Practical Carriage Building
Compiled by M.T. Richardson, Vol.1. 1891
Shrinkage of Hub Mortises
The general impression among wheel makers is that, because a hub shrinks so as to loosen the hub bands, the mortise in a hub naturally shrinks and tightens the spoke. This is a mistake. The shrinkage at the most is but a trifle, but such as it is, is in a direction—when the spoke is in the hub— not to affect the mortise except in depth, or in the line of the diameter of the hub, as shown by Fig. 187 (page 182).
The shrinkage that takes place in a mortised hub before the spokes are driven is an entirely different operation. It is a well established fact that timber shrinks, no matter how well it may have been seasoned, whenever a new surface is exposed, and that the shrinkage is greatest edgewise of the rings. It naturally follows therefore that when a hub is mortised numerous fresh surfaces are exposed extending to the center of the hub. The mortises, as shown by Fig. 187, represent about one-half the substance of the hub, and the cutting is in the direction to expose that part which is most susceptible to the action of the atmosphere. The effect therefore upon the mortises is to give the sides the form indicated (Fig. 188). In large mortises—one and a half to two inches—this concaving will increase the width at the center from one-thirty-second to one-sixteenth of an inch. The fiber at the front and back being held by the solid wood, yields but little, and in order to give a correct surface to the sides of the mortise, it must be cut away; otherwise the tenon cannot get the grip necessary to hold it firmly in the mortise. This being the case, it is evidently disadvantageous to mortise the hubs any great while before driving the spokes, for shrink they will, even if they have been seasoned for years; but if the spokes are driven immediately after the mortises are made, the surfaces which would be affected by the atmosphere are covered, and no shrinkage can occur.
If, after the wheel is finished, the hub shrinks, it will be in the same direction as if the mortises had not been made, and as that decreases the diameter, whatever change takes place is in a direction that will tend to tighten rather than loosen the spoke. But any hub that is at all fit for use is too dry to shrink to a perceptible degree after the spokes are in.