What is a Semi Tractor?

old 7 our truck

This is our semi tractor.  She pulls several different kinds of trailers, depending on the job.

A semi tractor is the pulling unit in a tractor trailer combination, the most common thing that you see on the roads.   It is commonly referred to as a truck. Tractors have a large diesel engine that can pull a lot of weight.  The standard tractor is licensed for 80,000 pounds.  There are some that haul far more.  A tractor itself is just a powerhouse.

The most common way of attaching a trailer is a fifth wheel.  This is a large plate with a slot in the middle.  Trailers have a pin that slides into this slot, and is locked in place by a large jaw.  This allows the tractor to pull, turn, and control the trailer.  There is a light cord that is plugged into the trailer, powering the trailer lights and syncing up the lights on the back of the tractor with the lights on the back of the trailer.  The last thing that connects the tractor to the trailer is 2 air lines.  A trailer has 2 air systems, one is used for braking and sometimes for actuating the trailer in some way, depending on the trailer.  The second is emergency brakes.  This is a failsafe, if something happens to the common air, so a driver can get stopped safely.  These tractors can have many names depending on their configuration .

Some tractors are called a daycab.   These are just a cab with the controls and seats.  Some tractors are equipped with a sleeper.  This is a compartment that gives a driver a bed in the truck.  Sleepers can be very small, like a cot, and others have full sized beds,  tv’s and even bathrooms!

A big semi truck is also called a tractor, a power unit, a truck or a semi.  These are all different names for the same piece of equipment.
  • A semi tractor is a power unit that has a fifth wheel plate that allows it to connect to a trailer.  There are many types of trailers, which we will discuss later.
  • A full sized semi truck has a motor that is big enough to haul 80,000 lbs or more in the US.
  • Semi trucks run on diesel fuel.  Most trucks average between 5 and 6 miles per gallon.  It takes a lot of fuel to pull these big loads all over the country.
  • Most trucks have 10 tires per truck.  The traction provided by all these tires is vital both for moving and stopping these vehicles.  Trucks can add axles in order to haul more weight in some states.
  • Some units have a bedroom on the back.  This is caller a sleeper.  A sleeper allows a driver to rest when other options are not feasible.
  • Driving a semi truck requires a special license called a CDL (commercial driver’s license).  Semi trucks and their drivers are subject to random inspections at designated ports along roadways, as well as by roaming compliance officers.
  • Drivers are subject to hours of service rules set up by the federal government.  These rules change often and can be very convoluted.
  • Semi trucks require special insurance,licensing, markings, paperwork, as well as being taxed specifically.
  • Driving a semi requires a person willing to be away from home for long periods of time, willing to deal with inclimate weather, willing to jump through bureaucratic hoops, willing to be alone for many hours everyday while staying alert.
What is a big semi truck?  It is a large piece of equipment that requires a certain kind of person to run it capably.

For now, just remember that a tractor is a pulling unit that can be hooked to many different trailers.  The trailer actually determines what job you can do…a tractor just provides the power to do the job.  And sometimes a place to live while you are doing it.

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  1. hi there!
    Ok I have to admit that I was pleasantly surprised with this post on semi-trucks! I found myself thinking “hmmm I did not know that” a few times while reading it 🙂 I knew some trucks were equipped with a cot or a small place to sleep but a full bed and a bathroom? wow that is impressive. Thanks for explaining the mechanism behind it all. I always wondered how the lights would be synchronized. Now I know 🙂

    • Most people are surprised at some of the creature comforts that are available in semi tractors these days. There are many comfort features to make being away from home more bearable, but there are a ton of safety features that are keeping trucks and truckers safer than ever…which of course means the driving public is safer as well. I will be addressing some more of these topics in future articles as well. Stay tuned!

  2. Good simple info here. People who are dating, married to or just interested in truckers and trucking should visit your site often. 🙂 My dad was a trucker so I know a fair amount about it 🙂

    • I appreciate the comments. I hope people check in often, I try to make the information here fun and informative!

    • The configurations of semi tractors is as diverse as the folks driving them…and that is a huge spectrum of humanity!

  3. Hello, Sis. This is really valuable information and gives me a better sense of the Semi Tractor. You see them on the road all the time, but I never really thought about how they work. Thank you for sharing your knowledge!

    • I am glad you found this article helpful. It is important to understand the machines you share the road with!

  4. I spent 35 years “trucking” and your article is very accurate and informative.
    31/2 years I drovefor a common carrier company. That means they hire their trucks to carry other companies’ product. 25% of their freight was from grocery producers to warehouses. This is one of the worst trucking jobs as you are forced to wait for no pay and then change product from the shipper’s pallets to the warehouse pallets.
    The best trucking jobs are hauling your companies own products.
    Most of my career I drove auto-transport for union companies. They paid for motel rooms and had good wages, benefits, and a pension. My rig was a truck and a trailer as the power unit had to be big enough to transport autos by itself.
    The long wheelbase and low center of gravity gave it good handling in slippery conditions. I was always home weekends as the specialized freight would take me up and back as opposed to going from point A to B to C etc.


    • Thanks for your insight. We did grocery hauls for a while. We hated the hours, the docks, the pallets. It was one of the worst trucking jobs we found. And delivering to grocery stores wasn’t much fun either. It is definitely not one of my top 10 fun jobs.

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