Sweep1 Framestyle Freeform

hallo at @pascal or all others:
what is the background of _sweep1 Framestyle = Freeform ?
(the option Framestyle = Roadlike is well explained in the help-files)
What is the background / how is the rotation of the cross curve defined - for Freestyle ?
I always thought, there is some relation between the unit tangent, normal and Bi-normal / osculating plane / Frenet-Serret-Frame.
But the following simple example doesn’t look like the the cross curves follows the osculating plane:

at the start the cross curve is parallel to the curve 's Normal, later it looks like a arbitrary Rotation.
Thanks for some info, that explains this result.
Sweep1_Freeform.3dm (47.0 KB)

@lowell - can you help? The Help file seems a little thin on this as well.


@pascal @lowell
still waiting for an answer - would be happy to get some news on this topic.
thanks and a happy 2020 - best tom

@dalelear - Do you have anything to share about Freeform sweep frames?

rhino 5 help:
“The Freeform option generally uses the rail-tangent cross-rail curvature direction.”
rhino 7 help:
“The cross-section curve rotates to maintain its angle to the rail throughout the sweep.”

with my little geometry-knowledge, and not being a native english speaker - either i still don 't understand both sentences,
or i would expect another result:
The cross section should stay in the same angle inside the curves Frenet-Serret-Frame.

sorry to reactivate this old topic, but i found another sweep1 issue here:

and still _sweep1 freeform does not behave in a nice and predictable way for freeform curves with a single cross-section.

so maybe someone can have a look at both aspects… thanks