Retarget Root motion animations Unreal 5.3

Skip to GUIDE section below to get on with it or read the back story below:
I have had a problem where a project updates from Unreal 4.27 to Unreal 5.3 where characters with root motion anims get screwed up. After a lot of futzing around this seems to be a solution. My problem may have been caused by the character being weird in some way (possibly strange rotations in the joints or something. I tried to export and reimport the fbx anims – that didn’t work. I tried to update from 4.27 to 5 – that didn’t work. What did work is retargeting the animations to a better, cleaner version of the character.)

The retargeting process in Unreal 5.3 is vastly improved now. However there are hidden settings based on context. The retargeting is documented here but in short, you make an IK rig for the source and target characters, setting up chains of bones.

Then you make a IK Retargeter which handles the mapping of the source chain to the target chain. Then select an animation(s) and it exports the retargeted anim on the target character.
I was foxed because my root motion was not getting carried over.

IK Retargeter window

Here’s how to fix it. Notice that even tho’ the Root has the actual animation, we need to set the Pelvis to the be the Retarget Root. Do this for both IK rigs, AND make a chain with just the Root in.

In the image above you can see this. Then
(1) select the Target then its Pelvis
(2) select the Root in the Chain Mapping pane
(3) this brings up a context sensitive Details panel where FK/Translation Mode needs to be set to Globally Scaled.

In the 3d preview window if we have an animation selected in the Asset Browser pain, we can see the red line of the root moving away from the origin.