Semi Trailer Basics

A trailer is simply an unpowered unit that is pulled by a powered unit. These are most often used for the transportation of goods. However, in the case of campers, they can also house living quarters. There are many different kinds of trailers, such as a bicycle trailer, motorcycle trailer, utility trailer, boat trailer, horse/livestock trailer, car trailer (1/2/3+ cars), car dollies, and so on down the line. These trailers can be small enough to be pulled by an economy car, or motorcycle, or large enough to require a one and a half ton pickup.

The one we are interested in here is the semi trailer. A semi trailer is trailer that does not have front wheels and is designed to have half or more of its weight supported by the tractor (or converter dolly). These trailers usually have front supports allowing the trailer to rest on these when not coupled to the tractor.  Although most all on-road trailers meet this definition, the term semi trailer is most often conserved for use with trailers heavy enough to be pulled by a semi truck.

Types of Semi Trailers

• Van or box trailer - These are your most common trailer. They have flat floor with walls and a roof all the way around sealing its contents from the weather and prying eyes. These trailers can come in many different lengths, such as 28.5 ft, 40 ft, 48 ft, 50 ft, 53 ft. The trailer can be built of any type of material, but typically you will see sheet metal walls and roof, and either a wooden or metal floor. The trailer is fitted with two long frame rails similar to that of the tractor. Up front you have the kingpin, and out back you have the axle(s).
• Reefer - These trailers appear to be nearly identical on the outside as a van trailer. However, at the very front of the trailer, they have a refrigeration unit. This unit is usually a small diesel engine that powers A/C compressors similar to what cools the interior of the truck for the driver. Reefers have temperature control that can keep the trailer very cold, or just cool depending upon what they are hauling.
• Tanker - A tanker is used for hauling liquid in mass quantities. They appear to be nothing more than a huge cylinder on wheels, and that is pretty accurate. Most tankers tilt toward the middle of the cylinder where the drain valve is allowing the tanker to easily dispense its payload. Many tankers have baffles to prevent the liquid from sloshing around and making the truck unstable. Some loads do not allow the use of baffles, so the driver must be more careful.
• Dry bulk - A dry bulk trailer resembles a tanker, but is used for hauling dry powder materials like sugar instead of liquid.
• Car carrier - A car carrying trailer (and usually the truck the pulls it) is a specially designed trailer to just haul cars. They usually have two levels allowing the cars to be stacked two high allowing more cars to be pulled at one time. The truck is usually modified to allow one car to be put above the cab/hood, on the roof.
• Livestock trailer - Whether it be for horses, cows, chickens, pigs, or whatever, a livestock trailer is a specially designed trailer just to haul livestock. They are built similar to a van trailer, but the walls are not solid allowing fresh air to enter the trailer. These trailers often have separate compartments, or allow the livestock to be secured in the trailer. Livestock moving inside the trailer (especially heavy animals, such a horses or cows) will offset the balance of the truck/trailer.
• Flatbed - A flatbed trailer is the simplest trailer in theory. You can think of it as a van trailer with the sides removed. A flatbed is nothing more than a solid floor with a bulkhead/headache rack at the front to protect the driver from his load. Flatbeds can generally haul anything that can be strapped to them, and allow for easy loading, and as thus are very versatile. However, the load is open to the effects of the weather, and extra precautions must be taken to be sure the load is secured.
• Lowboy - A lowboy trailer is very similar to a flatbed, except that once the trailer cleared the truck, the trailer drops really close to the ground. Typically the drop so close that they once again raise back up to clear the rear axles. These trailers drop low allowing tall objects to be hauled while still clearing power lines and bridges. They will commonly haul machines such as cranes, back-hoes, bulldozers, ect, but they can haul anything.
• Curtain sider - A curtain sider trailer is similar to a van trailer except that the side walls are a removable, waterproof curtain. This allows the trailer to be easily loaded from the side like a flatbed, and still have protective walls like a van trailer.
• Double decker - A double decker trailer is a trailer that has a moveable second floor. The reason for this is that pallets can only be stacked so tall, whether it be laws regulating it, such as in hazardous materials, or the weight of the pallets crushing the ones underneath. The adjustable second floor allows for more pallets to be stacked increasing the quantity of goods hauled. These trailers are usually a curtain sider allowing for easy loading, or a van trailer with a hinged second deck. The second floor usually swings into position and has 2 or 3 sections allowing for greater flexibility.
• Dump trailer - A dump trailer is usually very similar to the bed of a dump truck. They have a floor and walls with no roof allowing for overhead loading. The rear door acts like the gate on a dump truck opening as the body is lifted into the air allowing the cargo to be unloaded.

Fifth Wheels

A fifth wheel consists of a large, heavy steel plate affixed to the front of the trailer with a large steel pin in the middle. On the truck, there is another large steel plate, and in the middle of this plate is a locking hole in which the pin of the trailer goes into. The large plate on the trailer transfers the weight of the trailer onto the truck by use of the large plate on the truck. The pin, or king pin on the trailer does nothing more than lock the trailer to the truck. It doesn't support any weight.

This is in contract to the "bumper hitch" found on most pick-ups, in which the single ball holds up the weight and locks the trailer to the truck. The fifth wheel distributes the weight across a large area, and securely locks it.

Coupling and uncoupling is relatively painless. Unlike a ball hitch on a pick-up truck, the driver must just get close to being lined up. Most all fifth wheels are "self aligning" and will pull the trailer into the correct alignment. As you can see in the picture, the fifth wheel as a "V" shape to it. This is what it uses to guide the trailer. Although this does work, it is much easier on the equipment to be properly aligned to begin with.

To couple, the driver lines up straight in front of the trailer. You can't couple from an angle as this can destroy the landing gear on the trailer. The driver backs up close to the trailer. At this point, they must get out and hook up the air lines/glad hands, and wiring, and pull out the lock on the fifth wheel.

The driver then applies the trailer brakes and backs under the trailer. The king pin lock automatically locks when the trailer is fully engaged. The driver can usually feel when the trailer has locked. Most drivers at this point try to drive forward with the trailer brakes still locked. This tests the lock of the kingpin. At this point the driver must then raise the landing gear of the trailer. Uncoupling is basically the same. The driver must lower the landing gear and pull the lock on the fifth wheel to uncouple.

The main argument against fifth wheels is there lack of articulation. At one time, the fifth wheel was fixed on the truck's frame. The trailer was not allowed to articulate at all. Newer designs allow the fifth wheel to tilt to the front or back of the truck improving trailer stability on hills, bumps, and anything not smooth pavement. However, for off-road use this was not enough. There are now fifth wheels that can tilt in all directions to allow towing in off camber situations. However, many people still swear by goosenecks in these situations saying that they are much more stable in off-camber situations.

Fifth wheels are very strong. I, myself, was once amazed at the strength of a fifth wheel coupling. Coming home from work one night, I seen a truck that had slid off of the interstate into a large gully. The incline was very steep, probably around 60 degrees. The trailer was down in the gully perpendicular with the road. The incline was steeper than the fifth wheel could articulate. The trailer was holding the truck about 5 feet in the air. The king pin itself was strong enough to hold a 10,000 pound freightliner 5 feet off of the ground. I was amazed, and so was lots of other people as traffic slowed to a near halt

Axle Configuration

There are four common axle configurations used on our highways. However, depending on local laws and required loads, just about anything goes.

Tandem in the back is the single most common. This setup reaches 80,000 GVCWR, the maximum on most of our interstates. This setup is stable and effective.

Single axle in the back is second. This is usually reserved for smaller trailers, such as the 28.5 footer. Single axles trailers with single axles trucks are unstable and are not the preferred method. However, axles take up precious available weight, so when they can be left out, they are.

Spaced out tandem is probably third. The bridge formula is beyond the scope of this article, however, it basically dictates that the closer axles are together, the less weight they can carry. For example, two 20,000 pound tandems may only be allowed to carry 30,000 pounds if they are put right next to each other as with a normal tandem. By spacing them apart, they can then be allowed to carry 35,000 or maybe even truly double at 40,000 pounds. This law was designed to prevent overloading of bridges.

Based on the same idea as above, you have the tridem. Three axles in the back instead of just two spaced out.

Of course, anything goes. Some states have more relaxed laws regarding weight limits. This can make for some hilarious, but effective concoctions.