ukdodger wrote:Ok so I'm a bit thick but I dont understand this. This is a summery taken from a book on frame design: Fork rake is the perpendicular distance between the head tube centre line and the front wheel axis. ie, the amount by which your forks are bent forward. For any given wheel radius and head tube angle (that's the angle between the centre line of the head tube and the horizontal) there is only one fork rake that will achieve neutral steering. Ok that makes sense to me but here's the bit I dont understand - Neutral steering simply means that the forks will neither rise nor fall when turned.
astonishing??? I dont get it.
You don't get it because it's nonsense!
For a proper explanation of steering you need to read Bicycling Science by DG Wilson, however it's rather hard to get one's head around the mathematics, so here's the basics.
The steering geometry of all practical bicycles is such that the centre of gravity falls slighty as the steering is turned away from the straight ahead position. In other words: once the steering has been turned slightly, the weight of the bike tends to make it turn further. This tendency is resisted in the case of a stationary bike by tyre friction and when it's moving by the combined effect of tyre rolling drag and trail (as I decribed before and someone quoted above).
I suppose you could say that "Neutral steering" exists when when the force of gravity trying to turn the steering further is exactly balanced the effect of trail trying to straighten it up. But even that is not true. Both effects vary in a complex manner as the steering is turned, so they cannot exactly balance at every angle of turn - even before you add the further complication of lean angle!
In practice we find that in bikes with nice steering, the trail effect is greater than the gravity effect, so the bicycle has a slight overall tendency to keep going in a straight line in spite of the gravitational tendency of the steering to flop.
If you hold a stationary bicycle vertical and disturb the handlebars enough to overcome tyre friction (the gravitational flop effect is very slight for small angles of turn but gets stronger as the handlebars are turned further) you'll see that the steering comes to rest at something less than a right-angle of turn. That's the angle at which the bike is lowest. Turn the bars further and the bike begins to lift again.
It's a geometrical coincidence that if fork offset equals trail, the bike will resume its straight-ahead height when the steering is turned to a right-angle. Of course you never turn the steering anything like that far, but a person with a "little learning" in geometry, may possibly assume that if the bike is the same height at zero and 90 degrees of turn, then it's height doesn't change when you turn it between those angles. Unfortunately, as has been shown by those neat diagrams above, that is not the case.
However it is another coincidence that for typical bike head angles, the condition of offset = trail usually produces okay steering. Actually, this coincidence works better for the shallower head angles and more draggy tyres that were common in the past. With a steep 73 degree head and narrow racing tyres, much less offset is wanted in order to leave enough trail to avoid unstable steering.
And, as shown in the picture of the recumbent above, when head angle approaches vertical the forks must be offset backwards
(like a furniture castor) in order to produce the necessary trail.