is basically how far the front axle sits ahead of an imaginary line that runs through the steering axis. The most visible evidence of fork offset are the short extensions for the dropouts that house the front axle. Less obvious is that most fork makers also angle the crown slightly to provide part of the offset as well. When all bikes had 26-inch wheels and similar head angles, fork makers and frame designers settled upon 40-millimeters or thereabouts and that's where it remained - until the 29er appeared.
Gary Fisher liked 29ers, but hated the heaviness of the steering. The Fisher folk experimented with different offsets and discovered that 51 millimeters gave his 29ers a similar feel up front as a 26-inch bike. Fisher's 'G2 Geometry' was born and with it came more widespread knowledge of how the relationships between head angle, wheel size and offset play in the bike's steering equation. Today, there are a number of offset options available for production forks. RockShox, for example, offers 40, 42, 46 and 51 millimeter offsets.
In this chapter of 'To the Point,' Pinkbike attempts to clarify why fork offset changes with each wheel diameter and touch on how wheel size and head angle factor into the steering equation. What does fork offset do?
Fork offset is used to adjust the amount of 'trail' in the steering geometry. More offset equals less trail. What is trail?
The distance that the center of the tire's contact patch falls behind the imaginary point that the steering axis (center of the fork's head tube) passes through the ground. This is what causes the bike's front wheel to self-straighten when it is moving forward. More trail firms up the steering and adds straight-line stability. Less lightens the steering pressure at the handlebar and gives the bike a more nimble feel. Too far in either direction makes the bike difficult to control. How much trail is best?
The correct number varies with head angle and other factors, but somewhere between 80 and 90 millimeters seems to be the sweet spot that produces a light feeling at the handlebar, with a stable feel at speed, and with predictable cornering. How does wheel diameter affect trail?
As wheel-diameter increases, the axle moves farther back along the steering axis, which increases trail. With a 68-degree head angle, the difference between a 26 and a 29-inch wheel is 15 millimeters (.61 inches). How does the bike's head angle affect trail?
Trail increases as the head angle becomes slacker. How is fork offset used to adjust the trail to the correct amount?
Adding more offset reduces trail by moving the center of the tire's contact patch forward, closer to the steering axis. What is the relationship between fork offset and head angle?
If one of the offset options offered by the fork makers does not provide the frame designer with the desired amount of trail, he or she can change the head angle by a half degree or so to correct it. For instance: the slow steering of the 29er can be quickened by adding a slightly steeper head angle, then the designer can bring back some high-speed stability by choosing a fork with less offset to add some trail. The reverse is true for slack head angles, where the designer can erase some of the heavy wheel flop by adding offset and thus reduce the trail. Wouldn't an adjustable offset feature be the best solution for fork makers?
Not in the real world. Experiments with various fork offsets show that getting it wrong is far worse than riding a bike that is nearly perfect. Now that fork makers are providing options, for the most part, bike makers are getting it right. That said, if you see CNC-machined fork crowns on World Cup DH bikes, or stanchions that are noticeably angled on single-crown forks at the EWS, they are almost certainly fitted to alter the fork offset.Read More about this subject on Pinkbike