Rear-driver front forks.
Any of the four patterns above work exactly the same in the hands of the rider.
So much for the manner of obtaining slant of the head or pivotal connection, as we shall call it; but as to the amount of this slant it is desirable to obtain much more can be said.
The great system of castering, so knowingly discussed by some expounders of cycling faith, has in it really something of substantial importance. It is asserted that if the machine is so constructed that the line of pivotal connection strikes the ground in front of the point of contact of the wheel (see [Fig. 1]), a castering element comes into play which will cause the machine to retain its forward course, and enables the rider to go “hands off.” Note that the line a b strikes at c in front of d.
Supposed caster. Real caster.
I have observed many rear-drivers, and cannot see that this makes much difference; the various kinds seem to be equally well ridden, with respect to easy steering, if only the riders happen to be thorough experts; of course all sorts of theories in regard to the action of the steering have been advanced.
I take it that there is only one truly tenable theory of castering; this when applied will obviate “sensitiveness” completely and under all circumstances; it is as follows: The pivotal connection must be such that the line a b strikes in front of the point of support, as before spoken of, and it must also be so constructed and placed in such a position that no motion of the handle-bar will cause the machine to lower its centre of gravity. If by turning the handles any weight is lowered, you can depend upon it that the force of gravity, always tending to lower this weight, will inversely cause the handle-bar to turn. It will be noticed that when the machine stands upright the steering apparatus is not in a state of stable equilibrium; that is to say, the weight of the machine tends to shift the wheel, and it can hardly keep straight by means of such castering element as results simply from the line of the pivotal connection striking in front of the point of contact.
The necessary conditions are as follows (see [Fig. 2]): The pivotal line a b must strike at c in front of d, and the line a b c must be vertical in order that no motion on its axis can lower any weight when the machine stands upright. Now, it follows from these conditions that the head must be vertical and no part of the pivotal line in the rear of a vertical through the centre of the wheel.[6]