By trial and error I came up with the following:
Granny chain ring- good for climbing steep terrain where one is likely to sit all the time. This gear does not provide "torque on demand" if you stand up and it is ideal for obstacles of the type "if you stand you rear wheel will spin".
Great to use on a very steep terrain with the SECOND biggest ring in the back! You would likely to sit or hover over your seat.
Middle chain ring- good with tall obstacles where one will coordinate pedal push with a partial or full standing up motion. Also ideal for moment where short "torque on demand" burst is needed (e.g. to roll over a tall rock or root on otherwise relatively flat terrain).
Great with the BIGGEST or SECOND BIGGEST ring in the back.
WET CONDITIONS:
If the obstacle is not super steep and it is WET, I use the middle ring up front and SECOND BIGGEST in the back to avoid back wheel spinning. (during dry conditions, the BIGGEST ring in the back can be used).
For greater cardio-use middle ring up front (except for very steep terrain) and the BIGGEST or SECOND BIGGEST ring in the back.
For smooth (no bursts of energy) riding and a bit less cardio- use the granny ring up front and the BIGGEST or SECOND BIGGEST ring in the back.
