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BOTH the tractor-semi-trailer and the six-wheel vehicle have the same number of axles and wheels and each has its own particular advantages. They are seldom competitive if the transportation problem is analyzed properly and legislation does not unduly oppress either. The six-wheeler has the advantage over the tractor-semi-trailer of weight saving, more traction if four driving wheels are used, lower insurance rates, and it is free from any “jack-knifing” proclivities. The chief distinction in the construction of six-wheelers depends upon the types of axles used, whether they be dead or driving. There are five classifications in use today, ranging in various combinations all the way from three driving axles to one. The rear bogie unit may have two driving axles or a driving and a trailing axle. There is a natural resistance to turning in a bogie unit since the wheels do not roll tangentially when the vehicle travels around a curve.