THE STRESS OF THE CHAIN-PULL.
It is true, as alleged by the plaintiff in this action, that a severe strain is put by the driving on the frame of a chain-driver, increasing enormously as the load or resistance increases. It is true that by the laws of materials the yield is always in the direction of least resistance; this was understood by Autocrat Holmes’s old deacon, who reasoned, when about to construct his “one-hoss shay,” that “it’s mighty plain thet the weakes’ place must stan’ the strain” and if that does not stand the whole construction goes. The trouble with the bicycle frame is that it is a triangle, and ([as shown in the small sketch]) the line of draught is not parallel to the line of resistance. Suppose the resistance under trying conditions of grade and road is represented by a weight of 100; then it’s pedal, or crank, or axle or sprocket, or any piece in chain, or the spokes in the back wheel, or the frame itself, had a strength below 100, instead of the bicycle’s advancing along the road the rider’s power would expend itself in breaking or bending such weak part. The pull is a one-sided pull, necessarily tending to drag the frame around toward the side where the chain is, and frames have not always been made strong enough to resist entirely. Ten years ago, when the present type of bicycle was beginning to come in, a dealer whose interests were opposed made the most of this objection of side-drag, quoting the recent admissions of several English makers, thus:
DIAGRAM SHOWING ONE-SIDED CHAIN PULL.
“There is a great deal more strain between the two chain wheels than is generally understood, and experience has proved the necessity of a direct and rigid connection between these points.” (Starley & Sutton of Coventry, 1888 catalogue.)
“We would draw the reader’s special attention to our pattern for this year, as every one who has had experience in this type of machine knows that the bearings are of the utmost importance, the wear and tear being so much greater than on an ordinary bicycle. This has caused us to discard the old pattern pin-and-cone ball bearing, in which the cones and cups are continually going wrong, owing to the smallness of the bearing surfaces and the enormous strain upon them. As in all pin-and-cone ball bearings, the cone wears flat on the chain side, but in the bearings we have introduced it is impossible to do so. They are the same as fitted to the front wheel of our bicycles.” (Rudge Cycle Company of Coventry, 1888 catalogue.)
This was quite true. Bearings have since been modified to meet the case, and frames have been strengthened; yet a light frame may waste power, under trying conditions, by springing out of line without getting a permanent bend, and something more can probably be done in frame strengthening.
The reader will observe, on referring again to the cuts of the first [Columbia], where it happens to show more distinctly, that the frame is substantially two tubes crossing at right angles. Construction began in this manner in England, and the reason was that makers followed the “[ordinary],” to which they were accustomed. They took the familiar curved backbone, and made it straight, attaching it to the wheel in the old way by mortise and tenon, with a bolt passed through, as the cut clearly shows. They did not foresee the chain-pull, and (strange as it seems) actually did not at first always put on even the single tie-rod used in the Victor; later, a second tie-rod was carried to the rear wheel, thus making a truss, and then a second pair were placed above the backbone, thus further approaching the diamond. Now it was this identical construction of frame, as shown in the [Columbia] most distinctly, which caused Starley & Sutton (as just quoted above) to say that “there is a great deal more strain between the two chain wheels than is generally understood, and experience has proved the necessity of a direct and rigid connection between these points.” This strain had not only not been “generally understood,” but had apparently hardly been thought of at all. Yet the direct fork which those old Coventry makers called for was soon supplied, and the moral for which we relate this bit of history is the fact that the present frame of the chain-driver and the chain itself are the result of a long evolution process, in which every step has been suggested and proved by practical experience.