Semi-trailer axle is which part? What is the difference between the length of the wheelbase?

Axle, refers to the vehicle connected to the two wheels will be fixed on the vehicle to bear the weight of the body column, each pair of left and right wheels are connected by the axle.
Semi-trailer axles are generally used in the form of axle assemblies, which are often referred to as axles. The axle not only needs to bear the weight of the body, but also needs to bear the braking and maintain the function of vehicle driving, so its performance is particularly important for driving safety.
Semi-trailer wheelbase, refers to the front axle centre to the rear axle centre distance, through the same side of the vehicle adjacent to the centre of the two wheels, and perpendicular to the vehicle longitudinal symmetry plane of the distance between the two vertical lines.