A simple two-motor NXT car needs a non-powered slider, wheel, or wheels of some type. A single wheel is often a good choice, since it is simple to build and makes turning the car easier than a pair of non-powered wheels.
In fact, the "wheel" does not need to be a wheel at all; a slider can be a good substitute.
Four possible third wheels/sliders are shown below: a fixed wheel, a tow-ball slider, a knob-wheel slider, and a castor wheel.
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