[chrisguyw]

Interesting, thanks Chris.

I filled the central section with a mix of superglue and filler and then turned it off. The urethanes are glued.

Made quite a difference.

While filling the gap with a rigid substance will help alleviate the tire cupping, and will allow you to take less off the tire in the truing process (a good thing) it will not give the added grip/consistency benefit that a sponge insert (additional tire compression ) will.

Cheers
Chris Walker