New to towing? Start with our Start Here guide for a structured learning path, then use this glossary as your reference. Already experienced but want a quick refresher? This glossary covers every key towing term you need to know — from basic weight ratings to advanced setup concepts. Each definition links to a full in-depth guide where available.
B
Brake Controller
A device installed in the tow vehicle that activates the trailer’s electric brakes in proportion to the vehicle’s deceleration. Required in most states for trailers over 3,000 lbs. → Trailer Brake Controller Guide
Breakaway Switch
A safety device that automatically applies the trailer’s electric brakes if the trailer accidentally separates from the tow vehicle. Required by law in many states for trailers with brakes.
Bumper Pull
A standard ball-and-coupler hitch connection where the trailer attaches to a ball mounted at the rear of the tow vehicle. The most common hitch type for RVs, boat trailers, and utility trailers.
C
Coupler
The socket on the trailer tongue that fits over the hitch ball on the tow vehicle. Must match the ball size for a secure connection.
Curb Weight
The weight of the tow vehicle without passengers, cargo, or a trailer — but with a full tank of fuel and all standard equipment. Used as the baseline when calculating payload and GCWR.
G
GAWR (Gross Axle Weight Rating)
The maximum weight each axle on the tow vehicle or trailer is designed to support. Exceeding GAWR can cause axle failure and tire blowouts.
GCWR (Gross Combined Weight Rating)
The maximum allowable combined weight of the fully loaded tow vehicle plus the fully loaded trailer, as specified by the vehicle manufacturer. This is the “ceiling” that governs your entire towing setup. → What Is GCWR? (Full Guide)
GVWR (Gross Vehicle Weight Rating)
The maximum allowable loaded weight of a single vehicle — either the tow vehicle or the trailer — as rated by the manufacturer. For trailers, this includes the trailer’s own weight plus all cargo inside. → What Is GVWR? (Full Guide)
H
Hitch Ball
The ball-shaped component mounted on the receiver hitch that the trailer coupler attaches to. Common sizes are 1-7/8″, 2″, and 2-5/16″. The size must match the trailer’s coupler.
Hitch Class
A classification system (Class I through Class V) that rates receiver hitches by maximum towing and tongue weight capacity. Class III hitches are the most common for pickup trucks and SUVs.
Hitch Weight
The downward force applied to the hitch ball by the trailer coupler. Same as tongue weight. Should be 10–15% of the trailer’s total loaded weight for a bumper pull trailer.
P
Payload Capacity
The maximum weight a tow vehicle can carry above its curb weight, including passengers, cargo, fuel, and tongue weight from the trailer. Payload is often the first limit reached when towing — before towing capacity. → What Is Payload Capacity? (Full Guide)
Pin Weight
The downward force a fifth-wheel or gooseneck trailer exerts on the hitch. Equivalent to tongue weight for bumper pull trailers. Typically 15–25% of the trailer’s loaded weight for fifth wheels.
S
Sway Control
A device or system designed to dampen trailer sway. Can be a friction-style sway bar, an integrated weight distribution hitch with sway control, or an electronic stability control system. → Trailer Sway Control Guide
T
Tongue
The A-frame or straight section at the front of a trailer that extends forward and connects to the tow vehicle via the coupler and hitch ball.
Tongue Weight
The downward force the trailer’s coupler exerts on the hitch ball, measured in pounds. Proper tongue weight is 10–15% of the total trailer weight for bumper pull trailers. Too little causes trailer sway; too much overloads the rear of the tow vehicle. → What Is Tongue Weight? (Full Guide)
Towing Capacity
The maximum weight of the trailer that a tow vehicle is rated to pull. This number is set by the manufacturer and depends on engine, transmission, axle ratio, and hitch type. Note: towing capacity does not account for the loaded weight of the tow vehicle itself — that’s what GCWR is for. → Towing Capacity Explained
Trailer Sway
A fishtailing motion where the trailer swings side-to-side while being towed. The most common cause is too little tongue weight (less than 10% of trailer weight) or improper cargo loading. → Trailer Sway Control Guide
W
Weight Distribution Hitch
A specialized hitch system that uses spring bars to distribute the trailer’s tongue weight across all axles of the tow vehicle and trailer, rather than concentrating it on the rear axle. Required for many towing setups over 5,000 lbs. → Weight Distribution Hitch Setup Guide
WDH (Weight Distribution Hitch)
Abbreviation for Weight Distribution Hitch. See above.
Trailer Sway — Full definition
Trailer sway is the side-to-side oscillation of a trailer while towing, caused by improper load distribution, insufficient tongue weight, or excessive speed. If sway begins, ease off the accelerator — do not brake hard.
Weight Distribution Hitch — Full definition
A weight distribution hitch is a towing system that uses spring bars to redistribute tongue weight across all axles of the tow vehicle and trailer, leveling both vehicles and improving steering and stability for heavy trailers.
Learn More
This glossary is part of TowPro Academy’s ongoing effort to make towing education clear, practical, and accessible. If you want to go deeper on any topic, explore our full library of towing guides and the TowPro Academy online trailer towing course.
Towing Capacity
The maximum weight a vehicle is rated to pull behind it, as specified by the manufacturer. True usable towing capacity on any trip is determined by the most restrictive of: published towing capacity, GCWR minus loaded vehicle weight, remaining payload minus tongue weight, and hitch rating. See Towing Capacity Guide.
Trailer Brake Controller
An electronic device that sends voltage to a trailer’s electric brakes when the tow vehicle’s brake pedal is pressed. Synchronizes trailer braking with vehicle braking for safer, shorter stops. Required whenever a trailer has electric brakes. See Trailer Brake Controller Setup Guide.
Hitch Class (Class I–V)
A standardized rating system for trailer hitches defining maximum gross trailer weight (GTW) and tongue weight (TW) capacity. Class I (up to 2,000 lb GTW) through Class V (up to 20,000 lb GTW). Receiver size varies by class: Classes I–II use a 1¼-inch receiver, Classes III–IV use a 2-inch receiver, and Class V uses a 2½-inch receiver. See Trailer Hitch Classes Explained.
Surge Brakes
A self-contained hydraulic trailer braking system that activates automatically when the tow vehicle decelerates. The trailer’s momentum compresses a coupler-mounted actuator, which engages the hydraulic drum brakes. Requires no connection to the tow vehicle’s electrical system. Standard on boat trailers because the components are submersible. See Surge Brakes vs Electric Brakes.
ST Tire (Special Trailer)
A tire engineered specifically for trailer use. ST tires have stiffer sidewalls than passenger or LT tires to resist lateral flex during trailer sway and cornering. Most are speed-rated for 65 mph maximum. Should be inflated to the maximum pressure listed on the sidewall, checked cold. See Trailer Tire Guide: ST vs LT, Load Range & Pressure.
Load Range (Trailer Tires)
A rating indicating a tire’s maximum load capacity and required inflation pressure, expressed as a letter (C, D, E, F). Load Range C = 6-ply equivalent, D = 8-ply, E = 10-ply, F = 12-ply. Higher load range tires carry more weight but require higher inflation pressure to achieve their rated capacity. See Trailer Tire Guide.