[EricH]

Originally Posted by caffino54

where's the drive motor?

i was working on a design for this myself, but mine involves the drive motors being mounted inside the module housing itself

A coaxial crab, such as this one, has the motors mounted elsewhere. This is a slightly trickier form to implement because not only does the wheel turn on an axle, but the same axle (or one running down its centerline) is the power source for the wheel. It's kind of greyed out, but just at the top of the picture is another sprocket which would be used for the wheel power.