When running in worked ground many folks get by just fine with one OEM rubber and one Curvetine per row. PErsonally when running one of each I like to either run them both in the front hole or if staggered I like the rubber in front and Curvetine in the rear. CT in the rear makes it a bit more dominant.

The advantage of one rubber + one Curvetine in worked ground is that the rubber acts like a depth band and lets you apply a lot of spring pressure without going too deep.

As far as changing the spring goes, if you are using two Curvetines (or 2 other "spiked" closing wheels) on a JD 7200 or later into any worked ground I would definitely change to the lighter JD tailpiece spring. If you are using one Curvetine OR two Curvetines ALWAYS in to firm no till ground, then I would say changing to the JD half rate spring is optional but a good idea.

The disadvantage of one 12" std rubber + one 13" Curvetine is that in a notill or firm soil condition the rubber holds the CT up from getting down and closing the bottom of the slot where we like it. There is only 1/2" of radial difference between the rubber and the CT. In worked ground not so much of an issue it firm  not ill or stale seedbed it can be an issue. Here is a picture of what one CT + one rubber give you in stiff SE Wisc clay  notilling 30" beans into 30" cornstalks. Guess which side is the rubber and which is the Curvetine?

If you were a seed which side would you send roots to?  Now this is better than two rubber tires, but would really be a lot better with two Curvetines.

Jim at Dawn

(one rubber top plus one Curvetine bottom in heavy wet no till clay.jpg)

one rubber top plus one Curvetine bottom in heavy wet no till clay.jpg (97KB - 328 downloads)