Try to integrate BEMF wave shape to find out the magnetic flux wave shape. This is because of the following equation that links flux and emf.
Flux wave shape is dependant of mechanical desing (winding laying, bevel, etc.) of the motor.
However, I have never seen anything like a trapezoidal BEMF of real motor. There was a topic about this question here (but no answers).
That topic - http://www.rcgroups.com/forums/showthread.php?t=1645446
Also, paper almost on topic - web.mit.edu/scolton/www/SCThG.pdf
Sorry for my English