Also, remember that for a normal DC motor, the torque is unidirectional, for this to happen the current must reverse every 180 degrees through the use of a split ring commutator as you already know. Now, this means when the current reverses direction, the force it exerts due to the motor effect also changes direction. The reason why it’s constant though is because at any angle, the side of the coil AB is always perpendicular to the magnetic field.