An engineer is occasionally required to lay out and con struct a horse-race track, and therefore a consideration of this subject is not out of place here.
The intensity of centrifugal force varies inversely as the radius of curvature and directly as the speed; and therefore to secure a minimum effect of centrifugal force the amount of curvature should be as small as possible. and the radius of curvature should be as large as possible. The fastest track that permits the race to ter minate near the starting point, is two straight lines crossing each other at an angle and being connected at one extremity by a cir cular arc of long radius, since this track gives a large radius and minimum curvature. This form is known as a kite track (see § 427). The next best form is an oval, on which the races begin and end at the same point. An oval contains of curvature, while a kite track of the same length has considerably less curva ture. The most unfavorable condition for speed is one or more times around a small oval or circle.
The kite-shaped track requires more ground than the oval, does not afford the spectators quite as good a view of the race, and does not permit races longer than once around the track. The
half-mile oval requires less ground than the mile oval, gives the spectators a better view of the race, and is favorable for races longer than once around the track. A circular track is not so good as an oval, since a straight stretch is desirable for the start and finish. Half-mile oval tracks are most common. mile ovals are next, and kite tracks are least common. Straight tracks are con structed only for fast pleasure driving, when they are usually termed speedways.
The usual forms of tracks and the methods employed in laying them out will be described, and then some designs by the author will be presented.
Standard Oval.Mile Track. Fig. 70 shows the usual form of the mile oval, the only variation being in the width of the track itself. The dotted line represents the pole line, the line upon which the distance is measured, which is univer sally 3 feet outside of the inner edge of the track. The wire is usually 300 feet from the beginning of the curve, A. The and mile points are indicated by poles set at the inside edge of the track, in such a position that a line from the judges' stand cuts the pole line at the proper point.
In constructing the track, it is more convenient to mark the inner edge of the track than the pole line. The semi-circular ends may be laid out by any of three methods: 1. Amateur's Method. Fasten one end of a wire of a length equal to the radius, at the center of the semi-circle, and swing the free end around and establish as many points on the inside edge of the track as are desired. A wire is better than a string or rope, since it will stretch less.
2. Surveyor's Method. Establish eight points in each quad rant by laying off the lines and distances shown in Fig. 71. This method gives points on the curve about 66 feet apart, and is de signed especially for the land surveyor, who ordinarily uses a 66 foot chain or steel tape.