I have looked at the various brands of double universals. I see that some are available in two different lengths.
Xray has 50 mm and 52 mm.
RSD has wide and narrow.
Spec-r appears to have one length.
Roche appears to have one length.

According to the Xray manual, 50 MM is for foam and 52 mm is for rubber.

From what I read in this thread, If an RSD conversion is in my future, then I would need the narrow ones. Is this accurate?

I only will be racing rubber tires on carpet, if this makes a difference.

With the Xray being roughly $100.00 a set, I have not been able to find any of the others available in the US. I would like to have a set by next weekend.

Thanks.