The values shown in Fig. 169, page 640, were computed for a velocity of 48 feet per second, or a mile in 1 minute and 50 seconds. Should the speed vary materially from this value, the wheel will not stand exactly normal to the surface, and hence may have a tendency to slip; but the tires of a bicycle will not slip unless the angle be tween the plane of the wheel and the normal to the track is greater than the angle of repose. Experiments show that the angle of re pose for a bicycle rubber-tire sliding on a dry cement sidewalk is 23° 15', and practically the same value for a smooth dry wooden surface. For macadam, cinders, gravel, etc., the angle of repose is considerably more than the above. Therefore there will be no danger of the wheel's slipping, if the speed be increased until the wheel leans nearly 23° outside of the normal to the track, or if the speed be decreased until the wheel inclines nearly 23° inside of the normal. For the track shown in Fig. 169, page 640, the first posi tion of the wheel corresponds to a speed of a mile in 1 minute and 12 seconds, and the second to a mile in 4 minutes and 53 seconds. The former is the speed of the fastest steam-motor cycle, and the latter is lower than any probable bicycle race. Therefore if the super-elevation of the track shown in Fig. 169 is adjusted for a speed of 48 feet per second, or a mile in 1 minute and 50 seconds, it can be used for considerably faster races, and still be safe for slow races and amateur riding. However, it will not be possible to attain the highest speed unless the super-elevation is adjusted approximately for that speed, since otherwise part of the rider's attention and effort is required to balance his wheel.
It is the practice of some designers to compute the banking for one speed, and then construct the track with an arbitrary, fractional part of the computed value. For example, the banking of the Manhattan track was computed nominally for a 2-minute speed, and then constructed with a super-elevation equal to 60 per cent of the computed value. The actual banking is that required by a 2 minute and 34 seconds speed.
It has been proposed to make the surface of a bicycle race track on curves concave as shown in Fig. 170. In other words, it has been proposed to make the angle of inclination of the surface of the track greater at the outer edge than at the pole line. The claim is that such a surface would make it easier for one rider to pass another, since to accomplish this he must ride at a higher speed and hence would require a steeper inclination. This conclusion is wrong, since the effect of the increased radius of curva ture almost exactly counteracts the effect of the increased velocity (see equation (2), page 290). It is also claimed that the concave surface would prevent a rider from flying off the tracks, should he momentarily lose control of his wheel. This advantage is not important, since the banking is sufficient of itself to prevent such an accident. However, the concave surface would be an advantage on a track having low banking. A third objection to the concave surface is that it would be more expensive to construct. On the whole, a straight surface is probably the better.
The tendency to "fly the track" may be lessened by painting parallel guide lines on the surface of the track. This feature was used on the Manhattan track as described more in detail in § 1003.