wheel treads are not cylindrical, but conical. On idealised straight track, a wheelset would run centrally, midway between the rails. The example shown here uses a right-curving section of track. The focus is on the left-side wheel, which is more involved with the forces critical to guiding the railcar through the curve. Diagram 1 below shows the wheel and rail with the wheelset running straight and central on the track. The wheelset is running away from the observer. (Note that the rail is shown inclined inwards; this is done on modern track to match the rail head profile to the wheel tread profile.) Diagram 2 shows the wheelset displaced to the left, due to curvature of the track or a geometrical irregularity. The left wheel (shown here) is now running on a slightly larger diameter; the right wheel opposite has moved to the left as well, towards the centre of the track, and is running on a slightly smaller diameter. As the two wheels rotate at the same rate, the forward speed of the left wheel is a little faster than the forward speed of the right wheel. This causes the wheelset to curve to the right, correcting the displacement. This takes place without flange contact; the wheelsets steer themselves on moderate curves without any flange contact. The sharper the curve, the greater the lateral displacement necessary to achieve the curving. On a very sharp curve (typically less than about 500 m or 1,500 feet radius) the width of the wheel tread is not enough to achieve the necessary steering effect, and the wheel flange contacts the face of the high rail. Diagram 3 shows the running of wheelsets in a bogie or a four-wheeled vehicle. The wheelset is not running parallel to the track: it is constrained by the bogie frame and suspension, and it is yawing to the outside of the curve; that is, its natural rolling direction would lead along a less sharply curved path than the actual curve of the track. The angle between the natural path and the actual path is called the angle of attack (or the yaw angle). As the wheelset rolls forward, it is forced to slide across the railhead by the flange contact. The whole wheelset is forced to do this, so the wheel on the low rail is also forced to slide across its rail. This sliding requires a considerable