Centrifugal force is defined as P = WV2/Rg; however, this ‘overturning force’ is counteracted by the mass (W) of the vehicle, and may be expressed as P/W, termed the centrifugal ratio. Thus, centrifugal ratio:
where V is the design speed in m/s, g is acceleration due to gravity in m/s2 and R is the minimum safe radius in metres.
When V is expressed in km/h, the expression becomes:
Commonly used values for centrifugal ratio are:
Thus, if a value of P/W = 0.25 is adopted for a design speed of V = 50 km/h, then
The minimum safe radius R may be set either equal to or greater than this value.
The basic requirements in the design of transition curves are:
(1) The value of the minimum safe radius (R), and
(2) The length (L) of the transition curve.
The value R may be found using either of the approaches .