If you want to implement a successful mecanum drive I would really recommend that you use some sort of suspension. My team puts springs between the frame and the pillow blocks or wheel modules. Its incredibly easy to implement and it completely eliminates the whole rigid frame problem.
Here is the link to a picture of our suspension in 2011 and 2012 (you can see the springs right above the wheels).
http://www.chiefdelphi.com/media/photos/36399
https://www.facebook.com/photo.php?fbid=208337722601334&set=a.2064448461239 55.33843.136209953147445&type=3&theater
Edit: I just wanted to add, in 2011 our suspension had around half an inch of travel. In 2012 the suspension had closer to an inch of travel and we used bushings to make the movement smoother.