A practical treatise on coach-building historical and descriptive

by James W. Burgess

Another consideration is the height of the wheels. On level ground, draught is easiest when the centre of the wheel is a little lower than the point of draught, viz. the point where the traces are affixed to the collar; but this in practice would be found rather inconvenient, as very high wheels would be required, and consequently the height of the whole vehicle would have to be increased, causing great trouble and annoyance in getting in and out of the vehicle, and the driver’s seat would have to be raised to a corresponding height. Under equal circumstances a high wheel is more efficient than a low one, and requires less power to draw it; though it may be mentioned that a low wheel on a good and level road will do its work far better than a very much higher wheel on a rougher road. The sizes of the wheels of two-wheeled vehicles vary from 3 feet to 4 feet 6 inches.

It would be a very good thing if four-wheeled vehicles were to have the wheels of equal size, in order that the friction and power might be equal. But with the present mode of construction this is an impossibility, as we have only one mode of making the lock or turn. Therefore the height of the fore wheels must be regulated by the height at which the body hangs, so that the wheels may pass beneath it without striking, when the springs play. In practice this height varies from 2 feet to 3 feet 8 inches, according to the kind of carriage the wheels are intended for. The hind wheels vary from 3 feet to 4 feet 8 inches.

The next point is the dishing of the wheel, which is necessary for strength to take the strain off the nuts, to throw off the mud and prevent it clogging either the wheel or the body, and to give greater room for the body between the wheels without increasing the track on the ground. Whatever be the amount of dishing or coning, which varies from 1½ to 2½ inches, one rule should always be observed, viz. so to form the wheel that when running the lower spokes should maintain a true vertical position both in the fore and hind wheels. This is mainly accomplished by the dip of the axle, but if the fore and hind wheels have the same dish, they will take the same track along the ground. The dish of a wheel will be understood by referring to [Fig. 10], in which it will be seen that the extremities of the spokes are not in the same plane, thus forming a dish or hollow in the surface of the wheel.

Some ingenious persons have deduced from the foregoing that a wheel runs best on an axle having a conical arm (the arm is the extremity of the axle which fits into an axle-box in the nave or stock of the wheel), in which case the axle would not dip, but the wheel would be put on to a perfectly horizontal axle. The motion of a wheel thus placed would be anything but artistic, though there would not be so much friction on an arm of this sort as on an arm of the dipped axle. Dipping the axle is shown at [Fig. 10]. It merely consists in bending it so far out of the horizontal as to give the lower spokes a vertical position. But in practice this theory of the conical arm will not answer, inasmuch as curving the arm will reduce the front bearing surface so much that the oil would be squeezed out, and it would run dry, and the total amount of friction would be greatly increased. Long practice has shown that a cylindrical or slightly conical arm is the best that can be used.

Fig. 10.

We have now to settle the form, combination, and proportion of the springs. Springs which are laid on the axle at right angles have to carry the whole of the weight of the carriage, save only the wheels and axles. Where other springs are used in addition it is not necessary that the axle-springs should have much play. It will be sufficient to give them just so much play as will intercept the concussion caused by moving over a road. The strength of the springs must of course be adjusted to the weight they have to carry, for it is evident that if they be made sufficiently elastic to carry the weight of six persons, they will be found hard if only three enter the carriage. This is a disadvantage all carriages must labour under, for it is ridiculous to suppose that if a carriage is constructed to hold six that number will always want to use it at the same time. There would seem to be room for some improvement in the way of introducing springs adjustable to any weight, though, to give spring-makers their due, they do turn out really a first-class article in this respect; this is more noticeable because it is so recent. Light carriages are never so easy to ride as heavy ones, even when the springs are well adjusted, because on meeting with an obstacle there is not a sufficient resistance to the bound or jerk upwards of the spring, which makes riding in a light carriage over a rough road rather unpleasant.

The position of the front wheels next demands attention. As these have to turn under the body it requires some skill to fix them, and the play of the springs, the height of the axletree, and the height of the arch (the portion of the body under which they turn) have all to be considered. This will be more particularly described when dealing with wheel-plates.

The rule for the height of the splinter-bar, to which the traces or shafts are fixed, is that it should fall on a line drawn from the horse’s shoulder to the centre of the hind wheel. This, however, is not always convenient in practice, as the fore wheels regulate the height of the framing of the under carriage, to which the splinter-bar is fixed. The distance of the splinter-bar from the central pin, on which the wheel-plate and fore carriage turn, is regulated by the size of the wheels and the projection of the driving seat footboard.

All the above particulars are considered when setting out the full-sized draught, and all points capable of delineation are put on the board in some convenient part. In [Fig. 9] the outline is simply given, as to show everything would only confuse the reader. Such other details as are required are filled in after the draught has reached the stage shown in the figure.