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
information into their road regulations.
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
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
So next time you visit a tyre dealer, ask them if they can
produce a tyre standards manual. It makes good bedtime reading