While a four-wheeled motor vehicle will perform perfectly well for most purposes so long as two of its wheels are driven, writes Jeff Daniels, traction is much improved when all four wheels are driven. This improved traction allows poor terrain to be crossed more slowly and carefully, with less chance of becomingimmobilised, and confers the ability to negotiate steeper gradients. Full use of these abilities calls for significant drive torque to be available at very lowspeed, hence 4WD vehicles for "off-road" use are generally provided with a step-down transfer gearbox which allows the overall gearing to be significantly lowered when necessary.