FromTheDocs — Motion Blur and Expressions
Motion is evaluated at sub-frame intervals. If you use expressions to animate objects or deformations, you should use $T (floating point time) or $FF (fractional frame number) in the expressions instead of $F (integer frame number). Expressions that use $F will not show motion blur properly (since the expression will give the same result at every sub-frame).
source: http://www.sidefx.com/docs/houdini15.0/render/cameras















