Understanding how to measure trailer tire size is essential for ensuring proper fit, safety, and optimal performance. The tire size is a coded string of numbers and letters stamped on the sidewall, revealing critical information about the tire’s dimensions and capabilities. Deciphering this code allows you to confidently replace worn tires, upgrade your trailer’s performance, and maintain roadworthiness.
Understanding the Trailer Tire Size Code
The trailer tire size is typically represented as a series of numbers and letters, such as ST205/75R15. Each part of this code provides specific information:
- ST (Special Trailer): This designation signifies the tire is specifically designed for trailer use and constructed to handle the unique stresses of trailer applications. These tires have stiffer sidewalls to minimize sway and are built for higher load capacities than passenger car tires of similar sizes.
- 205 (Section Width): This number represents the tire’s section width in millimeters, measuring the widest point of the tire from sidewall to sidewall when mounted and inflated.
- 75 (Aspect Ratio): This percentage indicates the height of the tire’s sidewall as a proportion of its section width. In this case, the sidewall height is 75% of 205mm.
- R (Radial Construction): This letter denotes the tire’s internal construction. “R” indicates a radial ply construction, which is the most common type of tire construction due to its superior handling and durability.
- 15 (Rim Diameter): This number signifies the diameter of the wheel rim in inches on which the tire is designed to be mounted.
Therefore, in the example ST205/75R15, the tire is a Special Trailer tire with a section width of 205mm, an aspect ratio of 75, radial construction, and is designed for a 15-inch wheel rim. Knowing how to identify these components is crucial for correctly measuring and selecting replacement tires.
Finding the Tire Size Information
The tire size is always stamped on the sidewall of the tire. Look for a series of numbers and letters that follow the format described above. It’s typically found near the rim, often accompanied by other important information like the load range, maximum load capacity, and inflation pressure. If you’re having trouble finding it, rotate the tire until you locate the code.
Reading the Sidewall Markings
Besides the tire size, the sidewall contains crucial information. Pay attention to:
- Load Range (e.g., Load Range C, D, E): Indicates the tire’s load-carrying capacity at a specified inflation pressure. Higher load ranges correspond to greater load capacities.
- Maximum Load Capacity (e.g., 2000 lbs at 50 PSI): Specifies the maximum weight the tire can safely carry when inflated to the recommended pressure.
- Inflation Pressure (PSI): Indicates the maximum cold inflation pressure required for the tire to achieve its rated load capacity. Never exceed this pressure.
- DOT Code: A Department of Transportation (DOT) code that identifies the manufacturer, plant, and date of manufacture.
Practical Applications of Tire Size Measurement
Measuring trailer tire size is crucial for:
- Replacement: Ensuring you purchase the correct size tires when replacing worn-out ones. Using the wrong size can compromise handling, stability, and safety.
- Upgrading: Determining if you can safely upgrade to a different tire size to potentially improve load capacity or ride quality. Consult with a tire professional before making any significant size changes.
- Wheel Compatibility: Confirming that the tires you choose are compatible with your trailer’s wheel rims.
- Maintaining Safety: Ensuring that all tires on the trailer are the same size and load range for consistent performance and stability. Mismatched tires can lead to uneven wear and handling issues.
FAQs: Demystifying Trailer Tire Sizes
Here are 12 frequently asked questions designed to address common concerns and provide further insights into measuring and understanding trailer tire sizes.
1. What happens if I install the wrong size trailer tires?
Installing the wrong size trailer tires can lead to several problems, including:
- Reduced Load Capacity: The tires may not be able to support the weight of the trailer and its load, leading to premature wear or even tire failure.
- Handling Issues: Mismatched tire sizes can cause instability, swaying, and difficulty controlling the trailer, especially at highway speeds.
- Braking Problems: Uneven tire sizes can affect the braking system, leading to longer stopping distances and potential loss of control.
- Damage to the Trailer: Improperly sized tires can rub against the trailer’s fenders or frame, causing damage.
2. Can I use passenger car tires on my trailer?
Never use passenger car tires on a trailer. Trailer tires (ST tires) are specifically designed with stiffer sidewalls and higher load capacities to handle the unique demands of trailer applications. Passenger car tires are not built to withstand the stresses of towing and can fail prematurely, leading to dangerous situations.
3. How do I determine the correct load range for my trailer tires?
Determine the correct load range by:
- Checking the Trailer’s GVWR (Gross Vehicle Weight Rating): The GVWR is the maximum permissible weight of the trailer when fully loaded.
- Dividing the GVWR by the number of tires: This gives you the minimum load capacity required for each tire.
- Choosing tires with a load range that meets or exceeds this minimum: Always err on the side of caution and choose tires with a higher load capacity if you’re unsure.
4. What does the DOT code on my trailer tire tell me?
The DOT code provides information about the tire’s manufacturing details. It includes:
- Manufacturer and Plant Code: Identifies the manufacturer and the specific factory where the tire was produced.
- Tire Size Code: Identifies the tire size.
- Date Code: A four-digit code indicating the week and year the tire was manufactured. For example, “2024” means the tire was made in the 20th week of 2024.
5. How often should I check my trailer tire pressure?
Check your trailer tire pressure before each trip. Tire pressure is affected by temperature, so check it when the tires are cold (before driving). Use a reliable tire pressure gauge and inflate the tires to the pressure recommended on the tire sidewall or in your trailer’s owner’s manual.
6. What is the difference between bias-ply and radial trailer tires?
- Bias-Ply Tires: Have plies that run diagonally across the tire from bead to bead. They are generally less expensive but offer a stiffer ride and may wear faster.
- Radial Tires: Have plies that run radially from bead to bead, with stabilizing belts beneath the tread. They offer a smoother ride, better handling, and longer tread life. Radial tires are generally preferred for most trailer applications.
7. How do I convert inches to millimeters when measuring tire size?
To convert inches to millimeters, multiply the measurement in inches by 25.4. For example, a 15-inch rim diameter is equivalent to 381 millimeters (15 x 25.4 = 381).
8. Can I increase the size of my trailer tires for better performance?
Increasing the tire size can potentially improve load capacity or ground clearance, but it’s essential to ensure that the new tires:
- Fit Properly: Check that the tires don’t rub against the trailer’s fenders or frame when turning or loaded.
- Are Compatible with the Wheel Rims: Ensure the rim diameter and width are appropriate for the new tires.
- Don’t Exceed the Trailer’s Load Capacity: The total load capacity of the tires must be equal to or greater than the trailer’s GVWR.
- Consult a professional: Always consult with a tire professional before making significant size changes to ensure safety and optimal performance.
9. What is tire ply rating and how does it relate to load range?
Tire ply rating, although now largely replaced by load range designations, refers to the tire’s strength and load-carrying capacity. A higher ply rating (and subsequently higher load range designation) indicates a stronger tire capable of carrying a heavier load. Modern tires use load range letters (e.g., C, D, E) that directly correlate to the ply rating (e.g., 6-ply, 8-ply, 10-ply).
10. How do I prevent trailer tire blowouts?
Preventing trailer tire blowouts involves:
- Regular Inspections: Check tires for cuts, bulges, or uneven wear.
- Proper Inflation: Maintain the correct tire pressure.
- Load Management: Avoid overloading the trailer.
- Tire Age: Replace tires every 5-6 years, regardless of tread depth, as the rubber degrades over time.
- Proper Storage: Store tires properly when not in use to protect them from sunlight and extreme temperatures.
11. What is “tire feathering” and what causes it?
Tire feathering is a type of tire wear characterized by uneven wear patterns across the tread blocks, creating a sharp edge on one side of the blocks and a rounded edge on the other. It’s often caused by:
- Misalignment: Improper wheel alignment can cause tires to scrub against the road surface.
- Worn Suspension Components: Worn shocks, springs, or bushings can contribute to uneven tire wear.
- Aggressive Driving: Harsh braking or cornering can exacerbate tire feathering.
12. Where can I find reliable information about trailer tire sizes and recommendations?
Reliable sources of information include:
- Trailer Owner’s Manual: Provides the recommended tire size and load range for your specific trailer model.
- Tire Manufacturers’ Websites: Offer detailed specifications and recommendations for their tires.
- Tire Professionals: Consult with experienced tire technicians for expert advice and recommendations.
- National Highway Traffic Safety Administration (NHTSA): Provides safety information and regulations related to tires and vehicles.
By understanding the components of the trailer tire size code and addressing common questions, you can confidently select the correct tires for your trailer, ensuring safety, performance, and peace of mind on the road.