locomotives and cause additional maintenance on rail equipment and curved trunk itself. The sharper the curve
the more resistance it offers to the locomotive and the more maintenance it requires. Track maintenance men can
easily find how sharp a curve is with a 62-foot stringline cord. They simply stretch it between two points on the
inner side of the outside rail and measure from its center to the head of the rail. The number of inches measured
equals the degree of curvature. Most countries designate a curve's sharpness by its radius, but the United States
and Great Britain designate it by degree of curvature. If you know the radius length, you can find the degree of
curvature, and vice versa, by using the formula
where R equals the radius, D equals the
degree of curvature, and 5730 is a constant.
Section II. Superelevation
3.7.
GENERAL
While driving a car around a sharp turn, you have probably felt as if an invisible force were pulling you
toward the outside of the curve. Similarly, in riding a bicycle around a curve, you lean to the center of the curve
to keep your balance. These phenomena are a result of a characteristic associated with all moving bodies. It is
explained by the physical law which says that any body in motion tends to remain in motion at the speed and in
the direction it is going at any one instant. Bodies, whether automobiles, bicycles, or railroad cars, tend to move
in a straight line even though they are being steered or pulled around a curve. This tendency is known as inertia.
The combination of inertia and turning force has the effect of causing the body to roll over. Naturally, this must
be prevented.
The bicycle rider compensates for inertia by leaning to the inside of the curve; automobiles are protected
from rolling over by their springs, and, sometimes, by the banking of the highway toward the center of the curve.
All motorists know how much easier it is to drive around a banked curve than a flat one. Railroads provide the
same protection for trains by raising, or superelevating, the outside rail on curves. Raising the rail, or
superelevation, is done to overcome the effects of centrifugal force. That's the physical force that pushes a body
traveling in a circular path away from the center or axis of that circle.
How is superelevation used? What are the problems associated with it? What is the maximum that can
be used? These are the questions answered in the remaining paragraphs of this section.
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