[Fig. 5] shows a new machine of the Star pattern, with twenty-four-inch front steerer, F, and a thirty-nine-inch rear driver, R. The check in the momentum is not so radical as that shown in [Fig. 4], as the front wheel mounts the obstacle. The one short line below the curve shows the backward thrust.
The sudden check in striking an obstacle, with the machines last referred to, shows the necessity and enormous advantage of a forward give to the saddle support adopted in some of those patterns. This arrangement is not so necessary in the Ordinary, yet it would do no harm, for it will be seen that the large front wheel of the latter strikes the obstacle with quite a sudden upward curve and check in the momentum sufficient to justify its use.
In the Star, Eagle, and such other types the man is raised upon the obstacle entirely by the large rear wheel, which carries nearly all of the weight, as shown by the height of the curve; it raises beautifully upon the obstruction with little or no check in the momentum, the diverging lines showing about the same distance apart as at the base. It has been thought to be an advantage to reduce the weight upon the front wheel, but the importance is very much exaggerated; it will reduce the impact in dropping down from an obstruction, and will thus cause less annoyance in rough-road riding; still this does not alter the fact that the momentum in the man and part of the machine is not only stopped, but reversed backward, as shown in the diagrams. If the wheel were lifted entirely free from the ground before advancing upon the obstruction, it is obvious, then, that no harmful result would ensue, not so much because the jolt and impact in dropping off is obviated, but for the reason that the momentum forward is not interfered with. If the rider should run full force against a wall with his forward wheel, it would be of little consequence to him whether there was any weight upon it or not; it is not always a question of vertical disturbance or of the action of gravity that is of annoyance to the bicycle rider; it is sometimes better to have a heavy weight upon a wheel if it can be kept in contact with the obstruction, as, for instance, upon the front wheel of the Ordinary when it rolls off, as it will be seen that the curve shows a splendid contour by which to give a good pull on the machine.
[Fig. 6] shows the Kangaroo type, with a forty-inch front driver and an eighteen-inch rear wheel; this curve presents very little change from that of the Ordinary.
[Fig. 7] illustrates the Rover type, having two thirty-inch wheels with their centres forty-one inches apart, the saddle forty inches high and twelve inches in front of the vertical through the rear axle. The mere contour of the curve in the last figure mentioned would be somewhat misleading did the diverging lines not show that in the rolling off of the rear wheel the momentum is considerably checked,—that is, the saddle moves more slowly forward than the normal forward pace of the wheels, though there is no direct reversion of the momentum, as occurs in the Ordinary and some others.
In this connection let me call particular attention to a cardinal distinction with reference to the action in rolling upon and from an obstruction. If the wheels in descending hold the man back in order to remain in contact and thus roll off, it will, of course, result in a check of momentum exactly equal to that which would occur in such advance upon an obstacle, as would be shown by a similar curve in the opposite direction; but, as a matter of fact, the momentum being a certain amount, the effect is to cause the wheel to leave the obstruction entirely and not roll, but jump off, which result causes a great loss of energy and is sure to occur in rapid running. In this case the forward momentum gets no benefit from the potential energy acquired in mounting the obstacle, which shows the great necessity of proper springs such as will enable a man to swing forward slightly without rigidly drawing the machine after him. The object of the springs in this connection should be to hold the wheel in contact and permit it to roll instead of forcing it to jump off; if it rolls and is not carried off by the force of momentum, the energy will be given out in driving the machine forward instead of being lost in the vibration caused by impact when the machine strikes the common level. That is to say, the machine should roll off, but not hold the man back in order to do so; by proper springs the wheels remain in contact, while the man goes on at the regular pace of momentum. The liability of the rear wheel to jump off is a serious difficulty in the present Rover type of rear-driver; there is no reversion of the momentum, nor such a tendency to drop perpendicularly, as in the Ordinary, yet it drops a greater distance and is charged with more weight. This objection cannot be entirely remedied by any springs we now have in use; it requires a lively vertical as well as a horizontal amplitude in the motion of the springs, and they should be placed at the hub of the rear wheel in a manner similar to those used of late in connection with the front wheel. It will be seen from the diagrams that the curves shown by the front wheels leaving the obstructions are never such as would show any liability to jump off; advancing upon the obstruction must, in them, be mostly provided for.
In [Fig. 8] we have a machine provided with a thirty-inch front and twenty-four-inch rear driving-wheel. This is a modification of the Rover type recently favored by some English makers. The drop of the rear wheel is more radical than that of a full thirty-inch.
In [Fig. 9] appears a Dennis Johnson machine, with two wheels of the same size, having the seat low down and exactly midway between them. This is perhaps the easiest riding contrivance in so far as vibration, jolt, and shock are concerned. Observe the equable motion it displays. This machine was patented in England, as spoken of in an early chapter, seventy years ago.
It will be seen, from a general observation and study of all of the diagrams, that the best and most gradual curves are made by the front wheel in descending from, and by the rear wheel in advancing upon, the obstacle; hence it follows that the front wheel works against momentum more in ascending and the rear wheel more in descending.