update Manual

Adds details of switching behaviour

Author: cjakeman

Source/Documentation/Manual/physics.rst

@@ -259,12 +259,17 @@ wheel adhesion. The first model is based upon an algorithm by Pacha, whilst the
 uses an algorithm developed by Polach. The Polach algorithm provides 
 a more accurate outcome and facilitates the future inclusion of track conditions. 
 However due to the number of algorithm steps required to calculate the wheel adhesion 
-value, it is more CPU load intensive then the Pacha one. This can produce low 
-frame rates for the screen display in machines with low performance specifications. 
+value, it is more CPU load-intensive then the Pacha one. On low performance PCs, this would lower the 
+frame rate for the screen display to an unacceptable degree. 
 
-Hence OR automatically senses the CPU load, and switches to the Pacha algorithm at 
-high loads and to the Polach algorithm under lower CPU loads. In this way OR attempts 
-to support the operation of lower specification computers. When OR is using the 
+To avoid this, OR senses the frame rate and switches from the Polach algorithm 
+to the Pacha one as follows.
+If the frame rate falls below 30 fps, then a switch is made to Pacha until the frame rate
+recovers to more than 40 fps. If a switch to Pacha happens more than once in a 5 minute interval
+then it will persist for the rest of the session.
+
+In this way OR provides a more accurate algorithm whilst retaining 
+the original one for lower specification computers. When OR is using the 
 Pacha algorithm, the "Wheel Adh (Max)" values will both read 99%, whereas when the 
 Polach algorithm is being used these values will be around the expected values of 30-55%.
 
@@ -276,6 +281,7 @@ The heart of the adhesion algorithm is the slip characteristics (pictured below)
    :align: center
    :scale: 70%
 
+
 The *wheel creep* describes the stable area of the characteristics and is
 used in the most of the operation time. When the tractive force reaches
 the actual maximum of the slip characteristics, force transition falls