RPM cranks - review? I know some of you have 'em!

PeterD Just to jump on this thread for a slightly unrelated query...I have RPMs...they work fine so far, though i am thinking of changing the ring for summit smaller.
Currently running a 48/16 and with the wind on the up me knees are feeling it a tad. How do you calculate the BCD of the ring accurately to know that it'll fit. Im thinking of some GEBHARDT rings.

here - http://sheldonbrown.com/gears/