Practical Carriage Building
Compiled by M.T. Richardson, Vol.1. 1891
The Dishing of Wheels Part 2
When the wagon or cart is upon a level road, as shown by Fig. 145, the weight is equally distributed between the two wheels, and of course no horizontal strain is possible, especially when only a low rate of speed is maintained. On a side hill, however, provided the surface is rough enough to prevent the wheels from slipping, the strain is taken in its full force by the wheels, as shown in Fig. 146. If the lower wheel, as shown in this figure, is examined, it will be seen that the dished form, with the horizontal strain acting at the center of the convex side of the wheel and the resistance acting at the rim on the concave side, is the strongest form that can be adopted. Or, in other words, having exactly the same conditions that are illustrated and explained in connection with Fig. 144, the form and arrangement of parts is favorable to strength and stability. On examination of the upper wheel in Fig. 146, it will be noticed that exactly opposite conditions prevail. The strain acting at the center of the concave side and the resistance at the rim of the convex side produces the same relative arrangement of parts as is illus¬trated in Fig. 143. From this it would appear at first sight that what is best for one wheel is worst for the opposite. This, however, is an erroneous conclusion, for one important factor—the amount of effective horizontal strain upon each wheel—has been overlooked.
What I have termed effective horizontal strain depends on the adhesion of the wheel to the road acting in opposition to such strain. This adhesion is determined by the weight thrown upon the wheel, by the weight of the wheel itself, and by the nature of the road. If, for example, the road was a side hill, and smooth like ice, the horizontal strain, so long as the weight was not sufficient to destroy the surface, would be very small. If, on the other hand, it was rocky or otherwise rough, so the wheels did not slip at all, it would be very great. On a level road, as already stated, the strain would be practically nothing.
Referring again to Fig. 146, it will be seen that a vertical line from the center of gravity in the cart, instead of falling in the middle of the track, as in Fig. 145, falls on the side next the lower wheel. By this it is shown that the weight on the lower wheel is greater than that on the upper wheel. But I have already proved that the lower wheel, by its position and construction, is better adapted to receiving a strain than the upper one. Therefore it is fitting that the greater weight should fall upon it. The upper wheel is subjected in a less amount of horizontal strain, which compensates in a great measure for its defective construction for resisting strains.
Assuming that this explanation will be readily understood, and that it will be deemed by all satisfactory reason for dishing wheels, I will give brief consideration to the amount of dish.