Instead, I've assigned the 'vehicle' (lower half of the model) to the pelvis bone and gave it the appropriate weight map. However, I have added leg bones with no value or weight maps, to get it into the auto-rigger.

Now is the important part, actually brings this explanation to a close You need all of the basic bones of a biped human for the Mixamo auto-rigger, but you do not need to assign all of the bones to a group or weight map. Example: lShldr, lForearm, rEye, head, hip, lShin, etc. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of Mixamo. Mixamo alternatives are mainly 3D Modelers but may also be 3D Animators. Other great sites and apps similar to Mixamo are DAZ 3D, Poser, Xnalara and Ready Player Me. You can see how you need to label your character. The best alternative is MakeHuman, which is both free and Open Source. If you do not know this structure, I would invite you to download Daz Studio (free), load up a Genesis character and/or export it to FBX. I come from a Daz3D background, and I know that bone structure is compatible with Mixamo rigging uploader, so I've labelled my rigged character accordingly. Mixamo's auto-rig will automatically recognize certain bone structures. I will answer questions if you have any questions, but the audience is people with intermediate experience in 3D beyond iClone. Greetings, This is a very short explanation, rather than a tutorial, aimed at people with rigging experience.

Daz3d to mixamo