Computer models are available to predict the steering forces, which depend on vehicle and track characteristics
as well as the detailed wheel and rail profile shapes (Wilson et al., 1995). However, the theory
behind the models is complex and cannot be described here.
An understanding of how these contact patch forces arise can be gained by considering the difference
in rolling radius between the left and right wheels of Figure 34.5. With the wheelset centered on straight
track, if the left and right wheel rolling radii are equal (
R
), the wheelset can roll normally. However, if
the wheelset is shifted laterally to the left, the left wheel rolling radius increases to
R
L
while the right
wheel radius decreases to
R
R
. Because the two wheels must rotate at the same angular velocity, this
difference in rolling radius produces longitudinal micro-slip (termed creep) between the wheels and rails
which, in turn, leads to a longitudinal force on each wheel to steer the wheelset back to the center of the
track. In a similar manner, lateral forces are produced on each wheel if the wheelset runs at an angle to
the track. Equal and opposite forces are produced on the rails.
Creep is a natural consequence of having fixed wheels on a solid axle, and is defined with respect to
the forward and lateral wheel and rail velocities: