OTR tyre codes and star ratings decoded

An OTR code such as L-3 or E-4 is two pieces of information: a letter for the machine family the tyre is built for, and a digit for the tread type and depth. The stars alongside it describe the strength of the casing, not the tread. Match both to the machine and operating conditions.

This is a different naming system from the truck and agricultural sidewall, and it is the one used across earthmover, quarry, loading shovel and grader fitments. Below is how to read it, what TKPH governs, and the wheel safety that has to go with it.

The classification comes from TRA, the North American standards body for tyres and rims, and is mirrored in ETRTO practice in Europe — which is why the same codes appear on tyres from every source. The letter says what kind of machine the tyre is designed around.

You will also meet ML on mining and logging variants and LS on log skidder fitments. Treat those as specialist and check the manufacturer’s own data sheet rather than assuming they slot neatly into the four families above.

Machines that spend part of their working life on the public road — tippers, mixers and site-to-site haulage — are normally on truck fitments rather than OTR codes. Those sit under truck, bus and coach.

The digit describes the tread pattern class and, from 3 upwards, the depth. It is the number most often got wrong on a replacement order: L-3 and L-5 look similar in a photograph but have different tread specifications.

The rule of thumb the industry works to is that a deep tread carries around one and a half times the tread depth of the equivalent 3, and an extra deep around two and a half times. Take that as an indication of the family rather than a specification — the actual depth is on the manufacturer’s data sheet, and it varies by size and by maker.

On radial OTR tyres the old ply rating has been replaced by a star rating — *, **, *** and above. A star rating is a casing strength class, and each class has its own load and inflation table: more stars means a stronger casing that can be inflated harder and so carry more at a given size.

Two things follow, and both matter on a parts counter. You cannot read a load off the star rating alone — the star tells you which table to open, and the load comes from that table at the pressure you intend to run. And you cannot substitute down: a machine specified for a two-star casing will destroy a one-star casing, and inflating the weaker casing harder is not a fix, because the pressure ceiling is part of what the class defines.

Star ratings do not map cleanly onto ply ratings either, despite both being casing-strength shorthand. Where a customer is cross-shopping a bias tyre quoted in ply rating against a radial quoted in stars, compare the two load and inflation tables rather than the labels. The ply rating and load range guide covers the bias side of that comparison.

TKPH — tonne-kilometres per hour — is a measure of how much heat work a tyre is being asked to do. TMPH is the same thing in imperial units, ton-miles per hour. It exists because on a long haul road the thing that ends an earthmover tyre is not the tread wearing out or a rock cutting it; it is internal heat.

The site figure is the mean tyre load multiplied by the average working speed over a shift. Mean tyre load is the average of the loaded and unloaded figures, so a truck that runs out full and back empty gets credit for the empty leg. Average speed is shift distance divided by hours worked, waiting at the face and the tip included — which is why it is usually much lower than anyone guesses.

Check the manufacturer’s thermal duty guidance for the tyre and compare it with the site duty. Where the tyre’s rating is insufficient, options include: shorten the cycle or improve the haul road, reduce the load, build cooling time in, or move to a tyre with a higher rating — often a shallower tread rather than a deeper one, because rubber depth is what holds heat in.

Manufacturers also publish correction factors for ambient temperature and for cycle length, so a hot site or a haul road with no cooling stop needs the figure adjusted before it is compared with the rating. Use the maker’s own correction method rather than a factor from another supplier’s chart.

Both remain in production and each suits particular duties. Bias is not obsolete off the road.

Never mix radial and bias on the same axle or across a driven pair. The difference in rolling circumference and in deflection under load overloads one tyre and winds up the drivetrain.

Large OTR tyres sit on multi-piece wheels — commonly three and five piece assemblies made up of a base, a bead seat band, a side ring or flange and a lock ring, often with an O-ring seal. Those components are held in place by air pressure, and when one lets go the energy released is lethal. If you sell or fit these tyres, the following is the minimum:

• Fully deflate before touching anything. Remove the valve core and check air actually stops coming out — a blocked valve or a tyre frozen to the rim can hold pressure behind a valve that appears clear.
• Never strike, lever or attempt to seat a ring while the assembly holds pressure. Most multi-piece failures happen during assembly and inflation, not in service.
• Inflate in a cage or restraining device, using a clip-on chuck, a long hose and an in-line gauge, with the fitter standing out of the trajectory. The trajectory zone extends in front of the assembly and to the side — know where it is before you turn the air on.
• Never mix components. Rings, bands and bases from different manufacturers, different part numbers or different rim sizes may appear to fit and will not lock correctly. Match everything to the base’s stamped part number.
• Never weld, heat, straighten or otherwise repair a wheel component. Scrap it. Heat destroys the properties the component depends on, and a damaged lock ring can cause a fatal failure.
• Inspect every component every time the tyre comes off — cracks at the gutter, wear in the lock ring groove, corrosion, distortion, bent flanges. Gutter corrosion is the usual culprit on a machine that lives in a wet quarry.

In the UK this is work equipment, and health and safety law requires that only trained, competent people carry it out with suitable equipment. Take the Health and Safety Executive’s current guidance on tyre and wheel safety as the authority for your procedures, and the wheel manufacturer’s service manual as the authority for that assembly. Neither this page nor a supplier’s catalogue substitutes for either.

A typical fitment reads 26.5R25 L-3 **: 26.5 inch nominal section width, radial, 25 inch rim, loader and dozer service, rock tread, two-star casing. A bias equivalent might read 17.5-25 16PR L-3, with the ply rating standing in for the star class. The sidewall decoder below breaks down the dimensional part of either.