wheels, suspension

Hi
Can anyone point me to any web pages with information about:
a) the construction of powered wheels, used on the mars rovers etc
and
b) a good, clear diagram of the 4-wheel internal body averaging system used on the NOMAD antarctic meteorite rover.
There are lots of diagrams and pictures for the 6-wheel rocker-bogie eg.: http://www.magneticpie.com/LEGO/techNotes/MecTherMob.html (see the little picture with the orange background on the top right of the page)
http://www.frc.ri.cmu.edu/projects/lri/Luna/report/images/systems.bogie-exp.gif
I am looking for a similar thing, to help me understand the workings/construction of the 4-wheel system.
The intention, of course, is to try to recreate these great ideas for smaller and (much!) cheaper hobby robots.
Thanks Pete
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