FreightViking is reader-supported. We may earn commissions if you buy through our links. Also, as an Amazon Associate we earn from qualifying purchases.

How Do Semi Trucks Connect To Trailers? (Step By Step Guide)

A semi truck connects to a trailer via the Fifth wheel, which is locked in place automatically by a kingpin.

The airlines are then connected manually. We will take you through how to do this safely.

And also how to identify airlines and what could happen if you get the wrong height on the trailer.

Here’s a quick answer to how do Semi Trucks Connect to Trailers:

Semi trucks connect to trailers by means of a fifth wheel. It has a locking jaw for when a trailer aligns its kingpin through the gap. It locks the tractor and trailer together. Electrical and airlines connect a truck to the trailer to allow the driver to keep full control over both of them. 

What are the Parts of a Semi Truck and semi-trailer that Connect to each other?

The semi truck itself has a Fifth wheel, that when reversed onto a trailer, will connect to what is called a Kingpin. A skid plate that sits on top of the Fifth wheel then forms the connection.

The jaws close around the kingpin and lock them together.

Should your Semi Truck Connect to a Trailer when it’s Loaded or Unloaded?

As long as the load is secure and you follow the correct safety measures, you can connect your semi truck to a loaded trailer.

An unloaded trailer can be just as dangerous if not properly secured.

Be sure to take the time to do your checks and secure them safely.

Do you need to set the Axles of a Trailer Before you Connect to a Semi Truck?

It depends on the weight of your load. The axles will need to be adjusted to distribute the weight of your load to even out the truck’s overall weight if needed.

It can be done by either sliding the fifth wheel. Provided it is moveable, of course, or by sliding the trailer axle forward. So it picks up some of the excess weight.

It can cause extra wear on your tires, depending on how you distribute the weight.

How do Semi Trucks connect to Trailers?

You can follow the steps below to ensure a successful connection between the semi truck and trailer.

Inspect the Fifth Wheel

The Fifth wheel should be inspected for damage and missing parts. It should be correctly mounted and secured with a good amount of grease on the skid plate.

Check the wheel can be tilted to the correct height, and the safety locking handle is in good working order. If it is a sliding, Fifth wheel, check it has the right amount of play. And can lock in the required position.

Also, check the kingpin is not bent or damaged in any way and is well greased.

Make the Trailer ready for Connection

Aside from checking the kingpin, the trailer will need to have any load secured to stop anything moving. The area around the trailer should be checked for any hazards.

The trailer wheels should be choked or have their brakes applied. Be sure the tractor has enough room to approach the trailer as straight as possible. Check the trailer is at the correct height.

Lower or raise the landing gear as needed. It should be low enough to raise the trailer slightly when the tractor is back under it.

If it is too low, you could damage the trailer, too high, and it may not lock in place properly.

Position the Semi Truck

The semi truck and trailer should ideally be on a level surface. The tractor should be lined up directly in front of the trailer. The exterior mirrors can be used as a guide to slowly back up.

Keep checking your position to avoid any damage. Even the most experienced truckers make mistakes. Take the time to get out and check frequently.

Back up the Semi Truck Towards the Trailer

After lining up correctly and doing all your safety checks, get back in the cab and slowly reverse until you feel a slight jolt and hear a clang.

This indicates the trailer and tractor have made contact.

Place the truck in neutral and apply the parking brake. Inspect the Fifth wheel is correctly positioned and secured. Chock the wheels of the truck to avoid any unforeseen incidents.

Ensure the Trailer is at the Correct Height

After lining the truck up to meet the front of the trailer, back up until the fifth wheel gently touches the trailer. Get out and look to see how much space you have.

The fifth wheel should be around a quarter of an inch to half an inch higher at its lowest point than the trailer.

Raise or lower the landing gear to a height that will allow the truck to ease under the trailer slowly.

Coupling of Semi Truck to Trailer

The fifth wheel is a clever design that allows the truck to be connected to the trailer.

In the middle of the fifth wheel are locking jaws that clamp around a kingpin. The kingpin is part of the trailer.

These two inventions allow movement in the tractor and trailer, making it easy for the vehicle to navigate the roads.

Connect the Airlines to the Trailer

Ensure that you secure all airlines to prevent them from being caught and crushed before you backed the truck onto the trailer.

The airlines are located at the lower outer rear of the cab.

Just above the catwalk, check them for any leaks before connecting them. They are usually color-coded.

The Redline is your emergency brake line, the blue line is your service line, and the green is your electrical.

They connect at 90-degree angles and then twist down to lock-in. Lines can change in color from truck to truck.

Here is a really cool video on how to connect and disconnect a semi truck to a trailer:

How do you do Functional checks Before Taking Off?

To check the truck is securely connected to the trailer and everything is in working order, you should do what is known as a tug test.

Place the vehicle in gear, release the truck brakes, and slowly raise the clutch or press the gas.

Then you can release the trailer brakes and slowly drive forward a little. This checks the brakes are connected and working.

A final check would be to ask someone to check your tail, and brake lights work.

These are the two most important checks to make after ensuring the fifth wheel has secured the kingpin properly.

How much Space should be between the Upper and the Lower Fifth Wheel Plates?

The truck and its fifth wheel should have no space between the upper and lower fifth wheel when coupled with the trailer.

This is a theory test question on most CDL tests throughout the United States.

It is essential to maintain the fifth wheel to prevent it not working correctly. The jaw, throat, and pivot points can get clogged by the build-up of grease.

What are the Airlines that Connect from the Semi Truck to the Trailers?

The airlines that have to be connected when coupling a trailer are there, so you have complete control over the trailer’s brakes, and so the vehicles following you can see your brake lights and indicators.

The red line is for the trailer emergency brake. The blue is for the trailer service brake, and the green is for the electrics on the trailer.

What Will Happen if the Airlines are Crossed when you Hook up to an Old trailer?

Crossing the airlines and hooking up to an older trailer could mean that you don’t have trailer brakes. The older models didn’t have spring brakes.

No brakes on a trailer will put all the strain on the tractor’s brakes and depending on the weight of the load. The tractor may not be able to cope.

What are the Electrical Lines that Connect from the Semi Truck to the Trailer?

The green line is the electrical line to connect the braking system and signals up to the trailer from the truck. Make sure you check for any erosion to any part of the line.

This could show a serious fault in the truck’s electrics and prevent you from gaining full function of the trailers system.

How do you know if the Kingpin is Locked onto the Trailer?

To ensure the kingpin has locked securely onto the trailer, check the locking jaws have clamped around the pin.

Check the coupling has no space between the fifth wheel and the trailer apron.

Check the release handle is in the locked position.

How do you Raise or Lower the Height of a Semi Trailer?

The lowering or raising of the trailer is done through the landing gear. This is a wrench-type extension that is attached near the front of the trailer by the telescopic legs.

There usually are two gears for operating this bar, you can pull it fully out towards you to get a higher gear and make it easier to turn, or you can push it in fully to engage a lower gear.

It depends on how you are feeling.

What might happen if the Trailer is too High when you try to Couple?

If the trailer is too high when you attempt to couple it with the tractor, it may not couple properly. This can damage the kingpin and maybe even the locking jaws. 

If this goes unnoticed, it will cause further issues with the trailer itself. The trailer could slide out and away from the tractor.

Take the extra time to get it right and save any unnecessary headache or danger to yourself and everyone else.

Happy Trucking!