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Doug,
My vdg uses to rollers made from PVC and Nylon with a rubber belt. I
tried latex, but the real rubber one was much better. I'm not sure
exactly what the difference is. I may be able to find the link to the
source on ebay if you are interested.
Looking through various tribo-electricscale lists gives various
values for PVC, Nylon and soft/hard rubber. So I went for a set that
was roughly top middle and bottom in the lists. The combination works
well for me.
On my VDG both rollers have a crown roughly 1mm higher in the
centre spread over 210mm. I found it very easy to set up the belts as
long as there was some slip between roller and belt and the belt
material was fairly stiff, otherwise the belt ended up folded over
multiple times in the centre.