Interesting and well thought our hypothesis. I will add the following.
The vibration motors in the OG Droid & D2 are larger motors. I have seen both up close having had both apart. Also, the offset counterweight which the motor spins to cause the vibration is larger. Finally, the motor only has 1 weight.
The vibration motors in the RAZR and also therefore the RAZR MAXX are considerably smaller. Then, the RAZR motors have 2 counterweights, each on opposite ends of the motor's through shaft. Finally, the two weights are smaller than that of the D1 &D2. Again, I've seen the motor in my RAZR and have pics I'll post later.
The purpose was to reduce the footprint of the motor and allow effort a thinner design since the vibration motor is mounted in the bottom of the phone next to the microphone. In fact, the only reason why the RAZR couldn't be the same thickness along the full length is the rear camera which is pretty thick, the front camera which is somewhat thinner, the front and rear speakers, and the various jacks at the top.
So my theory is that the dual counterweight design was to help the smaller motor to make a stronger vibration. Unfortunately since the counterweights are smaller in diameter, they don't create enough centrifugal force to have the desired effect.
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