In order to take a header, a certain centre of gravity must get beyond a certain line. This centre of gravity will vary in position in different machines, and the modifications spoken of cause the line to move in a way which is, I think, sometimes overlooked.
If we discuss the crank Ordinary, it will be noticed that when the front wheel is stopped in its forward progress, the frame of the machine together with the rider and all other parts of the system revolve about the centre of the wheel and cause an action within the system, the same as that of the forward wheel revolving backward through the fork. Now, it is just when such backward motion is prevented, that the gravity line moves and alters the conditions, decreasing the liability to headers. If the forward wheel can revolve backward through the fork, then, in taking a header, the system, exclusive of the forward wheel, will revolve about a point in the wheel centre; but if it cannot so revolve, then the entire system, including the forward wheel, must all tend to revolve about the point of contact of the wheel with the ground. Now, it will be seen in the latter case, or anti-header machine as we shall call it, that as the system tends to revolve about the point of contact, such point will constantly change; in other words, the wheel must roll onward, and the point of contact will therefore advance.
Header action, smooth road.
In [Fig. 1], in the annexed diagram, we show the distance forward and upward, a to b, the rider must be thrown before he gets beyond the gravity line, g, in the Ordinary; [Fig. 2] shows the distance when the wheel will not revolve backward through the fork. In either case, the header is supposed to be taken on a smooth road and not against an obstruction; this can easily occur in vaulting into the saddle or in leaning too far forward. It will be noticed that the distance the rider is elevated, or, in other words, the amount of work done against gravity, is in both cases the same, but the distance forward he must be thrown is considerably greater in [Fig. 2]. This is for the reason that while the point of contact, h, with the ground remains the same in [Fig. 1], in [Fig. 2] the point rolls on to i. For more accurate illustration of the work to be done against gravity, and the distance forward the rider must be thrown, see the header curves in Figs. [4] and [5], farther on.
We see, then, that the advantage which the anti-header (No. 2) has over the Ordinary machine (No. 1) is not so very great when in both cases a smooth road is considered; when, however, we consider the element of an obstacle in the path, the difference is much more in favor of No. 2. Let us compare the action of both classes of machines against a four-inch obstruction. In all cases the action of No. 1 machine will be the same,—that is, the wheel will remain in contact at h, [Fig. 1], and the saddle will go on over, just as it does in the case of no obstruction at all. But in No. 2 the very act of taking the header must raise the entire weight and roll the system upon the obstacle, as shown in [Fig. 3].
Anti-header wheel action on obstruction.
The point of contact, h, over and beyond which the centre of gravity must be thrown, will not only move forward, as shown in Fig. 2, but will move to the top of the obstacle i, [Fig. 3]. Or, if the question is one of a rut or indentation in the surface of the roadway, No. 2 will be caused to roll partially or altogether out of the rut. Now, since the rider, by the action of his momentum and that of the machine, is rolled upon the obstacle or out of the rut, it is easily seen that if he is attending strictly to his work and is at all a skilful rider, he can, by a lively thrust upon the pedal at the opportune time, right himself and keep the drive-wheel rolling on, in which case the rear part of the machine will, in all ordinary cases, drop back upon the ground, from which, of course, it will have raised.
