EFFECT OF “SIDE-THRUST” UPON THE FRAME.
A question arises as to the effect of “side-thrust” as distinguished from “end-thrust” upon the frame (as well as the bearings) in chainless driving. This “side-thrust” is under new conditions and in a somewhat different way; but it is not a new thing—it exists in chain driving, because it always exists. When power is applied to any structure consisting of several movable parts put together, the part to which the impulse is directly applied pushes first on the parts next to it; they pass the push to other parts, and so along the line (as if the several parts were links in a chain) until at last, all the parts having refused to either break or be shoved out of place, the load aimed at is moved. In case of the bicycle this load is the moving of the structure and its rider along the ground. If any of the parts involved could break more easily than the movement along the ground is accomplished that breakage would occur, instead of the movement intended. This is only one example of the law, already stated, that the yield is always in the direction, or at the place, of least resistance—the weakest thing gives up.
It was said just now that the pinion on the forward end of the shaft tries to roll away from the gear wheel which pushes it; the pinions at the rear also try to roll away from each other. Thus they put a side pressure on their bearings, as stated, but the same pressure comes on the frame which holds the bearings. At the crank axle this tends to crowd the fork sides toward or from each other, according as the teeth on the large gear face in one direction or the other; at the rear the tendency is to separate the forks. This tendency is to simultaneously crowd upon the balls, to spread open the forks and to press the teeth of the gears into closer contact.
Since the roller-geared and the bevel-toothed types must meet the same pressure on the bearings, they are alike in this pressure on the frames. As already remarked, each of them escapes entirely the heavy pressure which the pull of the chain puts on the axles and their bearings; in place of this they get other and different strains, as just described.
Observe that we do not say these strains will not be successfully resisted—that would be prediction. Some further strengthening of the frame might perhaps be had, and in fact the doubling of the fork on the chain side of chain-drivers, to get additional stiffness, is not unknown in present English practice. It might even be suggested as a fair question, whether a new or somewhat modified form of frame ought not to have been devised for chainless driving instead of applying it to a form never intended for it.