Star Rating and Load Capacity Differences in OTR Tyres and Mining Truck Tires

Choosing the wrong OTR tyre for a mining operation is an expensive mistake. A tyre that cannot carry the load will overheat, wear unevenly, and fail prematurely. A tyre with too high a star rating for the application wastes money on capacity you will never use. The star rating system exists to help you match the tyre to the real-world demands of your haul roads, payloads, and cycle times. This guide explains how star ratings relate to load capacity, what the numbers actually mean, and how to select the right tyre for your specific mining conditions.

Key Takeaways

Star ratings indicate the ton-kilometer-per-hour (TMPH) capability of a tyre, not just its physical strength.
Higher star ratings mean the tyre can handle more heat generation from higher speeds and heavier loads.
Load capacity is also defined by the tyre's load index, ply rating, and inflation pressure—not the star rating alone.
Matching the star rating to your average haul speed and cycle time prevents premature heat-related failures.
Under-inflation is the single most common cause of OTR tyre failure in mining applications.
Always consult the manufacturer's load and inflation tables before specifying a tyre for a new site.

What You Need Before Starting

Before you can select the correct OTR tyre, you need to gather specific operational data from your mine site. This is not a desk exercise. You need real numbers from your haul roads and your trucks.

Average haul distance and cycle time: Measure the one-way distance and the total time for a full cycle, including loading, hauling, dumping, and returning.
Maximum vehicle speed: Record the top speed your trucks reach on the haul road, not the theoretical speed limit.
Payload per axle: Know the loaded weight distribution on each axle position. Front axles and rear axles carry different loads.
Ambient temperature range: Note the seasonal highs and lows at your site. Heat dissipation is harder in hot climates.
Current tyre specifications: List the size, ply rating, and star rating of the tyres currently installed on your fleet.

The star rating system is a standardized way to express a tyre's heat-handling capability. It is not a measure of physical strength or puncture resistance. A 4-star tyre can carry the same static load as a 2-star tyre of the same size, but it can do so at higher speeds or for longer continuous operation without overheating.

Step 1 — Understand What the Star Rating Actually Measures

What to Do

Learn the definition of the star rating before you compare tyres. The star rating is a relative index of a tyre's ton-kilometer-per-hour (TMPH) capability. TMPH is calculated by multiplying the average tyre load (in tons) by the average speed (in kilometers per hour) over a working cycle.

Calculate the average tyre load: (empty tyre load + loaded tyre load) ÷ 2.
Calculate the average speed: total cycle distance ÷ total cycle time.
Multiply the two figures to get the TMPH requirement for your operation.
Compare that TMPH value against the manufacturer's published TMPH rating for each tyre.

Why This Matters

The star rating directly reflects how much heat the tyre can dissipate. Heat is the enemy of OTR tyres. When a tyre runs hotter than its design limit, the rubber compounds degrade, the casing separates, and the tread peels off. A higher star rating means the tyre has been engineered with cooler-running compounds and a more efficient heat path through the sidewall and tread base.

Industry practice generally recognizes that a 2-star tyre suits slow, short-haul operations. A 3-star tyre handles moderate speeds and distances. A 4-star tyre is for high-speed, long-haul mining. A 5-star tyre is for the most demanding ultra-class truck applications. These are general guidelines, not universal standards. Always check the manufacturer's specific TMPH data.

Common Mistakes to Avoid

Assuming star rating equals load capacity: A 4-star tyre does not carry more static weight than a 2-star tyre of the same size. It carries the same weight at higher speeds.
Ignoring ambient temperature: A tyre rated for 4-star operation in a temperate climate may overheat in a tropical mine. Heat dissipation is slower when the air is hot.
Using the wrong TMPH formula: Some operators use maximum speed instead of average speed. That overstates the requirement and leads to overspending on higher-rated tyres.

Step 2 — Read the Load Index and Ply Rating Correctly

What to Do

The star rating tells you about heat. The load index and ply rating tell you about static strength. You need both sets of information to make a correct selection.

Locate the load index number on the tyre sidewall. This is a two- or three-digit code that corresponds to a specific load capacity in kilograms.
Find the ply rating, often marked as "PR" or "PLY RATING." This indicates the strength of the casing, not the actual number of plies.
Cross-reference the load index with the inflation pressure table in the manufacturer's data sheet.
Verify that the tyre's maximum load at the recommended pressure exceeds the actual axle load on your truck.

Why This Matters

The load index is a standardized industry code defined by the Tire and Rim Association (TRA) and the European Tyre and Rim Technical Organisation (ETRTO). For example, a load index of 180 corresponds to a capacity of 8,000 kg at a specific pressure. The ply rating, meanwhile, is a legacy measure that originally referred to the number of cotton plies in the casing. Modern radial tyres use steel belts and synthetic cords, but the ply rating remains as a comparative strength indicator.

A typical mining truck tyre might carry a load index of 189, which equates to 10,300 kg per tyre at the rated pressure. Multiply that by four tyres on a rear tandem axle, and you get a total capacity of over 41 tons per axle group. That is how a single tyre size can serve different truck classes—the load index and pressure determine the actual capacity.

Common Mistakes to Avoid

Confusing ply rating with actual ply count: A "42 PR" radial tyre does not have 42 physical plies. It has a casing strength equivalent to that many bias plies.
Ignoring the inflation pressure table: The load index only means something at a specific pressure. Drop the pressure, and the load capacity drops too.
Mixing brands without checking data: Two tyres with the same size and star rating from different manufacturers may have different load indices. Always verify the specific data sheet.

Step 3 — Match the Star Rating to Your Haul Cycle

What to Do

Calculate your site's TMPH requirement and compare it to the tyre's published rating. This is the step where most selection errors occur.

Record the loaded and empty weights of your truck on a weighbridge.
Time a complete cycle, including loading and dumping delays.
Calculate the average speed using the total cycle time, not just the haul time.
Multiply the average load by the average speed to get your TMPH requirement.
Select a tyre with a TMPH rating at least 10% above your calculated requirement.

Why This Matters

A mining truck tyre operates in a continuous cycle of loading, hauling, dumping, and returning. Each phase generates heat. The tyre must dissipate that heat faster than it accumulates. The TMPH calculation captures this dynamic better than any static load figure.

Consider a typical 100-ton haul truck. Loaded, it might weigh 170 tons. Empty, it weighs 70 tons. The average tyre load is roughly 15 tons per tyre. If the average speed over a 30-minute cycle is 25 km/h, the TMPH requirement is 375. A 3-star tyre with a TMPH rating of 400 would work. A 2-star tyre rated at 300 would overheat. This is why the star rating and load capacity differences in OTR tyres and mining truck tires matter so much in practice.

Common Mistakes to Avoid

Using maximum speed instead of average speed: This inflates the TMPH requirement and forces you into a higher, more expensive star rating.
Ignoring loading and dumping time: These add to the cycle time and lower the average speed. Including them gives a more accurate picture.
Selecting a tyre with exactly the required TMPH: You need a safety margin. Heat buildup varies with road conditions, grades, and ambient temperature.

Step 4 — Evaluate Tread Patterns for Heat and Wear

What to Do

The tread pattern affects both heat generation and wear life. Match the tread design to your haul road conditions.

For hard, smooth haul roads, choose a tread pattern with a higher contact area for even wear.
For soft or muddy conditions, select a deeper, more aggressive tread for traction.
For mixed conditions, look for a compromise pattern that balances wear and grip.
Check the tread depth and compound specifications in the manufacturer's data.

Why This Matters

Tread depth is a trade-off. Deeper treads provide more traction and longer wear life, but they also generate more heat because there is more rubber flexing and compressing. A deep tread on a high-speed haul road can overheat even with a high star rating. Conversely, a shallow tread on a muddy site will wear out quickly and lack traction.

The Long Haul Tyre range from Longmarch Tire, Roadlux Tire, and Supercargo Tire includes highway patterns designed for even wear on long, steady runs. These tyres use a high-strength tread rubber formula that extends service life while maintaining low heat generation. For mining applications, you need a pattern that balances these factors against the specific demands of your site.

Common Mistakes to Avoid

Choosing the deepest tread available: More tread means more heat. Match the depth to the actual haul distance and speed.
Ignoring the compound: A heat-resistant compound matters more than tread depth for high-speed mining operations.
Using a highway pattern off-road: The tread blocks are not designed for the stresses of loose rock and uneven surfaces.

Step 5 — Verify Inflation Pressure and Maintenance Protocols

What to Do

Set the inflation pressure according to the manufacturer's load and inflation tables. Then establish a daily inspection routine.

Check cold inflation pressure at the start of each shift.
Adjust pressure based on the actual load, not the maximum rated load.
Inspect for cuts, punctures, and sidewall damage after each cycle.
Monitor tread wear patterns weekly to catch alignment or pressure issues early.
Keep a log of pressure readings and tyre temperatures for each vehicle.

Why This Matters

Under-inflation is the leading cause of OTR tyre failure in mining. A tyre running at 80% of its recommended pressure loses roughly 20% of its load capacity. That overloads the remaining structure, generates excessive heat, and leads to rapid tread separation. Over-inflation is less common but equally damaging—it reduces the contact patch and causes center-tread wear.

The LM/R/SC 326 model from the Longmarch, Roadlux, and Supercargo brands features a unique bead structure design that provides good bead durability. This matters because the bead area is where heat concentrates under heavy loads. A robust bead structure helps the tyre maintain its seal and structural integrity even when operating at high TMPH levels.

Common Mistakes to Avoid

Setting pressure once and forgetting it: Tyres lose pressure naturally. Daily checks are non-negotiable.
Using the same pressure for loaded and empty runs: If your truck runs empty half the time, consider a pressure adjustment protocol.
Ignoring temperature readings: A tyre that runs 10°C hotter than normal is a warning sign. Investigate before it fails.

Step 6 — Compare Manufacturer Data and Certifications

What to Do

Before finalizing your tyre selection, compare the published data from different manufacturers. Look for certifications and test standards.

Request the technical data sheet for each candidate tyre.
Compare the TMPH ratings, load indices, and recommended pressures.
Check for compliance with TRA, ETRTO, and ISO standards.
Verify the star rating is backed by published test data, not just marketing claims.
Consider the manufacturer's track record in your specific application.

Why This Matters

The star rating system is not a global legal standard. It is an industry convention that manufacturers apply with some variation. A 3-star tyre from one brand may not have the exact same TMPH rating as a 3-star tyre from another. This is why you must compare the actual published data, not just the star symbols.

The Tire Education section of the Longmarch, Roadlux, and Supercargo website covers the strengths of all-steel radial tyres and the differences between radial and nylon tubeless designs. This resource is useful for fleet managers who want to understand the engineering behind the specifications before making a purchase decision.

Common Mistakes to Avoid

Relying on the star rating alone: Always verify the TMPH figure in the data sheet.
Ignoring the load index: Two tyres with the same star rating can have different load capacities.
Skipping the inflation table: The recommended pressure is part of the specification, not an afterthought.

Pro Tips for Success

Calculate TMPH for each haul route separately: A mine with multiple pits and different haul distances may need different star ratings for different trucks.
Monitor tyre temperatures with a pyrometer: Infrared temperature readings at the shoulder and tread center reveal heat buildup before failure occurs.
Rotate tyres between positions: Front and rear positions wear differently. Rotating extends overall fleet life.
Keep spare tyres inflated and stored properly: A spare that sits deflated for months is not ready for emergency use.
Train your operators on tyre care: Aggressive braking and cornering generate heat. Smooth driving extends tyre life significantly.

Frequently Asked Questions

What is the difference between star rating and ply rating?

The star rating measures heat dissipation capability, expressed as ton-kilometers per hour (TMPH). The ply rating measures casing strength and relates to the maximum static load the tyre can carry. A tyre can have a high ply rating and a low star rating—it can carry a heavy static load but only at slow speeds.

How do I calculate the TMPH requirement for my mining operation?

Multiply the average tyre load (in tons) by the average speed (in km/h) over a complete cycle. The average load is the midpoint between the loaded and empty tyre loads. The average speed is the total cycle distance divided by the total cycle time, including loading and dumping.

Can I use a higher star rating tyre to extend service life?

Yes, but only if the higher rating matches your actual operating conditions. A higher star rating tyre running well below its TMPH limit will run cooler and may last longer. However, you will pay more for the higher rating, so the cost-benefit depends on your specific haul cycle.

Does a higher star rating mean better puncture resistance?

No. The star rating is about heat management, not physical toughness. Puncture resistance depends on the tread compound, belt construction, and sidewall design. A 2-star tyre with a heavy-duty casing can resist punctures better than a 4-star tyre with a lighter construction.

Conclusion

The star rating and load capacity differences in OTR tyres and mining truck tires come down to one fundamental distinction: the star rating manages heat, while the load index and ply rating manage static strength. You need both to make a correct selection. Calculate your TMPH requirement, verify the load index against your axle weights, and set inflation pressures according to the manufacturer's tables. The process is straightforward, but it requires real data from your site—not guesses.

Start by measuring your haul cycles and weighing your trucks. Then compare the published TMPH ratings and load indices from the manufacturers you trust. For a deeper understanding of tyre construction and selection, explore the educational resources available from Longmarch Tire, Roadlux Tire, and Supercargo Tire. The right tyre, matched correctly to your operation, will deliver longer life, lower operating costs, and fewer unplanned downtime events. That is the real payoff for getting the star rating right.