How to draw the reverse arc of R85 in the blue cloud line as shown in the figure? Thank you.

Let the excircle radius of the big ring be R 1, the excircle radius of the small ring be R2, and the arc radius be R.

Make an arc with the center of the great circle as the center and (R 1+R) as the radius;

Make an arc with the center of the small ring as the center and (R2+R) as the radius;

Two arcs intersect at one point, which is the center of arc R.

Make a circle (or arc) with this center as the center and R as the radius, then this circle must be tangent to the excircles of the two rings.

Trim off the extra parts, and the rest is the arc of the cloud line in the picture.