I started the same topic a while back. No one seemed to have any ideas, I bought a couple of different tires, a Shinko, and some other even worse thing. I ended up grooving the factory cst for the rear. It definitely helped the flex, at 7 psi if was more flexible than the in grooved tire at 3 psi. (totally by feel so....) The defining moment was when my son was able to find traction on a section he just could not before the grooving.