zoomkat:
Use a little hot glue, strong but easy to remove.
I will give that a try next time I have to change a motor -- did buy hot glue and hot glue pistol some time ago, but never used it sofar.
Yesterday, after carefully increasing speed and runtime more and more, I did let Arduebot do a run at PWM=255 ... of course that could not be a linear run because of the high speed. I did do same PWM on both motors, the right motor forward, the left motor backward. I did a 120fps slowmo video of that run, and uploaded it to youtube:
The details of the sketch are described in the youtube video description, in the end the run got stopped unplanned by GND cable disconnected from LiPos, but at that time Arduebot was on an unplanned move already, so that power loss was the only reason nothing bad happened and Arduebot stopped a meter away before the bed. The video sequence with blue LED turned on after PWM=255 was reached reminded me on a pyro ground spinner 
Just before loosing power, still in acceleration, Arduebot did (more than) 4.05m/s circular speed.
What I thought was that Arduebot would rotate on center point of line connecting both motor a axis. I was proven wrong by reality, the animate .gif shows 5 frames of the 120fps slowmo, displayed forward/backward at original speed and three times slower. It can be seen that the rotation center is some centimeters away orthogonally from the point I thought would be center of rotation:
Here top and bottom view of Arduebot for reference to the animation:
Btw, I am impressed that even the 120fps cool camera from my son displays the blue LED as a line instead of a small circle because of the high speed (top part of last frame before power loss):
Hermann.