I'm learning along with the video:
"The Secret Language of Houdini from SideFX Houdini on Vimeo"
https://player.vimeo.com/video/240187599 [player.vimeo.com]

Not doing too badly - some of the details seem to have changed...
I'm not sure if the video shows a different version of Houdini? (I'm on 18.5)

in the video at timestamp 09:26
there is the following expression, used to rotate the ball as it rolls...

ch("../Translate/tz")/(2*+3.414*ch("../Rotate/ty"))*360

I have tried loads of things and googled but cannot get this to work.

I have also searched the forum.

Any tips/advice welcome - Thank you in advance.

Hey Duncan,

you have to make sure that your nodes are named accordingly. So your Null that translates is named "Translate" and the one that rotates is named "Rotate".
the partwill make the rotation 0 if your ty value of the rotated null called "Rotate" is 0.
Maybe that is causing your issue?

Cheers
CYTE

In any case, are you sure that the multiply *3,414 is correct? Should be pi-greco that's 3,14 and not 3,41