As an example, I cannot ever remember seeing preglued foams on spoked rims, (which would make the tyre flex more).
Im pretty sure the reason for this is to make them as firm as possible to prevent grip roll, and typically cars raced with foams have thicker chassis than rubber shod cars.
You would probably have to be (at least) a regular winner of the A main at club level to feel the difference between a hard or flexible rim on the track.
In low grip conditions, a wheel with more flex might help a little.
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