The following two things will help you prevent rollover, 1) keep the cargo as close to the ground as possible, and 2) drive slowly around turns. Keeping cargo low is even more important in combination vehicles than in straight trucks.
The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid. The foot brake sends air to all of the brakes on the vehicle (including the trailer(s). There is much less danger of causing a skid or jackknife when using just the foot brake.
When you have to back, start in the proper position. Look at your path and use mirrors on both sides and back slowly. Back and turn toward the driver's side whenever possible and use a helper.
The trailer should be low enough that it is raised slightly by the tractor when the tractor is backed under it. Raise or lower the trailer as needed. If the trailer is too low, the tractor may strike and damage the trailer nose. If the trailer is too high, it may not couple correctly.
When the trailer breaks away and severs the emergency air line, emergency brakes on the trailer engage. At the same time, the tractor bakes will remain operational.
To test that air reaches all the way to the back of the last trailer, you can open shut-off valves at the rear of the last trailer, on both the service and the emergency lines. You should hear the air escaping. Close the valves. If you do not hear the air escaping from both lines, check to see that the shut-off valves on the previous trailer/tractor are in the open position.
The air loss rate should be less than 2 psi in one minute for single vehicles and less than 3 psi in one minute for combination vehicles when no brakes are engaged.
Our of two units of comparable weight and length, a unit with fewer axles will have more rearward amplification and is more likely to roll over in a turn.
The emergency line (also called the supply line) line is always coded with red color, while the service air line is labeled with blue color.
Off-tracking causes the path followed by a tractor to be wider than the rig itself. Longer vehicles will off-track more. The rear wheels of the powered unit (truck or tractor) will off-track some, and the rear wheels of the trailer will off-track even more.
Combination vehicles are usually heavier, longer, and require more driving skill than single commercial vehicles. This means that drivers of combination vehicles need more knowledge and skill than drivers of single vehicles.
Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control (also called tractor protection valve control or trailer emergency valve), or place it in the “emergency” position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.
While inspecting coupling, check that the fifth wheel locking lever is in the lock position and that the safety latch is in position over locking lever (on some fifth wheels the catch must be put in place by hand).
Low-slung units (lowboy, car carrier, moving van, possum-belly livestock trailer) and single-axle tractor pulling a long trailer with its landing gear set to accommodate a tandem-axle tractor stand a higher chance of getting stuck on a railroad-highway crossing.
Newer trailers have spring brakes just like trucks and truck tractors; however, converter dollies and trailers built before 1975 are not required to have spring brakes. Those that do not have spring brakes have emergency brakes, which work from the air stored in the trailer air tank.
The fifth wheel jaws must close around the shank of the kingpin. If the coupling isn't right, don't drive the coupled unit, the trailer may come loose easily.
The following two things will help you prevent rollover, 1) keep the cargo as close to the ground as possible, and 2) drive slowly around turns. Keeping cargo low is even more important in combination vehicles than in straight trucks.
When the trailer breaks away and severs the emergency air line, emergency brakes on the trailer engage. At the same time, the tractor bakes will remain operational.
Combination vehicles are usually heavier, longer, and require more driving skill than single commercial vehicles. This means that drivers of combination vehicles need more knowledge and skill than drivers of single vehicles.
Off-tracking causes the path followed by a tractor to be wider than the rig itself. Longer vehicles will off-track more. The rear wheels of the powered unit (truck or tractor) will off-track some, and the rear wheels of the trailer will off-track even more.
