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Tyre Tips - Light Commercial Vans, Trucks and Buses

Tyre Tip 5 - Dimensions & Loads

Quite often questions are asked about how dimensions and loads of truck tyres are established by the various tyre manufacturers. Surprisingly, many people do not expect rubber based, flexible components to be closely controlled by industry regulations. This is obviously not the case -- in fact, there are a number of tyre and rim organisations around the world that publish criteria and are largely standardised between each other. Locally, we have the Tyre and Rim Association of Australia (T&RAA) publishing an annual standards manual that covers all types of tyres used in our market. They are affiliated with other world bodies such as the U.S. tyre and rim association (UST&RA) and the European tyre and rim organisation (ETRTO). Most countries adopt this
standardised information into their road regulations.


Click to Enlarge Click to EnlargeAll responsible tyre manufacturers contribute expertise and finances to these organisations to establish and maintain criteria enabling designers and end users to work to agreed dimensions, loads, nomenclatures, etc. This means that, say, a 295/80R22.5 tyre produced by any manufacturer in the world will be within a few percentage points of each other dimensionally and will carry the same load. This doesn't mean that construction or manufacturing methods are uniform --- quite the contrary, it is left to the individual manufacturer to accomplish their desired durability and treadlife by their own design and innovation, but within the confines of standardised dimensions and loads. However, because of this uniqueness between brands, it behoves the customer to consult each individual manufacturer for particular tyre information such as static loaded radius (spring rate) and rolling circumference. This should be taken into account when mixing and matching tyres, for instance in dual application, etc.


Returning to the published standards manual, the nominal size dimensions for diameter and section width are quoted with tolerances in the order of around four percent on section height and six percent on section widths to allow for production variations and service growth.


Loads, too, are standardised within the industry, so certain sizes will virtually always be classified into certain load carrying limits. The very basics of load carrying capability lie with two characteristics --- volume and air pressure. The product of these two items will give you the load capacity. Refer diagram 1. (Remember a tyre can be termed a flexible pressure vessel; it is the air inside the tyre that carries the load, not the tyre itself). The manuals contain charts on how much the tyre can carry at various inflation levels, and also allowances that can be applied for differing speeds. A worthwhile little "wrinkle" to understand is shown in diagram 2 where the sidewall marking of a 315/80R22.5 tyre displays the normal maximum load rating at "M" speed rating (130 km/h), but also exhibits an alternative improvement in load if the speed is reduced to "L" (120 km/h).


Rim configurations and widths must also be standardised via the tyre and rim committees to enable universal fitments to be achieved. For each size of tyre, there is an ideal (or design) rim width upon which the tyre design is established. In real life however, this rim width may not exist since most rims are manufactured in various inch increments. Therefore, the standards manuals quote a nominal (measuring) rim width followed by approved alternative widths and configurations.


So next time you visit a tyre dealer, ask them if they can produce a tyre standards manual. It makes good bedtime reading material!