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My effort to make a small syncro. The entire drivetrain (except for motor linkage) is within the height of the large turntables. CLick any image to open a more detailed view in another browser window.
This top view shows the entire drivetrain and steering assembly. The pulleys in the lower left steer and the remaining pullys (upper right) power the wheels. Steering is very simple: a crown gear positioned at the inside edge of the turntable engages the inner teeth of the lower half of the turntable. Propulsion is via 16-tooth bevelled gear on a vertical axle through the middle of each turntable.
I wanted to keep the vertical profile of the model as short as possible, so plaing the wheels inside the turntable side was a high priority. I placed small/wide rubber bands on wheel hubs to provide traction, otherwise the wheels just spun. It looks like the wheels are on the same axle, but this is not the case. Look at section 1.3.4 Syncro Drive of "Where Am I?" for an explaination. The outer teeth of the turntables don't mesh - but they don't turn either.
As you can see - the model is almost contained in the turntables. The crown gear and pullys project above and the wheels below. It is missing only the motors and RCX.
Here's the drivetrain motor attached.
Here's the steering motor attached.
Here's the RCX.