To learn that the leverage power of a high wheel is very great we need only consider the advantages gained by a large wheel in locomotives and in bicycles. In practice we find that wheels from 4 feet to 5 feet are sufficient for large carriages, and from 3 feet upwards for ordinary vehicles.
The present method of constructing all wheels which have to carry considerable weight will doubtless continue to prevail. The timbers of which they are composed, viz., elm for the stocks, ash and hickory for the rims, and oak or acacia for the spokes in Europe and America, have been found the best for many years. Wheels of the present make must also be dished. The tendency of a wheel is always to become upright, and when it becomes so by the gradual hammering out of the tyre to a greater length, and the gradual sinking of the spokes further into the nave and rim by wear and tear, then the wheel goes to pieces. If any one will watch the manner in which the materials of a mop become straight as it is twirled round, he will understand that the same centrifugal force compels wheels to become upright. Secondly, as a wheel runs along a road it is forced from side to side by the uneven surface, the uneven draught of the horses, and the rapid motion, which latter frequently causes the wheel to jump over the road instead of pressing equally over all its circumference. Then we dish our wheels, not only to keep the tyre tight as long as possible, but also to resist the lateral thrust, which in a perfectly upright wheel would soon force the spokes out of the naves. There is one exception to this rule in the American spider wheel; in this the spokes are not larger than a man’s finger, and being of elastic wood, bend like a reed under the lateral thrust, and recovering themselves again will endure, whilst the unbending oaken spoke would be unrooted.
The late Lord Palmerston, whose vigorous common sense grappled many a problem which would be supposed quite out of his line, was fond of talking about carriages. He considered that wheels should have the spokes dished, but that the arm of an axle-tree should be nearly straight, so that the outside rim of the wheel should be upright; and would illustrate the soundness of his views by quoting the wheels of artillery waggons and pony phaetons. It became necessary to remind his Lordship that we pitch the arms of the axles for two reasons; first, in order to keep the box of the axle pressing against the strong shoulder instead of against the weak linch-pin; and secondly, because the wheels, when they are wider at the top of the tyres than at the bottom on the road, will throw the mud away from, instead of into the carriage, or on the panels.