I really don't have 1 specific answer, but starting in 2012 all of their Limelight collection will have a Polymeric ABS substructure and base, it is strong, lightweight and will be recyclable at the end of its life. They state that this new base is similar but improved on what was featured on a couple models in '09 so that leads me to believe they just weren't 100% happy with the previous ABS material. The new polymeric ABS is supposed to handle extreme changes in temperature which in the end will minimize expansion/contraction as well.
