Rear 4 wheel truck bissel

[k36no4862002]

Hello all,
As some of you may remember I'm working on a UP Challenger. I'm in the process of building the rear truck and I have question over the self centering device.
On the rear corners of the truck is a curved rocker that takes the weight of the main frame, the curved thing sits in an upside down V.

So, does the curved thing roll in the V or is it fixed and the curve just pushes on the sides of the V when going round corners? I have seen some pics that make me think it's fixed, but these are not pics of a UP loco and I would like some idea of how this works before I start getting too involved in making this part.

Any advice would be very helpful.

Many Thanks
Paul

[Fender]

These are called heart rockers. The curved sides roll up on the vees. There are two pivot points on the bottom of the rockers (making them an upside-down "heart"), one of which must lift off of its seat, for the rocking to happen. There is a strong centering force that resists this lifting, but once the rocker tips beyond a point, this force is greatly reduced.
The idea behind this is to impart a strong centering force when running at speed to resist "hunting" and to guide the loco into and out of broad curves, but still allow the loco to negotiate sharper curves such as on yard trackage or wyes.
This was an improvement over swing links and heart links.