Brian Joss – Interpreting and understanding the detailed information on the sidewall of your vehicle’s tyres is important in helping you identify the right tyre and operating parameters for your car – an important consideration to achieve the best balance of tyre performance for acceleration, braking and cornering, as well as ride comfort, noise, rolling resistance, mileage and load capacity.
By way of example, we decode the sequence “225/45 R 18 95 H SSR MOE”. The number “225” stands for the width of the tyre in millimetres – so, in this case, the tyre is 225 mm wide. The “45” that follows is the aspect ratio – the height of the tyre sidewall as a percentage of its width, which equates to 101 mm for this tyre.
Next up is an “R”, which means “radial” – derived from the structure of the layers in the casing of the tyre, which run radially, or 90-degrees to the tyre circumference. The figure “18” indicates the wheel rim diameter in inches, while “95” is the load index, indicating the maximum weight the tyre can bear. In this example, the tyre is rated for 650 kg.
Then comes the letter “H” which is the speed index, where “H” indicates a maximum speed of 210 km/h. “SSR” identifies that this is a Continental “Self Supporting Runflat” tyre, otherwise shown as “RF” or “Runflat”.
The abbreviation “MOE” tells us this is a Mercedes original equipment tyre with runflat properties (E = extended mobility). The number of these manufacturer-specific codes is set to rise even further in years to come as more and more automakers submit their own unique specifications to the leading tyre manufacturers.
High-performance tyres often include “XL” after the size marking, or the word “Reinforced” on the sidewall. This refers to tyres with additional sidewall reinforcements, which are designed to cope with the additional forces created by high-speed driving, cornering, braking or load-carrying ability. “FR” is used on tyres with a flange rib, which incorporates additional rim protection features.
In the case of 4×4 tyres, “M+S” stands for “mud and snow” and is suited to operating in adverse conditions. Specialised winter tyres for passenger cars, which feature a snowflake icon, are not sold in South Africa due to construction and compounds which are not appropriate for our climate.
Along with this data, the tyre sidewall carries lots of other information, too. One important item for drivers is the production date, indicated by the DOT code. It’s made up of the letters DOT and two pairs of figures, separated by a forward slash. The first two numbers show the week the tyre was built; the last two indicate the year. So “36/16” means the tyre was built in the 36th calendar week (i.e. between September 5 and 11) in 2016.
This is important to determine the age of the tyre, as the rubber degrades over time. Fitment of tyres older than five years should be avoided in the interests of safety and reliable performance, and this can be further influenced by the manner and environment in which tyres are stored.
Tyres are the critical points of contact your vehicle has with the road, so it is important to ensure that they are regularly checked and maintained to ensure your safety. The condition of your tyres and the remaining tread depth are important factors in determining the ability of the tyres to maintain grip – specifically in wet driving conditions where the grooves, or sipes, in the tread pattern are designed to expel water and keep the tyre in contact with the road surface.
The legal tread limit on tyres that have tread depth indicators, or tread wear indicators (TWIs), in the main grooves of the tyre have a limit of 1.6 mm, while the limit on those without tread depth indicators is 1 mm. These indicators provide easy identification of the tyre having reached its tread limit once the tread is flush with the indicator bars, indicating that they must be replaced.
However, it is important to note that the braking distance in wet conditions for a worn tyre with a tread depth of just 1.6 mm is almost twice as long as that of a new tyre at 8 mm. Furthermore, when driving on wet roads, a wedge of water can build up between the tyre and the road surface, causing the tyre to be separated from the road by a film of water. This is known as aquaplaning, at which point the tyre no longer has any contact with the road surface, resulting in a total loss of grip for steering or braking.
The risk of aquaplaning is higher the faster you drive, and also the lower the remaining tread depth, as the tyre isn’t able to effectively disperse the water, thus leading to a far greater risk of an accident.
Continental also provides a Visual Alignment Indicator (VAI) – a wear indicator integrated on the tread itself. Two indicators on each side should show equal wear, indicating that the wheels are correctly aligned. Unequal wear indicates incorrect wheel alignment which will lead to premature tyre wear and higher fuel consumption.
CAPTION: Sidewall marking tell the story. Graphic: Continental/ Quickpic