ukdodger wrote:Thanks CJ. I dont fully understand what you are saying but in spite of that I still think the rising and falling bit does have to do with oversteering and understeering if only because it makes sense that if your weight shifts forward it will increase the turn and if backwards it will fight the turn. Gyroscopic and trail effects notwithstanding that is. According to the book I'm studying deliberate oversteer is built into the fork rake to compensate for the Gyro and trail effect.
Rising and falling surely does have something to do with it, which is why I said that statement in the book (Neutral steering simply means that the forks will neither rise nor fall when turned) was nonsense. I also think the book's use of the term oversteer may be misleading. And the suggestion that there is one perfect offset for a given head angle and wheel size is too prescriptive.
The only simple way to sum this up is to say that there's a whole lot of things going on when you steer a moving bicycle, especially when you also lean it into a corner. Some of these things (mainly gravity) try to make the steering turn further into the corner, whilst others (mainly the combination of trail and drag) try to straighten it up again. A bike has nice steering when these factors are near enough in balance for a range of lean and steer angles that correspond with typical cornering situations, whilst providing enough straight line stability.
Given the range of possible cornering speeds and radii, no geometry can be perfect in every situation. Any geometry will understeer a bit in some corners and oversteer in others. For any head angle there's a range of offsets that'll be good enough in most situations, better in some than others.
The right geometry nowadays tends to mean as little trail as you can get away with before the bike becomes vulnerable to shimmy. With modern super-light frames, that tends to be quite a lot of trail, which is why the standard offset for ready-made forks is only 43mm. It works okay provided the angle isn't too shallow, when the trail becomes so much that the bike is over-stable in a straight line, but with a disturbing tendency for the steering to flop side to side if you rock the bike when standing on the pedals.
Ideally you want the geometry that handles best for the way that you ride and that's not an easy thing to predict. Fortunately humans are so adaptable that we soon get used to and eventually prefer whatever kind of steering our chosen bike just happens to have.