Rake, trail and offset
All these terms refer to steering geometry so rake, trail and offset effect handling and are directly linked to each other, altering one will effect the other. So if you want to alter the handling, cornering ability or straight line stability, rake, trail and offset are some of the first things that you should look at.
Rake, which is sometimes referred to as steering head inclination, steering head angle or steering axis inclination, is defined as the rearward inclination of the steering head as measured from the vertical. Rake is always described in degrees. Although there are exceptions, the accepted practice is to consider a vertical steering head; one placed at 90-degrees to the pavement, as having a zero-degree angle. The rake angle effects steering ability, the smaller the rake angle then the easier the bike is to corner but will be less stable in a straight line. Consequently, the rearward inclination, or rake is normally in the 22 to 36 degrees range.
So, why inclining the fork rearward make it stable? The simple explanation is that when round objects, like wheels, have a force applied to their axis, they prefer to follow a course that’s in opposition to that force. So if the force applied by the rider was coming from directly overhead (zero rake), the wheel had tendency to rotate around its axis, rather than move forward.
However once that force changed direction and was applied in a forward direction the wheel preferred to move in the opposite direction. Furthermore, because the steering axis now contacted the ground at some point forward of the wheel’s vertical axis, it also created a levering affect on the wheel that tended to stabilize it when it was in motion. That lever is what we call Trail.
The distance on the ground between a straight line drawn through the center of the front wheel spindle and a line drawn through the center of the headstock axis is called trail, for the simple reason that it’s the distance that the center of the front tire’s contact patch trails behind the steering axis. The trail dimension is given in either inches or millimeters dependent on the manufacturer’s whim, and varies from 2 to 7-inches. Trail also affects the rear wheel, but since its affect is less pronounced, I mention it here only in passing.
Essentially, Trail is what gives our bikes directional stability at anything above a walking pace. Without it, the front wheel would try to revolve around itself like a broken shopping cart caster at the slightest provocation making the bike unrideable.
The length of the Trail lever determines how much effort it takes the rider to make the bike turn, and how much effort it takes to hold in the bike in a turn and it’s also largely responsible for the way our bike “feels” as we’re steering it, this is because the Trail acts directly against the front wheel, and that input is transmitted via the fork and handlebars directly to the rider.
Reducing the Trail dimension provides more input; and everything the front wheel does is telegraphed directly to the rider. Increasing the Trail dimension tends to dampen-out the feedback, and may make the front end feel a little vague.
The distance between a line drawn through the centre of the steering stem/ headstock axis and the centre line of the front fork tubes. Typically the offset inversely affects trail, if offset increases then the trail will decrease. Offset allows the frame designer to move the wheel forward or backward in relation to the steering stem, thereby increasing or decreasing the amount of Trail.
Rake, Trail and Offset relationship
If you increase the rake, the trail increases
If you increase the trail, the rake increases
If you increase the offset for both yokes then the trail decreases
If you increase the offset for only the top yoke/ triple tree then the trail will increase
If you increase the offset for the bottom yoke/ triple tree independantly then the trail will decrease