“A four-wheeled vehicle remains in a straight line when first the front pair of wheels are turned under, then by the effect of the draught the hind pair of wheels follow in a wider circle. To effect a turning we bring the front axle first in a corresponding direction with the desired turn.
“We make distinction between the moment of turning, or the angular position of the axles previous to the turning itself, and the effected turning of a vehicle round a centre or king bolt, according to the construction of the carriage part. The wheels have to be brought in a position corresponding with the direction of the turning. The body must be fully supported after the turning, and the front or dickey of a carriage must stand in a right angle to fore axletree.
“We have to consider a few points relative to the height of the front wheel, and the elevation of the body above the ground, which averages 30 inches. To give a front wheel its proper height (between 3 feet 4 inches and 3 feet 6 inches) and have it turn a full circle, we sweep the body at the required place, viz. put in the wheel house of a proportioned length, and a depth between 3 and 4½ inches.
“The front carriage part is fastened round the king bolt, turning that part horizontal. This action causes the front wheel to describe a circle, whose diameter is the width of the track; but as the wheel leans over at the top through the dish, we have a larger circle in the middle and top of the wheel. We, therefore, first find a top circle, having a diameter equal to the width between the highest point of the wheel, and a side circle following the termination of the cross diameter of the wheel, having as a centre the king bolt.”
Fig. 28.
In [Fig. 28], which is drawn to quarter-inch scale, the horizontal line A is the axletree, B is the wheel at rest, C is the wheel on full lock, D is the back of the arch, E shows the circle that the wheel will describe on the ground as it moves backward, and F is the circle the back of the wheel will describe in the air at the same movement. It will be seen by this that when the wheel is on half lock the back part of it will come in contact with the arch, and that when on full lock it will have travelled right away from it. It follows, therefore, that if we want to find out the right position for the perch bolt to occupy, we must not measure the circle the wheel will describe on the ground, but the one described in the air. We must, therefore, measure along the line F, and carry that measurement along to D.
The length of the line A to D is exactly 3 feet. Now the position of the perch bolt, or centre point on which the wheels lock round, need not be, and in fact very rarely is, in the same vertical line as the axletree. By compassing the beds or timbers on which the fore part of the body rests and through which the perch bolt passes, the centre of the circle described by the lock may be carried forward. Thus, if the beds be compassed 4 inches forwards from the straight line A along the axle, the centre will be carried forward 4 inches, the result of which will be, that when on the half lock the back part of the wheel will be carried away from the arch 2 inches, and that when on the full lock the wheel will stand 4 inches from the position it would occupy if the bed were straight. The dotted lines below F and E in the figure show the result of this difference in the shape of the bed; and it will be seen that to get a 2-inch clearance of the arch from the back of the wheel without carrying the wheel itself farther forward than 3 feet from the back of the arch, we must compass the bed 4 inches, the compass mark being to the centre of the substance of the bed.
In the fore-carriages for one-horse vehicles, and two-horse vehicles as well, the shafts are carried by “open futchells” (F, [Fig. 30]); and in the fore-carriages of two-horse vehicles the pole is carried by “close futchells” (F, [Fig. 31]).
A reference to the figures will make the following remarks more clear. The central circle is the wheel-plate, or, as the Americans term it, the fifth wheel. This is flat at the bottom and round on the top, and being fitted to the under part of the top carriage takes its bearing on the bottom carriage, and by its extended circular formation gives steadiness to the body when the carriage is running in a straight line, or when the fore-carriage is on full lock. These bearings are at the back ends and fore part of the futchells, and at those points of the bottom bed which are covered by the wheel-plate. The fore and hind bearings are of ash timber, and are necessarily circular in form.