_Answer._ It is, within certain limits. The nearer each other they are placed, the closer will the centre-pin of the truck be to the centre of the axles. The closer it is to the centre of the axle, the greater is the tendency of the wheels to become “slewed,” or to assume a diagonal position to the rails as represented in fig. 150, and thus increase the resistance and also the danger of running off the track. The increase of resistance from this cause, after the axles reach a certain distance from each other, is greater than the decrease from a closer approximation to the position of radii. In ordinary locomotives it is necessary to place the truck wheels from 5 ft. 6 in. to 6 ft. apart, in order to get the cylinders between them in a horizontal position. This distance apart works very well in ordinary practice. QUESTION 258. _What is meant by flange friction?_ _Answer._ It is the friction of the flanges of the wheels against the head of the rails. Thus if two pairs of wheels, _a a′_, _b b′_, fig. 151, be placed on a curve and rolled in the direction of the dart, the wheel _a_ will roll towards the outside of the curve until the flange comes in contact with the rail. As already explained, if two axles are parallel to each other, no matter whether the wheels are conical or cylindrical, they must slip laterally in order to roll in a curved path. As the flange must follow the curve of the rail, it forces the wheel laterally and thus compels it to roll in the curved path into which the rail is bent. As the wheel offers considerable resistance to sliding there is a corresponding pressure of the flange against the rail, and consequently the revolutions of the wheel produce an abrasive action between the two. This action is obviously increased with the distance between the axles, because, as has been shown, the lateral slip of the wheels is then greater than when they are nearer together. It is also obvious that if the wheels are parallel with the rails there will be no abrasive action of the flanges, but that the greater the angle at which the wheels stand to the rails the harder will the flanges rub against the rails, and the greater will be the flange friction. With the aid of geometry it can very easily be proved that the farther apart two parallel axles are, the greater will be the angle of the wheels to the rails on a curved track, and, therefore, the greater will be their flange friction. It must, however, be remembered that if the wheels are so close together that they are liable to become “slewed,” or assume a diagonal position across the rails, as shown in fig. 150, the angle at which the wheels would stand to the rails would thus be very much increased. It has therefore come to be a very generally recognized rule that the centres of axles should never be placed nearer together than the distance between the rails. QUESTION 259. _Is the flange friction of all the wheels of a truck the same on any given curve?_