Understanding the Parts of a Trailer & Why They Matter
Posted: April 27, 2023
Understanding the Parts of a Trailer & Why They Matter
Before towing a trailer for the first time, it’s important to know the various components that make up a trailer. In this guide, we’ll walk through the basic parts of a trailer. We’ll talk about what each part is used for and why it is important.
This guide is meant to be a helpful resource for those just learning to tow, as well as those who are looking to replace trailer parts on an existing unit.
Parts of a Trailer Explained
The trailer coupler is the forwardmost point on a trailer tongue, serving as the primary connection point to the tow vehicle. The coupler is the trailer-side of the connection, while the hitch and ball mount make up the vehicle-side. The coupler pivots around the ball to allow for smooth driving and turning.
Most trailer couplers are made up of a cup or receiver component to accept the hitch ball, a body component that attaches to the trailer, as well as a latching mechanism to secure the coupler onto the ball.
Types of Couplers
Trailer couplers vary in style and functionality, from their body design to their latch type. Some coupler body types include straight-tongue, A-frame and channel-style, referring to the type of trailer frame the coupler attaches to.
A-Frame vs. Straight-Tongue Coupler
Some types of coupler latches include the posi-lock latch with its adjustable lever, the easy-lock latch with a simple push-down lever, the sleeve-lock latch for a highly secure system, as well as the pin-style or no-latch system for innovative convenience (i.e. the CURT QuickPin coupler).
Some specialized forms of trailer couplers include the lunette eye used in pintle hitch connections, and the gooseneck coupler used in gooseneck hitch truck bed connections.
A trailer jack is a leg that is mounted on the trailer tongue that can be extended or retracted to allow the coupler to be raised or lowered. Jacks can be manually operated with a handle or electrically operated, using battery power.
Jacks come in all kinds of styles and capacities to accommodate virtually every trailer type. Some types of jacks include electric, smart, A-frame, swivel, jacks with wheels and heavy-duty.
For guidance on buying, installling, operating and maintaining your trailer jack, see The Ultimate Guide to Trailer Jack Replacement.
Some Types of Trailer Jacks
The trailer tongue is the front portion of the trailer frame that sticks out from the main body. Trailer tongues can have an A-frame shape or a single beam called a straight-tongue.
The forwardmost point of the trailer tongue is where the coupler is attached, the component that connects directly to the tow vehicle hitch. Sometimes tongue is used to refer to the coupler only.
Other components that are typically mounted onto the trailer tongue include the safety chains, trailer jack and wire harness, as well as a trailer tongue box, battery box, LP tank and other accessories, depending on the trailer type.
4. Wiring Harness
Trailers need electrical power to operate their lights and sometimes other systems like electric trailer brakes. This power is supplied by the tow vehicle through a wiring harness.
A trailer wiring harness usually has a standardized plug that connects to a socket at the rear of the vehicle. This plug can have four wires, five, six or even seven, depending on the trailer type and number of electrical systems. There are also different configurations, such as 4-way flat versus 4-way round.
The wiring harness typically courses its way through the trailer frame where the various wires branch off to the taillight lamps, running lights, brakes as well as other electrical systems and trailer lighting.
Types of Trailer Wiring
Even though trailer wiring has become quite standardized, it can be confusing when you start to discover how many different varieties there are. 4-pin flat, 6-wire square, 7-way RV blade – what does it all mean?
First, the number in each configuration corresponds to the number of wires that make up the connection: 4-way has four wires, 5-way has five wires, and so on. These wires are color-coded and are used for specific functions, based on the color: green is for the right turn signal / brake light. Brown is for the taillights. White is the ground connection, etc.
All Trailer Wiring Types Color-Coded by Function
Secondly, terms like way, pin, wire and prong are all synonyms. Simply put, they all mean the same thing. You can call it a 4-way, or you can call it a 4-wire, or you can call it a 4-pin – it’s the same configuration.
One caveat is that blade is more of a reference to the style of the individual contacts. Instead of a pin, each contact point is a flat blade-like piece of metal. On RV blade harnesses, the blades are arranged in a circle, which brings us to the final point.
Lastly, the terms flat, round and square are indicative of how the wires are arranged. In a flat configuration, the wire contacts are all laid out in a single, flat line. In round, the wires are in a circle. You get the idea.
Some Different Wiring Examples
When considering wire harness types and perhaps which one to buy as a replacement, the best course of action is to look at what you already have on your trailer. If it’s a 4-way flat, replace it with another 4-way flat. If it’s a 7-way RV blade, replace it with a 7-way RV blade.
5. Safety Chains
Safety chains are a backup connection between the tow vehicle and trailer. If ever the primary hitch connection should fail, the chains are designed to maintain the connection until the vehicle can come to a safe stop.
Trailer safety chains are relatively short in length – usually around four feet – and come in a variety of material and weight capacity options to accommodate different trailer types.
Two safety chains (or safety cables) should always be used on a standard trailer hitch setup. The correct way to connect safety chains from the trailer to the vehicle is to cross them once underneath the coupler.
A Note about Proper Safety Chain Hookup
It is typically recommended that you only cross your safety chains one time under the coupler. This allows proper chain movement while turning and creates a cradle for the coupler in the event of a disconnect.
If you twist the chains around multiple times, it could create a situation that the chains may bind while turning. In the event that your chains are too long, it is recommended that you use a safety chain hanger to keep them from scraping on the ground.
6. Side Marker Lights
The side marker lights on a trailer serve a very important purpose. First, they function as running lights, meaning any time your tow vehicle headlights are turned on, the side marker lights glow to bring more visibility for other drivers. Second, side marker lights function as turn signals, blinking in time with your tow vehicle signals or flashers, as well as the trailer’s taillights.
Different trailers have different numbers of side markers, depending on the overall size and length. The best side markers have LED lamps to be longer lasting, brighter and more weather-resistant. A typical replacement trailer light kit will include a pair of side markers, one for either side of the trailer.
7. Frame / Chassis
The frame / chassis are the bones of the trailer. They provide the basic structure onto which all of the other components, including the walls, axle, lights and so on attach. The structure is typically constructed from metal beams or posts welded together into a single piece.
Frame and chassis generally refer to the same thing – they are the main structure of the trailer. However, chassis can sometimes mean the lower portion of the structure, without the walls, while frame refers to the entire structure. Chassis is also more associated with heavier-duty trailers but can mean lightweight trailers, too.
8. Tie-Down Anchors
Tie-down anchors or simply tie-downs are an accessory found on many different trailer types but not all. They provide an attachment point for ropes, chains or straps to allow cargo to be better secured in the trailer.
Tie-down anchors can take many forms, from hooks and rings, to large stake-pocket-style anchors. Tie-downs also range in size and weight capacity to accommodate different size trailers and types of cargo.
9. Side Walls
The side walls of a trailer provide more security for the cargo inside. Some trailers come with 3’ high plywood walls, others with a low metal railing only. Still others come without side walls altogether.
Trailers without side walls or with a metal railing only can oftentimes be customized by adding walls made from plywood or another durable material. Stake pockets welded onto the trailer frame make it easy to construct walls or even accommodate detachable walls, using 2x4 studs. This also makes it easy to replace the side walls of your trailer if the wood ever rots out.
The bed of the trailer is simply the bottom surface or floor of the trailer body. Some trailers come with a wooden bed, others with a metal mesh or sheet metal construction. The type of cargo or materials you’re hauling should determine the type of trailer bed you look for.
Metal trailer beds can be more durable, but wooden beds offer the advantage of easier replacement if the material ever becomes too worn or rotted for safe usage.
All trailers have wheels. Trailer wheels are typically smaller than automotive vehicle wheels, but vary in size depending on the size and style of the trailer. The wheels of a trailer are mounted to its axle assembly, using the same style of lug nuts and studs that a vehicle wheel uses.
Some trailers have brakes built into the wheel hubs, particularly larger trailers. Trailer brakes can be electrically or hydraulically actuated, and those with electric operation require a wiring connection to the tow vehicle and a brake controller.
12. Axle Assembly
The axle assembly of a trailer is the primary structure on which the wheels of the trailer turn. An axle assembly is made up of an axle beam, hubs, suspension components and hanger hardware.
Trailer axles range in size and capacity to accommodate all of the different trailer types. There are single-axle configurations, tandem-axle with two axles, triple-axle and more. Axles can be spring or torsion in their suspension style. They can have a straight spindle or drop spindle. The springs can be underslung or overslung. They can have an idler hub or brake hub.
With so many different axle configurations, it can be overwhelming when shopping for a replacement. Fortunately, Lippert offers a number of resources when it comes to identifying your own trailer axle and choosing a replacement.
The fenders of a trailer serve as shields around the trailer wheels, helping to contain dirt, debris, water and other road elements from getting kicked up by the tires. Just like on a vehicle, the trailer fenders block this debris, deflecting it back toward the road and keeping it away from your cargo, the body of the trailer and other vehicles.
Trailer fenders are oftentimes constructed from metal and can take many different shapes, whether they hug the wheels in a clean curve or are angled for a squared off look. Some fenders are integrated with the body of the trailer, and others are flared, sticking out like the style of a flare-side pickup truck bed. There are also single-wheel fenders, tandem fenders, triple-wheel fenders and so on.
The gate of a trailer is its rear wall. On many models, particularly utility trailers and flatbed haulers, this gate features a downward hinging motion, providing open access to the trailer bed. On simpler trailers, the gate is nothing more than a panel that lifts off its mounting points to be set aside during loading. On others, the trailer gate actually serves a dual purpose, not only containing cargo but also acting as a ramp to drive various implements on board, such as a lawnmower, an ATV or even a car.
Trailer taillights are a vital safety component for all types of trailers. Just like a vehicle, the taillights of a trailer are designed to light up and alert other drivers when you are braking, signaling a turn or simply passing on the highway.
Taillights are located at the back of the trailer and require a wiring connection that runs through the frame, all the way to the front of the trailer tongue, where they plug into the back of the tow vehicle.
Taillights come in a variety of styles, including traditional incandescent configurations and more modern LEDs. Some have integrated side marker lights on the taillight lamps, and some are designed to be fully submerged in water, such as on a boat trailer.
All taillights should be DOT-approved for use on the road.
Disclaimer: These photographs, recommendations, and approximations are intended for demonstration purposes only and do not reflect the specifications of any particular tow vehicle, recreational vehicle, or trailer. Always consult the manufacturer’s Owner’s Manual.