Building a 4-wheel drive vehicle with steering always seemed like a difficult challenge to me. A few days ago I came across an interesting and compact design for a pendular suspension (thanks to TechnicBRICKs for highlighting this design). This is basically a 2-wheel module that incorporates a drive-train mechanism, a steering mechanism, and suspension. This clever and compact design is due to S. Ariel, who provides complete building instructions for this module. I used two of these modules to build a 4-wheel drive vehicle, with 4-wheel steering and a front suspension. It actually also has a suspension for the rear wheels as well, but the overall suspension was too soft so the vehicle wobbled, so I locked the rear suspension. Perhaps it would have worked better with 8 rather than 4 shock absorbers.

The vehicle is powered by two Power-Functions motors. It moves quite slowly, since the drive train uses a worm gear to rotate the differentials. But it is sturdy and strong. I was able to mount a NXT with batteries in front, along with a sound sensor and a grabber arm (with a NXT motor), and it still moved well.

I built the overall structure from several modules that snap together in about 3 minutes.

Check out the video to see how the vehicle moves, how its suspension allows it to overcome obstacles, and how the modules are put together quickly.