe clearance for a given top tube length) without the steering feeling unresponsive or having a tendency to "flop" onto lock at low speeds.
So, as all else being equal trail increases with wheel size, forks designed for 29" wheels often have more offset than for a 26" wheel - say 45 to 55mm as compared to 38mm for 26" wheels.
This also compensates for the slightly "slower" response of a larger wheel and permits a slightly slacker head angle (which has the added advantage of providing more toe clearance for a given top tube length) without the steering feeling unresponsive or having a tendency to "flop" onto lock at low speeds.
