CDL Vehicle Combination Practice Test

When backing a tractor under a trailer, it is important to understand the dynamics of the connection between the two. As the tractor backs up and makes contact with the trailer's kingpin, the trailer is designed to be lifted slightly. This lifting occurs due to the positioning of the tractor’s fifth wheel, which is responsible for connecting the tractor to the trailer.

As the tractor makes contact, the weight of the trailer and the force applied from the backing tractor causes the trailer to rise just enough to ensure that the kingpin is engaged securely in the fifth wheel. This slight lift is a normal part of the process and indicates that the connection is taking place as expected. It allows the fifth wheel mechanism to lock in properly, preparing the combination vehicle for safe travel.

Understanding this behavior is crucial for operators to ensure that they are safely connecting the two vehicles and to help prevent potential issues such as misalignment or incomplete coupling, which can lead to dangerous situations on the road.