When working on the LEGO tri-star wheel design I was unable to 
integrate a differential into the assembly without driving the 
meta-wheel the wrong way. What was needed was a differential
that had both of its outputs on concentric shafts which then
would connect to the meta-wheel and true-wheel gear train.
There doesn't seem to be a way to do this with LEGO elements
so I had to give up on that idea and then build the type of 
drive train used in REV 1.1 . Well, I've been thinking more
about the "diff-in-the-wheel" concept and have come up with
a really neat way to do this for robotics purposes. This is
to use a planetary gear set as the differential instead of
the usual bevel gear type.This flattens the assembly so that
it just adds one gear layer to the backside of the tri-star 
wheel assembly. In this arrangement the planetary gears are 
driven by the drive shaft, the ring gear is fixed to the 
meta-wheel, and the sun gear is driving the true-wheel gear 
train through the center shaft. The meta-wheel to true-wheel 
drive ratios are balanced by sizing the true-wheel drive gears 
appropriately.


color key :

Red   - Planetary gears - drive shaft connected to planetary
        gear carrier.

Black - Ring gear - fixed to meta-wheel gear cover.

Blue  - Sun gear - drives true-wheel gear train through
        center shaft

Magenta - True-wheel gear train.

The order of assembly would be something like this .

1. Outer meta-wheel.
2. Wheels.
3. Inner meta-wheel/true-wheel gear carrier.
4. True-wheel gear train with center gear fixed to center shaft.
5. Inner meta-wheel gear cover.
6. Planetary diff gears with ring gear fixed to gear cover and sun
   gear fixed to center shaft.
7. Planetary gear carrier/input shaft.
8. Planetary gear cover/input shaft main bearing.

Note - center shaft supported by bearings in planetary gear carrier and both meta-wheels.
The center shaft may be segmented to reduce the friction in the true-wheel gear train.

This design uses 3 inch diameter 1/10 scale RC car wheels and standard 48 pitch
gears. The rolling mode to stepping mode drive ratio is 1:1.